Planetary scientists used to believe that an exoplanet could be habitable by having a strong magnetic field. This vector field surrounding the planet can shield against ionized particles, X-rays, and ultraviolet radiation from the stellar wind.
This shielding mechanism happens on Earth, preventing harmful radiation from reaching living organisms on the surface. Meanwhile, Mars lacks a global magnetic field, causing initial inhabitants to live in underground caves and cavities to get protected.
The same mechanism can happen with exoplanets as they interact with their host star. In a new study, researchers from the University of Massachusetts examined the impact of electric currents on the ionosphere of an exoplanet popularly known as TRAPPIST-1e. The results of their research are discussed in the paper "Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid Orbital Motion through an Extreme Space Environment."
Earth-Like Exoplanet
TRAPPIST-1 is a cool red M-dwarf star located in the constellation Aquarius, around 41 light years away from Earth. This planetary system has seven known exoplanets, making it one of the most closely studied systems outside our own.
Three of the seven exoplanets are in the star's habitable zone, known for having surface temperatures where liquid water can exist. Since M-dwarf stars are cooler than the Sun, their habitable zones are much closer to them.
One of the famous exoplanets in this planetary system is TRAPPIST-1e, which was discovered in 2017. It is so close to its host that it orbits just 0.028 AU from it.
Aside from being rocky, this planet resembles the Earth in many ways. First, TRAPPIST-1e's average density is only 2% larger than the Earth's. It also has a surface gravity of 82% and an equilibrium temperature of just 9 K below Earth's.
READ ALSO: Are TRAPPIST-1 Exoplanets Habitable? Here's What Experts Say
Rapid Atmospheric Loss
Despite being one of the most interesting exoplanets discovered to date, TRAPPIST-1e is much closer to its star. This makes astronomers wonder if its atmospheric stripping due to stellar winds is much stronger. To answer this question, Ofer Cohen and colleagues modeled a phenomenon that could affect the planetary atmospheres of TRAPPIST-1e.
Previous studies revealed that stellar winds from TRAPPIST-1 can strip a hydrogen-rich atmosphere from its exoplanets through photoevaporation. In the new model, the researchers try to determine the impact of heating due to the planet's motion itself.
Since they do not have TRAPPIST's stellar wind and magnetic field measurements, the astronomers used validated models to calculate its energy output. The models also examined the star's solar wind as the changing magnetic field at the TRAPPIST-1e distance.
Using the estimates for the exoplanet's ionosphere's width, the study's result reveals that the heating energy flux in its upper atmosphere would vary from 0.01 to 100 watts per square meter. According to the researchers, these intense values can cause the atmosphere to escape and lead to a rapid loss, making TRAPPIST-1e uninhabitable. According to Cohen, the study indicates that exoplanets that are very close to their stars are likely to bare planets with no atmosphere.
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