How Many "Earth-Like" Planets Are There Really?

[Music] since the dawn of civilization we've looked up to the heavens and wondered how special is the earth does this planet represent the only seat for life the only place where self-aware collections of atoms ponder who and what they are or is the earth just one drop within a larger ocean a universe full of worlds teeming with life for millennia this has been a question limited to abject speculation and philosophical musings but not one of serious scientific inquiry but now in the modern day with all of our technical powers a statistical tools in our telescopic arrays could humanity finally answer a question which has haunted us since our very beginnings how common are earth-like worlds [Music] humanity has wondered how special our planet is since antiquity although many religious schools taught that the earth was unique there have always been opposing views for example the ancient Greek philosopher metrodorus of Chios speculated that the earth was akin to a single grain amongst a field of millet are those ancient schools of thought were really just speculation there was no evidence to point at and use to guide their reasoning it wasn't until the mid 20th century that science finally began to take seriously this question as to how common are planets like our own in 1961 Sir Frank Drake famously reasoned that the number of communicative civilizations in our galaxy could be estimated by multiplying a series of parameters together if we knew the value of each of these parameters then we would be able to calculate a direct answer thanks to decades of astronomical observations the first term in Drake's equation the rate of star formation is now known to a reasonable precision having crossed that term off the list astronomers started to shift their attention to the next term this is the fraction of stars with planets which is then followed by the number of habitable planets per system these two terms are often combined together into a single quantity usually dubbed eita earth the number of habitable planets per star the II to earth term is eminently observable whereas some of the other terms in Drake's equation such as the lifetime of a civilization are far more difficult for us to imagine being able to build an experiment to actually observe but when it comes to eat earth in principle all we have to do is look up to the sky in count up how many earth-like planets do we see and so institutions like NASA have pursued this approach vigorously designing their long-term astronomy strategy to first pin down e to earth and then perhaps eventually go on to tackle the next term in Drake's equation the fraction of habitable planets with life on them this philosophy motivated the construction of the famous Kepler mission a 95 centimeter Space Telescope designed specifically to measure e to Earth the frequency of habitable planets and thus advance Drake's equation by one new parameter Kepler was launched in early 2009 and enjoyed a six-month extension to its primary mission followed by several years in a second life observing different parts of the sky for even more planets make no mistake Kepler was a revolution altogether it observed over half a million stars detected 2700 new exoplanets as well as an additional 3,300 candidate signals still awaiting confirmation today this means that amongst the confirmed exoplanets that we currently know about over two-thirds of them were discovered by this 195 centimeter telescope and yet as hugely successful revolutionary and groundbreaking as this telescope was when it comes to its primary objective to determine the frequency of habitable planets that remains a controversial and undecided issue now that statement might surprise some of you in what way is it controversial you might ask because chances are you may have previously read headlines like this from the Washington Post saying that NASA estimates 1 billion earths in a galaxy alone and when you read that there's no uncertainty in the headline there's no maybe there's no controversy it's presented as if it is a settled issue and no doubts remain and yet why am i sat here insisting to you that this remains a controversial topic after all if you read it in a newspaper headline it has to be true right ok well rather than relying on newspapers let's turn to the peer-reviewed scientific publications on this topic and see what they report instead now Kepler didn't really finish its primary mission until 2013 but that didn't stop astronomers from trying to estimate eater earth this far back is 2011 where we get a value here of about 3% or point zero three habitable planets per star over the next three years you see a series of papers which estimate similar numbers even when the full data set becomes available but since that time the numbers have diverged with E to Earth being reported as high as 124 percent that's more than one earth-like planet her sun-like star so this gives us a range of over a factor of 100 and so you might be asking yourself how could it be the astronomers working with the exact same data set can end up with answers which differ by a factor of 100 as an analogy imagine that I asked these same researchers to go away and count up what fraction of the population are able to dunk a basketball you wouldn't expect them to come back to you with answers which differed by a factor of a hundred and it's not as if one team is getting an answer which disagrees with the rest each team can point to another as an example of someone who approximately agrees with them one possible explanation is that the teams are not agreeing on how to define what a habitable planet really is to come back to our analogy it would mean that they don't agree on the minimum height necessary to dunk a basket some teams might be using six foot three and others might think the minimum is six foot six and so when they make their counts they're actually counting different things and actually the same is true for a habitable planet a habitable planet is usually defined as that which lies within a certain range of orbital separations from the star and has itself a certain range of physical sizes but crucially it's actually very rare for these different research teams to use the same criteria consider first the range of allowed orbital distances for habitable planets often dubbed the habitable zone in the Solar System Venus is thought to be too close to the Sun giving rise to scorching temperatures and a runaway greenhouse effect Mars on the other hand is too distant meaning water is frozen and locked up in polar ice caps and so the Goldilocks earth is clearly habitable but how much could we