We're Finally Unlocking the "Cosmic Shoreline"

Added: 2026-05-24

[00:00:00]There is a concept that exoplanet astronomers won't stop talking about, but one that you have probably never heard of, the cosmic shoreline. So, today we're going to explore what it really is, why it's so important that the James Webb Space Telescope is spending a huge amount of time studying it, and why it might also answer one of the grandest mysteries about life in the universe. Way back when I was doing my PhD on exomoons, I remember journalists sometimes asking me how small could a moon be and still be habitable? Because clearly our moon, the moon, is too small to hold onto an atmosphere, and that's 1.2% of an Earth mass. I remember googling this question and stumbling across a nature paper by Darren Williams and colleagues in [music] 1997, who had estimated that the threshold is about 12% of an Earth mass. So, that's about the mass of Mars. It's a good starting guess, but even this might not be massive enough, because after all, Mars doesn't have a thick atmosphere. The present-day surface pressure on Mars is only about 0.6% that of our own. Just ask Arnold. Yet more, Mars is even further away from the sun than we are, and so it should actually be even easier for it to hold onto an atmosphere than for us. Now, you might wonder though, maybe the answer is that Mars just never formed a thick atmosphere to begin with. But that doesn't work, because the fact that we see so much evidence for past liquid water on Mars strongly indicates that its atmosphere was at some point billions of years ago much thicker. It just somehow lost that atmosphere over time. So, let's look at the basic physics of atmospheric escape to understand a little bit what's going on here. So, right off the bat, Mars has a lower surface gravity than that of the Earth. It's about 38% of a G. So, that means that it simply cannot hold onto stuff as easily as the Earth can. But gravity is only part of the story.

[00:01:56]Temperature matters, too. Because if the atmosphere is cold, then the kinetic motion of each gas particle is lower, and so gravity has an easier time keeping it bound. It's akin to why warm water evaporates faster than cold water.

[00:02:10]The random chance of a particle getting enough speed to escape the bulk is simply higher when the bulk warms up. In a planetary atmosphere, that simple particle-by-particle leakage is known as thermal escape, or more rigorously, Jeans escape. But upper when the atmosphere is blasted by intense high-energy radiation, it can heat up so much that the gas doesn't just leak away. It actually flows off the planet almost like a wind. And that more violent version is called hydrodynamic escape. The basic competition between gravity and heating helps explain why Titan, the largest moon of Saturn, has a thick atmosphere. In fact, it is the only moon in the solar system with a dense atmosphere. Despite the fact that Titan is only modestly larger than the moon, it actually has a thicker atmosphere than even the Earth. Why?

[00:02:59]Because it's so damn cold. It's 93 Kelvin, or -180° C.

[00:03:05]Now, besides from that nature paper back in 1997, there really wasn't a lot of interest in this question amongst astronomers until recently. You see, for a long time, most of the exoplanets being found were either Jupiter- or Neptune-size worlds, which they were so large that an atmosphere was a given.

[00:03:20]But today, we now know of many roughly Earth-size exoplanets orbiting in and around their stars' habitable zones. And so, the question has thus ripened. We want to know, do these seemingly Earth-like exoplanets have atmospheres, or are they barren wastelands, sterile, bare rocks in space, much like our own moon? Now, you might naively assume that well, the answer is, of course, duh. I mean, if the Earth has enough gravity to hold on to its atmosphere, then surely these other Earth-like exoplanets must too as well, right? Well, no, because most of these planets aren't really Earth-like exoplanets because instead of orbiting a Sun-like G dwarf star like we do, a lot of these actually orbiting red dwarfs or M dwarfs as astronomers [music] call them. These are stars that are between 1/10 to half the size of our Sun and it is their smaller size that explains why we've been able to find these rocky planets around them because when an Earth-sized planet transits a Sun-like star, the flux decreases by just 84 parts per million, hardly anything. But, for an M dwarf that's say 10 times smaller, the flux blocked out becomes a hundred times greater and that's within reach of our telescopes.

