Brown Dwarfs and Free Floating Planets: When You are Just Too Small to be a Star

good evening everyone my name is Andrew frack Noi I'm the astronomy instructor here at Foothill College in Silicon Valley and it's a pleasure for me to welcome everyone here in Smithwick auditorium as well as everyone listening to us or viewing us on the web to this lecture in the 13th annual Silicon Valley astronomy lecture series this series of public lectures is co-sponsored by the Foothill College astronomy program by NASA's Ames Research Center one of the key research centers that nASA has around the country by the SETI Institute the search for extraterrestrial intelligence Institute an organization devoted to understanding more about life in the universe and by the Astronomical Society of the Pacific a venerable organization devoted to the public understanding of science so on behalf of all those sponsors I'm delighted to introduce tonight's speaker one of my favorite people in astronomy professor Gabor basri he has been a professor at the university of california berkeley for more than 30 years teaching not just astronomy but also a course on science fiction with good science in it a topic I'm very interested in as well he is known worldwide as one of the discoverers of brown dwarfs which are the special topic of tonight's discussion and an expert on low mass stars and stars that are too large eyes are too small to succeed and that's going to be his topic tonight professor basri has used telescopes ranging from the giant Keck telescope in Hawaii the telescope's in space like the Hubble Space Telescope and the kepler telescope which is in orbit searching for planets around other stars in addition to his work in science he has long made the promotion of science and underrepresented communities a mission in his life and career and he is now the vice chancellor for equity and inclusion at the University of yeah at Berkeley tonight he will treat us to a discussion of the topic that is uniquely his own brown dwarfs and free-floating planets when you're just too small to be a star ladies and gentlemen Professor keyboard Bosley thank you so much Andy it's a pleasure to be back this is not my first appearance at this particular setting the last time I was here I think I was talking about what is a planet Mike Brown was a student in our department before he killed Pluto and he and I actually disagree about Pluto but we wrote an article together about what the definition of planet should be so we're friends I came into the subject of what a planet should be through the discovery of brown dwarfs so you know planets that definition of planets has two ends to it one of them is when are you too small to be a planet not not the title here and that's where the Pluto controversy came in but the other end of it is when are you too large to be a planet or too massive to be a planet and that was how I got into this this topic but that's not really what I'm going to talk about today today I'm going to try to sort of give you a feel for what has happened since we discovered brown dwarfs in 1995 what have we learned since then now a lot of what we've learned is is really physical I am an astrophysicist so I do like physical learning physical things about objects but that makes it a little bit harder to talk about so I'm going to I'm going to try to that tonight to give you actually a flair for the physical things that we're learning about these these very small objects and also a physical feel for why we don't consider brown dwarfs to be stars actually I'll argue that maybe we should but we'll get to that in a minute I want it to start with this see I should have figured out how to work this laser before I started well I maybe someone will come up and figure it out for me in the meantime in case you can't read the the caption on this this cartoon which appeared in The New Yorker not too long after we discovered brown dwarfs it says face it in this town either you're a star or you're just another brown dwarf and so the question has what why do we distinguish between these two so let me let me start with my kind of physical exposition here and what we what we want to talk about right now is what holds these things up so why are stars the size that they are and the brightness that they are what is it that actually fixes that it turns out that the size of the star and the luminosity of the star depend on how massive the star is and and so why is that and I think the the important thing that I want to talk about right now is is the question of what holes holds it up so what holds up a star is is really heat or thermal pressure more more particularly so the star has to be hot enough to cause the atoms and it's in its whole body to move hard enough to bounce against each other to create pressure and that pressure counteracts gravity and basically what determines how big a star is and how bright it is is when things come into balance so in the center of the star you have nuclear fusion going on the more you squeeze the center the more easy it is to have fusion and the brighter it might get and so if you squeeze it too much it gets to brighten the star gets too hot and it starts expanding out and then if you let that expand out too much it sort of cools off too much and then it doesn't have enough energy being generated and it comes back so stars find some kind of equilibrium based on that so that's how stars work brown dwarfs don't work quite that way and that's why people have talked about them as not really being stars so there's another way to support an object like this objects of this mass and brown dwarfs lie on the mask between sort of about 13 times the mass of Jupiter and about 75 times the mass of Jupiter which is about seven-and-a-half percent the mass of our Sun so these are small compared to our Sun you know less than 10% the mass of the Sun but they're kind of big compared to Jupiter so you know well over 10 times the mass of Jupiter it turns out there's another weird kind of pressure support you can get so electrons are what are called fermions which you don't need to know what that means what it means is though they don't really like each other that much and in particular electrons don't like to be too close to each other and they don't like to be in the same state either and so as you start jamming electrons together in the in the center of an object like like this they start trying to get away from each other basically and that creates a pressure and it's a pressure called Elect free electron degeneracy I don't want to talk about why why it's called degenerate it's not a moral statement but at any rate it's a little bit like you have a you know a cup of electrons and they don't they don't want to be in the bottom of the cup and so as you add electrons they they go up the sides and if you try to push on it they resist they don't want to all get crammed to the bottom of the cup so that's that's the the source of pressure that a brown dwarf has that kind of pressure doesn't actually depend on how hot it is that just depends on how how much you've squeezed it so what happens in stars is they're hot enough but the electrons managed to stay away from each other well enough that they don't get into this state but in brown dwarfs the object gets too dense in the center before it gets hot enough and the electrons then hold it up that way and it doesn't have to generate heat and so this is why brown dwarfs have it's sort of a different character than stars for most of their lives they're not they're under no obligation to generate any heat so they can cool off and what holds them up is free electrons see just to sort of complete the picture planets or people for that matter are really actually also supported by electron degeneracy of a sort you all probably are familiar with the this you know metaphor for an atom I should say where you have the electrons running in orbits around the nucleus what what determines chemistry is that also those electrons don't all all want to be in the ground state either they they're subject to those same rules and so when you have atoms with a bunch of electrons around them the electrons actually need to be stay away from each other and they form shells which are further and further out and that is actually what gives us our volume if you took the electrons and let them sit on top of the nucleus we would all shrink to tiny particles smaller than bacteria so we're almost entirely empty space us as people and our planet as well and that's because electrons don't really like each other but in the case of normal objects like we're familiar with the electrons are bound to nuclei whereas in a brown dwarf they're actually freely floating around so one thing I wanted to point out is in that kind of support if you add more mass to something it gets bigger right if you eat too much you get fatter if you dump a bunch of material onto a planet it gets larger that's not the same case when you have support by free electron degeneracy if I add more stuff on to it that wants to squeeze it harder and in order to for the electrons to resist it it needs to actually get denser so then they feel more more need to stay away from each other to hold up this extra mass so what actually happens for brown dwarfs and also white dwarfs is that if you add mass it actually gets smaller so it turns out that brown dwarfs are really smaller than Jupiter for example even though they have 10 or 20 or 50 times the mass of Jupiter the actual physical object would be smaller than Jupiter so and white dwarves which are basically dead star so our son will eventually become a white dwarf they have even more mass so that might have a whole solar mass in it but it'll be even smaller yet so actually white dwarves are more like the size of the earth and brown dwarfs are kind of the size of Saturn maybe okay the other thing I want to talk about is a basic characteristic of brown dwarfs is what happens on the inside of them it turns out so what this what this graph shows and then you know I'm going to show you various graphs it's not not that I want you to actually understand them I just want them there as a prop while I talk about the