move it around and keep it that way early work used a wide range of optical distances labeled here by The Associated orbital period in days rocky planets found anywhere between the red and blue lines were classified as habitable and thus would go on to count towards eater earth but leafing through the different publications we encounter at least ten different definitions for this critical zone this situation is then exacerbated by the issue of planetary size to early work considered anything between 80% to twice the size of the earth sufficient to be called a habitable planet but going forward very rarely do we see teams actually use the same size range with now eight different definitions existing in the literature so what is the right habitable range for planetary size and abhor separation what is the true criteria that we should be using well frankly we don't know and likely it's going to be impossible to conclusively give you an answer to that question until we start detecting life on those worlds for instance if I detected life on 1/2 earth-sized planet then I could confidently assert that half Earth size planets are indeed capable of supporting life and this should be classified as habitable but until we get to the point where we can do that we are really just guessing about this question and thus you can unfortunately expect a lot of disagreement about this going forward into the future with all of these different definitions floating around for habitable planets we can now easily understand how it could be that everyone might arrive at different answers for eita Earth but is this really the resolution does this factor completely explain the differences between each team or do they still disagree with each other even after correcting for this effect fortunately there is a way that we can make a fair like-for-like comparison between these different studies and really disentangle what's causing these differences but in order to do that we're gonna have to switch from e to Earth term to now remember that eateth was defined as the frequency of habitable planets around sun-like stars and gamma earth is different gamma earth is more like a rate in fact it is defined as the rate of planets per logarithmic unit of radius per logarithmic unit of period now on paper that looks like a very confusing definition but we can actually easily understand the difference between ETA and gamma earth by considering here another analogy let's imagine changing e to earth from the number of rocky planets within the habitable zone to the number of gas stations we pass in driving from the inner to the outer edge of that same zone this is the number that we are ultimately trying to measure for Drake's equation it's how many opportunities for life are there or here how many opportunities to fill up our cosmic car that we have ok so what does the gamma earth represent in this analogy gamma earth here would be the rate of gas stations along the road so in other words it's the number of gas stations per mile let's say gamma earth was point one as an example so if the habitable zone were say a hundred miles across then we would expect ten gas stations in this region now what's happening in astronomy is that everyone is effectively disagreeing about how long the stretch of this haploids own road really is some teams are using a hundred miles and others are using two hundred miles and so clearly if you double the length of road they're not surprisingly you come across twice as many gas stations or in other words habitable planets along your journey and so even if everybody is disagreeing about what really makes a habit or planet habitable in other words how long that stretch of road we is everybody should agree about the rate of gas stations along the road in other words in gamma earth and so rather than compared the e to earth values it's actually far more useful here to compare the gamma earth values going from 2011 up to this year we can see how this value compares between each study despite comparing the exact same quantity measured from the exact same data set there are still why or differences between the teams in fact the estimates vary by a factor of 140 or together and even after narrowing in on recent years we still see a large spread of a factor of 15 and honestly it is jarring to see some of the world's experts and apply statistical methodologies to astronomical datasets disagreeing with each other using the same data by such a large degree so what's going on how can they be arriving at such different answers let's say that we observed a hundred thousand stars and detected ten habitable planets on the face of it that would seem like each worth should just be the ratio of those two numbers or in other words point zero one percent but actually that's certainly low-balling it as Kepler can only detect planets which are aligned to our line of sight giving rise to an eclipse or transit event and so for every earth-like planet that we see transiting there's around another 100 which don't have the correct alignment and thus we need to adjust for this effect multiplying everything by hundred all of the teams should now agree that e to earth is about one percent now in that imagined example we considered that we had ten habitable planets which would be a decent Hall but in reality that's not what happened Kepler only found one planet that was within 20% of the Earth's size and orbital period that's kepler-452b now one detection is not much to work with it's very uncertain what the true rate really is in fact we would actually say that the number of detections was 1 plus or minus 1 even so we can take that one detection divide it by the hundred thousand stars that we observed and then multiply it by a factor of a hundred to correct for the nonaligned systems and that gives us a rate of about 0.1% but that number is surely too pessimistic because of course Kepler is not a perfect telescope and could have easily missed many earth-like planets hiding amongst those stars cases where the noise just overwhelms the signal in fact it turns out that earth sized planets that the Earth's orbital period are right on the hairy edge of what Kepler is capable of detecting in fact even kepler-452b has come in to scrutiny and criticism recently as perhaps not being a real exoplanet at all let's say that kepler-452b Israel though just for the sake of simplicity let's further imagine that for every real earth-like planet that we detect another 50 are still hiding in the data undetected so that would mean that there was about 50 habitable earths with the right alignment and now we multiply that by about a hundred to correct for other