Astronomers Find Evidence for 7-Year Stellar Cycle in Proxima Centauri

The Sun’s nearest stellar neighbor, Proxima Centauri, has a regular cycle of starspots, according to a team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Astronomical Observatory at the University of Warsaw.

Proxima Centauri. Image credit: Hubble / ESA / NASA.

Our Sun experiences an 11-year activity cycle. At the solar minimum, the star is nearly spot-free. At solar maximum, typically more than 100 sunspots cover less than one percent of the Sun’s surface.

A new study finds that Proxima Centauri undergoes a similar cycle lasting seven years from peak to peak.

However, its cycle is much more dramatic. At least a full 1/5th of the star’s surface is covered in spots at once. Also, some of those spots are much bigger relative to the star’s size than the spots on our Sun.

“If intelligent aliens were living on the recently-discovered Earth-like exoplanet Proxima b, they would have a very dramatic view,” said CfA astronomer Dr. Brad Wargelin, who is the lead author on the study.

Proxima Centauri is a small, cool, red dwarf located in the constellation of Centaurus, only 4.23 light-years from the Sun. It was discovered in 1915 by the Scottish astronomer Robert Innes.

The star is not visible to the naked eye and is only 1/10th as massive and 1/1000th as luminous as the Sun.

Proxima Centauri is also a flare star, meaning that convection processes within the star’s body make it prone to random and dramatic changes in brightness.

The convection processes not only trigger brilliant bursts of starlight but, combined with other factors, mean that the star is in for a very long life. Astronomers say it will remain middle-aged for another 4 trillion years, some 300 times the age of the current Universe.

An artist’s illustration depicts the interior of a low-mass star. Such stars have different interior structures than the Sun, so they are not expected to show magnetic activity cycles. However, B.J. Wargelin et al have discovered that Proxima Centauri shows signs of a 7-year activity cycle. Image credit: NASA / CXC / M.Weiss.

Dr. Wargelin and his colleagues were surprised to detect a stellar activity cycle in Proxima Centauri because its interior is expected to be very different from the Sun’s.

“We report on several years of optical, UV, and X-ray observations of Proxima Centauri: 15 years of All Sky Automated Survey photometry in the V-band and 3 years in the I-band, 4 years of observations from NASA’s Swift spacecraft, and 9 sets of X-ray observations from other X-ray missions (the NASA/ISAS Advanced Satellite for Cosmology and Astrophysics, NASA’s Chandra and ESA’s XMM-Newton space observatories) spanning 22 years,” the astronomers said.

“We confirm previous reports of an 83-day rotational period and find strong evidence for a 7-year stellar cycle, along with indications of differential rotation at about the solar level.”

“The existence of a cycle in Proxima Centauri shows that we don’t understand how stars’ magnetic fields are generated as well as we thought we did,” added Dr. Jeremy Drake, a co-author on the study and an astronomer at the CfA.

This artist’s impression shows Proxima b orbiting Proxima Centauri, which at only 4.23 light-years is the closest star to our Solar System. The double star Alpha Centauri AB also appears in the image between the exoplanet and Proxima itself. Image credit: M. Kornmesser / ESO.

The study does not address whether Proxima Centauri’s activity cycle would affect the potential habitability of the exoplanet Proxima b.

Theory suggests that flares or a stellar wind could scour the planet and strip away any atmosphere. In that case, Proxima b might be like Earth’s Moon – located in the habitable zone, but not at all friendly to life.

“Direct observations of Proxima b won’t happen for a long time. Until then, our best bet is to study the star and then plug that information into theories about star-planet interactions,” said co-author Dr. Steve Saar, also from the CfA.