basic physics behind it so this graph shows on the vertical axis temperature and on the horizontal axis is actually age the age of the object and it's it's a logarithmic scale so here is 10 million years where it says - - this is a hundred million years and zero there is a billion years so this is actually an evolution of the temperature in the middle of the object as a function of time for objects of different masses so what you can see is four stars which are the blue lines on top the central temperature that is how hot it is in the middle of the star grows with time and the scale there is in millions of degrees so up two three four five six seven million degrees in the center of the star and eventually levels off why does it level off well because of what we were talking about before the star finds the right equilibrium generates just enough energy for its size and its luminosity everything is happy and it stays there and that is really what we mean when we talk about stars we mean an object which is able to find an equilibrium in which it generates all the energy it needs in the middle of it in the nuclear burning core and finds a stable equilibrium and just kind of sits there for some amount of time okay I have a line there labelled hydrogen earning limits oh stars are mostly made of hydrogen and burning here means sort of thermonuclear fusion so once you cross that line going up you are like a hydrogen bomb in the middle you know actually the Sun is like you know hundred million hydrogen bombs going off all the time but has a lot of mass to support so you need a lot of energy and so once you get above that line you have thermonuclear fusion of hydrogen you may notice that the brown dwarfs which are it's kind of hard to see the colors here but it's the middle set of lines a few of them the highest mass brown dwarfs get over that limit briefly but they don't stabilize like stars do they start going back down why do they start going back down because their cores find enough pressure from electron degeneracy they don't need that that fusion and so it actually starts going back out again so that that is how people define the difference between stars and brown dwarfs okay and then below there there's a line called deuterium burning limit so it turns out in the Big Bang the universe was made of hydrogen to begin with because hydrogen is really just protons and then there were a few nuclear reactions in the first few minutes of the universe some protons were put together some of them turned into neutrons and we'll put together and they made what we call heavy hydrogen so that's not just a proton but it's a proton and a neutron there's a little bit of that you know kind of a hundred thousandths as much as there is regular hydrogen there's this deuterium out there also on the earth now you may have heard of heavy water reactors for example those used determine the deuterium burning ulema is much lower in temperature than the hydrogen burning limit because you have a neutron available I'm not going to get into the physics of that let's just say it's lower so you notice all the brown dwarfs are above the detering burning limit that's the sense in which I might say they're really actually stars too they just burn heavy hydrogen except there's not very much of it and then they run out of that and the ones which don't get above the hydrogen burning limit just turn over and the ones which even the ones which do also turn over but you could think of the dwarfs as being momentarily stars and by momentary here I mean a few million years the first few million years of the brown dwarfs life it is actually burning heavy hydrogen as shining partly because of that and then that kind of goes out and after that it fades what's not shown on here is the fate of stars like our Sun they eventually also run out of fuel and as I said form a white dwarf and it begins to fade too so stars also have this fading part of their life it's just that it takes place off this graph so it takes place for most stars and longer it's stars like the Sun and lower in mass for longer than the current age of the universe so it's not shown on here so you can quibble a little bit about whether brown dwarfs are not stars or whether they're just stars that only do the first nuclear burning phase but most people want to call them not stars I'm happy to go with that because then I get credit for having discovered a new kind of astronomical object thus the sub stellar domain okay this is basically what I said so this shows instead of the central temperature of the object it shows how bright they are their luminosity again as a function of time the same timescale on here and you notice the Stars everybody starts as bright as it's going to get that's because at the beginning of their life they're not actually living on nuclear burning they're living on gravitational contraction just the fact of the star squeezing together releases gravitational potential energy that's actually a better energy source than nuclear fusion it turns out and they're actually brighter at the beginning but when they contract to their equilibrium size then of course they stop contracting and that energy source goes away at that point so everybody is moving to fainter luminosity with time but you notice the stars over there on the right the blue lines they find a stable luminosity it's powered by hydrogen fusion and then they live for a length of time which depends on their ass actually depends on inversely on their mass so the less mass of the star is the less fuel it might seem to have the longer it lives this is this is like sustainability in the galaxy so you know if you drive a Prius you don't use up as much energy really as if you drive a Hummer and you can go longer on a tank of gas it's kind of that way with with stars as well so that the low mass stars actually can live for hundreds of billions of years even though they might only have a third or a fourth the mass of the Sun whereas the Sun kind of gives out after about 12 billion years the brown dwarfs you'll notice they're kind of flat at the beginning there the green lines there that's where the determining is happening so they do have a flat part and that's why I say you could call them stars then but then after that they're just cooling off and the red lines at the bottom here are those objects that are below the determine burning limit and I want to call those planets because they don't ever have any fusion at all and they start off bright due to contraction and they they just cool cool cool with time okay so that's that's an introduction to the physical characteristics of these objects a lot of what we have learned about them and what we're interested in is is was that but also what was their atmosphere like in particular as you saw brown dwarfs keep cooling with time they cool into the regime the temperature regime where young planets young giant planets like the young Jupiter the same temperature as a brown dwarf earlier in its career so we're very interested in looking at those things because I'm like planets which are hard to see because they usually found right near stars and stars are usually a lot brighter than the planet and that makes it hard to see it a brown dwarf is usually found out by itself so we can actually look at it and see what the surface is like was the atmosphere look like and I'll talk about some of the really interesting features of that but basically we're learning about also what do young planets look like I also talked a little bit later in the talk about the fact we actually see a few young planets directly now we can compare them to brown dwarfs in the same temperature so so that's a lot of the interest now I'm going to you know get a little more technical on you for a moment but because it's it's kind of interesting actually and and this also shows you why brown dwarfs or sort of planet like in a sense so the first plot on the upper left there is a plot of atmospheric transmission that's our atmosphere the Earth's atmosphere I'm going to talk about spectra so let me just explain what a spectrum is a spectrum it's a little like your radio dial okay you can turn your radio dial and you look you get different frequencies as you turn your dial and you know there's different stations at different frequencies and you can listen to one or the other or the other well light is like that in fact radio and light are actually the same thing it's just the light is higher higher energy higher frequency so what a spectrum means is let me look at some characteristic of either light or the things that absorb light as a function of wavelength or if you like as I turn the dial okay so the top panel there let's see if I can use my cursor can people see that the arrow there alright sorry I didn't bring a laser I should have brought one oops this top panel here shows the atmospheric absorption this is the Earth's atmosphere as a function of wavelength and you don't need to know what wavelength means all you need to know is that this blue band here is what we call the infrared so you can't see that with your eye it is a longer wavelength it turns out then your eye can see but you know if you put on night-vision goggles they work in the infrared and for reasons that I'll explain in a minute this yellow band right here that's what your eye can see so this little segment of this graph is the region in which your eye works that's the visible what we call the visible part of the spectrum and you know not surprisingly I don't know I just keeps going away not surprisingly the absorption in our atmosphere is pretty low there you knew that that's why you can see me right air doesn't absorb a lot of light in the visible part of the spectrum of course it's no accident that our eyes evolved to operate there where the air is pretty transparent so in the infrared you'll notice something kind of funny-looking here there are a bunch of peeks of absorption what does that mean that means if I try to look at this wavelength here the error looks really thick opaque it's like a dense fog you can't see very far in it but if I change wavelengths a little bit if I get off of this this band here and get into here oh