A newly discovered Neptune-sized planet orbits a young star in just over one Earth week. The planet, which is called AU Microscopii b (or AU Mic b for short), was found when scientists were analyzing data from the Transiting Exoplanet Survey Satellite (TESS) and NASA’s Spitzer Space Telescope. The discovery of AU Mic b will hopefully help scientists learn more about the evolution of newly formed planets as well as their atmospheres and their interactions with their host stars.
AU Mic b, which has a mass of approximately 58 Earths and is around 8% bigger than Neptune, is located about 32 light-years away from us and is part of a young star group that’s called Beta Pictoris Moving Group. Incredibly, AU Mic b zips around its host star every 8.5 Earth days.
AU Mic is a very young M dwarf star – forming only 20 to 30 million years ago – and it’s quite active as it spews out large irradiated flares. For comparison, our sun is approximately 4.5 billion years old, is bigger than AU Mic, and expels much smaller flares that are less dangerous to the planets in our Solar System. Since it’s so young, AU Mic shines mostly because of the heat it creates from the gravity pulling and compressing the star. More mature stars, such as our own sun, fuse hydrogen into helium which then creates energy (less than 10% of AU Mic’s energy is created by fusion).
Even though AU Mic b probably doesn’t contain any life based on its size and how incredibly active its host star is, scientists have already learned a lot about how young planets are formed. Bryson Cale, who is a doctoral student at Virginia’s George Mason University as well as a co-author of the study, said in a statement, “It's surrounded by a vast debris disk in which moving clumps of dust have been tracked, and now, thanks to TESS and Spitzer, [we know] it has a planet with a direct size measurement,” adding, “There is no other known system that checks all of these important boxes.”
Thomas Barclay, who is an associate project scientist for TESS at NASA’s Goddard Space Flight Center in Maryland and another co-author of the study, stated, “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.”
It actually wasn’t that easy to spot AU Mic b because of how active its host star is with countless sunspots in addition to emitting large flares. After the spacecraft witnessed AU Mic in July and August of 2018, experts had to eliminate the flares from the TESS data in order to determine that a planet was in fact orbiting the star. The team then analyzed more data taken by the Spitzer Space Telescope which ultimately confirmed the planet.
Another interesting development is that scientists believe that there may be other planets orbiting AU Mic. Peter Plavchan, who is an assistant professor of physics and astronomy at George Mason University as well as the lead author of the study, noted, “There is an additional candidate transit event seen in the TESS data, and TESS will hopefully revisit AU Mic later this year,” adding, “We are continuing to monitor the star, so stay tuned.” (A short video on the discovery of AU Mic b can be seen here.)