misaligned planets and we would end up with five thousand earths scattered amongst our 100,000 stars or about five percent of the sample but slow down for a second where did that number of 50 even come from how do I know that it's 50 and say not 5 or 500 well that point right there is basically where a lot of the disagreement is coming from if you think that Kepler is a near-perfect telescope then that one habitable earth that we have detected essentially represents the entire sample and thus you would conclude that eita earth is a very small number if however you think that Kepler is very inefficient that it actually misses many many earths despite them really being in the data then you would multiply that number of one habitable planet by some large number and end up with a much more optimistic estimate than eita earth and so really it comes down to how efficient do you think Kepler really is and clearly astronomers right now do not agree on this sometimes astronomers looking at this data try to give themselves a bit of help by assuming that the occurrence rate of planets is a smooth function in period and radius and sometimes they even assert that they know precisely what that function is now critically those are strong assumptions and they tend to overwhelm the constraints presented by that one single true detection and that's because the vast majority of Kepler's 5000 detected planets are shorter period larger planets in essence some of these teams have been taking the population properties of these closer in larger planets and extrapolating them into the window where earth-like planets live and it's deeply unclear what the correct method of extrapolation really is all we can really say is that what you choose for how you do that extrapolation clearly has a very large impact on the final answer this is most apparent when we compare these two estimates for gamma earth these two use the exact same data reduction but just elected to use two different perfectly reasonable methods of extrapolation nevertheless they end up with an answer which differs by a factor of six now none of this disagreement would have likely happened had kepler being a near-perfect telescope in that case it would have detected even smaller planets than the earth and even wider separations than that of the earth in that case I think everybody would have agreed but the fact is that kepler ends up with this fuzziness in its detection ability right at the sort of zone that we most care about for earth-like planets and because that fuzziness is in fact very difficult to quantify we end up with these wildly varying estimates for eita earth I would say that the best way to interpret these numbers right now is that eita earth is conservatively 1% but it could in fact be much higher depending on your assumptions now I know that's not very satisfying but it really is the best we can say right now but bluntly the question of our own world's uniqueness is still very much unresolved now I want to offer two caveats to the story that I've offered you so far the first is that all of the numbers I presented in this video have only been for sun-like stars we haven't talked about M dwarfs at all M dwarfs are by far the most numerous types of stars in the universe they are smaller than our own Sun and in fact it appears as though they have even more rocky planets in their hubzones of their stars than sun-like stars do but we really don't know how hospitable they are for life is in fact a great deal of uncertainty and debate right now about whether these words are even capable of having life on them the second caveat is that headlines like that of the Washington Post's are performing a second extrapolation on top of everything else that I've discussed thus far that is because they tacitly assume that the frequency of earth-like planets is essentially the same across the entire galaxy but it's important to remember that Kepler only looked at one tiny patch of the local galaxy and I am personally deeply uncomfortable with extrapolating that rate out to across say the galactic core or the outer rim of our galaxy now it does seem as though there are indeed many millions of habitable planets on this basis but it might not be numerous in the billions now I appreciate that it might seem frustrating to hear a video like this to help astronomers can't even measure this quantity that maybe you thought was a done deal but it's important to remember that detecting an earth-sized planet is an incredibly challenging technical feat when the Earth passes in front of the Sun for instance it only blocks out 84 parts per million of the brightness of our star and so it took space telescopes lofted up above the a sphere with the most advanced optics and instrumentation that we could muster to even have a chance of detecting these things and although gamma earth is swinging wildly between the different studies right now the same thing actually happened for Hubble's constant which describes the expansion of the universe early estimates were in fact way too high with large uncertainties but over the years the figures were revised and eventually consensus was found and finally one big topic that I deliberately didn't get into in this video is what does it really mean to be an earth-like planet anyway so far we've really just sort of called it something with the same sort of period and size as that of the earth but are there additional criteria about plate tectonics a magnetic field a large moon a nearby gas giant let me know what you think down below in the comments are the criteria for being a truly earth-like planet so thank you so much for watching this video everybody I really appreciate all of your guys time if you like this video then please do do all of the YouTube things make sure you hit subscribe you share this video and like this video and that just lets YouTube know that science videos like this are valuable and should be present on this platform so we really appreciate it if you do that so whether we are adrift in the universe as an isolated single earth-like planet or whether we are just one grain amongst a field of millet our sea wall in the next video stay thoughtful and stay curious you

Channel: Cool Worlds

Views: 158,007

Rating: 4.9177999 out of 5

Keywords: How Many Earths?, Number of Habitable Planets, Habitable Planets, frequency of habitable planets, frequency of earths, how many earths in galaxy, other earths, another earth, earth twins, earth 2.0, Gamma Earth, Eta Earth, Professor Kipping, David Kipping, Columbia University, Cool Worlds Lab, earth-like planet

Id: xEt941k2GAg

Length: 25min 26sec (1526 seconds)

Published: Fri Sep 27 2019