Stellar winds, magnetic activity and evaporating exoplanet atmospheres

An illustration of the TRAPPIST-1 system of seven planets around a dwarf star M. The star has a strong emission of X-rays and UV, as well as an ionized wind that can evaporate the atmosphere of a planet that orbits around. Astronomers have completed simulations using TRAPPIST-1 system parameters that reveal the complex possible consequences of a stellar wind in a planet’s atmosphere. Credit: NASA / JPL-Caltech

Most stars, including the Sun, generate magnetic activity that causes a rapidly moving ionized wind that also produces X-ray and ultraviolet emission (often known as XUV radiation). XUV radiation from a star can be absorbed into the upper atmosphere of an orbiting planet, where it is able to heat the gas enough to escape the planet’s atmosphere.

Dwarf stars M, by far the most common type of star, are smaller and cooler than the Sun and can have very active magnetic fields. Its cold surface temperatures make its habitable zones (HZ) close to the star (HZ is the range of distances at which the surface water of an orbiting planet can remain liquid).

Any rocky exoplanet orbiting an M dwarf at its HZ, because it is close to the star, is especially vulnerable to the effects of photoevaporation that can cause partial or even total removal from the atmosphere. Some theorists argue that planets with large envelopes of hydrogen or helium could be more habitable if photoevaporation removes the gas blanket enough.

The effects of XUV radiation on exoplanet atmospheres have been studied for almost twenty years, but the effects of stellar wind on exoplanet atmospheres are little known.

The Harvard-Smithsonian Center for Astrophysics (CfA) is a joint venture between the Smithsonian Astrophysics Observatory and Harvard College Observatory. Founded in 1973, the Harvard-Smithsonian Center for Astrophysics consists of six research divisions: Atomic and Molecular Physics; Optical and infrared astronomy; High energy astrophysics; Radio and Geoastronomy; Stellar, solar and planetary sciences; and Theoretical Astrophysics.

CfA astronomers Laura Harbach, Sofia Moschou, Jeremy Drake, Julian Alvarado-Gomez and Federico Frascetti and colleagues have completed simulations modeling the effects of a stellar wind on an exoplanet with a hydrogen-rich atmosphere orbiting near an dwarf star M. As an example, they use the exoplanet configuration in TRAPPIST-1, a cool dwarf star-M with a system of seven planets, six of which are close enough to the star to be in its HZ.

The simulations show that, depending on the details, the stellar wind can generate outbursts from a planet’s atmosphere. The team finds that both the star’s and the planet’s magnetic fields play an important role in defining many of the details of the output, which could be observed and studied through the atomic hydrogen lines in the planet. ultraviolet.

The complex results of the simulation indicate that the planets around the host stars of the dwarf M are likely to show a diverse range of atmospheric properties, and that some of the physical conditions may vary in the short term, leading to interpretations. observations of sequential exoplanet transits are more complex. The results of the simulation highlight the need to use three-dimensional simulations that include magnetic effects in order to interpret the observational results of the planets around the dwarf stars M.

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