In a new study published in the Astrophysical Journal Letters, astronomers investigated the effects the activity of the red dwarf star Proxima Centauri might have on the atmosphere of one of its planets, Proxima c.
An artist’s impression of the Proxima Centauri system. Image credit: Lorenzo Santinelli.
Proxima Centauri, the smallest member of the Alpha Centauri system, is an M5.5-type star located 4.244 light-years away in the southern constellation of Centaurus.
The star has a measured radius of 14% the radius of the Sun, a mass of about 12% solar, and an effective temperature of only around 3,050 K (2,777 degrees Celsius, or 5,031 degrees Fahrenheit).
Proxima Centauri is 1,000 times less luminous than the Sun, which even at its close distance makes it invisible to the naked eye.
It has a very slow rotation of 83 days and a long-term activity cycle with a period of approximately 7 years. Its habitable zone ranges from distances of 0.05 to 0.1 AU.
In 2016, the Earth-mass planet Proxima b was discovered by a research team led by Queen Mary University of London astronomer Dr. Guillem Anglada-Escudé.
The planet has a mass close to that of Earth and orbits its star with a period of 11.2 days at an average distance of 0.05 AU. It sits within the star’s habitable zone, where liquid water could theoretically exist on the surface.
Proxima b is susceptible to stellar flares, winds, X-rays, and other kinds of activity that could disrupt its atmosphere and possibilities for life.
These activities are linked to the strong magnetic fields in M-dwarfs, and they remain active in dwarf stars over much longer timescales than in higher-mass stars like the Sun, so that the cumulative exposures are commensurately greater.
Proxima b is probably subject to stellar wind pressures 10,000 times larger than those exertred by the Sun on the Earth.
In 2020, the second planet was discovered in the Proxima Centauri system after Dr. Mario Damasso from Italy’s National Institute for Astrophysics and colleagues spotted slight variations in the orbital velocity of Proxima b.
Followup studies determined that the planet, named Proxima c, was a 7-Earth-mass super-Earth and orbited at 1.5 AU every 1,907 days.
In the new study, Dr. Julián Alvarado-Gómez of the Leibniz Institute for Astrophysics Potsdam, Dr. Jeremy Drake from the Harvard & Smithsonian Center for Astrophysics and colleagues analyzed the effects the star’s activity might have on the atmosphere of Proxima c.
The astronomers constructed a comprehensive numerical simulation of the space environment of the Proxima Centauri system, including models for the stellar corona and realistic surface magnetic field configurations during the minimum and maximum activity states of the star.
Their results indicate that Proxima c experiences Earth-like conditions, at least in terms of stellar wind effects.
It is not known whether or not Proxima c actually has an atmosphere, but the new models indicate the conditions are not unduly corrosive and are favorable for the persistence of any atmosphere that does exist.
“Our results indicate that Proxima c experiences Earth-like conditions in terms of the dynamic pressure exerted by the stellar wind along its 5.3-year orbit, with minor variability due to the activity cycle of the star,” the researchers said.
“To investigate the relative effect of such conditions on the energy dissipation in the upper atmosphere (Joule heating), we also simulated a possible magnetosphere and ionosphere around the planet.”
“We found that even with a relatively weak planetary dipole field, the associated Joule heating of the upper atmosphere is negligible for Proxima c, due to a diminished interplanetary magnetic field at the distance of the planet.”
“Whether or not Proxima c currently has an atmosphere would depend on several factors, including its formation channel and evolutionary path.”
“Nevertheless, at face value the resulting conditions from our models do not appear to be unduly corrosive and should be favorable for the persistence of any extant atmosphere, supporting the prospect of fruitful future observing campaigns.”
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Julián D. Alvarado-Gómez et al. 2020. An Earth-like Stellar Wind Environment for Proxima Centauri c. ApJL 902, L9; doi: 10.3847/2041-8213/abb885
