Exoplanets Need to be Prepared for Extreme Space Weather, Chandra Finds

This artist’s illustration represents the results from a new study that examines the effects of X-ray and other high-energy radiation unleashed on potential exoplanets from a host star. As outlined in our latest press release, astronomers using NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton observed Wolf 359, a red dwarf that is only 7.8 light-years from Earth, making it one of the closest stars to the Earth other than the Sun.

The artist’s rendering shows Wolf 359 in the foreground and a potential planet in orbit around it in the background. Red dwarfs are the most common type of star in the Universe. They are much smaller and dimmer than Sun-like stars, which allows them to last for trillions of years. This would give planets in orbit around them ample time for life to form and emerge, which makes them particularly interesting to scientists looking for life beyond the Solar System.

In the new study, researchers used Chandra and XMM to study the impact of steady X-ray and energetic ultraviolet (UV) radiation from Wolf 359 on the atmospheres of planets that might be orbiting the star. They found that only a planet with greenhouse gases like carbon dioxide in its atmosphere and at a relatively large distance away from Wolf 359 would have a chance to support life as we know it around a nearby star. The planet is depicted with the heavy cloud cover expected from the effects of greenhouse gases.

Their work suggests that just being far enough away from the star’s harmful radiation would not be enough to allow a planet around Wolf 359 to sustain life. The team looked at the “habitable zone,” the region around a star where liquid water could exist on a planet’s surface, for Wolf 359. They found that an Earth-like planet in the middle of the habitable zone blanketed with greenhouse gases should be able to sustain an atmosphere for almost two billion years.

In addition to the dangers posed by the steady, everyday high-energy radiation from a star like Wolf 359, any orbiting planets would be subjected to occasional giant bursts of X-rays. Using observations with Chandra and XMM-Newton, astronomers discovered 18 X-ray flares, or outbursts, from Wolf 359 in under 4 days. Chandra data of Wolf 359 is shown in the inset. Extrapolating from these observed flares, the team expects that much more powerful and damaging flares would occur over longer periods of time. The combined effects of the steady X-ray and UV radiation and the flares means that any planet located in the habitable zone is unlikely to have a significant atmosphere long enough for multicellular life, as we know it on Earth, to form and survive. (Evidence suggests that it took at least 3 billion years for multicellular life to emerge on Earth.) The exception is the habitable zone's outer edge if a planet has a significant greenhouse effect.

The researchers used a special technique to estimate the energetic UV radiation from Wolf 359 using Chandra. The team looked at the difference in radiation measured with the High Resolution Camera (HRC) using two different filters: first, a thick filter that allows only X-rays to be detected, and second, a thinner filter that allows both X-rays and UV radiation to be detected. There are currently no specialized space missions for studying the most energetic ultraviolet radiation.

These results were presented at the 245th meeting of the American Astronomical Society in National Harbor, MD and are being prepared for publication in a journal. NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.