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.
The hot Jupiter exoplanet Halla could once have orbited two stars that interacted via mass transfer. Image credit: Tobias Roetsch / gtgraphics.de.
“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 critical in this process.”
At a distance of 0.5 AU to its star, Halla resembles warm or hot Jupiter-like planets. These gas giants are thought to have started their lives in orbits much further out from their stellar hosts before migrating inward.
However, in the face of a rapidly evolving host star, such an origin story becomes an extremely unlikely survival pathway for the planet Halla.
“We just don’t think Halla could have survived being absorbed by an expanding red giant star,” Dr. Huber said.
A more plausible theory for the planet’s survival is that it never faced the danger of engulfment.
“The system was more likely similar to the famous fictional planet Tatooine from Star Wars, which orbits two suns,” said University of Sydney’s Professor Tim Bedding.
“If the Baekdu system originally consisted of two stars, their merger could have prevented any one of them from expanding sufficiently to engulf the planet.”
“This is because the two stars would have ‘fed’ off each other during the transition from hydrogen burning stars to what Baekdu is now, a helium burning red giant star.”
A third possibility is that Halla is a newborn planet: that the violent collision between the two stars produced a gas cloud from which the planet formed.
In other words, the planet Halla may be a ‘second-generation’ planet in the star system.
“This system shows that core-helium-burning red giants can harbor close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems,” the researchers said.
The team’s work was publsihed in the journal Nature.
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M. Hon et al. 2023. A close-in giant planet escapes engulfment by its star. Nature 618, 917-920; doi: 10.1038/s41586-023-06029-0
