We get worried enough here on Earth when the sun pops off a large solar flare. There’s always a concern that it could temporarily take down a poorly protected power grid or damage orbiting satellite electronics. Last March 24th, a flare ten times the largest flare ever seen on the sun exploded at Proxima Centauri, a red dwarf star just 4.24 light years away and the closest star to Earth.
The flare increased Proxima Centauri’s brightness by 1,000 times over 10 seconds. It was preceded by a smaller flare with the whole event lasting fewer than two minutes. It was observed by ALMA, the Atacama Large Millimeter/submillimeter Array, which uses multiple radio dishes to gather light between the far infrared (which we sense as heat) and microwave energies (the “light” that cooks our food in the microwave oven).
Proxima was no worse for the wear, but the powerful burst almost certainly sent a searing blast of radiation to Proxima Centauri b, a small planet orbiting 4.6 million miles (7.5 million km) away from Proxima with an estimated mass of about 1.3 times that of Earth. The storm was 4,000 times stronger than the worst recorded at Earth in part because Proxima b orbits 20 times closer to its parent star than the Earth orbits the sun.
Even though the planet orbits within the star’s habitable zone, where liquid water would be stable on its surface, the cumulative effects of this flare and others over the lifetime of the system has probably rendered the planet uninhabitable.
“Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water,” said Meredith MacGregor, an astronomer at the Carnegie Institution for Science who co-led the study that appeared in the Feb. 26 Astrophysical Journal Letters.
Proxima Centauri is only about 14 percent as large as the sun and if moved to the sun’s position in the center of the solar system would measure just 4 arc minutes across (about as round as a crescent moon is thick). We’d look up to see a small but brilliant orange-red dot in the sky. Red dwarfs are smaller, cooler stars than the sun but magnetically active with large sunspots and powerful flares. Stellar flares happen when stored magnetic energy is released in a great burst, producing an eruption that hurls clouds of subatomic particles at high speed into space and produces deadly X-rays and gamma rays.
Earth’s magnetic envelope called the magnetosphere protects our atmosphere from what the sun doles out, but seeing what happened at Proxima makes you wonder how long our defenses would last if we swapped stars.