X-rays emitted by a Sun-like star may provide valuable information about whether its planetary system will be hospitable to life, a new study suggests. Queen’s University Belfast astronomer Rachel Booth and co-authors looked at 24 stars similar to the Sun, each at least one billion years old, and how their X-ray brightness changed over time.

The X-ray emission from a star comes from a thin, hot, outer layer, called the corona.

From studies of Sun’s X-ray emission, solar astronomers have determined that the corona is heated by processes related to the interplay of turbulent motions and magnetic fields in the outer layers of a star.

High levels of magnetic activity can produce bright X-rays and UV light from stellar flares. Strong magnetic activity can also generate powerful eruptions of material from the star’s surface. Such energetic radiation and eruptions can impact planets and could damage or destroy their atmospheres.

Since stellar X-rays mirror magnetic activity, X-ray observations can tell scientists about the high-energy environment around the star.

Booth and her colleagues from Queen’s University Belfast, Aarhus University and the Harvard–Smithsonian Center for Astrophysics (CfA) used data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton to see how the X-ray brightness of stars similar to the Sun behaves over time.

Specifically, they examined 24 stars (including 16 Cyg A&B, 40 Eri A&B, Gliese 176, Gliese 191, Proxima Centauri and Alpha Centauri B) that have masses similar to the Sun or less, and ages of a billion years or older.

The rapid observed decline in X-ray brightness implies a rapid decline in energetic activity, which may provide a hospitable environment for the formation and evolution of life on any orbiting planets.

“This is good news for the future habitability of planets orbiting Sun-like stars, because the amount of harmful X-rays and UV radiation striking these worlds from stellar flares would be less than we used to think,” Booth said.

“This result is different from other recent work on Sun-like and lower mass stars with ages less than a billion years. The new work shows that older stars drop in activity far more quickly than their younger counterparts.”

“We’ve heard a lot about the volatility of stars less massive than the Sun, like TRAPPIST-1 and Proxima Centauri, and how that’s bad for life-supporting atmospheres on their planets,” said co-author Dr. Katja Poppenhaeger, from CfA and Queen’s University Belfast.

Astronomers have observed that most stars are very magnetically active when they are young, since the stars are rapidly rotating. As the rotating star loses energy over time, the star spins more slowly and the magnetic activity level, along with the associated X-ray emission, drops.

“We’re not exactly sure why older stars settle down relatively quickly. However, we know it’s led to the successful formation of life in at least one case — around our own Sun,” said co-author Dr. Chris Watson, also from Queen’s University Belfast.

“One possibility is that the decrease in rate of spin of the older stars occurs more quickly than it does for the younger stars.”

“Another possibility is that the X-ray brightness declines more quickly with time for older, more slowly rotating stars than it does for younger stars.”

The study is published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).

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R.S. Booth et al. 2017. An improved age–activity relationship for cool stars older than a gigayear. MNRAS 471 (1): 1012-1025; doi: 10.1093/mnras/stx1630