now it's very transparent I can see Sivir really well why is it that way well this actually also has to do with climate change and the greenhouse effect and so on if you look at the next panel down this one notice what it says here you probably can't read it it says water vapor water vapor so obviously the Earth's atmosphere has water vapor in it and water vapor has this funny pattern of absorption as a function of wavelength where it has these regions where it absorbs a lot and then if you move in wavelength it suddenly it's transparent and then it absorbs a lot again and so on that and you notice that the next one down here is called carbon dioxide this is the one everybody's all exercised about you know we put more carbon dioxide in the atmosphere we're going to get more of a greenhouse effect what does that mean it means those bands that absorb are going to be stronger well you can see that if we didn't already have water in our atmosphere water would be much more of a concern of ours that you know that's that's a pretty major greenhouse gas there that has lots of big huge blocks that block infrared light block heat and would make our planet quite a bit hotter fact it does make our planet quite a bit hotter carbon dioxide doesn't look that bad by comparison but the answer is we already have the water vapor and so now we're adding carbon dioxide the earth would be frozen if you didn't have that greenhouse effect from water vapor so we're actually too far away from the Sun to have liquid water on our surface if you didn't have a greenhouse effect the earth would be quite frozen I would have stayed frozen its entire life so the greenhouse effect due to water is already taken into account that's what makes this planet comfortable and then it's carbon dioxide that we're worrying about now well what's my point why do I care about that with respect to brown dwarfs well brown dwarfs are cooler than stars so if you look at the bottom here these are the amount of power now the amount of power that a star produces as a function of wavelength again four stars of different temperatures so this yellow one is meant to represent our Sun you notice the power is pretty strong right here at the visible okay again not not exactly an accident yeah maybe a little bit of an accident but at any rate everything works out nicely the Sun puts out lots of power in the visible part of the spectrum the atmosphere is transparent to that and our eyes work there so it's all good as I go to cooler objects this is a thousand degrees this is 600 degrees 300 degrees 300 degrees this is the Kelvin scale this is about the temperature of the earth okay so you you put out power like this in the infrared this is 10 microns which is a unit of wavelength if you like so we're all sitting here shining right now at these wavelengths so if I turn off all the lights it would be dark but dark invisible light if our eyes worked at 10 microns we'd see everybody glowing in fact you're all about you all put out about 100 watts all the time but mostly at those wavelengths brown dwarfs live in sort of this part of the spectrum so they're like a thousand degrees or a couple thousand or the cool ones are a few hundred degrees so they put out most of their power out here well away from the visible that's why for visible light brown dwarfs are really faint they're putting their they're putting all their juice out here they're not putting out hardly anything over here in the visible so we want to observe brown dwarfs you want to look at them with telescopes or whatever you're a lot better off if you can move over to the infrared they're going to be substantially brighter but if I move over to the infrared I've got to deal with those absorption bands as well because I'm if I'm on the ground I'm going to be looking up through our atmosphere on a brown dwarf they're cool enough that water vapor also forms amazingly enough you know it's hot enough that it's steam really there's plenty of steam in the atmosphere of a brown dwarf the steam has the same pattern they're so it's hard for the light of the brown dwarf to come out where those absorption bands are high the brown dwarfs pretty opaque and it's easy for the light to come out in between them so the brown dwarf tends to shine out in the in the holes in the spectrum there which happened to be exactly the same holes in our atmosphere so it's like there are windows in the atmosphere the brown dwarf these are windows and wavelength where the light comes out and they happen to be lined up with the windows in our atmosphere so the light can come down and so the spectrum of a brown dwarf as seen from the earth and as as seen at the brown dwarf even looks like this and hopefully you now understand why it's got this really strange peaked appearance this is what the spectrum of the Sun looks like up there nothing like that Suns too hot it wipes out any molecules so there's no water vapor in the Sun so it doesn't have to deal with that that source of blockage but a brown dwarf can look like this and we can see it just because our atmosphere actually is somewhat similar so water vapor is the controlling thing in both in both instances I don't know if that was comprehensible to this audience probably at least some of you if you want to ask me more about that hold your question and ask it at the end okay so I said brown dwarfs cool off over time so we should expect to see brown dwarfs of different temperature maybe if I didn't point it out also it depends on what mass you are so if you notice the mass scale over here the the most massive star shown is 200 Jupiter masses which the Jupiter's about a thousandth the mass of the Sun so that's that's about point two solar masses 20% the mass of the Sun is the top line there the break between stars and brown dwarfs you see is some are in the 70s so like point as I said 7% of the mass of the Sun is where you go from stars to brown dwarfs but you notice the mass scales going down so if I go to any particular place here in luminosity or it could be in temperature I can find the same place for a higher mass object later so if I drew a line as a vertical line across I mean a horizontal line across I would say let's say I did it here at minus 4 I'd intersect planets first but really young planets then I'd get into the brown dwarfs but they'd be a few hundred million years old and then if you know if I'm high enough I'm then I'll actually hit really low mass stars really you know a billion years or a few billion years into their life all at the same luminosity or all at the same temperature so it's tricky in this domain you really really if you want to understand the object you really need to know what mass it has but also how old is it that's something that doesn't really hold for stars but it definitely holds in the substellar domain so this is a it's a source of confusion even for astronomers so we'll talk about brown dwarfs of different temperatures and they'll tend to think that oh the cooler one is actually lower mass well that would be true if they were all the same age but they're not all the same age so it's not necessarily true anyway we've been talking about spectra so let me just mention that astronomers like to classify stars by their spectra and we call those classifications spectral types and it's a really weird system it doesn't you know it's letters but it doesn't go in order it's OB a FG you know because it doesn't really make sense and it also didn't go far enough to cover brown dwarfs so you know nobody knew brown dwarfs and the coolest star is only so cool and so the spectral sequence ended at M and one of the exciting things and that sequence was was put together in the in the early 20th century and then nothing happened really until nine 95 or so actually was 96 I guess and in fact my group was the first to formally propose that we need another spectral class and the next spectral class would be L right next to em but for no particular reason and so we have L dwarfs which are the next tranche down in temperature from the M stars and then it didn't take long at all another year or two before people started to find even cooler brown dwarfs whose spectra are look different enough that we said alright ok we'll have another spectral type T so then you have the T dwarfs and it was hypothesized there needs to be one more after that because of the molecules that are in these objects so it turns out that I show this here a little bit an M dwarf if you look in the visible part of the spectrum the features you see so you see these these little cliffs in the spectrum and they're labeled with weird really weird molecules so this is vanadium oxide it's a pretty pretty strange one there's not a lot of vanadium in the star but it turns out that vanadium oxide creates one of those absorption features that I was talking about with water earlier and actually influences the spectrum this is titanium oxide so M stars the M spectral class is actually characterized by that it has bizarre molecules with heavy heavy metals and oxide heavy metal oxides is what actually sets the shape of the spectrum this is these are El Dwarfs here you see that doesn't really look anything like that we get this huge feature here and an even huger one out here which which is so so absorptive that it's really hard to see any light in it but we know it's there that one is actually one you're familiar with that you go outside and look at the streetlamps the yellow ones those are sodium lamps sodium it turns out in the Eldar wharfs it is the alkaline atoms that create the spectrum this feature here is not sodium but aciem and these next big things are cesium and rubidium so those of you who know your periodic table will recognize that it's the alkali elements which are creating the spectrum of an elder Worf now why is that well it's really cool the it turns out there are a couple of things that go on these heavy metal oxides actually start glomming together and making heavy heavy metal oxide grains so it's actually getting dusty in the atmosphere of the brown dwarf and so then they don't absorb the light the same way and to kind of disappear from the spectrum they're still there but now there