Why Do We Care About Exomoons? – Tyler Gordon (UW)

they're ducks in the club I am sorry to say that I'm not going to be talking about plants today because that's our on top hop on top but all right so I have spoken at this event before about exome ins that time I talked about how we might find EXO mins today I'm going to talk about why we might be interested in finding them so that seems a little bit backwards to you seems backwards to me too but here we are alright so the first question of course is what is an XML and the answer is pretty simple it's the min of an exoplanet so we stick the EXO prefix on there to denote that it's not our solar system um now I want to motivate this by talking a little bit person about what we've learned about exoplanets in order to show you that our lack of knowledge about EXO means represents a pretty significant gap in what we know about planetary systems outside of the solar system in general so in the last 20 years we really learned a lot about exoplanets we've learned something about the abundance of exoplanets Vimal talked a bit about that so in order to highlight that I'm going to show you an animation this is an animation done by evening cruise to the graduate of our u-dub astronomy ph.d program and this just shows all of the kepler planets so I'm going to go ahead and play this this is all the planets found by the Kepler mission on orbiting and it's superimposed on the planets of the solar system so I'll just let you watch this for a minute and bear in mind as you're watching this and this isn't even all of the exit planets that we've discovered this is just the independence that we discovered with the Kepler mission there's quite a few more than this but this gives you an idea of the kind of magnitude that we're talking about and so this of course is not all of the independence that there are either this is just the echo pendant that we found in one patch of sky and as Michael mentioned during his talk we now think that about the order of one stars in our galaxy host planets of some sort at least one planet we've also learned something about the diversity of exoplanets so this is a plot that I pulled off the NASA exoplanet archive just on Monday so this is pretty current again this isn't all of the exoplanets this is just for once for which we have a measurement of the radius and a measurement of the equilibrium temperature of the planet the equilibrium temperature of a planet is just a function of the distance of the planet from its star and the brightness of the star you can think of that as a measurement of the temperature of the surface of the planet if it doesn't have an atmosphere and I'm gonna highlight some planet populations in this image so the first planet population I'm going to highlight our hot Jupiters these are very massive planets gas giants that are orbiting close to their stars so close to their stars but a lot of beads you can see a very large radii and that's because the heat of the star pops up the atmosphere over here we have sort of regular jupiter-like planets that are further from the star and thus are puffed up like the hot Jupiters are below those we have neptune-like planets and we could probably stop divide those if we want it to into hot Neptune's over on the right and cooler Neptune's on the left we have what I'm calling here lava worlds which are sort of terrestrial science planets that are close to into the star and that's very hot and then Earth's and super Earths which are the sort of planets that we look at if we're concerned about habitability so this illustrates some of the different types of exoplanets that we've been discovering and we've also started to learn something about the formation and evolution of exoplanets and to illustrate that what I have here are pictures from the planetary disks and these are taken with Alma which is an array of radio telescopes these are really high-resolution images of disks around stars that are in the process of forming and you can see some gaps in these discs so the bright areas are where there's material the darker areas are where there are gaps in these discs and one possible reason for these gaps is that narrow planets forming in these discs so some researchers have done work here they've simulated the formation of proto planets in a in a disk like this and we've highlighted they've highlighted kind of a few of those discs here that have structures that look similar to what we actually observed with Alma suggesting that it's possible to form form structures like this by forming planets in the disk okay so that's what we've learned about exoplanets that's not everything we've learned about exoplanets by any means but that illustrates some of the things we've learned in the last few decades so what about enzone is what we learned about then we haven't learned anything about extra business because we haven't found any EXO business and why haven't we found any accidents well then ill astray tweye these are difficult to find and why we don't have any yet I'm going to talk a little bit about the transit method of detecting exoplanets so this is an animation showing a planet moving in front of its star what we do is monitor very carefully the brightness of the star that's what you're seeing plotted along the bottom of this animation and we can detect a dip in the brightness as the planet walks some of the line of the star now say this planet has a moon orbiting it we see as you would expect kind of a double dip where the planet blocks sunlight or where the moon blocks sunlight and the planet blocks a lot more light and if you always see these two dips kind of near each other then that suggests that maybe these objects are orbiting each other that it's a that it's a moon orbiting a planet but moons we expect tend to be small at least smaller than the planets otherwise we probably wouldn't call it mins so let's look at what happens when I add in what I just showed you with an idealized image I didn't have any nodes in it now in reality stars vary a little bit in brightness and additionally there's instrumental noise and noise that just has to do with counting small numbers of photons so when we add that in we have something that looks like this and you can see really clearly that planet Transit because it's a big planet I think that's a planet like the size of Jupiter or something that I put in there but this moon that's a really large moon - that's a mean like the size of Neptune or something like that but as large that system it is it's much smaller Canton is completely lost in the noise so this is what's so far prevented us from detecting accidents assuming they're out there