Possible atmospheric destruction of a potentially habitable exoplanet

Astrophysicists studying a popular exoplanet in its star’s habitable zone have found that electrical currents in the planet’s upper atmosphere could create enough heating to expand the atmosphere enough for it to leave the planet, likely leaving it uninhabitable.

Until now, planetary scientists believed that a habitable planet needed a strong magnetic field surrounding it to act as a shield, directing ionized particles, X-rays and ultraviolet radiation from the stellar wind around and away from it. its atmosphere.

This is what happens on Earth, preventing dangerous radiation from reaching life on the surface, and what does not happen on Mars, which now lacks a global magnetic field, meaning that the first inhabitants of the red planet will probably have to live in caves and underground cavities. for sun protection against wind.

The new research, by Ofer Cohen of the Lowell Center for Space Science and Technology at the University of Massachusetts Lowell and colleagues, published in The Astrophysics Journalexamined whether electrical currents generated in the ionosphere of the exoplanet Trappist-1e would cause the atmosphere to warm and expand enough that it could dissipate away from the planet’s gravity and be lost to space .

TRAPPIST-1e is an M dwarf star located in the constellation Aquarius, approximately 41 light years from Earth. Its planetary system, which has seven observed exoplanets, is the most studied system outside our own solar system.

Three of these planets are in the star’s habitable zone, with surface temperatures where liquid water could exist. Since M dwarfs, which make up about 70% of the stars in the universe, are cooler than our sun, these areas are much closer to these stars.

Trappist-1e, an exoplanet discovered in 2017, orbits just 0.028 AU from its star (where 1 AU is the average distance between the sun and Earth; Mercury orbits about 0.4 AU). Rocky and Earth-like, its average density is only 2% greater than Earth’s and its surface gravity is 82%. In addition, its equilibrium temperature is 246 Kelvin, only 9 K below that of the Earth.

These properties make Trappist-1e one of the most interesting exoplanets discovered to date. But is there an atmosphere? Because it is located much closer to its star, the atmospheric destruction by stellar winds should be much stronger than, for example, that of Mercury, which has no atmosphere.

Previous work has shown that Trappist-1’s stellar winds could potentially rob its exoplanets of a hydrogen-rich atmosphere through photoevaporation, but the complexity of the modeling means these planets could have a multitude of atmospheric environments.

But another potential stripping mechanism occurs when external charged stellar winds impact the ionized upper atmosphere. In previous work, Cohen and others found that when the conductance and impedance of each are similar in magnitude, the three Trappist exoplanets e, f, and g could experience direct current (DC) resistive heating of up to at 1 watt per square meter, 1% of incoming solar irradiation and 5 to 15 times the stellar energy from extreme ultraviolet radiation. Such “Joule heating” could potentially rob the atmosphere of any of these planets. (On Earth, Joule heating is approximately 0.01 W/m².)

Cohen and his colleagues modeled a second phenomenon that could also impact Trappist-1’s planetary atmospheres: warming due to the movement of the planet itself. Alternating electric currents (AC) will be generated in the planet’s upper atmosphere when it encounters a changing stellar magnetic field as the planet orbits its star (Faraday’s law of induction).

Nearby planets orbit very quickly (the orbital period of Trappist-1e is only 6.1 Earth days) and the rapid change in the background magnetic field leads to the generation of strong ionospheric currents which dissipate and potentially create heating very high, which they call tension. Joule controlled heating.

Because astronomers do not have measurements of Trappist-1’s stellar wind and magnetic field, the group used validated physics-based models to calculate its power output, solar wind, and magnetic field evolution at the Trappist-1e distance. Using reasonable estimates of the width of the Trappist 1e ionosphere, its conductance, and the magnitude of the changing magnetic field, their results show that the flow of Joule heating energy in the planet’s upper atmosphere would vary by 0.01 to 100 W/m.²a significant amount of heating that may be greater than that due to extreme ultraviolet and 1-10% of the planet’s stellar energy flow.

They conclude that such intense values ​​could cause strong atmospheric leakage and “could lead to rapid loss of the atmosphere.” This means that astrobiologists and others should take Joule heating into account when considering the habitability of an exoplanet.

“It is likely that both mechanisms work together in nearby exoplanets,” Cohen said. “Therefore, our work (and our knowledge of the solar system) may suggest that exoplanets located very close to the star are likely bare planets, without atmosphere.”

Cohen notes that their work has a political element, as many teams study the atmospheres of Trappist-1 planets. The James Webb Space Telescope (JWST) has already begun observing planetary atmospheres in this system (finding none), and there are plans to do more. “It can be a bit of a waste of resources if there’s no atmosphere for studying,” Cohen said.

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