Astronomers are pushed to explain why a far-off planet still exists

A far off planet which should have been destroyed in a stellar apocalypse mysteriously still exists. Astronomers think they know why.

University of Sydney astronomers believe the planet Halla, pictured in this artist’s impression, may have survived because the twin stars which it orbited collided and merged before they had the chance to engulf the planet.

It’s the planet that shouldn’t be there. University of Sydney astronomers are pushed to explain why Halla, a Jupiter-like planet, is still in a close orbit around its star Baekdu when current astronomical theory says it should have been destroyed.

Dan Huber and Tim Bedding, who are part of an international team that discovered the anomaly, say that Baekdu, a star near the end of its life, would normally have already expanded and engulfed its planet.

Never before have astronomers seen a planet like Halla, which has apparently survived a stellar apocalypse.

“It was a complete surprise. It’s a planet that shouldn’t exist,” said Dr Huber.

The reason is that when stars like Baekdu, billions of years into their life, exhaust the hydrogen fuel that powers their fusion energy, their brightness dims and they contract in size.

But the gravitational energy of their contraction heats them again and ignites the helium in the core that was created by hydrogen fusion, giving the star a second life as a “red giant”.

The star expands to 100 times, or even 1000 times, its former diameter, destroying any close by planets. Halla, being only half the distance from Baekdu as the earth is from the sun, would have been right in the danger zone.

University of Sydney astronomer Dan Huber.

Halla, which is about 520 light years from earth, is so unusual that it led to a paper published on Thursday in the prestigious science journal Nature. Titled A close-in giant planet escapes engulfment by its star, Dr Huber and Professor Bedding are authors.

In the paper, they offer a Star Wars-like explanation for Halla’s continuing existence, postulating that, like the fictional planet Tatooine in the movie, Halla once orbited a double, or binary, star. In other words Baekdu was once two stars locked in a close orbit before the pair violently collided.

It’s not a far-fetched explanation because more than half of stellar systems have multiple stars.

The astronomers postulate that the double stars were very close to each other, with an orbital period of days. When one of them ran out of hydrogen fuel, it expanded into a red giant and violently swallowed the other.

“This merger caused the red giant stage to be shorter than it normally would be and that, we believe, saved the planet,” Dr Huber said.

Another possibility is that Halla is a recently formed planet that coalesced out of the cloud of gas spun off when the two stars merged. Dr Huber said this was plausible because planets could form quickly compared to a star’s lifetime.

To test these hypotheses, he said that astronomers need to find more planets like Halla. Or they could do an analysis of Halla’s atmosphere, which may be possible with the new James Webb Space Telescope, and the composition of Halla’s atmosphere might uncover its history.

Professor Bedding said more research might reveal other ways to explain the mystery. “That’s how science works,” he said.

Although Halla has been lucky so far, Dr Huber and Professor Bedding say that, in the long run, it’s doomed. A red giant stars like Baekdu, which is in the final stage of its stellar evolution, will expand again and destroy Halla.