Astronomers now know of around 5,000 planets in orbit around other stars. A handful of these are both Earth-sized and in the habitable zones of the stars they orbit, where the temperature is right for liquid water. But many candidate Earth-sized worlds are in orbit around red dwarf stars, much smaller and cooler than our own sun. Astronomers estimate that two-thirds of stars near Earth are red dwarfs. They range from 8% to 55% the mass of the sun, 10% to 60% the sun’s radius, and only 0.05% to 7% the sun’s luminosity.
Due to their small size, it is easier to detect Earth-sized planets orbiting red dwarfs. The closest star system to the sun, Proxima Centauri, is a red dwarf, and it harbors three close planets, one of which is an Earth-sized planet in the habitable zone. To be in the habitable zone, the planets need to be much closer to their stars than we are to the sun. The problem is that red dwarfs can produce significant X-ray emission, and often have large flares of radiation and eruptions of particles in so-called coronal mass ejections (CMEs).
Giant Planet 50 Times Closer than the Earth to the Sun
To try to assess the risk, Dr. Eike Guenther of the Thueringer Observatory and colleagues intensively monitored low-mass stars where flares might take place. In February 2018, they observed a giant flare from the star AD Leo, located 16 light years away from our Sun in the constellation of Leo. AD Leonis – one of the most active flare stars known with emissions that have been detected across the electromagnetic spectrum as energetic as X-rays – has a giant planet orbiting 3 million kilometers away (50 times closer than the Earth to the sun), and it may have Earth-sized worlds further out in its habitable zone.
The astronomers are working to establish what the flare did to the known giant planet and any hypothetical planets further out. Their initial results suggest the giant planet was unaffected, and that unlike similar events on the sun, the radiation flare was not accompanied by a Coronal Mass Ejection. CMEs are large expulsions of plasma from a star’s corona, channeled by the star’s intense magnetic fields.
This is potentially good news for life further out, as CMEs are thought to have a role in stripping away the atmosphere of smaller planets. From their monitoring, the team believe CMEs are generally less common in smaller stars.
Not Great Places for Life
On the other hand, the X-ray radiation is dangerous. According to Guenther’s team, these would cut through the atmosphere and reach the surface of an Earth-like planet. Life on land would be badly affected by a stellar flare and might only survive in the oceans.
“Astronomers are mounting a global effort to find Earth-like worlds, and to answer the age-old work suggests planets around the commonest low-mass stars are not great places for life, at question of whether we are alone in the Universe. With sporadic outbursts of hard X-rays, at least on dry land,” says Guenther.
Some scientists suggest that giant radiation flares could deplete the ozone layer of a planet by 94% for two years and could even be fatal for all life. If they are right, then talk of ‘Earth 2.0’ may be premature.
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Royal Astronomical Society
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