Extract from ABC News
Around 520 light years away lies a planet that should have been destroyed by its ageing star.
Key points:
Halla is the first 'survivor' planet to be discovered
The Jupiter-sized planet is orbiting so close to its ageing red star that it shouldn't exist
Scientists don't know how it survived destruction, but it may have once orbited two stars like Tatooine from Star Wars
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.
When the red giant swallowed the white dwarf, it triggered an early switch to burning helium, which would have shrunk the star.
"It changed the chemical make-up of the star and led to the star not becoming as large as it would have been," Dr Huber said.
Measurements by team member Ben Montet, from the University of New South Wales, shows there are much higher than expected amounts of lithium in Baekdu's surface, which supports this hypothesis, since lithium can be produced when stars merge.
Another more speculative explanation is that Halla is a new planet created by the merger of the two stars.
According to this idea, the collision between the two stars produced a cloud of gas that formed Halla, which would then be termed a 'second-generation' planet.
Astrophysicist Rob Wittenmyer at University of Southern Queensland welcomed the research.
"The most exciting thing is always when we find something that should not be there," said Dr Wittenmyer, who was not involved in the research.
"Here we see a planet that defied the expectation."
"That is the most fun part of science — when we get a 'what happened'? moment … So I am keenly interested in this work as a clever way to explain such an unusual planet."
Planetary science expert Charley Lineweaver said "the coolest thing" for him was that scientists were using a planet to give insight into the history of a red giant.
He hopes research of this kind will one day help settle uncertainties around the fate of Earth when the Sun ages and becomes a red giant itself.
"The more we know about red giants and the planets around them, particularly the ones that are very close, that gives us constraints on how big red giants get."
Searching for exoplanets
The findings could have implications for the search for exoplanets.
Red giants are not normally the target for exoplanet searching, Dr Hon said.
"People normally gravitate towards Sun-like stars."
But exoplanets may appear around stars we least expect, and red giants should not be passed over so easily, the researchers suggest.
Surveys of stars outside our Solar System suggest that more than 50 per cent have a companion star. Perhaps, thanks to interactions between stars, more planets may exist around ageing stars.
Astrophysicist Orsolo De Marco of Macquarie University said there was a view that there would be few planets around evolved stars, like the red giant Baekdu.
"It seems that there is now mounting evidence that either planets manage to escape destruction or that there is a way to form planets from the ejecta of stellar interaction," Professor De Marco said.
"These are exciting times."
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