[00:04:38]But, there's a catch. Although M dwarfs might be great for detecting planets, they might be terrible places to actually live around because they are often much more violent than stars like our Sun. For example, the nearest star to us after the Sun is Proxima Centauri.

[00:04:54]That's a small M dwarf star about 4.2 light-years away and it actually does have a rocky planet in the habitable zone of that star. So, great news. But, unfortunately, we also know that star spits out these huge super flares several times per year, any of which would easily strip the planet of its ozone layer and possibly even threaten the entire atmosphere at large. So, look, we could plausibly imagine life surviving on a planet without an ozone layer. I mean, on the Earth that certainly was true for billions of years, but no atmosphere whatsoever?

[00:05:30]That seems like a showstopper because think about it, no atmospheric pressure means liquids would be exposed to vacuum and would immediately boil off. Now, yes, a tardigrade might be able to survive like that in its tun state for a few weeks, but it can't go on like that indefinitely. It eventually needs liquid water in order to survive. So, this [music] isn't a critical. We need a way to distinguish between rocks and Earth's between barren and hospitable. Before I explain how the cosmic shoreline does exactly that, let me first take me to thank the sponsor of today's video.

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[00:09:15]And crucially, most of that high-energy radiation doesn't actually make its way down to the surface. Most of it hits the upper atmosphere where it heats up the gas particles, it puffs up the atmosphere, and potentially drives atmospheric loss. So, Zahnle and Catling then tried plotting the cumulative XUV radiation that various solar system planets and moons have received as a function of each world's escape velocity. So, you can see here that worlds with thick atmospheres like Jupiter, Saturn, and Neptune, they live towards the bottom right of this graph, which makes sense. These planets receive relatively little radiation from their star, and they have huge escape velocities, meaning gas is strongly trapped. Vice versa, look at Mercury and the Moon, which of course don't have atmospheres. They live towards the upper left of this diagram. So, those are worlds that are blasted by the Sun, but have pathetic escape velocities. The two authors argued that for solar system bodies, a good choice for this dividing line is a power law, such that the XUV intensity is proportional to escape velocity to the fourth power. And that is motivated by the physics here.

[00:10:20]However, it was hard to guess what the [music] Y-intercept of this dividing line should really be from first principles. So, they simply selected it by hand, such that this line, known as the cosmic shoreline, goes through Mars, which serves as an anchor point. And the reason here is that Mars is sort of a world on the threshold of having a thick atmosphere, given that it probably did once have one in the past, but has now lost it. Now, if we didn't know of any exoplanets, that would basically be the end of the story. There'd be nothing else to say, but fortunately, we do. We have many exoplanets, and indeed, for many of them, we know whether they have atmospheres or not, thanks to telescopes like the Hubble Space Telescope and James Webb. So, here are all of the exoplanets that we can put on this plot.

[00:11:01]The green points are planets which we have detected an atmosphere, and the orange ones are those which observations indicate a bare rock, no atmosphere. So, right off the bat, you can see the original cosmic shoreline proposed by Zahnle and Catling, it doesn't work that great. There are way too many green points, that is, planets with atmospheres above the cosmic shoreline, where they really shouldn't be.

[00:11:22]Now, we can actually try to move this line a little bit, though, because remember, Zahnle and Catling just kind of guessed that offset, but it really doesn't help that much, because you still not only have Mars now way below the cosmic shoreline, but also this guy LTT 1445 AB. And as we'll see, that exoplanet is actually a real problem on this plot. Some quick background on this planet, it is 2.7 Earth masses and a back-of-the-envelope calculation predicts a temperature of 430 Kelvin.

[00:11:50]Remember that number. Now, the way that we know that it is likely airless is thanks to some clever James Webb observations led by Wycherley and colleagues. They observed the system as the planet dipped behind the star. So, the flux slightly decreased as light from the planet itself was temporarily hidden from view. What we call a secondary eclipse. Because JWST is an infrared telescope, that light is all basically heat. So, the size of that dip tells you the temperature of the planet, or really the temperature of the side of the planet facing us, which here is the day side. And Webb reported that the temperature was a scorching 525 Kelvin. Now, that kind of excess temperature is exactly what you'd expect if this was a bare rock. I mean, to see this, just look at the moon. The temperature swings there from 100 Kelvin to 400 Kelvin between the night and day side. Why? Because it has no atmosphere.

