Breakthrough for Exomoon Hunting

do planets outside of our solar system exoplanets have moons what might these exo moons be like could they be stranger than the ones that we find in our own solar system could they even host life for over a decade we've been hunting for exomoons using a technique that i helped to develop but then why hasn't it delivered a zoo of exo moons by now i've spent the last couple of years deeply thinking about this and today i'm excited to share with you new research that has just been published that might finally change the game exo moon hunting version 2.0 the study of exo moons is a subject close to my heart and one which my own career has been intertwined with for better or for worse so in this video i'm going to tell you about the history of this subject and my own new paper from a personal perspective now i have to say that to me it's always been strange that everybody working on exoplanets isn't jumping up and down fanatically about this subject i mean we're talking about discovering an entirely new class of world something not seen since the first exoplanet discovery decades ago just in our own solar system moons are remarkably diverse and fascinating from the volcanic tormented surface of io to the plumes of enceladus from the methane lakes of titan to the subsurface ocean of europa just imagine just imagine how outrageously diverse exo moons will be the mind boggles at the possibilities yet it has to be said that exo moons is a field with its back up against the ropes in my own team we have applied for federal funding for a [Music] moon survey multiple times but not once have we succeeded and believe me that's not unusual i know others work on exo moons or in a similar situation constant rejection is a part of modern academia it feels like getting smacked around the back of the head over and over again and sometimes it feels difficult to get back up look research funding is tight across the board but exo moons are not just going to magically happen unless we invest we could accomplish so much with more resources this is in part why we started our own crowdfunding campaign which you too can assist by joining the cool worlders to help change this in the last decade we've been scraping by and around 70 exo plants have been carefully surveyed for exo moons out of a total of more than four thousand just the tip of an iceberg but even so there is one compelling exo moon candidate in the literature a super-sized moon that we discovered called kepler-1625bi as someone who helped develop the very methods that we use to look for exomoons lately i've been wondering whether we're missing something though a piece of the puzzle that could change everything but before we can get into that we need to go back the year is 1999. just a handful of exoplanets have been discovered and none yet transiting their star it was in this year that a pair of french astronomers wrote a theoretical paper that got quietly tucked away in the supplementary portion of a european astronomical journal in it the authors paula sateretti and her advisor john schneider considered a range of different methods by which one might hypothetically detect exomoons including the idea of what would become known as ttvs when i started graduate school in late 2007 there was growing interest in this ttv concept which wasn't just limited to moons ttv stands for transit timing variations which measures the time difference between consecutive transits or more simply eclipses of a distant exoplanet imagine a distant alien world looking at our home star the sun through their telescopes if they had the right alignment to us then one might expect them to see a transit of our home planet the earth in front of the sun once every 365 and a quarter days this should happen like clockwork but the earth's position gets slightly perturbed by the gravitational influence of our moon which means that in fact the aliens would see the earth transit sometimes two and a half minutes earlier than expected and sometimes two and a half minutes later those wobbles which we call ttvs could thus be used by the aliens to detect our moon i was fascinated by this brilliant idea by this pair of french astronomers but as i studied it more carefully i realized that it had two major problems first moons orbit planets too fast let's again think of the earth's moon which completes an orbit once every 27 days around the earth now our imagined alien watching the earth transit from afar would only see those transits once every 365 days but during that time the moon has completed just over 13 revolutions when looking at the earth's ttv wobbles the aliens would be able to tell that the moon's period could be about 1 13th of that of the earth's but they would also be able to see that about half of that number would work equally well or indeed one quarter in fact they'd find hundreds of periods that could all work this happens because the sampling rate the frame rate if you like is once per 365 days which is much slower than the underlying cause which is the moon's motion around the planet technically we call this undersampling and under sampling is not game over we could still tell that there was a moon there potentially but we wouldn't be able to say anything about its orbit or its mass so this is frustrating but not crippling but the second problem is perhaps the real killer it's not just moons that wobble planets other planets do as well so if one sees a ttv how is one supposed to tell if it's due to a moon or another planet especially given that we've already established that ttvs don't even let you figure out a unique solution for the putative moon's