Some 48 light-years away, in the constellation Cetus, lies an ice-covered planet straight out of Star Wars central casting. One hemisphere always faces the host star, creating a small melted ocean on the permanent day side that gives it the appearance of a gargantuan eyeball.
What’s more, this watery orb, almost twice as large as Earth, may be our best shot at finding habitable conditions elsewhere in the universe.
That, anyway, is the most up-to-date depiction of LHS 1140 b, which we can't see directly. The planet (so named because it orbits a red dwarf star called LHS 1140), was discovered in 2017, and even then it seemed an auspicious place for life.
Now, in a recent paper, researchers at the Université de Montréal have upped the ante. After analyzing data from the James Webb Space Telescope, they concluded the planet is more likely a rocky, super-Earth than a gaseous mini-Neptune and that it probably has a nitrogen-rich atmosphere — two crucial factors for the emergence of biology as we know it.
Habitable Planets
Astronomers have documented more than 5,700 exoplanets since 1992, and others could potentially harbor life.
“But in all of that,” says Charles Cadieux, lead author on the paper and a doctoral student at Montréal, “there are only three planets in the habitable zone that we know that have an atmosphere: It’s Earth, it’s Mars, and LHS 1140 b could be the third one.”
Jason Dittman, who led the team that discovered the planet and was not involved with Cadieux’s work, said the new findings make a strong case for a “super significant” allotment of future telescope time.
“This is the planet we should seriously consider investing a lot of our precious resources into,” he says. “It’s definitely promising.”
A Potentially Habitable Super-Earth
Scientists have a complicated understanding of LHS 1140 b because they’ve never seen anything like it up close. In our own solar system there’s an immense gap between Earth (the largest of the rocky inner planets) and the gas giants Neptune and Uranus, which are four times larger with a fundamentally different makeup.
“Everything in between,” Cadieux says, “we’re not really sure about the composition of those planets.”
At 1.7 times the size of our planet, LHS 1140 b sits squarely in between. And which category it falls under makes all the difference. Without some kind of surface — present on rocky planets but absent on gaseous ones — there would be nowhere for life to gain a foothold.
To discern between the two options, Cadieux and his colleagues observed LHS 1140 b’s “transit,” the period when it passes between us and its star. By measuring how the star’s light spectrum changed as it shone past the planet, they were able to rule out the presence of the hydrogen-rich atmosphere that’s characteristic of gas giants. This method, known as transmission spectroscopy, confirmed that LHS 1140 b is likely more super-Earth than mini-Neptune.
Read More: What Exoplanet Is Closest to Earth?
The Possibility of Water and Atmosphere on LHS 1140 b
Just as importantly, their measurements offered the first hint of atmosphere on a potentially habitable exoplanet, with certain spectral features best explained by the light-scattering effect of nitrogen molecules — the same phenomenon that’s responsible for blue skies here at home. These results are tentative. But if LHS 1140 b’s atmosphere is indeed dominated by nitrogen, it would mean the planet boasts one of Earth’s life-sustaining hallmarks.
It also seems likely to boast another: water. Despite its status as a super-Earth, LHS 1140 b is much less dense than expected for a planet with an Earth-like composition, suggesting that a whopping 10-20 percent of its mass may be water, though mostly in ice form — because the planet is locked in synchronous rotation with its star, just like the moon is with Earth, there’s always a frozen side.
Unfortunately, we can’t see how much liquid water there might be directly. But Cadieux’s climate simulations (which assume Earth-like levels of nitrogen and carbon dioxide) produced a bull’s-eye ocean nearly 2,500 miles in diameter — about half the size of the Atlantic Ocean — and a balmy 68 degrees Fahrenheit.
Read More: Space Telescope Begins To Reveal the Secrets of Exoplanet Atmospheres
Comparing Exoplanets
Even as JWST observations have boosted the profile of LHS 1140 b, they’ve tempered excitement around another set of potentially habitable worlds: the seven exoplanets orbiting TRAPPIST-1. Discovered in 2017, the same year as LHS 1140 b, the whole lineup is rocky and roughly Earth-sized, and several lies within the “Goldilocks zone” that would allow surface water in liquid form.
“It’s an awesome system,” Dittman says, but early JWST results have been deflating. Based on analyses of TRAPPIST-1 b and c, the inner planets appear devoid of both atmosphere and water. “If this holds true all the way out, then maybe we’re in trouble.”
Meanwhile, LHS 1140 b keeps looking more enticing. Its star is less active than TRAPPIST-1, meaning it receives less X-ray and ultraviolet radiation — which can strip away an atmosphere — than the planets in that system.
But TRAPPIST-1 also has a big advantage; the star is so small that its planets are much larger in comparison, boosting the transmission of the spectroscopy signal. (Granted, the intense stellar activity of TRAPPIST-1 can also interfere with that signal, making it hard to interpret the atmospheric data). In other words, even if LHS 1140 b is the better candidate for an atmosphere, it’s harder to observe.
Dittman accepts this cosmic catch-22 with a scientist’s composure, “You’re dealt the planets you’re dealt.”
Read More: 6 Exoplanets in our Universe That Could Support Life Other Than Earth
Deciding Where to Search for Life Beyond Earth
These considerations leave the astronomical community with a tough decision. In the search for habitable exoplanets, where should they look, and for how long? As Dittman noted, Webb's time is precious; thousands of scientists from around the world vie for it each year. The telescope is expected to survive perhaps another two decades, but with its lens spread across so many projects, exoplanetary researchers must choose their targets wisely.
When it comes to observing the atmospheres of distant planets, Dittman says, “you don’t get many bites at the apple” — in the case of 1140 b, only four of its hour-long transits are visible to the telescope each year. “Basically, you need to hit every single one of those hours going forward,” he says. “We’re at this moment where if we don’t start now, it just won’t happen.”
Jennifer Lotz, the director of the Space Telescope Science Institute (which oversees the operation of the JWST and Hubble space telescopes), announced last month that she’d allocate 500 hours of Webb time to investigate atmospheres on rocky exoplanets in nearby solar systems. It’s not yet clear which planets the project will cover or how the time will be split among them.
It’s worth remembering that the JWST has only examined a handful of these planets so far. As it continues scouring the universe for signs of life, it may turn out that atmospheres abound on rocky, temperate planets, adding to the list that begins with LHS 1140 b. But for now, Cadieux is betting on the eye in the sky.
“In terms of habitability,” he says, “I think it will still be very unique.”
Read More: Why Do Astronomers Look For Signs Of Life On Other Planets?
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Nature. A temperate rocky super-Earth transiting a nearby cool star
The Astrophysical Journal Letters. Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS
NASA. Discoveries Dashboard
NASA. Largest Batch of Earth-size Habitable Zone Planets Found Orbiting TRAPPIST-1
Space Telescope Science Institute. STScI Initiates a Concerted Search for Atmospheres around M-dwarf Exoplanets
Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.
