X-rays, at least, no problem for planets orbiting Alpha Centauri AB

More than a decade of observations show the two main stars in the nearby Alpha Centauri system are not pumping out dangerously high levels of X-rays – good news for any exoplanets that might be there. Image: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zdeněk Bardon/ESO

More than a decade of observations by NASA’s Chandra X-ray Observatory indicates any planets that might be orbiting the two sun-like stars known as Alpha Centauri AB are not being bombarded by X-ray levels that would be harmful to life.

Alpha Centauri is the closest star system to Earth, a scant 4.3 light years away, or 41 trillion kilometres (25 trillion miles). It is made up of three stars, two of which – Alpha Cen A and B – are very similar to the Sun and very close together.

A third component, known as Proxima Centauri, or Alpha Cen C, is a much smaller red dwarf that orbits the AB pair 10,000 times farther out than Earth’s distance from the Sun. Proxima Centauri is the closest of the three to Earth, is an active red dwarf that frequently releases powerful X-ray flares and likely would prove hostile to life.

But Chandra observations show Alpha Cen A provides a milder X-ray environment than the Sun with Alpha Cen B only slightly worse.

“Because it is relatively close, the Alpha Centauri system is seen by many as the best candidate to explore for signs of life,” said Tom Ayres of the University of Colorado Boulder. “The question is, will we find planets in an environment conducive to life as we know it?”

The Chandra observations are “very good news for Alpha Cen AB in terms of the ability of possible life on any of their planets to survive radiation bouts from the stars,” said Ayres. “Chandra shows us that life should have a fighting chance on planets around either of these stars.”

An Earth-size planet has been detected orbiting Proxima Centauri, but no exoplanets have yet been found around Alpha Cen A or B, in part because the two stars are currently so close together as viewed from Earth. But Chandra has observed the pair every six months since 2005, capturing a detailed look their X-ray emissions.

The data show any planets orbiting Alpha Cen A would be exposed to a lower average dose of X-rays than planets orbiting the Sun. Emissions from Alpha Cen B are about five times higher than the Sun’s, but still within habitable limits. For comparison, planets orbiting Proxima Centauri would receive average levels about 500 times higher than Earth experiences, 50,000 times higher during a major flare.

Ayres presented the Chandra results at the 232nd meeting of the American Astronomical Society in Denver, Colorado.