properties back in 1999 we didn't know of many exoplanets but already by 2007 it was becoming clear that they were common and multi-planet systems too so it stood to reason that planet planet tvs should actually be abundant and that was bad news for exo moons it was 12 years ago 2008 that as a freshman graduate student i wrote a paper that has since become my most well-cited one in which i described a method to finally overcome these problems the method to finally reveal exomoons the solution struck me one day at my desk at university college london while staring at that original paper by the french team it was then that i realized that if the planet's positions are periodically changing back and forth causing these ttvs then that means that the planet speed must also be changing between transits you can't have one without the other and that in turn means that if the planetary speed is varying back and forth then some transits will last a little bit longer and some shorter transit duration variations or tdvs were born what made this idea so exciting was that it seemingly solved both of these major problems that i mentioned earlier about ttvs let me break that down first a remarkable outcome of the mathematics was that the ratio of the ttv to tdv amplitude happened to yield the orbital period of the moon so that means that even if your signals are under sampled you can't measure the frequencies reliably doesn't matter you can still measure their amplitudes and thus use that to get the orbital period of the moon first problem solved second though even better the unique signature between ttv and tdv could only be explained by exomoons remember that ttvs are caused by changing planetary position its position changes whereas tdvs are caused by changing planetary speed think about those same two properties for a swinging pendulum when the pendulum has its maximum position or offset the speed is zero just for a moment it seems to hang perfectly still in contrast when the pendulum is moving at its fastest the position is zero it's right in the middle and so we can see that position and velocity share this unique intimate relationship and in the same way moon ttv and tdvs have a unique phase signature coming off the back of that paper i remember feeling on top of the world it was a wonderful sensation and the impact and potential of this idea was certainly starting to gain traction more broadly too i remember haphazardly bumbling my way through my first on-camera interviews telling them about this exciting idea so the difference with this is is that the velocity of the planet can also be speeded up and slowed down due to the moon so we use both the position and the velocity of the planet to determine if there's a moon there in the following few years i built upon this by developing software that could model ttvs tdvs and even the transits of the moon itself i have enormous gratitude to my phd advisor at the time dr giovanna tinetti who gave me the latitude to explore my own ideas like this and in those days with nasa's kepler mission on the horizon a technique in hand it felt like the doors would now open for an era of exomoon detection during the remainder of my phd i wrote some more papers refining these ideas but also explored other topics in the field of exoplanets such as atmospheres eccentric planets and even led the discovery of a couple of hot jupiter planets myself and yet as i approached my graduation in 2011 i realized that i had to make a choice because postdoc applications were generally expected to focus on a single scientific goal a fork in the road lay ahead of me much like what has happened to many of us in life at one point or another on the one hand there was searching for exo moons something that truly excited me and i'd helped to invigorate but it was also a field with zero detections and zero guarantee of success i remember many of my peers at the time saying to me david what if you detect no moons after years of trying your career would be over and then they'd gently urge me towards the safer path just follow the crowd and work on topics like detecting hot jupiters look it might not be as exciting but many teams are busy doing this you'll easily find a nice safe secure job did i really want to roll the dice with exo moons gamble my entire career like that i have to say i wondered maybe they were right maybe it was time to stop dreaming of exo moons to pack away those boxes to put childish things behind me and get a serious job like developing incremental improvements to planet hunting methods i got a lot of good advice though from my mentors at the time dr giovantinetti and gaspabakos who encouraged me to pursue that which i really enjoyed and was excited about i realized that i would have to live with this choice for better or for worse picking the exo moon path was one of the most terrifying choices i'd ever made but something about this field it captivated me it lit me up it reminded me of why i fell in love with science as a child i decided that i had to try else i might live my life with a regret that would haunt me throughout my days not knowing what would have happened had i at least tried in 2011 i moved to harvard university as a newly selected carl sagan fellow which just meant all the more to me since the namesake was a personal hero of mine it wasn't easy to leave behind my home the united kingdom but if i was ever going to look for exo moons i needed to be close to the best data in the world and that meant nasa's kepler mission i had a small amount of research funding as part of my sagan fellowship which was supposed to be spent on things like a laptop travel publications and