The three boundaries divvied up the solar system planets equally well. But the XUV and sunlight shorelines cut very different swaths through the population of rocky planets orbiting M dwarfs, with more worlds falling on the airless side of the XUV dividing line.
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“We don’t know exactly where the cosmic shoreline sits for the M dwarfs,” said Eliza Kempton, an astronomer at the University of Maryland. And it’s important to figure that out, she said, because before you can try to find signs of life in habitable worlds, you need to ask the question: “Do the planets that we can observe with JWST have atmospheres in the first place?”
A Map of the Shoreline
This question captured headlines and the popular imagination in 2017, when astronomers spotted a batch of seven roughly Earth-size planets spinning around the red dwarf star TRAPPIST-1. The unusual planetary system seemed like the perfect alien hunting ground. Not only were three of the planets in the star’s habitable zone, all seven would eventually be within reach of JWST. And by scouring the planets’ atmospheres, astrobiologists hoped to search for the spectral fingerprints of life.
Eliza Kempton, an astronomer at the University of Maryland, is searching for atmospheres on planets orbiting an important class of stars called M dwarfs.
Of course, that would only be possible if these planets had air.
The easiest way to detect an atmosphere is by taking a planet’s temperature, said Jacob Bean, an astronomer at the University of Chicago. By comparing the temperatures on a planet’s day and night sides, as JWST is now doing for hot, rocky worlds, scientists can infer whether an atmosphere distributes heat across the planet.
Initially, when astronomers used JWST to measure the temperatures of TRAPPIST-1 b and c, the system’s innermost planets, they could rule out thick atmospheres — not a surprise, because these worlds fall on the airless side of all three shorelines. Kempton, Bean and their colleagues have also ruled out thick atmospheres on other planets such as Gl 486b and GJ 1132b — both hot, rocky M dwarf planets that Zahnle’s plots suggested would be airless.
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However, more recent observations of TRAPPIST-1 b are consistent with either a bare — but geologically active — surface, or a hazy carbon dioxide atmosphere. And JWST observations of a cooler planet, called LTT 1445 A b, are similarly “murky,” Bean said. But this world could be instructive; it falls on opposite sides of the XUV and sunlight shorelines and could help point toward which one matters.
For other cooler worlds like ours, Bean said, finding air requires a different technique.
When a planet passes between its star and Earth, the star’s light briefly shines through any atmosphere that might exist. Astronomers can then sift through that starlight and look for spectral fingerprints that hint at an atmosphere’s composition. But this method, called transit spectroscopy, sometimes has a hard time distinguishing cloudy atmospheres from absent ones, and stellar messiness like sunspots can further complicate interpretations.
Using this method, Lustig-Yaeger and Stevenson are assessing the cosmic shoreline by aiming JWST at five cooler, rocky worlds, including chilly TRAPPIST-1 h. So far, they’ve published observations of four worlds that sit close to the total sunlight shoreline but are deep within the XUV shoreline’s airless desert. Two look airless, but a thin, hazy or cloudy atmosphere might have escaped detection. The other two show hints of steamy atmospheres, but these observations could be just as easily explained by sunspots.
Undoubtedly, finding atmospheres on small alien worlds is a challenge. But atmosphere hunters are confident they can meet it with JWST. By starting with hot rocks that are easier to observe and then refining their methods, “we can get down to planets that straddle where we think the cosmic shoreline is,” Kempton said, “and map out where that boundary is.”
Life on the Cosmic Beach
Even though scientists are hunting for the cosmic shoreline, some — including Zahnle — also recognize that the concept is probably an oversimplification. It ignores the amount of air that planets start out with. It assumes that escape is all that matters. It also assumes that if a planet loses its atmosphere, it loses it permanently.
Reality is likely much more complex. The cosmic shoreline is probably less of a tidy fence and more of a wild borderland, said Joshua Krissansen-Totton, a planetary scientist at the University of Washington. Notably, his computer models of planets around M dwarfs suggest that they can regain lost atmospheres over time.
“Just because there’s enhanced loss doesn’t necessarily mean that these planets end up airless,” he said. Instead, he said, an atmosphere on an older planet is a complex function of a planet’s evolution and its starting conditions.
Zahnle agrees. “It’s the usual question of nature versus nurture,” he said. “Of course the answer is nature and nurture.”
Regardless of whether the cosmic shoreline is a neat divide or a fuzzier boundary, it has important consequences for our understanding of life in the universe.
Beyond the solar system, 70% of the stars in our galaxy are M dwarfs — systems that have often been framed as “galactic real estate for habitability,” Bean said. If M dwarfs inevitably blast away their planets’ atmospheres, that real estate won’t be all that real, after all. Depending on what JWST finds, the search for life’s atmospheric fingerprints could start now. Or it might need to wait for the next generation of space-based observatories, decades in the future, to hunt for biosignatures in the atmospheres of Earth-like worlds.
As the hunt for the cosmic shoreline shows, learning anything at all about exoplanets is still enormously difficult. But the multitude of exoplanets offers one undeniable advantage — numbers. We’ll never fit a solar system into a lab flask, but we don’t have to. The universe has made a lot of worlds, each its own experiment in planet formation.
“That,” Bean said, “is the promise of extrasolar planets.”
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