Around 520 light years away lies a planet that should have been destroyed by its ageing star.
The Jupiter-sized planet dubbed Halla orbits an ageing star called Baekdu, in the ‘Little Bear’ constellation Ursa Minor.
Discovered in 2015, it circles close at about half the distance between the Earth and the Sun.
Today in Nature scientists confirmed the star is a red giant that should have at some stage swelled up to a point where it engulfed and destroyed the planet.
The same fate is expected for Mercury, Venus, and likely Earth, as the Sun ages and expands.
But Halla survived and is the first of its kind to be discovered.
“This is a case where the planet in the system we report should really not exist, from what we understand about stars and planets,” said lead author Marc Hon, a NASA Hubble Fellow based at University of Hawaii’s Institute for Astronomy.
So how did it survive, and what does this mean for life, the Universe and everything?
How do we know Halla still exists?
The first thing the researchers did in their study was to check that previous studies hadn’t made a mistake, said second author, observational astronomer Daniel Huber now at the University of Sydney.
“One way that could have explained this whole puzzle very easily is that the planet is not really there,” said Dr Huber.
But after monitoring a wobble in Baekdu over 15 years they found a pattern that could only have been caused by the tug of the planet’s gravity on the star.
“So we confirmed the planet was really there.”
The team also probed Baekdu’s innards, by measuring variations in the brightness of the star.
From that they determined Baekdu was no longer burning hydrogen and had switched to burning helium, meaning it was near the end of its life and had gone through a phase of expansion that should have engulfed Halla.
Modelling confirmed that the star should have expanded to 1.5 times larger than the orbit of the planet before shrinking.
So how could a planet be orbiting this close to a red giant with a helium core?
One explanation the researchers ruled out was that as the red giant expanded, it lost mass and gravity, allowing Halla to orbit further out and dodge engulfment.
The discovery that Halla had a stable circular orbit argued against this idea, Dr Huber said.
Their next explanation has echoes of Star Wars.
Tatooine-like planet?
Like the fictitious Star Wars’ planet Tatooine, the authors propose the real life Halla may have orbited two suns.
They think Baekdu could have formed through a collision and merger of a smaller white dwarf star and a larger red giant, which circled each other.