hiring summer students i spent mine heavily on computing building my own personal cluster just dedicated to finding exomoons at this point i was all in i decided that i'd make up the difference for travel expenses through my own salary skip on students and just publish in the cheaper journals all that mattered was finding exomoons as i ran out of money i started a small crowdfunding campaign to purchase another machine to expand the computing capabilities further gasper backhoes had a secret room in the basement of the department that few and knew about and he let me store these loud whirring machines down there with his you know those days were so wild because unlike exoplanets there was no standard recipe or procedure for how to detect an exomoon everything had to be devised on the fly in real time in that first year i put together a team of astronomers to help me look for exo moons and we called ourselves the hunt for exomoons with kepler h e k project i couldn't pay anyone but it didn't matter these amazing people were willing to help me just because they believed in what we were doing i was so excited about what we're trying and maybe you two have experienced that that kind of energy and excitement when you start a new project but i think a good lesson from life is to remember that not everybody will see it as a recent arrival at harvard many of my connections were happening at a somewhat famous regular event called ssp coffee this is a bunch of star and planet focused astronomers sipping coffee each morning and discussing the latest papers i remember during one of those first coffees sort of timidly walking down the corridor towards a corner that then led to where the coffee was and as i walked down the crowd i could hear voices and even laughter of my colleagues in the coffee area i remember feeling excited to get to meet them and make new acquaintances but then as i got closer i could hear that the subject of their laughter was me as they referred to me as the moon guy that crazy british dude who thinks that we're gonna start discovering exo moons okay it wasn't exactly the welcome that i've been hoping for but look i don't resent anybody for laughing me back then because at the end of the day i knew what i was getting myself into when i made this choice i'd been hearing this all along the road you know sometimes you just have to look straight ahead even if it feels very lonely chasing your dream i'll see about a job there what job stock broker stock folk yeah not an astronaut in those first few years we surveyed dozens of exoplanets for moons using a sophisticated algorithm that combined ttvs tdvs and even the transits of the moon itself ttvs are generally the easiest detect of these three but recall that hidden planets can also cause these so we set our detection criteria bar that at least two of these three effects had to be found before we could claim an exo moon the thinking was that let's make sure that our first claims are solid and unambiguous but time after time we came up blank and of course a series of no detections doesn't feel good but the truth was that if exo moons were generally smaller than the earth then we likely wouldn't see them because kepler itself was only designed to find earth-sized worlds part of me was a little bit surprised by this string of neural detections because exoplanets were so diverse that surely weird super-sized moons would be common too now unfortunately the universe wasn't kind enough to grant us with a cosmos full of huge just juicy fat moons to detect but i remained optimistic because hey we'd only still surveyed a few dozen planets at this time just a drop in the ocean this series of no detections actually built confidence in the community that we knew what we were doing we weren't going to jump the gun and claim a signal prematurely we were taking a methodical and careful approach to our search in those first few years i worked round the clock to get the project going to me every time we looked at a new exoplanet there was a chance of striking gold and that excitement drove me to push the project as hard as i could during this time i shared a house with some other international students and would almost always be working over the weekends the holidays and late after work in my bedroom on the project i remember for thanksgiving dinner that year i had a pot of ramen noodles hunched over my laptop most i worked on a new candidate sometimes i'd forget to eat or sleep and so much so that i remember one day one of my housemates found me collapsed in the corridor blacked out from exhaustion and he told me look you need to take a few days off and he was right but i couldn't i wouldn't look i wouldn't expect or recommend anyone anyone to work that way and now i have a much healthier work-life balance but back then i knew it was all on me the string of no detections was definitely not easy to stomach though into about the third or fourth year of the project i remember being at an evening event at harvard with many famous exoplanet astronomers from the local area at one point in the evening a professor and i were discussing our research and i was summarizing our latest exomoon paper that describes some 40 new detections he looked right at me and kind of shook his head with a confused smile and said how do you keep going david how do you keep working on exo moons after so many no detections and i kind of laughed and had a little smile and i looked back at him and said something like look it's simple as long as i keep trying there's a chance of success [Music] flash forward to today a decade of moon hunting there is really just one compelling exomoon candle