An exoplanet identified in 2017 as one of the most promising places for extrasolar life to flourish, just got even more promising—and a whole lot weirder.
The alien world LHS-1140b shows signs of being an “eyeball” planet, with an ice-covered global ocean and a single iris-like region about 4,000 kilometers (about 2,500 miles) long that permanently gazes at its host star.
“Of all the currently known temperate exoplanets, LHS-1140b could be our best bet to one day indirectly confirm liquid water on the surface of an alien world outside our solar system,” says astrophysicist Charles Cadieux of the University of Montreal. “This would be a significant milestone in the search for potentially habitable exoplanets.”
LHS-1140b, whose discovery was announced only a few years ago, has a radius approximately 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 its star much more closely than Earth, completing an entire orbit in just 25 days.
If the star was even like the Sun, it would be 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 cold enough to freeze all surface water, but not close enough to evaporate into oblivion.
Even so, the close proximity means the exoplanet is likely to be tidally locked. Then its rotation period comes into close step with its orbital period, so that the same side always faces the star. It is the same phenomenon we see with the Earth and the Moon, and why we never see its far side from the Earth.
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Being in the habitable zone does not automatically mean that it has the necessary conditions for life. To learn more about the chemistry of LHS-1140b, we need to look into its atmosphere, if it has one. And that’s what Cadieux and his colleagues did, harnessing the power of JWST.
At less than 50 light-years away, the system is close enough to us that we can gather detailed information about how the light changes as the exoplanet passes between Earth and the star. Some starlight passes through the atmosphere; in doing so, some wavelengths are absorbed or amplified by the atoms inside. Exactly which atoms work can be determined by which wavelengths are affected.
In this way, scientists were able to preliminarily detect the presence of nitrogen, the dominant component in the Earth’s atmosphere. If LHS-1140b were gassier, like tiny little Neptune, it would have an atmosphere richer in hydrogen. The presence of nitrogen suggests a secondary atmosphere—an atmosphere that formed after rather than with the exoplanet’s birth.
In a study published last year, the team also combined the density and radius of LHS-1140b to calculate its density. They got a number of 5.9 grams per cubic centimeter. This is not dense enough for a world that is made purely of stone; given its size, either a mini-Neptune or an ocean-covered water world is best suited. If we exclude mini-Neptune, we are left with a global oceanic exoplanet.
However, with tidal locking in mind, this global ocean may not look like what you might think. The side that is permanently turned away from the star could be cold enough to freeze. Only the spot directly facing the star would be warm enough to thaw, resulting in a world that looks like a ghostly eye floating in space.
However, this spot could reach a very pleasant 20 degrees Celsius (68 degrees Fahrenheit) at the surface – warm enough for a thriving marine ecosystem.
We don’t know for sure what’s going on, but it looks like the most promising candidate we have to date for an exotic alien ecosystem outside our planetary neighborhood, so you can bet your bottom dollar there’s a lot more to come. he was staring right back at that weird (possible) eyeball.
“Detecting an Earth-like atmosphere on a temperate planet pushes Webb’s capabilities to its limits – it’s doable; we just need a lot of time to observe,” says physicist René Doyon of the University of Montreal.
“The current indication of a nitrogen-rich atmosphere requires confirmation with additional data. We need at least one more year of observations to confirm that LHS 1140b has an atmosphere, and probably two or three more to detect carbon dioxide.”
The research was accepted by The Astrophysical Journal Lettersand is available on arXiv.