Of all the potential super-Earths (terrestrial exoplanets more massive than Earth), an exoplanet orbiting a star just 40 light-years away in the constellation Cetus may be the most similar one found so far.
When exoplanet LHS 1140 b was first discovered by NASA’s James Webb Space Telescope in late 2023, it was assumed to be a mini-Neptune. After analyzing data from those observations, a team of researchers led by astronomer Charles Cadieux of the Université de Montréal suggests LHS 1140 b is more likely a super-Earth.
If this planet is an alternate version of our own, its relative proximity to its cool red dwarf star means it is likely a giant snowball or a largely frozen body with a substellar (the region closest to its star) ocean that makes it look like a cosmic eyeball. It is now thought to be the exoplanet with the best chance of having liquid water on its surface, and thus perhaps even habitable.
Cadieux and his team say they have found “intriguing evidence for a [nitrogen]-dominated atmosphere in a habitable zone above Earth” in a study recently published in The Astrophysical Journal Letters.
Sorry, Neptune…
In December 2023, two transits of LHS 1140 b were observed with the NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument aboard Webb. NIRISS specializes in detecting exoplanets and revealing more about them through transit spectroscopy, which picks up the light from an orbiting planet’s host star as it passes through the planet’s atmosphere and travels toward Earth. Analysis of the different spectral bands in that light can then tell scientists about the specific atoms and molecules that exist in the planet’s atmosphere.
To test the previous hypothesis that LHS 1140 b is a mini-Neptune, the researchers created a 3D global climate model, or GCM. This used complex mathematics to examine different combinations of factors that make up a planet's climate system, such as land, oceans, ice and atmosphere. Different GCMs of a mini-Neptune were compared to the spectrum of light observed via transit spectroscopy. The mini-Neptune model typically involves a gas giant with a thick, cloudless or nearly cloudless atmosphere dominated by hydrogen, but the spectral bands from this model did not match NIRISS observations.
With the possibility of a mini-Neptune largely ruled out (though further observations and analysis are needed to confirm this), Cadieux's team turned to another possibility: a super-Earth.
An Earth far from Earth?
The spectra observed with NIRISS were more consistent with GCMs of a super-Earth. This type of planet would typically have a thick nitrogen or CO2 layer.2-rich atmosphere surrounding a rocky surface on which water, either frozen or liquid, existed.
The models also suggested a secondary atmosphere, an atmosphere formed after the original atmosphere of light elements (hydrogen and helium) escaped during the early stages of a planet's formation. Secondary atmospheres are formed by heavier elements released from the crust, such as water vapor, carbon dioxide, and methane. They are usually found on warm, terrestrial planets (Earth has a secondary atmosphere).
The most significant Webb/NIRISS finding that did not match the GCMs was that the planet is less dense (based on measurements of its size and mass) than expected for a rocky world. This is consistent with a water world with about 10 to 20 percent of its mass composed of water. Based on this estimate, the researchers believe that LHS 1140 b could even be a Hycean planet: an ocean planet that has most of the characteristics of a super-Earth but an atmosphere dominated by hydrogen instead of nitrogen.
Because the planet orbits so close to a dim star that it is tidally affected, some models suggest it is a largely icy planet with a substellar liquid ocean on its day side.
While LHS 1140 b could be a super-Earth, the Hycean planet hypothesis can ultimately be ruled out. Hycean planets are susceptible to the runaway greenhouse effect, which occurs when enough greenhouse gases build up in a planet's atmosphere and prevent heat from escaping. Liquid water will eventually evaporate on a planet that can't cool itself.
While we're getting closer to discovering what kind of planet LHS 1140 b is, and whether it could be habitable, more observations are needed. Cadieux plans to continue this research by comparing NIRISS data with data of other super-Earths previously collected by Webb's Near-Infrared Spectrograph, or NIRSpec, instrument. At least three transit observations of the planet with Webb's MIRI, or Mid-Infrared instrument, are also needed to ensure that stellar radiation isn't interfering with observations of the planet itself.
“Given the low visibility of LHS 1140b, several years of observations may be needed to detect its potential secondary atmosphere,” the researchers said in the same study.
Could this planet really be a frozen exo-Earth? The suspense will last for a few more years.
The Astrophysical Journal Letters, 2024. DOI: 10.3847/2041-8213/ad5afa
