The red dwarf star "Wolf 359" (Wolf 359), which is only 7.8 light-years away from us, may harbor planets, but extreme X-ray and ultraviolet radiation make it difficult for planets to inhabit. Only planets far from their stars with thick greenhouse gas atmospheres have a chance of sustaining life. Frequent X-ray flares add to this challenge, potentially stripping away the atmospheres of habitable zone planets before life is born.

This illustration describes a new study examining how intense X-ray and ultraviolet radiation from the nearby red dwarf star Wolf 359 affects potential exoplanets. Using data from Chandra and XMM-Newton, the researchers found that only distant planets with thick greenhouse gas atmospheres could potentially support life. Image source: NASA/CXC/SAO/M.Weiss

Planets orbiting other stars could face extreme space weather, according to a new study using data from NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton. The study examined how X-ray radiation affects potential exoplanets around the most common types of stars in the universe.

Astronomers have found that for a planet to have a chance of supporting life, it needs to have a thick atmosphere of greenhouse gases like Earth and be far away from its host star to avoid harmful radiation.

Meet Wolf 359: A Nearby Red Dwarf Star

One of the stars studied, Wolf 359, is a red dwarf with about one-tenth the mass of the Sun. Red dwarfs are the most abundant stars in the universe and can burn for billions of years, theoretically enough time to nurture life. Wolf 359 is only 7.8 light-years away from our solar system and is one of the closest stars to our solar system.

Scott Wolk of the Center for Astrophysics | Harvard & Smithsonian (CfA), who led the study, said: "Wolf 359 can help us unlock the secrets of stars and habitability. It is so close to us and it is in such an important class of stars - it is a great combination."

Searching for exoplanets around Wolf 359

Because red dwarfs are the most common type of star, astronomers are constantly trying to find exoplanets around them. Astronomers using optical telescopes have found some evidence that two planets orbit Wolf 359, but these conclusions have been questioned by other scientists.

Walker added: "While we don't yet have evidence that Wolf 359 has planets around it, it seems very likely that it hosts multiple planets. This makes it an excellent testbed for studying what planets would experience around such a star."

Walker and his colleagues used Chandra and XMM to study how much stable X-ray and extreme ultraviolet (UV) radiation - the most energetic type of ultraviolet radiation - Wolf 359 would emit to possible planets around it.

This image from Chandra was used along with XMM-Newton data to study how X-ray and ultraviolet radiation from Wolf 359 affects potential exoplanets. Researchers have found that only distant planets with thick greenhouse gas atmospheres might still be habitable. Image source: NASA/CXC/SAO/S.Wolk, etc.; Image processing: NASA/CXC/SAO/N: NASA/CXC/SAO/N.Wolk

They found that Wolf 359 produced enough harmful radiation that only planets with greenhouse gases such as carbon dioxide in their atmospheres - relatively large distances from their stars - could possibly support life.

"Merely being far away from a star's harmful radiation is not enough to make it habitable," said co-author Vinay Kashyap, also from the CfA. "The planets around Wolf 359 would also need to be covered in greenhouse gases like Earth."

To study the impact of high-energy radiation on the habitability of candidate planets, the team considered the star's habitable zone - the region around a star where liquid water might exist on a planet's surface.

Because red dwarfs are much less luminous than the Sun, the outer limit of Wolf 359's habitable zone is about 15% of the distance between Earth and the Sun. Neither of the system's two candidate planets is within Wolf 359's habitable zone; one is too close to the star and the other is too far away.

The video shows a young, low-mass, relatively cool star called a red dwarf, like Wolf 359. Because of its youth, the star is very active and flares several times a day. These flares are so hot that they eject as much mass as a small moon. Around the star, we could see a planet with roughly the same mass as Earth. Because the star is so cold, the planet would need to be very close to its host star - only 2.5 million to 5 million miles - to have a chance of maintaining a temperature on its surface that would produce liquid water. For comparison, the Earth orbits the sun at a distance of about 93 million miles.

Ignazio Pillitteri of the National Institute of Astrophysics in Palermo, Italy, a co-author of the study, said: "If the inner planet does exist, the X-rays and extreme ultraviolet radiation it is exposed to will destroy the planet's atmosphere in about a million years."

The team also considered the effects of radiation on as-yet-undiscovered planets in the habitable zone. They concluded that planets like Earth in the middle of the habitable zone should be able to maintain atmospheres for nearly 2 billion years, while planets closer to the outer edges could do so indefinitely, helped by the warming effects of greenhouse gases.

X-ray flares: Life's fatal obstacle

Another huge danger for planets orbiting stars like Wolf 359 comes from X-ray flares, occasional bursts of bright X-rays on top of the star's steady daily output of X-rays. Combining the Chandra and XMM-Newton observations, we found that Wolf 359 had 18 X-ray flares over 3.5 days.

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

The findings were presented at the 245th Meeting of the American Astronomical Society in National Harbor, Maryland, and are being prepared for publication in the journal Science.

NASA's Marshall Space Flight Center in Huntsville, Alabama, oversees the Chandra X-ray Observatory program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center manages science operations in Cambridge, Massachusetts, and flight operations in Burlington, Massachusetts.

Compiled from /ScitechDaily