Exoplanet that probably survived its own demise found in space: how it could happen

Dmytro IvancheskulLife
Halla is where it shouldn't be. Source: Tobias Roetsch/gtgraphics.de/ESO/L.Calçada/ OBOZREVATEL collage

A team of Korean astronomers discovered the planet 8 Ursae Minoris b orbiting the star, which by all accounts simply cannot exist there. The researchers speculate that it was absorbed by the luminary, but somehow managed to survive.

The study, published in the journal Nature, was reported in an article for The Conversation by University of Sydney astronomer Daniel Huber. Both the star and the planet were discovered in 2015, and in 2019 they were given their names, Baekdu and Halla, after the highest mountains on the Korean peninsula.

The planet has about the same mass as Jupiter and orbits about half the distance of Earth from the Sun. But it wasn't the distance that surprised scientists.

Baekdu, scientists believe, has already gone through a red giant evolutionary stage. In this stage, the star's core shrinks and its envelope expands. As a result, the star can grow to more than 100 times its original size. For example, in about 5 billion years our Sun will also expand so much that it will swallow Mercury, Venus and possibly even the Earth.

Subsequently, the core of the star gets so hot that it starts melting helium and the star shrinks to about 10 times its original size, after which it burns steadily for tens of millions of years.

Now scientists have obtained new data on the star thanks to NASA's Transiting Exoplanet Survey Satellite (TESS) space telescope and were very surprised to find that the star has already begun melting helium in its core.

"The discovery was puzzling: if Baekdu is burning helium, it must have been much larger in the past, so large that it would have consumed the Halla planet. How is it possible that Halla survived?," Huber writes in amazement.

He explained that scientists first set out to check the reality of Halla itself, which they discovered through the Doppler Oscillation method, which is based on the gravitational interaction between the star and the planet. This method, as explained by the astronomer, does sometimes produce erroneous results, as scientists are misled by long-term variations in the behavior of the star itself.

However, years of observations have confirmed that Halla does exist.

Scientists then came up with two scenarios that could explain the current situation with Baekdu and Halla.

As Huber explained, at least half of all the stars in our galaxy did not form alone, like our Sun, but do so as a binary system. This could have saved Halla, since the merger of the two stars could have prevented one of them from expanding to a size large enough to absorb a planet.

"If one star became a red giant on its own, it would absorb Halla, but if it merged with a companion star, it would go straight to the helium-burning phase without becoming large enough to reach the planet," the scientist explained.

Another scenario is that Halla could be a newborn planet, emerging from a cloud of dust and gas formed by the collision of Baekdu and its companion star.

"Whichever explanation is correct, the discovery of a nearby planet on a helium-burning red giant star demonstrates that nature finds ways for exoplanets to appear where we least expect them," the astronomer summarized.

Previously OBOZREVATEL also reported that scientists have found a strange space object that is hotter than the Sun. It is neither a planet nor a star.

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