Impossible Moons

[Music] in the last 25 years we've discovered over 4000 exoplanets planets orbiting other stars new exoplanet discovery x' have become normal even routine so much so that they rarely even make the news anymore and yet as wonderful as these discoveries are planets are just one part of the equation that describes a solar system for example in our own solar system there are eight planets but a staggering 205 moons around those eight worlds in fact if you add up the surface area of all of those moons it's almost three times greater than the land area of the earth an enormous amount of interplanetary real-estate for our descendants to explore and these moons are truly fascinating from the volcanic flows of Io to the plumes of Enceladus from the liquid ethan lakes of Titan to the subsurface ocean of Europa these moons have a unique ability to excite our imagination and spark scientific curiosity and so it's tantalizing to dream about possible moons around the 4000 exoplanets discovered thus far what treasures they must hold how strange and wonderful many of them surely are how spectacular the secrets of these small worlds could be yet these dreams remain firmly hidden from realization because despite a decade of effort trying to look for these things we still don't have a catalogue of X image they have been persistently elusive in fact today we only know of just one EXO moon candidate an object that we found in my team called Kepler 1625 be I to be real with you I sometimes feel like I'm somehow personally responsible for this failure at least in part a decade ago I wrote my PhD thesis on the topic of EXO means where I devised methods to detect them in future missions and the idea was really quite simple if a planet has a moon around it and that moon should gravitationally tug on the planet and cause it to wobble about in space for planets which eclipse their star so-called transiting planets these wobbles will cause the timing of these transit events to apparently very so for example if an alien watched the earth transit in front of the Sun the transit would be expected to occur once every 365 days but the actually causes the earth to sometimes transit two and a half minutes too early and other times two and a half minutes too late and so using those variations an alien could figure out that the earth has a moon these transit timing variations or TT these emerged as an exciting path forward for detecting exomoons yet more these wobbling planets not only cause tea TVs but also the wobbles caused the speed of the plan to change and thus the duration of the transits to vary giving us transit duration variations or T D these so after finishing my thesis I have to admit to being pretty optimistic that the combination the fusion of these two techniques T TVs and T DVS together would really solve the XO moon problem and give us many detection this optimism was heightened by the launch of NASA's Kepler space telescope around the same time which I predicted would be capable of detecting Mars mass exomoons using these techniques things looked great I even launched a project to search for them could the hunt for eggs and moons with Kepler XA means we'll be playing an increasing role in our study of planetary systems in the future I think we will discover potentially hundreds of XA means in the future now exomoons was pretty niche as a field and nip still is there really was just me and a few others thinking about how to detect them and so I sometimes wonder was my enthusiasm and optimism for how many detections we might find somehow off-putting for new people thinking about joining the field they might reasonably think you know if kipping is going to detect all of these moons what's the point in me trying to join it he's just gonna find them all I won't be able to get any and that's definitely not desirable because no field benefits from a monopoly everyone makes mistakes and misses things certainly me included and so the more eyes we have on the prize to try and look for these things the better our chances for success but the fact is that there really weren't any of the teams apart from us who were doing a systematic survey for XO means and so when we failed to come up with a catalogue of these things I think it really kicks the wind out of the sails of this emerging field and once Kepler was done it is now no longer functioning and taking data I can totally understand why many of us might have pessimism about the future of EXO moon hunting I think what we need are new ideas new thoughts and approaches going back to the original theory dusting off and seeing if there are ways to improve it to give us the best chance possible to find these exomoons at last and I'm certainly hoping that other people will join that effort but you know I'm taking some responsibilities as myself and going back to some of those original papers and seeing through things we can do to improve our chances so what went wrong why is it that Kepler didn't find a bunch of moons one effect surely coming into play here is that most of the kepler plants that were discovered orbit their stars in very tight orbits much closer than mercury orbits the Sun even that's certainly not good for means but there's still plenty of planets found further out where it is a little bit more surprising that we haven't seen anything and in our EXO moon survey we specifically focused on those cooler longer period worlds hence the name of our team and this YouTube channel the cool worlds lab but even amongst these cool worlds we don't seem to have found an abundance of X but the only one we have is this one EXO moon candidate so why [Music] remember what I told you earlier one of the principal ways proposed silico EXO means is tea TVs transit timing variations wobbling planets now soon after Kepler launched it didn't take long for to find these t TV effects in fact Kepler found a lot of them hundreds of systems exhibiting T TVs a veritable gold mine of XM moons but we soon realized that many of these t TVs were very clearly caused by planet planet interactions and not planet moon interactions this wasn't that surprising after all Neptune in our own solar system was discovered by seeing Uranus wobble around in space all the way back in the 19th century and don't get me wrong these planetary interactions have been wonderful for learning about planetary masses and architectures but the sheer volume of these things poses a major problem to those of us like myself who are interested in looking for exomoons how can we tell whether the wobbles that we see are due to a planet or due to a moon it's a bit like trying to detect a small faint LED light residing within a field of flashlights the signals there is just buried under a wave of spurious effects one way we can distinguish between planets and moons is to run a detailed computer simulation of the two hypotheses and see which one of them most closely matches the data that we have and that has largely been a strategy that we have used in the last five years unfortunately it is simply very inefficient to do this to give you some context if I take just one of these 4,000 exoplayer and run the moon simulations on my desktop computer it can take months or even years for the simulation