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The Exoplanet That Should Not Be

Australian astronomers discover a Jupiter-sized planet orbiting its star at about one-twentieth the orbital radius of Mercury.
Artist's impression. Image: ANU

The exoplanet HATS-6b is simply too big. With a size roughly that of Jupiter, the planet orbits its host star at a distance of about one-tenth that of Mercury, completing a single year every 3.3 Earth-days. Said star, HATS-6, is classified as an M-dwarf, a cool and quite dim variety that's about one-twentieth the brightness of our Sun. The relationship, according to currently accepted theories relating the sizes, masses, and orbits of planet and stars, should be impossible, according to the Australian National University researchers who first discovered and described the unlikely pair.


"It must have formed further out and migrated in, but our theories can't explain how this happened," offers ANU astronomer George Zhou in a statement. The discovery was revealed in observations of the repeated dimming of HATS-6 characteristic of a transiting planet (a planet passes between us and the star, and we see a dip in brightness), first via robotic telescopes at the ANU Siding Spring Observatory and then by the Magellan Telescope in Chile, which confirmed the initial findings. A backyard astronomer in Perth, Australia named T G Tan also lent a hand, Zhou notes.

"The planet has a similar mass to Saturn, but its radius is similar to Jupiter, so it's quite a puffed up planet. Because its host star is so cool it's not heating the planet up so much, it's very different from the planets we have observed so far," Zhou said. "The atmosphere of this planet will be an interesting target for future study."

The discovery is reported in the current issue of the Astronomical Journal, but an earlier open-access version of the paper can be viewed at the arXiv pre-print server.