are dusty grains they kind of created just a haze and the thing that does shape the spectrum is now the alkali metals which have very low excitation 'z these are very cool objects so these are the things that show up and then in the tidak warps it gets to be that very peaky thing that I showed you earlier so now it's you know basically water and methane and those kinds of things that set the spectrum and the T dwarfs and people said you know if we get to the last spectral class we're going to start getting ammonia clouds those are like what Jupiter has because we're going to be bette down so cool that we're starting really into even solar system planet temperatures so we one of the big things that's happened since we discovered brown dwarfs we've done a lot of study like this and we've seen what these atmospheres look like and we've characterized them as I show on the right they're down in temperature and what temperature are they so the elder wharfs you can see occupy from about 2200 Kelvin down to about 1500 Kelvin and then there's what's called the LT transition which is a little weird objects sort of are all the same temperature and yet they look different and I'll explain why in a second and then below that is these T dwarfs whose spectra are dominated by water and methane water and methane of course in Jupiter's atmosphere as well so what's going on well especially at the LT transition what's actually going on is clouds clouds are forming we've got these dusty metals in the atmosphere they they form clouds and then some of this some of these other molecules start forming clouds now obviously this picture is not a picture of a brown dwarf it's actually an artist's rendition of Jupiter but they're they're pretty related actually brown dwarfs and Jupiter so we have these really strange clouds these are clouds not made of water droplets or even methane droplets or anything like that they're made of droplets of heavy metal oxides and they'll rain sometimes and you know you might get iron droplets raining out so it's hot it's hot compared to the earth so you know it's a couple thousand degrees but it's really cold for stars so stars don't do this stars don't have clouds on them but brown dwarfs do have clouds on them and and we can see the effects of that and the LT transition is actually a place where the cloudiness is changing which makes the the spectrum look different without changing the temperature very much and so people do the I'm not going to explain these models except to show that this is this is kind of looking at the color of the object one spectral band relative to another if you do it right you you can see things get kind of organized so the the M stars are up here in this color color plot then here are the Elder wharfs and then the T dwarfs head off over this way these blue points are the T dwarfs but theory didn't really expect them to do this and these lines on here are models that people do with these objects in which they include cloudiness and in fact the basically it gets gets cloudier and cloudier or is good you go this way and so people now play this game where they see these points on here and they say well this this things you know somewhat cloudy and this is more cloudy and these are pretty cloudy and you ought to be able to tell maybe if it's got different levels of cloudiness on it maybe there's some weather you could actually see now of course remember these are these are all distant objects we only see a point of light so we always have to analyze them through these slightly indirect methods but isn't this isn't very indirect actually so here on this top plot is the brightness of a particular brown dwarf near the L T transition over time and the it's labeled from 0 to 1 that's the that's the that's the essentially the longitude so from 0 to 1 is a complete turn of the object and it says below it you can't read it says P equals seven point seven days I'm sorry seven point seven hours excuse me seven point seven hours so you may you may or may not know that our giant planets are pretty rapid rotators Jupiter spins in about ten hours Saturn Uranus Neptune they're all in the sort of seven to eleven twelve hour orbital I mean spin period range that means their day lasts for ten hours or seven hours or whatever so another commonality between brown dwarfs and giant planets is brown dwarfs have a tendency to be spinning that fast as well their their spin periods are measured in hours so you can watch during a night and see whether you see any change as the brown dwarf spins once it might spin all the way around in one night of observation and what that plot up there shows you is the brightness varies and it varies in a systematic way so it looks like one side of the brown dwarf is brighter than the other side and then it does it again this picture on the right here is an infrared picture of Jupiter and it shows you what's probably happening why we're seeing this kind of observation so Jupiter as you all know has has clouds too and and it has sort of clear spots so you know it's all there's no solid surface so if you if the clouds clear you see deeper clouds but these objects are organized so that the deeper you go the hotter it looks so on the infrared the bright parts of Jupiter there are places where it's less cloudy I can see further into Jupiter and it's hotter and if it's hotter it's brighter and so that's what those bright bands are so you can look at Jupiter and you can say okay the bright areas are areas where the clot the upper clouds have cleared and I'm seeing down to another lower cloud deck well I'll suppose the other side of Jupiter didn't have that particular cloud pattern on it maybe it was more filled in then that side would be darker than this side or if it was more cleared it would be brighter and as Jupiter spun even if you couldn't see that you would see this and so we do believe that we see weather on the brown dwarfs and then over time like over a few days the the pattern the depth of the dips and so on changes slowly that's that's got to be the weather changing on the brown dwarf so that's that's another very nice way in which they they sort of look like planets in a sense so that over here there's a model of people have done from this data trying to figure out you know what what is the clearing and what's the temperature difference so this is this on this axis is temperature difference and on that axis is the actual temperature of the object so it looks like you know a temperature difference of maybe three or four hundred degrees so you can figure out how much further in you're looking to get that kind of temperature difference how much clearing there must have been and the object itself is you know something like 1200 1200 Kelvin 1200 Kelvin is kind of like what's in your toaster oven we when you're toasting so if you look at these things people used to think they would look like a dull red but as I've explained to you their spectra are significantly more complicated than that the sodium and potassium absorption and so on basically takes out the middle of the spectrum so you have your I would see something at the red end and they would see a little bit something of the blue end and that's why we when we draw them what and they were miss named they probably should have been called purple dwarves so that you know the person who who named them brown dwarves is is Jill tarter who has been the research director at SETI Institute right here for a long time and she was thinking these are cool enough that they'll form dust and she was right about that and then she was thinking well dust is no brown whatever and that's how they got called brown dwarfs but but actually they're they're more like purple dwarves okay so let me tell you a little bit other kinds of things about the brown dwarfs for one thing they're not all alone so brown dwarf binaries are certainly found the upper picture there is a picture I took this is at the Keck telescope I'm about to try to take a spectrum of the brown dwarf it's that let's see I guess I could use the cursor for this one it's this little little bright dot here and that black stripe just below it is the the slit of the spectrograph so I'm about to move it into there and take a spectrum that's how we how we get this kind of data but you notice there's this this big bright whoops there's this big bright thing above it that's an M star so that's a low-mass star it's probably a third the mass of the Sun you see how much brighter it looks than the brown dwarf and it's only probably a thousandths as bright as our Sun is so the brown dwarf is really really faint so that that's one of the reasons they weren't found for a long time they're they're hard to find this is a Hubble Space Telescope picture we took of one of the first brown dwarfs that was an elder Worf that we were looking at and in fact we looked at three 3l Dwarfs to test whether any of them were were had companions and two of them showed up with companions and it turns out the third one had a companion - it was hiding behind so they were - lined up for even Hubble to resolve them but a few years later it moved enough that you began to see both of them also so that's not that uncommon but it's not as common as with stars so what we have learned is that there is something like 20% of the time if you're looking at a brown dwarf you're probably looking at - brown dwarfs that's more like closer to 50% for stars like the Sun okay and the other thing we've learned and this has been learned mostly from our searches for planets around stars you know especially the wobble technique in which you're actually just looking for the planet to be tugging on the star and you're just trying to measure the motion of the star well if it was a brown dwarf there it would pull a lot harder right and you know if it's a one jupiter-mass planet that's not nearly as effective as a xx jupiter-mass brown dwarf so those techniques would have uncovered any brown dwarfs that were orbiting solar type stars you know even at much greater distance than the planets they found and for the most part we don't find those so this is the so-called brown dwarf desert so I don't want you to get the wrong