In August, astronomers declared to great fanfare that they had discovered a 'second Earth' orbiting nearby star Proxima Centauri, four light-years away.
Orbiting in the star's habitable zone, the Earth-sized planet Proxima b piqued the interests of experts because they believe it has a temperature suitable for liquid water to exist on its surface. Although this, of course, doesn't guarantee it could host alien life, evidence of water could increase the chances of it significantly.
Now, further to this, researchers from Harvard have found Proxima Centauri may be more like our own Sun than previously thought.
On first appearance, Proxima Centauri seems nothing like our Sun. It's a small, cool, red dwarf star only one-tenth as massive and one-thousandth as luminous. However, it is sunlike in one surprising way: it has a regular cycle of starspots.
Starspots (like sunspots) are "blotches" on a star's surface where the temperature is a little cooler than the surrounding area. These areas are driven by magnetic fields which can restrict the plasma on the surface and create spots. The more the magnetic field changes, the more this affects the number and distribution of starspots.
Our Sun goes through an 11-year activity cycle. At the solar minimum, the Sun is practically spot-free. At solar maximum, typically more than 100 sunspots cover less than one percent of the Sun's surface, on average.
The study shows that Proxima Centauri goes through a similar cycle lasting seven years from peak to peak. But it's much more dramatic. At least one-fifth of the star's surface is covered in spots at any one tie and some of those spots are much larger relative to the star's size than the spots on our Sun.
"If intelligent aliens were living on Proxima b, they would have a very dramatic view," said lead author Brad Wargelin of the Harvard-Smithsonian Centre for Astrophysics (CfA).
To make the initial discovery of Proxima b, researchers observed Proxima Centauri using the Harps spectrograph and the ESO 3.6-metre telescope at La Silla in Chile, while others also kept an eye on the closest star to Earth, apart from the Sun, as part of the Pale Red Dot campaign.
The 'Earth-like' planet Proxima b discovered by Pale Red Dot
The cool, red dwarf star is part of a triple system of stars in the neighbouring solar system of Alpha Centauri, approximately 25.2 trillion miles from Earth and is also the target of humankind’s first attempt to travel to another star system, the Starshot project.
The astronomers involved in the recent study said they were surprised to detect such stellar activity in Proxima Centauri because it was believed its interior was very different from our Sun's.
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"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," said Smithsonian co-author Jeremy Drake.
The study does not address whether Proxima Centauri's cycle would affect the potential habitability of Proxima b. Theory suggests flares or a stellar wind could potentially strip the planet of its atmosphere suggesting Proxima b might be like Earth's Moon – in the habitable zone, but not habitable.
"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," explained co-author Steve Saar.