the other possibility is that they don't think this which seems pretty unlikely because when you look around our solar system you see a lot of moons right there's a lot of moons in our solar system so it seems kind of unrealistic to imagine that these don't exist out there somewhere okay so that's all great why do we care about X and moons that's what I'm actually here to talk about today and so I mean I find like three reasons we care about accidents and then I'm gonna walk through each of them in more detail the first reason we might care about X and minutes is that they can tell us a lot about how planets and planetary systems form and evolve over time the second reason is that the presence or absence of the next to meet my effect that habitability of the planet into four bits and the final reason is that X events themselves might actually be palpable Michael talked a little bit about that now go into some detail about that as well okay so so diving into the first point here what connects and moons tell us about how planets form and of all in order to illustrate one way that they can tell us something about the history of a planetary system I have a little not quite an animation a series of diagrams the this is a planet in the center with two moons orbiting it say say it's early in the lifetime of this planetary system and the planets haven't settled down into nice stable orbits yet maybe there's planets buzzing around and undergoing kind of close encounters what we call planet planets scattering events and that's this yellow planet that's about to undergo a close encounter with this happy little planet moon system and when that happens of course everything interacts with each other gravitationally and in this case what I'm illustrating is news being thrown out of their orbits around this planet out in the interplanetary space they might be thrown away from the system altogether out of danger stellar space another thing that can happen is one of those be captured by the other planet that's passing by there's all sorts of things that can happen but the bottom line is this disrupts the planetary system and you're most likely to end up losing those moons so what that allows us to do is say something about the history of the planetary system based on whether or not we observe Entenmann's in that system if we don't observe exomoons in a planetary system that might suggest that there's been a lot of these planet planet scattering events going on that could have caused those planets to lose their mins if we do see a lot of moons in the system that might suggest that a system has had a relatively calm history and happened to experience a lot of these events that would cause those means to be flung off away from their planets ok so what else connect them in tell us about the history of planetary systems well in order to answer this part of the question I'm going to talk a little bit about the ways that means in form there's kind of three basic ways that moods can form the first is coaccretion this is how we think that Jupiter and Saturn's large moons form the second is captured this is how we think that Neptune's moon Triton formed and the third is a giant impact which is how we make Earth's moon form and I'm going to talk a little bit about each of those because each of these results in different observable characteristics of the moons that are formed via that mechanism so tell accretion this is a scenario where much much like planets secrete out of the disk around the forming star you and have moons accreting out of the certain planetary disc around the planet as the planets form so this is why it's called coaccretion because the planet and the moon are treating together out of the disk of material around the star now what this gives us is a moon with a total mass of about one ten-thousandth the mass of the planet or a series of moons with about 110 thousand the mass of the planet that they form around this comes from both observation this is about what we see for Jupiter's moons and for savage large moon Titan this is also what comes out of simulations formation on the other thing that we get from this is that we expect that the moon's that result from this formation process would orbit in the same direction as the planet rotates because they have the same initial angular momentum as they're all forming out of the same disk okay so capture captures just what it sounds like this is where you have some a small body that moves past a larger body is captured by this gravity and pulled into orbit this is what we think happened with pregnant to icy body from the outer solar system that was captured by Neptune and finally impact this is what we think happened with the with the earth in the moon this is a process in which some impactor in this case I another proto planet a little bit smaller than the earth called Theia would have impacted the earth sent a bunch of material from the earth out into orbit around the earth which would have been accretive back into a moon and what this gives us is a moon with a with a mass ratio that's much larger than the case for coaccretion so our moon is about 1/100 of the mass of the Earth which is much larger than that one mm figure that you get for coaccretion and then the other thing you find is that that moon should be left orbiting above the planets equator I actually forgot to mention I'm gonna pop back real quick to capture the observable results of this capture process our been abou that was captured by another planet wouldn't necessarily orbit in the same direction of the planet rotates and that's because it has been captured coming in from any direction with any initial orientation with respect to the rotation of the planet and that also means that it might be misaligned with other moons in the system okay so this means that by observing on the size of moons potential XM into the different planetary system by observing their size and the way that they orbit their orbital characteristics we can say something about how moons are formed and which of these processes are dominant weather which processes are dominant depends on something else about the system things like that okay so now what about how accent moves impact planetary habitability it's in order to talk about that I first have to talk about this concept of obliquity so obliquity is just a fancy word for the the tilt of a planet axis away from its orbital axis so the planets spin axis in this case this is the diagram here the Earth spins on an axis that is tilted with respect to the plane of its orbit by 23 degrees this is what's responsible for seasons on earth so right now for instance as you can