idea though the brown dwarf desert only means there's a kind of lack of brown dwarfs which orbit reasonably close to stars like our Sun so it's not very likely we would have a solar system in which there was a brown dwarf in around the sun-like star but it's not it's much more common that they would be in orbit around a low-mass star or in orbit around each other okay now I want to spend a little bit of time on what has been the most excitement in the brown dwarf game over the last couple of years which is not too surprisingly a NASA mission what's great about a NASA mission is it goes into space where you don't have to worry about this water absorption in our atmosphere anymore okay so what you'd love to do and what they did do is get a get an infrared telescope up in space the telescope itself of course you don't want to be warm because that glows in the infrared so you have to cool the telescope down to sort of liquid helium temperatures and you want and you needed infrared detectors that were big arrays of detectors like the ones you have in your cell phone for a visible light and those have only recently really been been available so that's why this mission didn't happen before but so here's what the wise mission looks like you see it's not that big it's kind of the size of a Volkswagen bus or something and most of it is actually a liquid helium tank to keep the telescope cool and that only lasted for you know less than a couple years but during that time was able to scan the entire sky several times and the idea now you now you're experts on spectroscopy so you can you can get this idea pretty quickly the idea is to go into the infrared so here's one micron and ten microns again remember the earth sort of emits over here so do brown dwarfs that are only a few hundred degrees hot right so let me let me put some put my own filters onto the spacecraft and design them cleverly so I'll put one filter here so it'll pass light in this part of the spectrum that'll measure the brightness of things that are sort of the temperature of the earth or a couple times the temperature of the earth things that are hotter will look even brighter in in this band until until the peak of the wavelength moves too far away then they'll start getting dimmer again over there but the main trick is here let me take one of these big bright bands that come out of the brown dwarf I'll put a filter right on there so that that's a little like our own atmosphere okay so our atmosphere let's threw light at that at that wavelength so does this spacecraft filter but the trick is now I can afford to put another filter right next to it where the Earth's atmosphere totally knocks out the signal but I'm in space so I can measure the cig right adjacent here where it you know brown dwarf also knocks out that signal okay great though if I ratio these two filters I can detect all the objects that have a really big peak here because I know that it's not right right here but it is bright here okay where's on the earth I wouldn't have any idea what what happened here because like I can't see anything at that wavelength so that's the that's the essential trick of the the wise mission it's not that it's a wise thing to do although it is a wise thing to do but it stands for wide field Infrared Survey Explorer okay and that mission flew in the last few years it actually had another band out at 22 microns for even cooler stuff but that wasn't brown dwarf science okay so here now I we've talked about spectral type a little bit so here the M stars here the L dwarfs here the T dwarfs and here's why all the way over here so this is this is really a temperature scale going from warmer to cooler except that on a stellar scale you know the stars are all out here so we're into the sub stellar temperature range here and this is that ratio of those two clever filters the the ones that find the peak okay and so this is what you find you measure that ratio and that ratio changes as a function of spectral type and these are all the the black ones are all the brown dwarfs that Weiss found the purple ones are ones that had been found earlier so you can see why is it sensitive to these really cool things that we really were having a lot of trouble finding before and the other thing is wise is an all site survey so you're going to find everything that's at least bright enough for it to see that has these really strange colors and pick out especially the the really cool down to the why why spectral range and so this is how you do it these are false-color little postage stamps out of the out of the all-sky picture right and you just adjust the colors so that when the when the filters have the ratio that you want it looks green and when they don't you know it looks blue or white or red or whatever and you so you see a green dot at the beginning at the middle of each of these posters stamps those were all white dwarfs so you know it's not not that quite that simple but of course you can so you can think of it I if I adjust the wise data properly and produce an image of the sky I just have to find all the green dots you know that's the basic idea now then you can go back and study those objects with other instruments so for example this is a detailed spectrum of a wide Wharf and a really cool tea dwarf the difference isn't that big so the the white dwarf is the green and the tea dwarf is the red you notice the red is taller than the green there but a little less tall over here that's the kind of subtle stuff that you know astrophysicists make their business on and then you can go to two models and say well why is why did that ratio change and you'll notice that there are some labels here so this is methane so we know there's a methane absorption there and a methane absorption here and a water absorption that's kind of there also so you'll notice the the brown dwarf doesn't get much light out in that part of the spectrum and then it starts to to transmit again but then there's another methane band here and the ratios of those bands and the intensities of them depend on temperature and so that's how we can take a spectrum like this and turn it into physical properties of the object itself at any rate the the first y dwarfs were then discovered by by wise so here's you know here a couple record setters this is the cold snown brown dwarf this is a picture with the mighty kick okay in the in the infrared so that you try to make it bright the red circle you may not be able to see it the red circle around a little tiny black dot that's the wide Worf these other objects the blight is it's a negative so the black things or other objects in there so it's incredibly faint even with Keck even in the infrared and you probably never would have found it if Weiss had located it but this thing is pretty cold okay this is as a temperature and so we have to model the spectrum so this this red line here is a model spectrum this thing is way too faint to actually take a spectrum like that all we get we get the brightness of it at a few colors here and then try to see which one sort of fits the model best that's so the black dots are are the actual measurements that those wavelengths and the the other two colors are what you would turn the model into so it's a bit of a tricky business still and people aren't super confident what the temperature is but it's something like 350 Kelvin well what's the temperature of the earth the temp there in Kelvin the temperature of the earth is 270 Kelvin so this thing is not a whole lot hotter than the earth is but it's a it's a free-floating object and now getting back to the beginning of the lecture is it a planet or is it a brown dwarf well that depends on how old it is which we don't know okay so if it's young enough it's a planet and if it's too old it's a brown dwarf but either way it's really cool really cold list and they actually have measured the parallax of it already so the parallax is basically the motion the apparent motion of that object in the sky caused by the orbit of the earth going around the Sun so it's 11 parsecs away which is like 35 light years away something like that but then we also have the closest round dwarf which actually turns out not to be one brown dwarf but - this is quite recent this was announced earlier this year also discovered by Weiss and then followed up by ground-based telescopes this one's only six and a half light-years away so those of you who know this stuff will know that the very closest star to us is Proxima Centauri which is an M star which is about 4 light years away so this isn't much further away it's the third closest object currently known to our solar system it is not a t dwarf or a white dwarf it's actually an L 8 and T 1 this is this is a wise image of it Weiss doesn't have a great spatial resolution because it's operating in the far infrared but then you can take a Keck picture and actually in this case you see two like that picture I showed you earlier so it's 2 - brown dwarfs temperatures of sort of 1,300 and 1200 Kelvin again don't know how massive they are because we don't know how old this thing is but best guess based on how it's moving through the galaxy and so on something like 50 and 60 Jupiter masses for those those two objects their their apparent separation on the sky we do aren't actually already have a parallax for it so that's how we know it's six and a half light-years away so those are only about three au apart so that's like the asteroid belt and the Sun something like that that's how far apart they are and their orbital period is something like 20 years and the the South African Large Telescope actually took the first resolved spectrum of them so that's how we know the spectral types so those are the two spectra down there you notice this this big potassium absorption we talked about earlier so that that's an L Dorf and here you can see the two separate components so they got them both so everybody's pretty excited about this thing right now it was unclear whether the very closest thing to the Sun would be a star or a brown dwarf you know the way we're thinking now that they're probably about a few percent as many brown dwarfs as there are stars so that would