Astronomers find zombie planet that ‘shouldn’t exist’

The newly-discovered planet Halla managed to survive the demise of its star Baekdu

Scientists have found a new planet that shouldn’t exist, after it seemed to miraculously survived the violent death of its star.

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

But the new study offers hope that at least some of these planets are able to 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 up to 1.5 times the distance between it and Halla, engulfing the star, before shrinking back down to its current size.

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

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

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

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

Those observations confirm that the planet had been in its stable orbit for over a decade, and that it really existed. “Together, these observations confirmed the existence of the planet, leaving us with the compelling question of how the planet actually survived,” said IfA astronomer Daniel Huber, second author of the study.

But scientists still do not know how it survived. One possibility is that it started on a larger orbit before moving closer to its star, but astronomers believe that is unlikely.

Another is that Baekdu was actually once two stars. They may have merged during their death, sparing Halla from being merged at all, by stopping them getting big enough to engulf it.

A separate possibility is that Halla was actually born out of the collision of the two stars. That might have produced a gas cloud that actually gave birth to Halla, and so it may be the result of the demise of its star rather than a survivor of it.