to finish there are billions of possible moon configurations to try to speed this up we use NASA's Pleiades supercomputer which actually appeared in the movie the Martian although we don't log into the computer by physically rocking up at the data centers in the movie and so this takes time and not just supercomputer time but our own human time as well to look through the results of these simulations and interpret what we have and so in the last five years we have been limited by human manpower computational resources that we have only really been able to look through about 50 or so exoplanets with these detailed moon simulations what we need are faster methods techniques which can very quickly discern what is worth looking at and what is maybe more valuable to spend that precious computer time on enter and new paper entitled impossible moons okay what am I talking about here a new paper relook Sat the landscape of all of these ttv planets and just asks right off the bat can we throw out some of these planets as being potential moons is there a subset of these for which we can say with great confidence there is no way that an EXO moon could possibly produce the signal that we are looking at to do this we really dust off those original equations from my PhD thesis and plug in extreme parameters parameters so extreme that they're impossible so for example the TTB effect gets larger when the moon is further away from the planet it's really just the law of pivots so the further out the moon is the more leverage it has to jerk the planet around moons have a very well-defined limit for how far away they can be called the hill radius if you put a moon any further out than this then the gravity of the Sun would destabilize the orbit and so you'd actually lose the moon so let's put our impossible moon just beyond this threshold and second let's make that moon as heavy as it could possibly be because the heavier the moon then the greater its gravitational influence on the planet and so almost by definition we can say that the heaviest moon possible is one which has the same mass as the planet that's because if you make the moon any heavier than the planet then the moon really becomes the planet and the planet becomes the moon and so the ratio of the masses is still less than one and so putting these together allows us to calculate an upper limit a ceiling for how big these ttv effects could possibly be due to annex a moon it varies from planet to planet but it's typically of order of hours and remarkably many of the real Kepler players have T TVs above this level in other words they are impossible moons and so when it comes to moon hunting we can just throw these guys out there's no point even considering them this diagram from the paper shows what I'm talking about here are 2416 planets discovered by Kepler for which we observe T TVs for each planet we can use this logic to calculate a minimum possible orbital distance of a hypothetical moon so if these plants have a moon then that moon would have to be at least this far away to explain the T TVs that we see you'll notice that I've divided the orbital distance here by the planets hill radius so it's easy to see how feasible a moon really would be the red points represent systems which are right up against this hill radius or even beyond it whereas the black points are comfortably inside adding these are we find one and 79 of the kepler planets can be flat-out rejected as possible moons now that's assuming a maximum allowed distance of one hill radius but it turns out that only retrograde moons can be stable beyond half a hill radius moons like those around Jupiter and our own moon are all pro-grade that means they orbit the same direction around the planet that the planet orbits around star if we impose the extra condition that only pro-grade moons are allowed then this ends up killing nearly 500 of the planets our paper introduces a second technique for killing moons as well now remember that moon should cause both a TT v and a t DV so if you have a system which shows a TT v and you don't see a t DV that's immediately suspicious for the moon hypothesis and in fact you can use it to completely rule out an x mm sadly only 708 kepler plants have T TVs available right now but using those we can reject another 40 planets as being impossible moons what's neat is that there's almost no overlap with the moons that we killed using the previous technique so using these two strategies together and using maybe the more conservative assumption that only pro-grade moons are allowed this actually ends up killing nearly 30% of all of these kepler planets as being possible moons I'm kind of happy with that number it's enough that it will kind of make it more efficient for our future searches but not too big that it's removing a vast swath of all of the possible exomoons out there what's attractive about these impossible moon tricks is that it really doesn't require hours of computer time within just a few seconds we can eliminate many planets and direct her resources more efficiently to where it's needed so what's next well we've already started using this in our own xmu hunting surveys along with another even more powerful development that I will tell you about in a future video together these two developments really represent the first significant improvement in EXO moon theory that we've had in a decade and I'm really excited to see what they can do for us in the near future right under our noses many of these Kepler ttv systems could really be genuine exomoons we just need to figure out which ones are real and which ones are caused by planets instead it's a thrilling time to work on X image there's just so much we don't know and we're literally figuring out the methodology right now you know sometimes students ask me whether the gold rush era of exoplanet discovery has passed and maybe they wish they'd been born in 20 years earlier so they could have played a pivotal role in those early formative years of exoplanet discovery and I don't know the answer to that maybe there is some truth to it but 4x a means we can say confidently that the gold rush era of EXO means is absolutely ahead of us and not behind EXO means will be a game-changing revolution one that's been on the tip of our fingertips for the last decade yet just out of reach this is the time for not just my team but for others too to join in and try out new ideas searching different techniques it's through the variety of these ideas and human ingenuity that we will succeed infinite diversity in infinite Nations so until next time stay thoughtful and stay curious [Applause] [Music]

Channel: Cool Worlds

Views: 80,233

Rating: 4.9327817 out of 5

Keywords: Exomoons, Exoplanets, Extrasolar moons, alien moons, alien worlds, kepler moons, kepler planets, transit timing, TTVs, TDVs, transit timing variations, astronomy research, exomoon discoveries, exomoon research, impossible moons, impossible exomoons, cool worlds channel, professor kipping, david kipping

Id: 2tteEkzJKiI

Length: 18min 15sec (1095 seconds)

Published: Sun Apr 12 2020

Reddit Comments

[u/Gelfling86]

Does this guy have anything to say about the prediction that the moons of gas giants cannot be larger than Mars?