mean the odds are that it closest thing as a star but that wasn't known 10 years ago and so and this one's close so so I you know on the other hand I think Weis probably has found all the clothes brown dwarfs I don't think everyone is the date is not fully reduced but that this might be it this might be the very closest brown dwarf to us okay and then I did also want to talk about what I would call free-floating planets briefly now if you want to see free-floating planets you want to catch them young right we saw that the younger you catch them the brighter they are and the hotter they are so the best place to look is where stars are just being born now and so that you know people will recognize the Horsehead Nebula this is what's called the Sigma orianna's star formation region this has been one of the one of the happiest hunting grounds for what we would call free-floating planets so you know very red objects although you can never trust colors on a astronomical picture but they they actually are very red and since we know the age of the stars in this very young cluster we we presume that planets of the same age we can infer a mass based on their luminosity and these things are less than the detering burning limit so they are sort of ten Jupiter masses a two Peter masses six Jupiter masses so it looks like the same cluster and conditions that form stars work all the way down into the brown dwarf realm and even beyond into what we would call free-floating planets now whether those planets were born free or they were they were escapees from a planetary system is still a bit of a question so I do want to also show you a really exciting system that has been discovered in the last few years this really Wiggy looking thing here is a star but it's been blocked by an instrumental it's called a coronagraph so you try to block off all the light from the star you don't quite succeed but you get rid of almost all of it leaving this weird thing here but now you see and this is an infrared picture one two three four objects which are very red very faint and we kind of know the age of that star actually and so you can infer the masses of these these things and three of them are in the sub determine burning realm so I would call them bonafide planets and the fourth one B there is like twenty Jupiter's well this is throwing people for fits okay first of all though it's I mean it's nice they you can observe them obviously you can take a spectrum of each one that's been done quite recently as well you can't really see the comparison spectra on here but they're there of spectral type L and T so it's what I said earlier the the brown dwarfs and the planets will pass through the same temperature ranges and young one young planets will look like same as brown dwarfs but they're not quite in the same place on that plot there those green dots there and that's another one of these cloudy diagrams but where you sit on that diagram also depends on the surface gravity of the object so if these are planets they're not going to have as high a surface gravity as brown dwarfs of the same temperature and so they they move a little bit over as predicted by theory and they look relatively less cloudy but that hasn't been tested yet but what's really weird about this system is you've got these four objects and in fact we've been observing them while if you can begin to see them move so they're in orbit around the star so that looks like a planetary system thing is these are 100 au and more a from the star and all the theories of planet formation say it's really hard to build a big planet that far from the star never mind four of them one of which is a brown dwarf even bigger and so I would say right now this is a theoretical conundrum for for planet formation folks or star formation folks we never see we see quadruple star systems but they're never like this they're always two stars are close to each other two stars are close to each other and those two they're going around each other like that so it's a hierarchical quadruple this one is definitely there's a massive star in the center and four little guys going around it so it's a raid like a planetary system but at least one of those is definitely a brown dwarf and you know in my article with Mike Brown we we talked about the fact that we'd live we'd love to classify things cleanly into how they're made and so on but nature may not cooperate with us and I would say probably didn't there well I've talked about the that one so so how do you make them anyway well our classic picture of planet formation is you when you make when you make a store you always end up with a disc around it and that we see that observational E and the disc it turns out I mean it's turning out that the Kepler results you're telling us the same thing once you get a disc you almost certainly get planets that is why we don't know all the details of planet formation it looks pretty good at this point to say that if you've got a disc you probably have planets but how do you get really big planets there are two competing theories one is you well you make little planets and then you keep piling stuff on until you get a big planet that's sort of a classic planet formation theory the other possibility is the disk has either some kind of instability in it or maybe a vortex or something in which material ends up being collected in one place pretty rapidly and you can build up a you know jupiter-mass object that way so that's called gravitational collapse these are computer models over here of things forming this one forming in the classical way so it got built up from smaller things but now it's eating its getting material out of the disk they tend to open up a gap in the disk but that's not really stable and stuff flows in across the gap and to streams and actually I've studied a t-tauri store where we see that happening on a stellar scale so it definitely happens in the upper plot what happens is they did they just made a disk which was sort of substantive enough that it was pretty unstable and it basically just collapses and makes a bunch of reasonably high mass objects but as a physicists are not settled on these models people argue vociferously about whether that upper model has unrealistic conditions or not or they did the computer simulation right so that's kind of where we are at the moment meanwhile nature is you know giving us the truth to that we have to test against the general population brown dwarfs probably forms the same way of stars and this is a computer simulation again of a interstellar cloud with turbulence in it that's dense enough that it goes gravitationally unstable in places where the turbulence just happens to to pile enough stuff up and all those white dots in there are stars or brown dwarfs that have formed out of this cloud due to turbulence and you can see material still flowing in along filaments and it then it gets into this dense region and starts making stars or brown dwarfs and they get they get kind of the right fraction of brown dwarfs out of models like this for those of you who've seen pictures of the universe making galaxies this might look similar and it is it's a it's it's the same kind of process basically and then the other the other consideration with brown dwarfs is maybe they maybe they just get kicked around by stars and they that's why they don't end up as real stars so so those same models which which produce stars and brown dwarfs and when you look in detail what happens is you get the star forming region you can actually see two two stars forming at the center of that disk there and they trigger further instabilities and another object forms and then what happens is you end up with a couple of pairs there and one or two objects get thrown out as all this complicated orbital dynamic dynamics goes on you can't see this very well but this is the orbital tracks of objects in those simulations so it's a mess right stuffs coming in forming interacting with other stuff and brown dwarfs or I should say the smallest objects are the ones most subject to ejection and what some people are arguing about is maybe what happens is you're starting to make a star and then that thing gets ejected before it's able to collect up it's it's due and material and it would have been a star but it got thrown out too early that's why it's a ground to work the other possibilities you just make brown dwarfs and then as they interact with the multiple stars near them they end up being the ones thrown out just because they're they're easier to push around essentially so maybe you know maybe they're it's just because they're wimps and so then they stay wimps like that okay so that's that's a little tour through the the things that we're learning about substellar objects since they were discovered so in less than 20 years since I mean these objects were hypothetical in 1994 we've seen all the way down to planet like temperatures even almost earth-like temperatures at this point we've seen objects being born we've seen objects that have cooled over the edge of the galaxy there's a lot of really interesting stuff happening right now about clouds on brown dwarfs and weather and and and actually the difference between L dwarves and T dwarfs might have partly to do with the sky clearing so you'd have cloud L dwarfs or cloudy and T G works or not not cloudy and we see them as companions to stars to each other and to nobody and this is kind of where we're going for the next few years so cloud formation it turns out even on the earth is not all that well understood that's one of the headaches in these climate climate change models is people don't actually know how to model clouds that well so maybe we'll learn something here that will help us with cloud modeling and you know how that depends on mass and temperature and so on and then the white dwarfs we really have just begun to scratch the surface in the last few months so there's a lot a lot left to do on the Y dwarf thing and finally as I was talking about at the end there are a lot of connections between brown dwarfs and planets but you know what are they exactly are they related to formation or not you know our our the atmosphere is different