Astronomers find a zombie planet that “should not exist”

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The Independent
Astronomers find a zombie planet that “should not exist”
Andrew Griffin
June 28, 2023, 9:07 pm
Astronomers find a zombie planet that “should not exist”
Astronomers find a zombie planet that “should not exist”
Scientists found a new planet that shouldn’t exist, after it apparently miraculously survived the violent death of its star.

Many planets, including our own, will face virtually certain doom when their stars reach the end of their lives and engulf them. When our own Sun dies, for example, it will expand 100 times and swallow the Earth.

But the new study offers hope that at least some of those planets may survive. The newly discovered world, a Jupiter-like planet known as Halla, managed to survive the demise of its star Baekdu, in what should have been certain death.

Astronomers found the planet and discovered through follow-up observations that Baekdu had previously expanded into a red giant. When it did, it would have inflated to 1.5 times the distance between it and Halla, enveloping the star, and then shrunk to its current size.

Despite that dramatic and violent event, Halla managed to stick around, subsisting so astronomers could see it using telescopes in Hawaii.

“Planetary submergence has catastrophic consequences for the planet or the star itself, or both. The fact that Halla managed to persist in the vicinity of a giant star that otherwise would have engulfed it highlights the planet as an extraordinary survivor,” said Marc Hon, lead author of the study.

The findings were published today in a new paper, ‘A close-in giant planet escapes engulfment by its star’, in the journal Nature.

Halla was found in 2015, using what scientists call the “radial velocity method,” which monitors how stars move and uses it to understand how they might be dragged along by planets orbiting them. In the years since, scientists have discovered that the planet must have been engulfed by the star, and have made follow-up observations to better understand the planet.

Such observations confirm that the planet had been in its stable orbit for more than a decade and that it really existed. “Together, these observations confirmed the planet’s existence, leaving us with the compelling question of how the planet actually survived,” observed Institute for Astronomy astronomer Daniel Huber, second author of the study.

But scientists still don’t know how it survived. One possibility is that it started out in a larger orbit before coming close to its star, but astronomers think that’s unlikely.

Another is that Baekdu was actually two stars. They may have fused together during his death, preventing Halla from merging at all, by preventing them from growing large enough to engulf him.