Exoplanet dodges violent outburst from star and survives engulfment

Astronomers hypothesized that Halla managed to survive despite its star undergoing a cataclysmic transition that should have engulfed this neighboring planet.
Mrigakshi Dixit
The ‘hot Jupiter’ planet, Halla, could once have orbited two stars that interacted via mass transfer.
The ‘hot Jupiter’ planet, Halla, could once have orbited two stars that interacted via mass transfer.

Tobias Roetsch/GTGRAPHICS.DE 

In a star system about 520 light years away, a Jupiter-like exoplanet has survived a violent outburst from its host star, baffling scientists.

The fortunate planet has been dubbed Halla (8 UMi b), and it orbits a massive star called Baekdu — located in the Ursa Minor constellation. 

An international team of astronomers hypothesized that Halla managed to survive despite its star undergoing a cataclysmic transition that, in theory, should have engulfed this neighboring planet.

“Engulfment by a star normally has catastrophic consequences for close orbiting planets. When we realized that Halla had managed to survive in the immediate vicinity of its giant star, it was a complete surprise,” said Dan Huber, second author of this study from the University of Sydney, in a press release

Earth can't escape this fate 

After using all of its fuel, our Sun is predicted to reach the end of its existence in five billion years. When that happens, it will dramatically expand to 100 times its present size. In the process, it will swallow Earth and other nearby planets (Mercury, Venus) in the solar system.

Numerous exoplanets orbiting distant stars throughout the cosmos encounter a similar destiny. Halla, on the other hand, appears to have survived this catastrophic situation. 

Additional observation confirmed the survival 

Halla was first discovered by a team of Korean astronomers in 2015. The researchers studied Baekdu using NASA's Transiting Exoplanet Survey Satellite (TESS) data.

The study revealed that the star is presently fueled by helium, indicating that it has already entered the red giant phase and depleted its hydrogen fuel reserves for nuclear fusion.

Technically, the star should have engulfed the exoplanet during that phase of expansion. 

“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,” added Huber.

From 2021 to 2022, the researchers used the W. M. Keck Observatory and the Canada-France-Hawaii Telescope on Mauna Kea, Hawai'i, to confirm their survivability theory.

Data was collected to ascertain whether the star had a larger size compared to the current orbit of the planet. These fresh measurements verified that the planet's almost circular orbit, which takes 93 Earth days to complete, has been steady for more than a decade. 

“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. 

How did the planet survive?

The team has put forth multiple possibilities to explain how this fortunate planet managed to survive such a cosmic endgame.

According to one explanation, it was never in danger of being engulfed. Halla likely originally orbited two stars, and a violent collision between them resulted in the birth of Baekdu.

“The system was more likely similar to the famous fictional planet Tatooine from Star Wars, which orbits two suns,” said 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 theory is based on the fact that the twin stars would have slurped stellar material off one other, preventing expansion, and allowing Halla to escape engulfment.

Another potential explanation is that Halla is a newly formed planet or a second-generation planet, originating from the remnants of stellar gas clouds resulting from a violent collision between two stars.

The surprising discovery of Halla demonstrates that the field of exoplanet science holds far more complexity than previously anticipated. And this unique system could further expand our comprehension of the birth and demise of planets and stars beyond the solar system.

The findings have been published in the journal Nature.

Study Abstract:

When main-sequence stars expand into red giants, they are expected to engulf close-in planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris b orbits a core-helium-burning red giant. At a distance of only 0.5 AU 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. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems.

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