About 48 light years away, in the constellation Cetus, lies an ice-covered planet that appears straight out of star Wars Central casting. One hemisphere is always facing the parent star, creating a small molten ocean on the permanent day side, giving it the appearance of a gigantic eyeball.
Moreover, this body of water, almost twice the size of Earth, may be our best chance of finding habitable conditions elsewhere in the universe.
The latest image of LHS 1140 b (named after the star’s orbit around a red dwarf star called LHS 1140) was discovered in 2017and even then it seemed like a promising place to live.
Now, in a current paperResearchers at the Université de Montréal have upped the ante. After analyzing data from the James Webb Space Telescope, they concluded that the planet is a rocky super-Earth rather than a gaseous mini-Neptune, and that it likely has a nitrogen-rich atmosphere – two crucial factors in the emergence of biology as we know it.
Habitable planets
Astronomers have documented more than 5,700 exoplanets since 1992, and others could potentially host life.
“But despite all this,” says Charles Cadieux, the study’s lead author and a doctoral student in Montreal, “to our knowledge there are only three planets in the habitable zone that have an atmosphere: Earth, Mars, and LHS 1140 b could be the third.”
Jason Dittman, who led the team that discovered the planet and was not involved in Cadieux’s work, said the new findings strongly support the need to allocate more telescope time to observations in the future.
“We should seriously consider investing a large part of our precious resources in this planet,” he says. “It definitely has great promise.”
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 a huge gap between Earth (the largest of the inner rocky planets) and the gas giants Neptune and Uranus, which are four times larger and have fundamentally different compositions.
“For everything in between,” says Cadieux, “we’re not really sure about the composition of these planets.”
At 1.7 times the size of our planet, LHS 1140 b falls right in between. And which category it falls into makes all the difference. Without some kind of surface – which is present on rocky planets but absent on gas planets – life would have nowhere to take hold.
To distinguish between the two possibilities, Cadieux and his colleagues observed the “transit” of LHS 1140 b, the time it takes for it to pass between us and its star. By measuring how the star’s light spectrum changed as it passed the planet, they were able to rule out the presence of the hydrogen-rich atmosphere that characterizes gas giants. This method, known as transmission spectroscopy, confirmed that LHS 1140 b is likely a super-Earth rather than a mini-Neptune.
Read more: Which exoplanet is closest to Earth?
The possibility of water and atmosphere on LHS 1140 b
Just as importantly, their measurements provided the first evidence of an atmosphere on a potentially habitable exoplanet, with certain spectral features best explained by the light-scattering effect of nitrogen molecules – the same phenomenon that accounts for our blue skies. These results are preliminary, but if LHS 1140 b’s atmosphere is indeed dominated by nitrogen, it would mean that the planet has one of Earth’s life-supporting features.
And it probably has another feature: water. Despite its status as a super-Earth, LHS 1140 b is much less dense than you’d expect from a planet with Earth-like composition. That suggests a whopping 10 to 20 percent of its mass could be water, though mostly in the form of ice. 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 directly how much liquid water there might be. But Cadieux’s climate simulations (which assume Earth-like nitrogen and carbon dioxide levels) predicted a circular ocean nearly 4,000 kilometers in diameter – about half the size of the Atlantic – and a comfortable 20 degrees Celsius.
Read more: Space telescope begins to reveal the secrets of exoplanets’ atmospheres
Comparison of exoplanets
Although the JWST observations have improved the profile of LHS 1140 b, they have dampened the excitement surrounding another group of potentially habitable worlds: the seven exoplanets that TRAPPIST-1The entire formation was discovered in 2017, the same year as LHS 1140 b. It is made of rock and is about the size of Earth. Several parts lie within the “Goldilocks Zone” where surface water in liquid form could exist.
“It’s a fantastic system,” says Dittman, but the initial results from JWST were disappointing. Based on analyses of TRAPPIST-1 B And Cthe inner planets appear to have neither atmosphere nor water. “If that’s true for all planets, then we may be in trouble.”
Meanwhile, LHS 1140 b is looking increasingly tempting. Its star is less active than TRAPPIST-1, meaning it receives less X-ray and ultraviolet radiation – which can destroy 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, which amplifies the transmission of the spectroscopy signal. (Admittedly, TRAPPIST-1’s intense stellar activity may also interfere with this signal, making it difficult to interpret the atmospheric data.) In other words, even if LHS 1140 b is the better candidate for an atmosphere, it is harder to observe.
Dittman accepts this cosmic dilemma with the calmness of a scientist: “You get the planets you get.”
Read more: 6 exoplanets in our universe that could enable life outside of Earth
Decision on where to search for life outside of Earth
These considerations present the astronomy community with a difficult decision. Where and for how long should they look in the search for habitable exoplanets? As Dittman noted, Webb’s time is valuable; thousands of scientists from around the world vie for it each year. The telescope is expected to last for another two decades or so, but with its lens spread across so many projects, exoplanet researchers must choose their targets carefully.
When it comes to observing the atmospheres of distant planets, Dittman says, “you don’t get a lot of chances” – in the case of 1140 b, only four of its hour-long transits are visible to the telescope each year. “Basically, you have to capture every single one of those hours in the future,” he says. “We’re at a point where if we don’t start now, it’s just not going to happen.”
Jennifer Lotz, director of the Space Telescope Science Institute (which oversees the operations of the JWST and Hubble space telescopes), announced last month that she 500 hours of Webb time to study atmospheres on rocky exoplanets in nearby solar systems. It is not yet clear which planets the project will cover or how time will be divided between them.
It’s important to remember that JWST has only studied a handful of these planets so far. As it continues to search the universe for signs of life, it may find that rocky, temperate planets have plenty of atmospheres, adding to the list that begins with LHS 1140 b. But for now, Cadieux is banking on the eye in the sky.
“In terms of habitability,” he says, “I think it will still be unique.”
Read more: Why do astronomers look for signs of life on other planets?
Article Sources
Our authors at Discovermagazine.com We use peer-reviewed studies and high-quality sources for our articles. Our editors verify scientific accuracy and editorial standards. Review the sources used for this article below:
Cody Cottier is a contributing writer at Discover who enjoys 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.
