Of all the potential super-Earths—terrestrial exoplanets more massive than Earth—out there, an exoplanet orbiting a star just 40 light-years away in the constellation Cetus may be the most similar yet found.
The exoplanet LHS 1140b was thought to be a mini-Neptune when it was first discovered by NASA’s James Webb Space Telescope in late 2023. After analyzing data from these observations, a team of researchers led by astronomer Charles Cadieux of the Université de Montreal suggest that LHS 1140 b is more likely super-earth.
If this planet is an alternate version of our own, its relative proximity to its cool red dwarf star means it would most likely be a giant snowball or mostly frozen body with a substellar (the region closest to its star) ocean that makes it look like a cosmic eye bulb. It is now thought to be the exoplanet with the best chance for liquid water on its surface, and therefore could even be habitable.
Cadieux and his team claim to have found “teasing evidence for a [nitrogen]-dominated atmosphere on the habitable zone of a super-Earth” in a study recently published in The Astrophysical Journal Letters.
Sorry, Neptune…
In December 2023, two transits of LHS 1140b were observed by the NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument on board Webb. NIRISS specializes in detecting exoplanets and finding out more about them through transit spectroscopy, which captures the light of a host star orbiting a planet as it passes through that planet’s atmosphere and travels toward Earth. Analyzing the different spectral bands in this 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 scientists created a 3D global climate model, or GCM. It used complex mathematics to examine the various combinations of factors that make up the planet’s climate system, such as the land, oceans, ice and atmosphere. Several different GCMs of mini-Neptune have been compared with the light spectrum observed through transit spectroscopy. A model for mini-Neptune typically includes a gas giant with a dense, cloudless or nearly cloudless, hydrogen-dominated atmosphere, but the spectral bands of this model do not match the NIRISS observations.
With the possibility of a mini-Neptune mostly ruled out (although further observations and analysis will be needed to confirm), Cadieux’s team settled on another possibility: a super-Earth.
Earth far from Earth?
The spectra observed by NIRISS were more consistent with super-Earth GCMs. This type of planet should typically have dense nitrogen or CO2-a rich atmosphere surrounding a rocky surface that had some form of water on it, either in frozen or liquid form.
The models also suggested a secondary atmosphere, which is the atmosphere formed after the original atmosphere of light elements (hydrogen and helium) escaped during the early stages of planet formation. Secondary atmospheres are formed from 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 data that did not match the GCM was that the planet has a lower density (based on measurements of its size and mass) than expected for a rocky world. This is consistent with a water world with a mass that is about 10 to 20 percent water. Based on this estimate, scientists believe that LHS 1140 b could even be a Hycene planet—an oceanic planet that has most of the attributes of a super-Earth, but with an atmosphere dominated by hydrogen instead of nitrogen.
Because it orbits a dim star close enough to be tidally locked, some models suggest a mostly icy planet with a substellar liquid ocean on its dayside.
While LHS 1140 b may be a super-Earth, the Hycean planet hypothesis could ultimately be ruled out. Hycean planets are prone 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 eventually evaporates on a planet that cannot cool itself.
While we are getting closer to finding out what kind of planet LHS 1140 b is and whether it could be habitable, more observations are needed. Cadieux wants to continue this research by comparing NIRISS data with data on other super-Earths previously collected by Webb’s Near-Infrared Spectrograph, or NIRSpec, instrument. At least three transit observations of the planet using Webb’s MIRI, or Mid-Infrared Instrument, are also needed to ensure that stellar radiation does not interfere with observations of the planet itself.
“Due to the limited visibility of LHS 1140b, several years of observation may be required to detect its potential secondary atmosphere,” the researchers said in the same study.
So could this planet really be a frozen exo-earth? The tension will last for several years.
The Astrophysical Journal Letters, 2024. DOI: 10.3847/2041-8213/ad5afa