Neptune-Sized Planet Discovered Orbiting Young Star Known For Massive Flares

Astronomers have discovered a Neptune-sized planet orbiting a unique young star that is popular for its massive temper tantrums. The newly discovered planet is so close to its star that it completes an orbit in just over a week.

Star system AU Microscopii (AU Mic) is a nearby system just about 31.9 light-years away whose star is quite young at 20 to 30 million years old. In fact, its M dwarf star is an infant compared to our Sun, which is about 4.5 billion years old.

It is so young that it is still surrounded by the debris disk left over from when it was formed, and, for years, scientists have been trying to search for planets around it. However, the search is hard because of the star's massive flares. In fact, some of its solar flares in July and August 2018 were more powerful than the strongest flares recorded from our Sun.

This makes it tricky for telescopes to distinguish the dips and spikes in brightness that could signify a potential planet.

Thanks to data from NASA's Transiting Exoplanet Survey Satellite (TESS) and the now-retired Spitzer Space Telescope, the authors of a new study, published in the journal Nature, discovered a Neptune-sized planet orbiting very close to AU Mic. Called AU Mic b, the planet is so close to AU Mic, it can complete an orbit every 8.5 days.

"We think AU Mic b formed far from the star and migrated inward to its current orbit, something that can happen as planets interact gravitationally with a gas disk or with other planets," study co-author Thomas Barclay of the University of Maryland, Baltimore County said, in a feature from NASA.

What's interesting is that another system, Beta Pectoris, is of the same age as AU Mic and is also surrounded by debris, but its planets are over 50 times bigger and take considerably longer to orbit their star at 21 and 3.3 years.

"By contrast, Beta Pictoris b's orbit doesn't appear to have migrated much at all. The differences between these similarly aged systems can tell us a lot about how planets form and migrate," Barclay said.

In the future, more TESS observations could help identify possibly more planets in AU Mic, which is considered a "touchstone system" because it is so close to Earth and therefore allows scientists to study how stars and planets form and evolve. In fact, scientists have been considering AU Mic a possible home of planets, but it is thanks to TESS and Spitzer data that they found the first one.

To celebrate the discovery, AU Mic is now featured in a new NASA poster as a part of its Galaxy of Horrors series, which was initially released on Halloween of last year. In the poster, entitled "Flares of Fury," AU Mic b can be seen quite close to its host star that is erupting in violent flares.