A exoplanet What was identified in 2017 as one of the most promising locations for life outside the solar system has now become even more promising – and a lot weirder.
The alien world Evidence suggests that LHS-1140b is an “eyeball” planet, with a global ocean covered in ice and a single, iris-shaped region about 4,000 kilometers (about 2,500 miles) in diameter that permanently stares back at its parent star.
“Of all the currently known temperate exoplanets, LHS-1140b may be our best chance to one day indirectly confirm liquid water on the surface of an alien world outside our solar system,” said astrophysicist Charles Cadieux of the University of Montreal. “This would be a major milestone in the search for potentially habitable exoplanets.”
LHS-1140b, whose discovery was announced just a few years ago, has a radius of about 1.73 times that of Earth and 5.6 times its mass; larger than our own planet, but still small enough to be considered a terrestrial world. It also orbits much closer to its star than Earth does, completing a full orbit in just under 25 days.
If that star were anything like the sun, it would be far too close for life. Instead, it’s a cool, dim red dwarf – so the distance between the star and the exoplanet is right in what we call the habitable zone. That’s not so cold that all the surface water would freeze, but also not so close that it would steam away into oblivion.
Still, its proximity means the exoplanet is likely tidally locked. That’s when its rotational period aligns with its orbital period, so the same side always faces the star. It’s the same phenomenon we see with Earth and the Moon, and why we never see the far side of Earth.
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Just because it’s in a habitable zone doesn’t necessarily mean it has the conditions to support life. To learn more about LHS-1140b’s chemistry, we need to look into its atmosphere, if it has one. And that’s what Cadieux and his colleagues have done, using the power of JWST.
At a distance of just under 50 light-years, the system is close enough to gather detailed information about how light changes as the exoplanet passes between Earth and its star. Some of the starlight passes through the atmosphere; as it does so, some wavelengths are absorbed or amplified by atoms inside. Exactly which atoms are at work can be determined by looking at which wavelengths are affected.
By doing so, the researchers were able to tentatively identify the presence of nitrogen, the dominant ingredient in Earth’s atmosphere. If LHS-1140b were more gaseous, like a tiny Neptune, it would have an atmosphere richer in hydrogen. The presence of nitrogen suggests a secondary atmosphere—one that formed after the exoplanet was born, rather than with it.
In a study published last year, the team also combined the density and radius of LHS-1140b to calculate its density, coming in at 5.9 grams per cubic centimeter. That’s not dense enough for a purely rocky world; given its size, the best fit is a mini-Neptune or an ocean-covered waterworld. If we exclude mini-Neptune, we’re left with a global ocean exoplanet.
However, if you take tidal locking into account, this global ocean might not look like you think. The side that permanently faces away from the star would be cold enough to freeze. Only the bit that directly faces the star would be warm enough to thaw, resulting in a world that looks like a creepy eyeball floating in space.
However, temperatures on that patch of land could reach as high as 20 degrees Celsius (68 degrees Fahrenheit), warm enough for a thriving marine ecosystem.
We don’t know for sure what’s going on, but it seems like the most promising candidate we have yet for an exotic alien ecosystem outside of our own planetary neighborhood. So you can bet there will be a lot more people staring at that strange (possible) view.
“Detecting an Earth-like atmosphere on a temperate planet pushes Webb to the limit of its capabilities. It is achievable, we just need a lot of observing time,” said physicist René Doyon of the University of Montreal.
“The current indication of a nitrogen-rich atmosphere needs confirmation with more data. We need at least another year of observations to confirm that LHS 1140b has an atmosphere, and probably another two or three years to detect carbon dioxide.”
The research has been accepted in The letters of the astrophysical journaland is available on arXiv.