An artist's interpretation of the interior of a low-mass star, such as the one seen in an X-ray image from Chandra in the inset. Image Credit: NASA/CXC/M.Weiss

Proxima Centauri More Like Our Sun Than Initially Thought

Astronomers recently announced that the nearby star Proxima Centauri hosts an Earth sized planet in its habitable zone. The planet is called Proxima b. On the face of it, Proxima Centauri seems nothing like our Sun. It is a red dwarf one thousandth as luminous as the Sun and only one tenth as massive. New research however shows that it is like the sun in one startling way: it has a steady cycle of star spots.

Like sunspots, star spots appear as dark blotches on a star’s surface. Created by magnetic fields, the spots indicate areas where the temperature is not as hot as the surrounding area. A star is made of plasma, which consist of ionized gases. Magnetic fields restrict the plasma’s flow and creates spots. The number and distribution of star spots is affected by changes to a star’s magnetic field.

Our Sun has an activity cycle of 11 years. At solar maximum, more than 100 sunspots typically cover less than one percent of the Sun’s surface. At the solar minimum, virtually no spots are seen.

The new study finds that Proxima Centauri has a similar cycle, but it lasts seven years from peak to peak. Its cycle is however much more dramatic. Some of those spots are much bigger compared to the star’s size than the spots on our Sun. At solar maximum, a full one fifth of the star’s surface is covered in spots at the same time. Lead author Brad Wargelin of the Harvard-Smithsonian Center for Astrophysics (CfA), notes that if intelligent aliens were living on Proxima b, they would have a dramatic view of this phenomenon.

Astronomers expected Proxima Centauri’s interior is to be very different from the Sun’s. They were therefore surprised to detect a stellar activity cycle in it. The Sun’s interior remains relatively still, but the outer third experiences a churning motion called convection, comparable to water boiling in a pot. These two regions rotate at different speeds. Many astronomers believe the shear arising from this difference causes the Sun’s magnetic activity cycle.

proxima centauri x-ray
An X-Ray image of Proxima Centauri – nearest bona fide star. Image Credit: Chandra/Harvard/NASA

As the interior of a small red dwarf like Proxima Centauri is expected to be convective all the way into the star’s core, it shouldn’t experience a regular cycle of activity.

Smithsonian co-author Jeremy Drake explains that the existence of a cycle in Proxima Centauri shows that astronomers don’t fully understand how stars’ magnetic fields are generated.

The study does not report on whether Proxima Centauri’s activity cycle would affect the potential habitability of the planet Proxima b or not. One theory advocates that a stellar wind and flares, both of which are driven by magnetic fields, could scrub the planet and strip away any atmosphere. If that is the case, Proxima b might be located in the habitable zone, but it won’t be friendly to life, just like Earth’s Moon.

Co-author Steve Saar reckons the researchers’ best bet now is to study the star and plug that data into theories about star-planet interactions, as direct observations of Proxima b won’t happen for a long time.

The team noticed the activity cycle when combining space based X-ray measurements by several missions with ground based observations from the All Sky Automated Survey.  Mission data from Chandra, Swift, and XMM-Newton was included in the study.

Their results have been published in the Monthly Notices of the Royal Astronomical Society.