Proxima Centauri surprises with starspot cycle

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The nearby star Proxima Centauri shows signs of a seven-year activity cycle – weird behaviour for a low-mass star.
NASA / CXC / M. Weiss

Proxima Centauri, our stellar neighbour that recently was discovered to host a planet within its habitable zone, seems to also boast a starspot cycle – unusual for star so small and dim.

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Using ground-based observations from the All Sky Automated Survey combined with space-based X-ray measurements by several missions, including Swift, Chandra, and XMM-Newton, Brad Wargelin of the Harvard-Smithsonian Centre for Astrophysics in the US and colleagues found the star, which is 12% the mass of the sun and 0.15% its brightness, is marred with dark blotches every seven years.

The work, which will be published in the Monthly Notices of the Royal Astronomical Society, suggests astronomers don’t know as much about Proxima Centauri’s dynamic innards as they thought.

Our sun undergoes an 11-year sunspot cycle. The most recent maximum was around 2014, when dark, cooler regions – sunspots – dotted its surface.

Behind their appearance is the sun’s magnetic field, thought to be generated as its outer third layer roils like water boiling in a pot while its interior remains fairly static, and the two layers rotate at different speeds.

This means that, instead of the relatively nice uniform magnetic field we have on Earth, the sun’s magnetic field changes. And sometimes, those magnetic fluxes manifest as cooler patches – or sunspots.

Smaller stars are thought to lack the multi-layer rotational shear that generates the sun’s magnetic field, so shouldn’t have starspots.

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But Proxima Centauri does – lots of them, too, and on a shorter cycle. Each seven years, up to a fifth of its surface is darkened by starspots. In comparison, the sun’s spots generally cover less than 1% of its surface.

“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,” says study co-author Jeremy Drake, also from the Smithsonian.

So what does Proxima Centauri’s activity mean for life on its planet Proxima b?

The researchers don’t say, but our sun throws out flares and a stream of charged particles called the solar wind which are driven by its magnetic field.

If the same happens at Proxima b, there’s a good chance its atmosphere has been stripped away, leaving it in the habitable zone, but not habitable.

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