Of all of the potential super-Earths—terrestrial exoplanets extra huge than Earth—on the market, an exoplanet orbiting a star solely 40 light-years away from us within the constellation Cetus could be probably the most just like have been discovered to this point.
Exoplanet LHS 1140 b was assumed to be a mini-Neptune when it was first found by NASA’s James Webb Area Telescope towards the tip of 2023. After analyzing knowledge from these observations, a staff of researchers, led by astronomer Charles Cadieux, of Université de Montréal, recommend that LHS 1140 b is extra prone to be a super-Earth.
If this planet is an alternate model of our personal, its relative proximity to its cool crimson dwarf star means it could more than likely be a gargantuan snowball or a principally frozen physique with a substellar (area closest to its star) ocean that makes it seem like a cosmic eyeball. It’s now regarded as the exoplanet with the very best likelihood for liquid water on its floor, and so may even be liveable.
Cadieux and his staff say they’ve discovered “tantalizing proof for a [nitrogen]-dominated environment on a liveable zone super-Earth” in a examine just lately revealed in The Astrophysical Journal Letters.
Sorry, Neptune…
In December 2023, two transits of LHS 1140 b had been noticed with the NIRISS (Close to-Infrared Imager and Slitless Spectrograph) instrument aboard Webb. NIRISS focuses on detecting exoplanets and revealing extra about them via transit spectroscopy, which picks up the sunshine of an orbiting planet’s host star because it passes via the environment of that planet and travels towards Earth. Evaluation of the totally different spectral bands in that gentle can then inform scientists in regards to the particular atoms and molecules that exist within the planet’s environment.
To check the earlier speculation that LHS 1140 b is a mini-Neptune, the researchers created a 3D international local weather mannequin, or GCM. This used advanced math to discover totally different mixtures of things that make up the local weather system of a planet, equivalent to land, oceans, ice, and environment. A number of totally different GCMs of a mini-Neptune had been in contrast with the sunshine spectrum noticed by way of transit spectroscopy. The mannequin for a mini-Neptune usually includes a gasoline big with a thick, cloudless or practically cloudless environment dominated by hydrogen, however the spectral bands of this mannequin didn’t match NIRISS observations.
With the potential of a mini-Neptune being principally dominated out (although additional observations and evaluation will probably be wanted to verify this), Cadieux’s staff turned to a different risk: a super-Earth.
An Earth away from Earth?
The spectra noticed with NIRISS had been extra in keeping with GCMs of a super-Earth. One of these planet would usually have a thick nitrogen or CO2-rich environment enveloping a rocky floor on which there was some type of water, whether or not in frozen or liquid kind.
The fashions additionally advised a secondary environment, which is an environment shaped after the unique environment of sunshine components, (hydrogen and helium) escaped throughout early phases of a planet’s formation. Secondary atmospheres are shaped from heavier components launched from the crust, equivalent to water vapor, carbon dioxide, and methane. They’re normally discovered on heat, terrestrial planets (Earth has a secondary environment).
Essentially the most vital Webb/NIRISS knowledge that didn’t match the GCMs was that the planet has a decrease density (primarily based on measurements of its measurement and mass) than anticipated for a rocky world. That is in step with a water world with a mass that’s about 10 to twenty p.c water. Based mostly on this estimate, the researchers assume that LHS 1140 b may even be a hycean planet—an ocean planet that has a lot of the attributes of a super-Earth, however an environment dominated by hydrogen as an alternative of nitrogen.
Because it orbits a dim star carefully sufficient to be tidally locked, some fashions recommend a principally icy planet with a substellar liquid ocean on its dayside.
Whereas LHS 1140 b could also be a super-Earth, the hycean planet speculation may find yourself being dominated out. Hycean planets are susceptible to the runaway greenhouse impact, which happens when sufficient greenhouse gases accumulate in a planet’s environment and forestall warmth from escaping. Liquid water will finally evaporate on a planet that can’t cool itself off.
Although we’re getting nearer to discovering out what sort of planet LHS 1140 b is, and whether or not it might be liveable, additional observations are wanted. Cadieux needs to proceed this analysis by evaluating NIRISS knowledge with knowledge on different super-Earths that had beforehand been collected by Webb’s Close to-Infrared Spectrograph, or NIRSpec, instrument. A minimum of three transit observations of the planet with Webb’s MIRI, or Mid-Infrared instrument, are additionally wanted to ensure stellar radiation just isn’t interfering with observations of the planet itself.
“Given the restricted visibility of LHS 1140b, a number of years’ value of observations could also be required to detect its potential secondary environment,” the researchers mentioned in the identical examine.
So might this planet actually be a frozen exo-earth? The suspense goes to final a number of years.
The Astrophysical Journal Letters, 2024. DOI: 10.3847/2041-8213/ad5afa