all feel the northern hemisphere is tilted away from the Sun and that makes it colder up here than it is in the southern hemisphere now if if there's zero obliquity say the the earth was not tilted by 23 degrees with respect to thought relaxes but rather had its orbital axis and it spin axis pointing in the same direction in this case we wouldn't have any seasons because no matter what time of year it was every point on the earth would experience the same length of day and the same length of time they'd just be 12 hours each if we wouldn't have season Falls now if we have a moderate tilt we do experienced moderate seasons and additionally there are portions of the earth near the North and South Pole that at some point in the year experienced only darkness or only sunlight so you're probably all familiar with this idea that that the North Pole is completely enshrouded in darkness throughout the winter and the South Pole is completely sunny this time of year the situation will be reversed in 12 months and the North Pole will be in the Sun all of the time and the South Pole will be in darkness all of the time but what happens if we tilt took a planet completely have a very very large obliquity so this is an ability of 90 degrees well in this case we have a situation where each hemisphere of the planet is at some period of the year going to have only darkness or only light so in this situation I you can get very extreme seasons now in light of our previous talk I would like to note that this is not the same as being tidally locked right so if your Title II locked your rotational period is the same as your orbital period and you always have one face of your planet facing facing the star in this case you would have in this case you would have a scenario where one face of the planet would be facing the star in the summer and in the winter it be facing away from the star because that planets not going to turn as it orbits in the same direction as it orbits its kind of tumbling end over in the long it's orbit but with its spin axis always pointing in the same direction so this is a slightly different scenario although here here pictured at the southern source or the summer solstice in the northern hemisphere at this time of year you have the same kind of a situation where half of the planet is completely sunny all day long and the other half is completely dark all day long and so similarly this can result in some pretty extreme weather it's pretty extreme seasons which which we think might not be so great for life for a number of reasons although as Michael mentioned having a finally locked planet doesn't necessarily mean that your planet is uninhabitable so it might be the same for a planet like this that has a really extreme obliquity there are ways that it could very likely still be habitable and but there are other ways in which it might not be so conducive to life and so what happens with planets is that they experience torques from other bodies in the planetary system right they're not orbiting in isolation they're having gravitational interactions with all the other bodies in that system and those torques cause the obliquity to vary and over time that obliquity tends to vary pretty extremely they tend to take on just about all allowed values of obliquity so a planet about through periods of time where it has very moderate seasons in the know also goes through periods of time where it has very extreme seasons and this is where moons come in and moons might be able to stabilize those obliquity variations so this is some work done I it back in the 90s that suggested that that the presence of the Earth's moon kept the earth from undergoing those very very extreme obliquity oscillations which suggest that potentially for an exoplanet having a moon might increase its have an ability of course as sometimes happens in science somebody else came along a little bit later and said well maybe having a moon doesn't actually keep you from undergoing extreme obliquity oscillations maybe that happens anyway whether you knew or not somebody else came along and said or maybe it does keep you stable with respective liberties but maybe that doesn't even matter because the timescales over which these variations occur are so long that you've plenty of time for life to form and above and live happily between periods of extreme weather so this is a little bit up in the air but I would argue that regardless of the outcome here it so matters whether you have a moon that's still something that you would want to know about a planet if you were trying to characterize the Tabata ecology okay and since since we're kind of doing a Star Trek thing tonight I thought I should pull some Star Trek into this somehow and the way that I'm gonna relate this to Star Trek is by talking about I'm talking about this idea that tides had actually could actually contribute to the origin of life on our planet in the first place so what I have here is a screenshot from the Star Trek The Next Generation episode all good things the series finale it's a great episode but you should watch the rest of the series first so hope so here we have the the omnipotent alien Q has brought the car back in time to witness the origin of life on Earth in a pool of goo and it's a little bit hard to see here the contrast isn't so great the Q's reached his hand amethyst is full of goo that's full of biological molecules that are about to undergo a reaction and for the first time and in the history of life on Earth which doesn't exist at this point um undergo a chemical reaction and become self-replicating and that's eventually going to lead to the evolution of life and so this this idea the role that tides could play in this is that if this kind of interaction is happening in say a tight one you have these biological molecules to be done at high school and you would need periods full of concentration you would need that kite pole to dry out in order to concentrate those biological molecules bring them into proximity with each other where they can undergo these kind of reactions and then you would need periods of flooding in order to wash out some of the products of that chemical reaction and infuse it with new molecules and that's a process that would rely on tide so if you have this happening at a time for you would need tights racing to watch out the the products of that chemical reaction to infuse it with new material and then you would need the tights to recede and allow evaporation to concentrate those molecules so that they can undergo more reactions and of course all of that relies on you not having any you know tachyon induced space-time anomalies that