Whether or not it remains habitable, Earth itself has a hard limit to its lifespan – in about five billion years, the Sun will run out of hydrogen fuel to burn and start fusing helium instead. That will cause it to swell up into a red giant star, expanding its size and swallowing up Mercury, Venus and Earth. Eventually it’ll blow off those outer layers of material and leave behind a white dwarf that’ll gradually cool to the background temperature of space over trillions of years.
A similar fate awaits countless worlds orbiting other stars. But now, astronomers have discovered a “zombie” planet that seems to have survived this kind of cataclysm, against all odds. The dramatic story takes place in a star system that’s relatively close to Earth, only around 520 light-years away.
The lucky planet has been named Halla, and it orbits a giant star called Baekdu in the Ursa Minor constellation. Baekdu is almost 11 times the width of the Sun, but only contains 1.6 times its mass, and Halla orbits it quite closely, at a distance less than half that between Earth and the Sun.
In the new study, astronomers examined the star in more detail and found that it was already fusing helium, indicating it’s previously gone through a red giant phase. By their calculations, in that time Baekdu would have swelled far beyond Halla’s current orbit.
“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,” said Dr. Dan Huber, second author of the study. “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.”
So how did it survive being engulfed? The scientists say that the most likely explanation is that it didn’t. Instead, they hypothesize that Armageddon was averted by a second star that used to exist in the system. As both stars aged, they would have slurped material off each other, preventing expansion, until they eventually merged into what Baekdu is now. This process would involve no growing and no engulfing.
Another option is that Halla hasn’t been there all that long. When the two stars collided, they could have created a gas cloud that cooled and then formed Halla, as a new “second generation” planet in the system.
Whatever happened, it’s an intriguing system that could teach astronomers much about the life and death of planets and stars.
The research was published in the journal Nature. Animations of both the merger and newborn scenarios can be seen in the videos below.