when they're low gravity and high gravity all that kind of stuff so it's still a very exciting field a lot of people are in it now and the Weiss catalog is has you know been sort of just released so you can expect a lot more results over the next few years thank yes you mentioned the mass and the size of the brown dwarfs and from that I can understand that the gravity at the surface would be quite high not maybe like the black hole but quite high so I would assume that anything that falls on the surface is flat and immediately in the surface is smooth and moreover all those clouds should just fall immediately and be flattened and there shouldn't be any clouds in fact so I don't understand okay thank you for that question was the surface gravity must be pretty high on brown dwarfs because they're so dense and condensed and that's true surface gravity is very high I guess what I neglected to to make clear is like giant planets they aren't solid at all there is no solid surface it's just a gaseous body and the clouds form at different layers like they do on Jupiter Matthew remember Jupiter has no surface either it's just clouds all the way down until it's not really clouds anymore it's just a dense hotter and hotter plasma so so same with brown dwarfs except they're they're hotter than Jupiter so clouds form where clouds form I mean the only ones we can see are the ones far out enough for us to see them but you know there's stuff going on below that so that there's an optical surface below which you can't see but that's not a physical surface but I think that takes care of your your questions about things getting flattened also on the left you talked about some pretty heavy elements vanadium and things like that in and there isn't a lot of fusion going on in these brown dwarfs so where did that come from and if a brown dwarf didn't start with any of that stuff what what would it be like yeah well that's so where it came from is the interstellar medium and where that came from was supernova explosions so the the vanadium titanium and so on were products of a prior supernova explosion which was then floating in the interstellar medium and got incorporated into the brown dwarf and you know if it seems weird to you I should point out that if you look at the spectrum of our Sun almost all the spectral lines you see are due to iron and yet the Sun is not made of iron it's just that these heavy elements have a lot of spectral features which show up and so they dominate the spectrum and yet they're a tiny constituent of the body itself so like the Sun the brown dwarf is almost entirely hydrogen but the spectrum is you know a different matter and then you asked well what if the brown dwarf didn't have them and people do do that because presumably when brown dwarfs were being formed right at the time when stars were first being formed there hadn't been time enough to pollute the interstellar medium with all these heavy elements in that case what happens is you don't have all these atmospheric blockers that I talked about so you see deeper in basically so the object ends up looking smaller and hotter and the spectrum is much simpler but it's still the same object basically but it would it would have a hotter spectrum that was simpler yeah my question is related to that in in the in the model you had where they're forming in a star forming cloud you would I would expect the same elements to be in that cloud and you know in the in the development of a star versus development of a brown dwarf what happens to the elements it is that a difference in their composition no they're all made the same so so this is something it took astrophysicists a while to figure out so you take the same star you turn up the temperature to ten thousand degrees and you get an a star spectrum all you see is hydrogen lines so there you can say oh this thing's made of hydrogen you turn it down to five thousand degrees and now you see a lot of iron lines okay but it's the same object it's just that the hydrogen lines are are masked by this absorption of iron which was there before but actually the thing was too hot the iron was ionized and the hydrogen was was nice and strong now the hydrogen has mostly drained to the ground state because it's cooler so that's not giving you big lines and the irons present so and then as you keep cooling you start getting molecules forming in the in the atmosphere so and the M stars is the first place where molecules actually start dominating what it looks like so you get vanadium oxide and titanium oxide those are as I say em there's hardly any vanadium or titanium in the thing but it forms these oxides and they blanket the spectrum with absorption lines even though there's hardly any of it it is more opaque than all the real stuff and so it's kind of sitting in a layer above it hiding what's really there but the composition is the same in all cases right they're all made from the same interstellar medium yeah hi one thing you said that that puzzled me which is I know that small stars are much more common than large stars and from all the talks I've been going to from Kepler results small planets are looking much more common than big planets and planets are looking more common than stars but you're saying that brown dwarfs are less common than M class cars so there's this dip and I'm wondering if maybe just we haven't detected them or if there's a theoretical basis is the universe old enough for closely formed pairs to have spun down to become M stars or what no that's a really good question so the difference between star the frequency of stars the frequency of planets is likely a formation difference so you don't make planets the same way as you make stars for the most part although there's probably some overlap of that which is what I was talking about but for the most part the distribution of planet sizes is just divorced from the distribution of star sizes because they're made differently and in fact they're made around stars but the distribution of star sizes is peaked so the as you said the small stars are more common than the the massive stars about a third of a solar mass looks to be the peak of the format of the mass distribution and after as you go smaller than that it goes down again and actually it turns out there's a very nice theoretical argument for that turn turns out if you if you say that I need five distinct ingredients to do star information and I don't need to say what they are even and each of those ingredients has a sort of bell curve distribution to it you know but the five are all necessary to there's then the central limit theorem yields a mass distribution which is what's called log normal which which is basically like a bell curve but in logarithmic space so that'll have a peak you know now where the peak is well that depends on what the actual processes are and how they're distributed so it's a third of a solar mass for for whatever reason but it falls off on both sides of that and so that's why the brown dwarfs are not as not as common as the Stars thank you sir my question sort of feeds into that I mean I understand from globular clusters that we found some planet inside globular clusters that were formed in a different processes that you know the sweeping out process but it's coagulation of material so I was wondering if you could expand on that how do you what is the terminology would use for the formation of a planet inside a globular cluster and brown dwarfs if it's not a sweeping out process do you have a name for it and then yeah I guess I'm just happy with that well so so we're hope I gave you a little skepticism about whether we really know how planets are formed or how these these objects are formed when we see them around stars like the system I showed you is basically a puzzle but but they're two fundamental mechanisms that I didn't fully name one is called core accretion in which you start with a core and you start putting stuff on top of it and the other ones called gravitational collapse or gravitational instability although it might be fluid instability at anyway those are those are the two basic processes and so I don't think anybody really knows the the globular cluster planets which are I think you're talking about ones that are still orbiting stars yeah so so the difference between globular clusters and today's our information mechanism or places is there's there there are a lot less heavy elements in globular clusters they were formed much much earlier in our galactic history so it's possible that the that the gravitational collapse mechanism was favored at that time and maybe you mostly got bigger planets if you got them at all whereas now it's easy to to make lots of small things and then try to collect them together because you have more heavy elements to work with but we don't know that you know every time planet formation folks have predicted something the observations have contradicted them basically and so I I'm not a theorist but I don't trust and I think it's great you know to make predictions and then the observers can go see if it's right and then the theorists always adjust the theory to match that but then something else comes up so that's kind of where we are with with formation theory yeah you mentioned that there are dwarfs that are approaching earth-like temperatures and we know that there are likely that be planets that also have earth-like temperatures but the planet is going to be close to it the Sun and we'll have a lot of higher energy wavelengths hitting it right and presumably would occur in the dwarf and I wonder if there's any speculation with respect to the friendliness to creating life under those two quite different conditions yeah we need the high-energy wavelengths coming in as part of the process I haven't actually thought about life on brown dwarfs but as Andy knows you know I'm a really open thinkers ought to think about it but but of course they have the brown dwarfs temperature it has cooled to that point and it's still generating all that heat internally the earth's temperature is coming from the Sun basically so that's fundamentally different and of course the groundwork has no solid surface and whereas the