would prevent that kind of interaction from happening so so we can okay hi now I'm going to talk about the habitability of EXO mins and top first we have to talk about the habitable zone of course and Michael already covered some of this in his talk and so thanks for doing some that work for me all right so so the idea of the Goldilocks zone this is the this is the distance from the planet at which your earth sorry the distance from a star at which your planet would be not too hot to have liquid water on its surface and it would be warm and not put all that liquid water wouldn't be frozen it would be just right you can support liquid water on the surface and not something that we think is possibly necessary for life but of course the situation is different for accidents because accidents aren't just orbiting star they're orbiting a planet as well and this changes things primarily via their title interaction again this is something that we just heard about tonight and so this is audio and IO is very close to Jupiter and as a result of experiences pretty severe tidal interactions during part of its orbit it's closer to Jupiter and it's stretched out and buried another part of its orbit it's further away so it's less stretched in this process of stretching and then relaxing and stretching and relaxing and heats up the interior hiding all that he has to get out somehow so you have pretty extreme volcanism happening all over the surface of Io and we can see on the limb of Io on the top there you can see a volcanic plume on which the New Horizons mission caught in action so this isn't great for life of course because the entire surface is constantly being recycled through the interior and there's volcanoes everywhere and there's sulfur and there's not really much of an atmosphere it's not great you wouldn't want to live there probably nothing else does either but if we move out a little bit further we have the Europa which adds Michael disgust is is a world that's full of water in its interior so we think it has this subsurface ocean that's have to just warm enough to remain liquid by the same tidal interaction that makes IO so inhospitable okay so now I'm gonna illustrate this kind of in the form of a diagram and this is sort of the top down version of what Michael showed us so here I have Jupiter representing some some gas giants of exoplanets somewhere and around it and we might have moons orbiting so I'm illustrating with Venus here a moon that's too close in to the planet so it's experiencing too severe tidal interaction and it's it's gonna look something like IO or maybe something like Venus in our solar system which is too close to the Sun except in this case the heat would be provided not by the Sun but by a tidal interaction with the planet that it's orbiting now if you move out further as long as you're in the house Google's own you can move out further and further away from the planet and and your your ends engine might be habitable as long as it's far enough away that it's not experiencing a severe tidal interaction but the really cool thing about exit mins is if you consider an exoplanet that's outside of the habitable zone where a planet by itself would not be habitable you still might be able to support and how to hold them much like your up in our solar system and far outside of the how the cool zone in our solar system yet still has liquid water by virtue of its title interacting with Jupiter you have a range of distance around an exoplanet around the gas giant exoplanet where potential moon could have just enough tidal heating to be habitable so in that case you have something like so here this is a Horsemen of Endor from star wars so we might see something like this or again since we're kind of focusing on star trek in this talk I have a picture that you've seen already tonight and it's a this is this is the moon on and/or which is that have no home world of the Andorians so so hopefully someday we'll be able to find exum is out there in the universe and maybe on some of those accidents there's life and maybe that bike looks just like this alright so to summarize this all basically EXA moons are cool or or maybe warm enough to be habitable and we haven't found any yet we want to find some because they'll help us understand exoplanets and because they could affect the habitability of exoplanets and finally because they might themselves actually be possible and that would be really neat alright thank you [Applause] okay so the question was how are we gonna find the effectiveness and that's a good question the way we're going to find them is by figuring out how to do a better job of eliminating the noise that prevents us from finding them so we're gonna have to do a better job of finding really really small transiting objects essentially yeah that's that's a good question and I really don't know the answer to I I don't think a lot about atmospheres and I think it would probably depend a lot on how much you are getting through the surface yeah I don't have a great answer to that question fun to think about that yeah and yes so I think potentially it depends on what accidents are out there and how big they are whether or not they're detectable with something like the James Webb telescope I would say that it's certainly worth worth giving a try it's it's gonna be you know win win it montes and it's going to be probably the best tool out there for looking for X any chances yeah yeah yeah so um see so so your question was how much do we do we know about these exoplanets um yeah we know quite a lot about some of them uh we certainly know so what we get from the transit method is primarily the radius of a planet and its orbital period if we can look at radial velocities induced on the star by the planet so that's the radial velocity method of detecting exoplanets but that can also be used to further characterize them and that gives us constraint on their mass so with those two things we can get something about their density and that tells us something about possible compositions of a planet we can also from transit so you can do something called transit spectroscopy where we look at how it has an atmosphere that atmosphere blocks different amounts of light at different wavelengths so we can get something like a spectrum from that and put constraints on the atmosphere of the planet and that is at the present about as good as we can do there are Sun there are a few planets for which we can very crudely map out the heat pattern across the surface of it and that tells us something about their rotation and not not a ton though uh-huh yeah all right if there's no more questions thanks [Applause]