Planet lives on despite violent transition of host star to red giant

Astronomers are mulling over theories about a planet which looks to have survived being engulfed by its dying – and therefore expanding – star.

In five billion years, our Sun will exhaust its hydrogen fuel and begin burning helium, leading to its transition to a red giant. It will expand to about 100 times its size, and engulf Mercury, Venus and possibly the Earth in hot plasma, destroying its planets.

But is it possible that this won’t be the end for them?

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For the first time, astronomers have made the surprising discovery of a planet which has lived on beyond this scenario after observing Jupiter-like gas giant 8 Umi b, also known as “Halla,” which closely orbits its host star Baekdu, in the ‘Little Bear’ constellation Ursa Minor.

Binary star system red giant with gas giant and gas cloud
Portrayed is the violent merger between two stars that may have formed the helium-burning giant star Baekdu. The merger debris forms a disk from which the planet Halla formed, enabling the planet’s unlikely survival around the star. Credit: W. M. Keck Observatory/Adam Makarenko.

Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers concluded that Baekdu is burning helium rather than hydrogen, meaning the star had expanded into a red giant.

The international team, including researchers at the University of Sydney, found that Halla survived Baekdu’s violent transition.

Their findings are published in Nature.

Man with beard smiling in jacket in front of observatory on a hill
Dr Dan Huber at the summit of Mauna Kea, Hawai’i. Pictured in front of the Canada-France-Hawaii Telescope, which was used in this research. Dr Huber is an ARC Future Fellow at the University of Sydney.

“Engulfment by a star normally has catastrophic consequences for close orbiting planets,” says co-author Dr Huber, an Australian Research Council Future Fellow at the Sydney Institute for Astronomy. “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 [Halla] completely in the process – before shrinking to its current size.”

Baekdu is nearly 11 times the radius of our Sun and 1.6 times its mass. It is approximately 520 lightyears from Earth.

Halla was first discovered in 2015 by Korean astronomers. It is a type of planet known as a “warm” or “hot” Jupiter – these gas giants are thought to have started life in orbits far from their host stars before migrating toward them, becoming hotter along the way.

Researchers conducted additional observations in 2021 and 2022 to confirm Halla’s existence as it was already understood that the star would at one point have been larger than the planet’s orbit.

Sure enough, the new data confirms the planet’s 93-day, nearly circular orbit has been stable for more than a decade.

“Together, these observations confirmed the existence of Halla, leaving us with the compelling question of how the planet survived,” says lead author, Dr Marc Hon from the University of Hawai‘i.

It seems this is a trick question, and its solution may be related to Star Wars of all things.

“We just don’t think Halla could have survived being absorbed by an expanding red giant star,” Dr Huber acknowledges.

A more plausible theory is that the planet never was engulfed.

Artist’s animation showing one of the possible explanations as to how planet Halla managed to avoid getting swallowed by its host star Baekdu. In this scenario, Baekdu may have originally been part of a binary system comprising a red giant star and a white dwarf star. The two stars then merged, which would have prevented the red giant from expanding large enough to engulf Halla. Credit: W. M. Keck Observatory/Adam Makarenko.

“The system was more likely similar to the famous fictional planet Tatooine from Star Wars, which orbits two suns,” explains University of Sydney 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.”

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The team believes 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.

Artist’s animation showing an alternative explanation for planet Halla’s existence. In this scenario. Baekdu was originally a binary system comprising a red giant star closely orbiting a white dwarf star. The two stars then merged, and the violent collision between the pair released debris from which the planet Halla formed. Credit: W. M. Keck Observatory/Adam Makarenko.

Another possibility altogether is that Halla is a new planet, born out of a gas cloud produced when the two stars collided. Most stars in the galaxy come in pairs – or even higher-numbered systems. How planets form in such binary stellar systems is still a mystery, but it is possible that it is the reason Halla exists after Baekdu’s violent transition.

The upshot is that, unlike Halla, in five billion years it still looks like Earth is in danger of saying farewell.

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