- The exoplanet LHS 1140 b – in its star’s habitable zone – might have an atmosphere and an ocean of liquid water. It is also relatively nearby, only 48 light-years from Earth.
- It might be what’s called an eyeball planet, with a single liquid ocean surrounded by ice. Or it might be entirely ice-covered, with an ocean below the ice, similar to Jupiter’s moon Europa or Saturn’s moon Enceladus.
- Either way, further observations are needed. But if confirmed, it would be the first potentially habitable ocean world found outside our solar system.
Meet the intriguing exoplanet LHS 1140 b
A new study from researchers at the University of Montreal suggests that exoplanet LHS 1140 b is most likely a super-Earth. That would make it more massive than Earth, yet lighter than our solar system’s ice giants, Neptune and Uranus. This distant world lies in its star’s habitable zone, the zone in which liquid water might exist on a planet’s surface. And the researchers said on July 8, 2024, that the planet is likely a water world. There are, they said, two possibilities, given their current understanding. LHS 1140 b may have a global ice-covered ocean, similar to Jupiter’s moon Europa or Saturn’s moon Enceladus. Or it might have a vast “eyeball” liquid ocean on its surface, surrounded by ice. Creepy as it sounds, scientists call such hypothesized worlds eyeball planets.
This new analysis, made from data gathered by the James Webb Space Telescope and other space-based telescopes, tentatively hints that LHS 1140 b has a nitrogen-rich atmosphere like Earth’s. If so, then computer models favor the eyeball ocean scenario.
These results make LHS 1140 b one of the most promising exoplanets found so far in the habitable zone of its star. And it’s relatively close, too, at only 48 light-years away.
The research team published the intriguing peer-reviewed findings in The Astrophysical Journal Letters on July 10, 2024.
Mini-Neptune or super-Earth?
LHS 1140 b has been the subject of debate among planetary scientists. They thought it might be a super-Earth. But it also might be a mini-Neptune, resembling our solar system’s planet Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans.
Previous observations of LHS 1140 b had been inconclusive. Was it a mini-Neptune with a deep hydrogen atmosphere? Or a super-Earth, a rocky world larger than Earth with a possible thinner, more Earth-like atmosphere and water? Or even a Hycean planet, with a deep global ocean and hydrogen atmosphere?
The newest results favored the super-Earth interpretation.
What astronomers did know was that LHS 1140 b orbits within the habitable zone of its red dwarf star. That is the region around a star where temperatures could allow liquid water to exist on a planet. In addition, they also measured its radius and mass. The planet is 1.7 times the size of Earth and 5.6 times its mass.
Now, the researchers say that the mini-Neptune scenario has been strongly excluded. That leaves LHS 1140 b as being some type of water world. But what kind, exactly?
Nitrogen atmosphere on an ocean world?
The astronomers combined the new data from Webb with previous data from Spitzer, Hubble and TESS. It showed that the planet is less dense than it should be if it were just a solid rocky world like Earth. In fact, 10-20% of its mass may be composed of water. That would mean a deep global ocean, either under a hydrogen atmosphere (a Hycean water world) or a more Earth-like atmosphere of nitrogen.
Webb revealed that LHS 1140 b does not have a thick hydrogen atmosphere. That’s why the mini-Neptune hypothesis was ruled out. However, it also means that the planet is unlikely to be a Hycean water world, either. Instead, LHS 1140 b’s ocean may be covered by ice, at least mostly.
Intriguingly, Webb’s analysis does hint at a nitrogen atmosphere on LHS 1140 b. The paper stated:
Our retrieval analysis identifies tentative evidence for a cloud-free N2-dominated atmosphere on LHS 1140 b. Our tantalizing evidence for a N2-dominated atmosphere on a habitable-zone super-Earth raises a resounding call for additional observations of LHS 1140 b to confirm this result.
This is a great thread about the discovery on X, by Ryan MacDonald at the University of Michigan:
? An atmosphere on a super-Earth in the habitable zone? ???
We're excited to share our new @NASAWebb observations of the super-Earth LHS 1140b.
THREAD https://t.co/fXds4K3arL pic.twitter.com/jTYcGOlnyW
— Ryan MacDonald (@MartianColonist) July 8, 2024
An eyeball planet?
So, if it is indeed a water world, as Webb suggests, then the data indicate two possibilities. One is a global ocean covered by a layer of ice, similar to the moons Europa or Enceladus in our solar system. The other is that LHS 1140 b is still mostly ice-covered, but with a large eyeball ocean on the side facing its star. That’s because the planet is thought to be tidally locked to its star, so the same side of the planet always faces the star. Indeed, scientists have hypothesized about such planets – also called eyeball planets – existing before, but LHS 1140 b could be the first one actually discovered.
To be sure, that finding still needs further confirmation. But if it holds up, then the researchers say that the planet probably does have an eyeball ocean. More specifically, the researchers calculated that the ocean would be about 2,500 miles (4,000 km) across, or about half the surface area of the Atlantic Ocean. This would be open ocean, like on Earth, surrounded by the ice covering the rest of the planet. The simulation models of this ocean even indicate that the temperature in the center of the ocean could be a balmy 70 degrees Fahrenheit (20 C).
A major milestone
But indeed, any kind of ocean on LHS 1140 b would be an exciting discovery. Lead author Charles Cadieux, a Ph.D. student at University of Montreal’s Trottier Institute for Research on Exoplanets (iREx), said:
Of all currently known temperate exoplanets, LHS 1140 b could well be our best bet to one day indirectly confirm liquid water on the surface of an alien world beyond our solar system. This would be a major milestone in the search for potentially habitable exoplanets.
More observations needed to confirm ocean world
Right now, the results from Webb strongly support LHS 1140 b being an ocean world with a tentative nitrogen atmosphere. However, further observations are needed to fully confirm this. Astronomers need to take additional eclipse and transit measurements of the planet, which could take several years. They also want to look for carbon dioxide in the atmosphere, as this would further support the findings. René Doyon, also at iREx, said:
Detecting an Earth-like atmosphere on a temperate planet is pushing Webb’s capabilities to its limits; it’s feasible; we just need lots of observing time. The current hint of a nitrogen-rich atmosphere begs for confirmation with more data. We need at least one more year of observations to confirm that LHS 1140 b has an atmosphere, and likely two or three more to detect carbon dioxide.
Finding exoplanets with liquid water is an important step toward determining which ones may actually be habitable, by earthly standards. Will LHS 1140 b be the first one? Only time and more observations will tell, but this is a tentatively exciting and promising discovery.
Bottom line: NASA’s Webb space telescope has revealed that exoplanet LHS 1140 b is a super-Earth ocean world. It may have an ocean on one side or a global ice-covered ocean.
Source: Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS
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