[00:12:47]There's no air to recirculate heat around the planet and balance that temperature out. The other possibility is a Venusian carbon dioxide-rich atmosphere, a huge greenhouse that boosts the temperature. [music] But thankfully, Webb's spectroscopic abilities also strongly disfavor [music] that, too. But I mean, look at it. 1445 does kind of ruin this nice cosmic shoreline plot. Not deterred though, astronomers have tried to salvage it in a couple of ways. The first is pretty simple, just forget that original proposal by Zahnle and Catling that the slope of this line must be four and instead just make it whatever fits the data best. And a pair of Spanish astronomers did exactly that using Mars and the exoplanet 55 Cancri e as their two anchor points. By the way, 55 Cancri e is kind of wild. It's a planet so close to its star that day side is about 3,000 degrees Celsius, meaning it's like Mustafar with a liquid magma ocean bubbling away and outgassing a truly alien atmosphere. Now this re-sloping idea, it does help with many of the points in the plot, but not so much for our friend 1445, which is now way below this so-called empirical cosmic shoreline. But maybe there's a better way of fixing this. Remember that many of these exoplanets are actually orbiting M dwarf stars for which estimating that critical number, the cumulative XUV history, is really non-trivial. You see, these stars are pretty complicated. They can stay active for a long time, they flare frequently, and they spend their youth in a bright pre-main sequence phase. So what that means is that even if we take two planets that today have roughly the same amounts of radiation, the M dwarf planet might have endured a much harsher atmosphere loss history. Emily Pass and colleagues suggested exactly that, and they developed a refined prescription for the cumulative XUV radiation of these small M dwarf planets. Now it only works for stars more than about a third of that of the sun, so filtering on those we get this plot. [music] And then applying the new correction moves some of these planets up a bit to here. Okay, so if we use the empirical cosmic shoreline, this actually almost works. We could imagine making further refinements to this XUV calculation to perhaps make this even cleaner, but really what we need are just simply more planets straddling this shoreline. And that's where JWST comes in.

[00:15:08]Again, you see, for flagship telescopes like Webb, a small fraction of its total time can be allocated at the director's discretion, known as DDT. And it's from such DDTs that we got the awesome Hubble Deep Field, the radical notion to stare the emptiest patch of the sky that we could find and simply just see what it might look like in high resolution. And here too, the director has stepped in and decided to spend 500 hours of precious telescope time to investigate whether the rocky planets straddling that cosmic shoreline have atmospheres or not. Link down below in the description for this program. So, if we turn back to our plot for the M dwarfs, the square points here are planets scheduled to be observed with James Webb. Three of which we have no idea what the answer is and they live right on the border. And remember how Webb used secondary eclipses to figure this out for our friend 1445? Well, the idea here is just to rinse, wash, repeat for a bunch of planets. And what is really cool is that this program has no proprietary period, meaning anyone can download this data as soon as Webb obtains it. So, have at it. At the end of the day, the cosmic shoreline addresses one of the most important questions that we can ask about rocky exoplanets. Do they have atmospheres? If we find that many of the rocky habitable zone exoplanets around M dwarfs live north of this cosmic shoreline, then it's kind of game over for them. They don't have biospheres. And that in turn may answer another very deep question, why don't we live around an M dwarf?

[00:16:42]Because after all, they outnumber sun-like stars by about 30 to 1. If you've not seen this before, it's known as the red sky paradox and you definitely want to check out our early video to learn more about this really profound issue. But surely, the coolest thing about this whole story is that we can even answer these questions. Webb might not be able to detect life on an exoplanet, but it can answer the question just before that one, whether they have atmospheres. And it will do so in the next couple of years.

[00:17:11]And I think that is incredibly awesome.

[00:17:13]So, until next time, stay thoughtful and stay curious.

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