Astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) and Spitzer Space Telescope have discovered a planet about the size of Neptune orbiting AU Microscopii, the second closest pre-main-sequence star.
An artist’s impression of the planet AU Microscopii b. Image credit: NASA’s Goddard Space Flight Center / Chris Smith, USRA.
AU Microscopii (AU Mic), also known as Gliese 803 and HD 197481, is located 31.9 light-years away in the southern constellation of Microscopium.
The star is just 22 million years old and is a member of nearby collection of stars called the Beta Pictoris moving group, which takes its name from a bigger, hotter A-type star that harbors two planets.
AU Mic is surrounded by a relatively rare edge-on disk of debris extending from about 35 to 210 AU (astronomical units) from the star.
“AU Mic is a small star, with only about 50% of the Sun’s mass,” said Dr. Jonathan Gagné, an astronomer in the Institute for Research on Exoplanets at the University of Montreal.
“These stars generally have very strong magnetic fields, which make them very active. That explains in part why it took nearly 15 years to detect the exoplanet, called AU Mic b.”
“The numerous spots and eruptions on the surface of AU Mic hampered its detection, which was already complicated by the presence of the disk.”
AU Mic b has a radius of 0.4 Jupiter radii and a mass of less than 0.18 Jupiter masses.
It orbits its parent star once in every 8.5 days at a distance of only 0.07 AU.
“AU Mic is a young, nearby M dwarf star. It’s surrounded by a vast debris disk in which moving clumps of dust have been tracked, and now, thanks to TESS and Spitzer, it has a planet with a direct size measurement,” said Bryson Cale, a doctoral student at George Mason University.
“There is no other known system that checks all of these important boxes.”
“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,” added Dr. Thomas Barclay, a research scientist at the University of Maryland, Baltimore County and a project scientist for TESS at NASA’s Goddard Space Flight Center.
“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.”
“There is an additional candidate transit event seen in the TESS data, and TESS will hopefully revisit AU Mic later this year in its extended mission,” said Dr. Peter Plavchan, an astronomer at George Mason University.
“We are continuing to monitor the star with precise radial velocity measurements, so stay tuned.”
The team’s paper was published in the journal Nature.
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P. Plavchan et al. 2020. A planet within the debris disk around the pre-main-sequence star AU Microscopii. Nature 582, 497-500; doi: 10.1038/s41586-020-2400-z
