Exoplanets near red dwarfs face extreme space weather

Red dwarfs are very compact luminaries, but in spite of this, they have flares all the time, which produce a large number of high-energy particles. Recent studies of the star Wolf 359 confirm this.

Wolf 359. Source: phys.org

Red Dwarf Wolf 359

Planets around other stars should be prepared for extreme weather conditions, according to a new study from the Chandra and XMM-Newton X-ray observatories that examined the effects of X-rays on potential planets around the most common type of stars.

Astronomers found that only a planet with greenhouse gases in its atmosphere like Earth’s and at a relatively large distance from the star under study would have a chance of supporting life as we know it.

Wolf 359 is a red dwarf with a mass that is about a tenth of the mass of the Sun. Red dwarfs are the most common stars in the Universe, and they live for billions of years, providing enough time for life to develop. With a distance of only 7.8 light-years, it is also one of the closest stars to the Solar System.

“Wolf 359 may help us unlock the mysteries of stars and their suitability for life,” said Scott Wolk of the Center for Astrophysics at Harvard and Smithsonian Institutions (CfA), who led the study. “It’s so close and it belongs to such an important class of stars—it’s a great combination.”

Exploring the neighborhood of Wolf 359 with telescopes

Since red dwarfs are the most common type of stars, astronomers have been persistently searching for exoplanets around them. Using optical telescopes, astronomers found some evidence for the existence of two planets orbiting Wolf 359, but these findings have been challenged by other scientists. 

“While we don’t have proof of planets around Wolf 359 yet, it seems very possible that it hosts multiple planets,” Wolk added. “This makes it an excellent test bed to look at what planets would experience around this kind of star.”

Wolk and colleagues used Chandra and XMM to study the amount of persistent X-ray emission and extreme ultraviolet radiation — the most energetic type of UV radiation that Wolf 359 emits to possible planets around it.

They found that Wolf 359 produces enough harmful radiation that only a planet with greenhouse gases like carbon dioxide in its atmosphere — and located at a relatively large distance from the star — would be able to support life.

Exploring the habitable zone

To study the effect of energetic radiation on the habitability of candidate planets, the team looked at the habitable zone, a region around a star where liquid water could exist on the planet’s surface.

The outer limit of the habitable zone for Wolf 359 is about 15% of the distance between Earth and the Sun, since the red dwarf is much less bright than the Sun. None of the candidate planets in this system are in the luminary’s life zone: one is too close to the star and one is too far away.

“If the inner planet is there, the X-ray and extreme UV radiation it is subjected to would destroy the atmosphere of this planet in only about a million years,” said study co-author Ignazio Pillitteri of the CfA and the National Institute of Astrophysics in Palermo, Italy.

The team also looked at the effects of radiation on still undiscovered planets within the habitable zone. They concluded that an Earth-like planet in the middle of the habitable zone should be able to maintain an atmosphere for nearly two billion years, while a planet near the outer edge could exist indefinitely, thanks to warming caused by greenhouse gases.

X-ray flares and their dangers

Another major danger to planets orbiting stars like Wolf 359 is X-ray flares, or random bright bursts of X-rays, in addition to the star’s constant, daily radiation. A combination of observations from Chandra and XMM-Newton detected 18 X-ray flares from Wolf 359 over a period of 3.5 days.

Extrapolating from these observed flares, the team expects much more powerful and destructive flares to occur over longer periods of time. The combined effects of constant X-ray and ultraviolet radiation and flares mean that any planet in the habitable zone is unlikely to have a significant atmosphere sufficient for multicellular life to form and survive as we know it on Earth. The only exception is the outer edge of the habitable zone if the planet has a significant greenhouse effect.

According to phys.org

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