des so there's been a lot of speculation about whether Jupiter for example might have life on it we know that Jupiter as you go in you know that the surface of Jupiter the outer layer is pretty cold because it's 5 au from the Sun but as you go in you reach a layer where it's exactly room-temperature and there's plenty of water vapor and we see a lot of organic molecules floating around and people have certainly speculated whether you can form life if you don't have any solid surface anywhere in other words the problem is that that these things are convecting so and you know you're at this really lovely conditions but then you know a downdraft pulls you into super high pressure and temperature so even the cool ones are not no solid surface yeah no no brown dwarf has a solid surface okay that's a that's an absolute statement I could it makes no solid surfaces okay thanks right yeah I guess with my mental model that temperature says you're at a temperature where this is a gas and you get cooler and it goes to liquid as we're talking about these gases no surface and just gas for do they eventually cool to the point that they do start to go through phase transitions and collapse or does it stay to be clouds of gas all the way home and then I have a I guess the second question is I'm thinking about the the right side of that spectral temperature graph here showing if something is the temperature of a planet and maybe larger than we traditionally think of a planet are we saying it's a ground or because it once was a brown dwarf or because it's that big I'm using a physical definition for brown dwarf which just is an object which is massive enough to have determine burning but not so massive that it can have stable hydrogen burning so that that just sets a massless oh it's under 13 Jupiter masses I'm going to call it something other than the brown dwarf and if it's over 75 Jupiter masses is going to be a star so that's the definition I'm using in terms of liquids I mean yes as things cool the most refractory things condense first and that's what's happening in the the m-dwarf 2l dwarf transition so the titanium droplets and the iron droplets and so on they're they're becoming liquid but not not a pool just droplets basically making clouds and then as you get cooler other molecules form and then they form droplets and so on by the time you get down to the coolest brown dwarf you've got ammonium clouds and water clouds and you know stuff we're more familiar with if you keep keep cooling that I mean the the lowest mass brown dwarf is 13 Jupiter masses the age of the galaxy is 10 billion years so there is a limit to the coolest possible brown dwarf and that wouldn't be a lot cooler than the one that we found basically so that's kind of where that's that's as cold as you could have gotten right now but if you you know keep waiting for another 50 billion years then it'll be you know as cold as Jupiter maybe and but Jupiter's still not doesn't have oceans or you know it's still clouds and stuff I mean people planets like Uranus and Neptune probably have you know a significant fraction of the planetary radius is occupied by something you might want to call an ocean it's it's you know a lot of water and but it's under such high pressures that it's not clear you really would want to call that a liquid exactly so we don't have good words for you know the interior of Jupiter's is metallic hydrogen is it solid liquid crisp you know we don't our words that we use don't really apply to states of matter like the in the interiors of these things yeah as I understand it that the brown dwarfs are super compacted gravitationally collapsed degenerate matter yeah it's least in the inside yeah well it's hard to see this as a potential planet since planets are not made of collapsed matter I'm just asking you whether you think that there's an object which at the end up at stellar life does not collapse into super dense bodies and we may normal matter such as we know here on earth no I mean we know what happens to stars at the end of their lives so stars which are less massive than about sort of eight times the mass of our Sun end up as white dwarves now you could argue about whether a white dwarf ends up being a crystalline structure they do you know in theory in theory anyway so they actually think it can end up a stellar sized diamond in a sense so that the material in a white dwarf is carbon and oxygen so the carbon is compressed far harder than diamonds on the earth and and that could have a solid surface at some point neutron stars which is what you get sort of between eight solar masses and some number like twenty or something like that people do they talk about the crystalline structure of a neutron star or the crystalline surface and these dark quakes that we see on neutron stars do imply that the surface cracks basically as the neutron star contracts so I you could call those solid surfaces although the gravity is you know ten to the eight times higher than then on the earth so so the gentleman who asked about things getting flattened would have a really good point if he was talking about white dwarves or neutron stars you put anything there it'll get flattened out into a month monomolecular layer and then above 20 solar masses or so you get a black hole which doesn't have a solid surface and that's what you get so it's either white dwarves neutron stars or black holes those are those are the stellar products and brown dwarfs are really like white doors but they're much lower in mass which means they're bigger and they're not going to crystallize nearly as quickly so you wouldn't have any in the current current universe and you probably have to wait you know a trillion years before we want to talk about it yeah I have a question about the electron degeneracy the picture that I'm getting from you is that we have electrons that are filled from the lowest energy levels up to up to whatever is needed all right so there so we have levels that are filled up to let's say well this would be like a very conductive object we fill up to a Fermi level beyond that you'd have free electrons on the outside and if the thing is rotating so you can have an extraordinarily a very high magnetic field that's generated by a very rotating highly conductive object well it's a little trickier than that so first of all the electrons don't know about the state of every electron and the whole object they know about the electrons within some sphere of influence but they do they do get relativistic in the in the interior so so they have the the ones that are you know squeezed down of all the lower energy states are squeezed up to high enough energy states that they end up moving relativistically but that's happening in a certain domain and then it's you know it's happening everywhere but the but it's not as though you have to have an electron which is at a higher state than any electron in the whole ground or if it's just any electron that's within its sphere of influence basically and that's happening on the interior but as you get high enough in the and more I should say as you go far out enough and on the brown dwarf the gravity is getting less and the temperature is getting less and you get back into normal states of matter at some point so then then you just have regular free electrons they're not degenerate and so it's more like Jupiter as you get towards the outside of the brown dwarf and of course the parts we're looking at our really normal matter so we have water vapor and stuff like that so that's that's nothing like a degenerate gas so it's it's really deep in the interior where this degeneracy pressure operates but that's the place where you would get nuclear fusion and it's not needed because the electron degeneracy provides enough pressure already well if they're degenerate down there then wouldn't they also be it would have to be very very cold as well well it depends on what you mean by cold you throw it in Amex sense sorry in a thermodynamic sense they're not moving there is well they're moving now they have to move as I said they move but the high energy ones move at relativistic speeds electrons won't sit still in that situation you know just the ground state electron even that won't sit still but the ones that are higher energy their higher energy in part means they're going to be moving faster so they're but they're forced to move faster not because I add heat but just because there's so many damn electrons around them you know it's the the quantum effect of having a lot of fermions in a small space that makes them move so fast it's a strange effect it's not something that normal life has anything to do with but it but we know it happens so it's it's pretty weird but that's what that's what makes them work and that's why they're called substellar objects because stars stars interiors are not like that they're thermal thank you and let's thank dr. Bob thank you excellent we'll see everyone in May drive carefully

Channel: undefined

Views: 80,743

Rating: 4.59129 out of 5

Keywords: astronomy, science, astrophysics, stars, planets, Gibor Basri, Basri, free lectures, infrared astronomy, definition of a planet, exoplanets, stellar evolution, Star

Id: zXCDsb4n4KU

Length: 92min 53sec (5573 seconds)

Published: Sun Jun 02 2013

Reddit Comments

[u/alllie]

I wonder who's downvoting this without watching it. Cause it's pretty long.

Hoped I'm not being brigaded.

[u/anju0730]

Thank you for this! In fact the entire play list seems amazing! Will definitely watch them all :)

[u/alllie]

Dr. Gibor Basri (University of California, Berkeley) The least massive star is six times heavier than the most massive known planet. In between is the realm of the mysterious "brown dwarfs." The first of these was discovered only in 1995, the same year astronomers found the first planet beyond our solar system. Since then we have found hundreds of each, and new techniques are giving us even more power to probe the properties of these enigmatic bodies. Dr. Basri, one of the discoverers of brown dwarfs, summarizes the progress we have made in understanding the domain of cosmic objects that don't qualify as stars.

Brown dwarfs have weather!