that has emerged kepler 1625 b i that object is endlessly fascinating and it exhibits two of the three possible detection effects thus satisfying our detection criteria but the dream wasn't just to find a single exomoon but to find many don't get me wrong i'm thrilled that alex ticci and i found that thing but until we have dozens of exomoons it's going to be very hard to really understand if that particular example is real typical or anomalous in much the same way that the first hot jupiter detection was met with widespread skepticism until dozens of them are identified for context one approach is just to scale up what we've already been doing rather than surveying tens of planets why not survey thousands but frankly that's somewhat impossible without a greater investment of resources that historically we've just not seen a big part of the problem is just the sheer computational and human time requirement necessary to scan through billions of possible configurations and try and find a match to the data now we developed this sophisticated algorithm that could account for all of the known effects that exomines caused that's ttvs tdvs and the moon transits he knew about all of those and thus should be the most sensitive moon hunting tool possible it was the most sophisticated it could be but in a way that was kind of the problem it was so sophisticated that it took simply too long for us to run it on each planet at least without current resources what's really striking is that we know of literally hundreds of exoplanets exhibiting ttvs they are everywhere and yet more in our last exo moon paper which we did a video about we showed that the vast majority of these are actually fully consistent with being caused by exomoons but could equally be caused by hidden planets when we started this hunt recall that i set this detection criteria that we had to find at least two independent pieces of evidence for an exo moon before we would claim one and that's actually exactly what happened with kepler-1625 but the thing is for any given hypothetical exomoon it's in general going to be far easier to bag just the ttv effect in isolation rather than this requirement of getting two independent lines of evidence why well consider tdvs first first off tdvs are generally smaller than their ttv counterparts moreover it's a velocity effect so it only gets large when the moons whip around quickly which only happens for close-in moons a bit like a coin falling down a spiral wishing well but when the moon gets close to the planet the ttv effect gets tiny because that is more analogous to the lore of pivots scaling with greater separations and at the same time when the moon is close in its transits get mixed up with a planetary signal making it difficult to separate them okay so no luck there but what about the combination of ttvs with moon transits well again ttvs get larger for widely separated moons but moons widely separated from their planet they only need a small amount of tilt in order to avoid transiting the star most of the time again making it difficult for us to get both effects simultaneously so this is a pretty frustrating situation i think we have hundreds of known planets exhibiting these ttv motions and we can even prove that the vast majority of them are consistent with being caused by exomoons it's just that the moons aren't freakishly big enough to cause two effects which we can detect in the data they might just leave behind one and so they're ambiguous now all of that is perfectly consistent with our observations and indeed physically quite plausible it just means that freakishly large moons aren't very common in a way we've kind of come full circle here we began our story trying to solve an ambiguity if you see a ttv how do you know if it's due to a moon and myself and indeed others thought that ttvs and moon transits would solve this but frankly it was optimistic because it assumed that moons would be so big and the data so good that we would regularly detect two of these three signatures in the absence of that we're just back at ambiguity tdv's and moon transits failed to live up to their promise and i have to live with that i've spent the last two years thinking a lot about this maybe we've spent the last decade working in the wrong direction largely because of me i felt convinced that either tdvs or the moon transits would break this ambiguity that plagues ttvs but it's proven to be optimistic to expect to detect these secondary effects at least with existing data and so i've been reflecting back at those early years wondering could i have missed something maybe there's still a chance maybe if i brush off those old dusty x-ray moon theories just one last time i might see something something staring us right in the face from the very beginning of the story under sampling since 2008 the community has basically thought of under sampling as a pain in the neck a hindrance preventing us from clearly seeing exo moons moons orbit planets many revolutions between each transit and that means it's impossible to uniquely figure out the exomoon period on the face of it that indeed seems like a bad thing so what treasure could possibly be in that trash sometimes inspiration hits you at strange times imagine staring at the passenger side window in a car ride down the highway your eyes fixate on a car in the next lane matching your speed you know that the wheels are spinning forward but they seem to be moving backwards of course your eyes are deceiving you you see your vision has a fixed frame rate of about 70 images per second but the wheels are spinning far faster than that and so to you the wheel's motion is under