The giant planet 8 Ursae Minoris b (also known as Halla) orbits the core-helium-burning red giant star 8 Ursae Minoris (Baekdu). At a distance of only 0.5 AU (astronomical units) from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 AU.
“Engulfment by a star normally has catastrophic consequences for close orbiting planets,” said Dr. Daniel Huber, an astronomer at the University of Sydney and the University of Hawai’i.
“When we realised that Halla had managed to survive in the immediate vicinity of its giant star, it was a complete surprise.”
“As it exhausted its core hydrogen fuel, the star would have inflated up to 1.5 times the planet’s current orbital distance — engulfing it completely in the process — before shrinking to its current size.”
Baekdu is located apprximately 532 light-years away in the constellation of Ursa Minor.
The star is nearly 11 times the radius of our Sun, with 1.6 times its mass.
The giant planet Halla was discovered in 2015 by astronomers using the radial velocity method, which measures the periodic gravitational tug of the orbiting planet on its star.
Following the discovery that the star must at one time have been larger than the planet’s current orbit, the astronomers conducted additional observations from 2021 to 2022 using the W.M. Keck Observatory and the Canada-France-Hawaii Telescope on Mauna Kea, Hawai’i.
The new data confirmed that the planet’s 93-day, nearly circular orbit had remained stable for more than a decade and that the radial velocity changes observed in the star must be due to this orbiting planet.
“Together, these observations confirmed the existence of Halla, leaving us with the compelling question of how the planet survived,” said lead author, Dr Marc Hon from the University of Hawai‘i. “The observations from multiple telescopes on Maunakea were…
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