Of all the possible exoplanets—terrestrial exoplanets larger than Earth—except an exoplanet orbiting a star only 40 years away in our galaxy of Cetus may be the most similar one found so far.
Exoplanet LHS 1140 was considered a mini-Neptune when it was first discovered by NASA’s James Webb Space Telescope in late 2023. After analyzing data from their observations, a team of researchers , led by astronomer Charles Cadieux, Université de. Montréal, suggests that LHS 1140 has the potential to be a supermassive Earth.
If this planet is one of our own, its close proximity to its cool red star means it will likely be a snowball or frozen body with a small interstellar ocean (a region of near its star) which makes it look like. the eye of the universe. It is now thought to be the exoplanet that has the best chance of liquid water on it, and therefore potentially habitable.
Cadieux and his team say they found “surprising evidence for a [nitrogen]-controlled by the atmosphere in a habitable super-Earth” in a study recently published in The Astrophysical Journal Letters.
Sorry, Neptune…
In December 2023, two transits of LHS 1140 b were observed by the NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument aboard Webb. NIRISS works hard to detect exoplanets and reveal more about them through transit spectroscopy, which captures the light of a planet’s orbiting star as it passes through that planet’s atmosphere and toward Earth. Analysis of the different spectral bands in that light can now tell scientists about the specific atoms and molecules present in the planet’s atmosphere.
To test the previous hypothesis that LHS 1140 b is a miniature Neptune, the researchers created a 3D global climate model, or GCM. This used complex calculations to analyze the various elements that make up the earth’s climate, such as land, oceans, ice and atmosphere. Several different GCMs of mini-Neptune were compared to the observed brightness of the transit telescope. A mini-Neptune model usually includes a gas giant with a dense, cloudless or nearly cloudless atmosphere dominated by hydrogen, but the spectral bands of this type did not match the NIRISS observations. .
With the possibility of a mini-Neptune ruled out (although more observation and analysis will be needed to confirm this), Cadieux’s team turned to another possibility: the super-Earth.
Earth away from Earth?
The NIRISS observations were more consistent with the upper Earth GCMs. This type of planet would have dense nitrogen or CO2-a rich atmosphere covering a rocky area in which there was some kind of water, either frozen or liquid.
The models also suggest a secondary atmosphere, which is the atmosphere that formed after the first atmosphere of light elements, (hydrogen and helium) escaped during the first stages of the formation of the planet. The secondary atmosphere is made up of heavy elements from the surface, such as water vapor, carbon dioxide and methane. They are usually found on warm, terrestrial planets (Earth has a second atmosphere).
The most important Webb/NIRISS data that disagreed with the GCMs is that the planet has a lower mass (based on measurements of its size and mass) than expected on Earth and stones. This corresponds to a watery earth with a mass of about 10 to 20 percent water. Based on this estimate, researchers think that LHS 1140 may be a hycean planet — an ocean planet with many of the characteristics of a high Earth, but with an atmosphere dominated by hydrogen instead of nitrogen.
Because it orbits a star so faint that it can be well locked, some models suggest an icy planet with a liquid ocean during the day.
Although LHS 1140 may be the largest Earth, the hycean planet hypothesis may finally be dismissed. Hycean planets are prone to the damaging greenhouse effect, which occurs when enough greenhouse gases build up in the planet’s atmosphere and prevent heat from escaping. Eventually the liquid water will evaporate on a planet that cannot cool itself.
Although we are getting closer to determining what kind of planet LHS 1140 b is, and whether it is habitable, further observations are needed. Cadieux wants to continue this research by comparing the NIRISS data to other high-resolution data previously collected by Webb’s Near-Infrared Spectrograph, or NIRSpec, instrument. At least three observations of the planet by Webb’s MIRI, or Mid-Infrared instrument, are also needed to ensure that the star’s rays do not interfere with the view of the planet itself.
“Due to the low visibility of LHS 1140b, several years of observation may be required to detect its second atmosphere,” the researchers said in the same study.
So could this planet really be an icy exo-earth? The effect will last for several years.
The Astrophysical Journal Letters, 2024. DOI: 10.3847/2041-8213/ad5afa
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