sampled what you are seeing isn't the wheel's true velocity but rather what's known as an alias now if you're anything like me in that moment your eyes might light up as you realize that exo moons are exactly the same okay so let's work through what this means first if we know the planets period and the moon's period then we can write down an equation for exactly what the aliased ttv period would be takes a bit of math and a lot of coffee but here's the answer that i derived but in reality we don't know the moon's period rather we see the alias period and we know the planet's period and we're trying to figure out the moon's period now trying to rearrange this equation and invert it reveals what we always suspected there just isn't one possible exomoon solution that works there are hundreds right now you're probably thinking david this sounds like we're back to where we started this doesn't really seem to have helped us very much and you're kind of right but now that we have this mathematical understanding of what's really going on we can actually use it to dig deeper so let's imagine conducting an experiment let's simulate a population of a million exomoons out there in the cosmos and let's give each of them a random orbital period around its planet what do i mean by random good question for now let's just assume that they have a uniform distribution in terms of their separation from the planet using the formula mentioned earlier we can take each of these random moons and calculate what their alias ttvs would be it turns out to be this a kind of exponential looking function that piles up at the shortest measurable period also known as the nyquist rate which equals twice the planetary period in fact 50 of all of our simulated moons have a ttv period between two and four planetary periods that's pretty interesting and actually a very narrow range but before we get too caught up in this recall that i assumed a uniform distribution in the exomoon separation we don't really know if this is a reasonable thing to do so let's mix it up let's try alternatives like a uniform in reciprocal period or uniform and log separation or proportional to separation radically different assumptions now here comes something truly amazing are you ready no matter what distribution one assumes one always finds the same spread of ttv periods that is incredible this is potentially hugely exciting it means that half half of all exomes in the entire universe will appear in our data as a ttv in this narrow range a range that i call the exomoon corridor what's truly mind-boggling about this is that that statement is true even though we don't yet know what the distribution of moons in the universe even is yet that is wild if that wasn't exciting enough things get even better let's take the catalog of all cases of proven planet planet ttvs these are systems where both planets fortuitously transit and they exhibit timing variations that let us prove unambiguously they must be planet planet ttvs amongst these the ttv periods hardly ever land in the exo moon corridor in fact only one percent of the time do they fall in this range that's kind of nuts for once the universe seems to be helping us out so as you can probably tell i am very excited about this paper which has just been published and you can find a link down below in the description if you want to read it look before this you see a ttv and nothing but a ttv and you're pretty much stuck you can't tell if it's a moon or a planet or what it is but now if you see a ttv in this narrow range you can immediately elevate it as a high priority target for exome analysis as a little cherry on top we applied this to the xa moon candidate kepler-1625b and indeed find that it has a short ttv cycle almost exactly at the median period expected for exomoons as exciting as this is we are just getting started everyone there is now hope of using ttvs in isolation to assign statistical probabilities to signals being exomoons we might not ever end up with 100 confidences but if we can find a large number of say 80 confidence signals that alone would prove that exo means were abundant which very much mirrors how exoplanets are typically statistically verified these days through a similar approach all of this is of course going to take work but dang this feels like the most exciting methodological development in the hunt for exomoons in a long time i'm hoping we can find federal funding to support this move this forward and i will try my hardest on that front but even if that never happens then still we can move this dream forward through your support support the cool world's team and you're not just supporting a youtube channel you're supporting a cutting-edge research group working on the hunt for exo moons interstellar travel and the search for life in the universe so you know what i'm okay with being called the moon guy and if anyone ever tries to mock your dream don't let them sway you look it might take a month a year or even a decade but as long as you keep trying there is still a chance of success and when it comes to exo moons i promise you that i will push hard on this even if you else will together we can make this happen we can make the dream of exo moons a reality so until the next video stay thoughtful and stay curious [Music] you

Channel: Cool Worlds

Views: 219,225

Keywords: Exomoons, Extrasolar moons, exoplanets, new exoplanet research, new exomoon research, exomoon breakthrough, habitable exomoons, astronomy, space, nasa, nasa kepler, nasa exoplanets, nasa exomoons, kipping, cool worlds, astrophysics

Id: voLPBjqTSao

Length: 34min 40sec (2080 seconds)

Published: Sat Dec 12 2020