Stars tend to get violent when they’re about to die.
Take our Sun, for example: in about 5 billion years, after running out of hydrogen, it will become unstable and begin to collapse from the inside out.
It’ll then expand, swallowing up any planet that happens to be orbiting nearby – Mercury, Venus, and potentially even Earth – before retracting into a smaller, cooler, and continuously fading version of its old self, now alone in a planetary graveyard of its own making.
No nearby planet can survive this process – or at least, so we thought.
A new international study, published today in Nature, has confirmed the existence of a planet more than 500 light-years away that seems to have survived this deadly expansion, against all odds.
This planet, 8 Ursae Minoris b – also known as 8 UMi b or ‘Halla’ – is orbiting close to its dying red giant star, well within the zone that would have been wiped clean during the star’s expansion phase.
“This is a planet that shouldn't exist,” says Dr Ben Montet, Scientia Senior Lecturer in UNSW Science and co-author on the paper.
“It should have been ingested by its star.”
Astronomers have known about Halla since 2015, when a study using the radial velocity method – a technique that analyses the slight gravitational tug a planet can have on its host star – suggested a planet was orbiting a star called 8 Ursae Minoris (also known as 8 UMi or ‘Baekdu’).
New research led by University of Hawai’i and co-authored by UNSW confirms this discovery, showing that Halla’s 93-day, nearly circular orbit has remained stable for over a decade. These findings were based on observations made from the Canada-France-Hawaii Telescope and the W. M. Keck Observatory, two telescopes on Maunakea, an active volcano in Hawai’i, over 2021-2022.
The team also used asteroseismology – that is, the study of a star’s oscillations to uncover its internal properties – to show that the star Baekdu is burning helium in its core: a phase that red giants reach only after they’ve swelled up and consumed nearby planets.
This makes Halla the only planet we know of closely orbiting a star in this late stage of life.
“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,” says lead author Dr Marc Hon, a NASA Hubble Fellow based at University of Hawai’i’s Institute for Astronomy (IfA). Dr Hon was previously based at UNSW, where he both completed his PhD and taught in the School of Physics.
The findings led the team to ask more questions. Is the planet breaking everything we knew about stellar-planetary evolution?
Or could there be another, equally bizarre, reason behind its survival?
Investigating the impossible
When peculiar things happen in space, astronomers try to seek answers by following the trail of weirdness, says Dr Montet.
It didn’t take them long to spot something strange.
“One of the weird things about this star is that it has a lot of lithium in its atmosphere,” he says.
“This is unusual as most stars don't have lithium – it burns too easily in stellar atmospheres. Lithium is often seen in young stars, but only about 1 per cent of older red giants.”
One of the leading theories to explain why some older stars are mysteriously lithium-rich is that they gained their lithium later in life, via some kind of interaction with another star.
Could this same interaction have also happened to Baekdu?
The team have pieced together several scenarios of how Halla might have survived – and the leading theories start with Baekdu.
“We believe Halla exists today because its host star Baekdu was previously two stars – a binary system – that merged into one,” says Dr Hon.
“The merger could have happened from several to tens of million years ago.”
In one survival scenario, a merger between these two stars could have restricted either one of the stars from expanding wide enough to engulf Halla, helping the planet narrowly escape.
Another scenario is that Halla wasn’t in danger in the first place, simply because it didn’t exist before the star’s expansion phase. Instead, it might be a second-generation planet, born from the ingredient-rich gas cloud created from the two stars’ violent collision.
The team also found evidence there might be a third actor at play. A long-term trend in the star’s radial velocity signal suggests there’s another nearby object that has a slight gravitational pull on Baekdu – but whether it’s a star or planet, and if it played a role in somehow influencing Halla’s current orbit, is still a mystery.
The findings wouldn’t be possible without stellar observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), which helped the team use asteroseismology to ‘look inside’ the star, Baekdu, and confirm it was in the post-expansion phase of its life.
“This astonishing result really hinges on the fact that we have been able to ‘see’ inside the star that it is now burning helium in its core,” says Professor Dennis Stello, co-author and previous PhD supervisor of lead author, Dr Hon.
“The evidence for this is revealed through ringing sound waves inside the star that we have been able to measure and interpret using similar techniques as geoscientists use to learn about the Earth’s interior from studying earthquakes. However, we instead use star quakes.”
What we know about Halla
Halla is a Jupiter-like gas giant nestled in the Ursa Minor (or ‘Little Bear’) constellation, which is visible only from the Northern Hemisphere.
If you caught a ride there on the Voyager 1, the fastest human-made object in the universe travelling at 61,500 kilometres per hour, it’d take about 9 million years to get there.
Once you arrive, though, you’d probably want to stay on board. As Halla is made up of swirling gas, not only wouldn’t there be anywhere to land, but its surface temperature is also likely to reach about 1000 Kelvin, or 730 degrees Celsius – making it hotter than any planet in our Solar System.
One of the reasons behind this heat is the planet’s closeness to its star, Baekdu. It orbits closely, at about half the distance between Earth and the Sun.
“The star itself will be about 20 times the apparent size in the sky as our Sun,” says Dr Montet.
“This is about the size of your fist held at arm’s length.”
But if you visited the planet before the potential stellar merger, you would’ve had a much different sight: like Tatooine, the famous planet from Star Wars, there would have been two suns in the sky.
Are there any other survivors out there?
Halla is the first planet of its kind we know of to escape engulfment, but that doesn’t mean it’s the only one.
This study suggests that more survivalist planets could be out there.
“Planets are still surprising us with their apparent resilience by turning up in ‘forbidden’ systems,” says Dr Hon.
“Our study shows that not all close planets are doomed at the hands of their host star when the star begins to grow old and swell.”
Dr Montet says there are about 1000 other lithium-rich red giant stars out there that we know of – and an exciting opportunity now could be to search them for nearby planets.
“There aren't that many of these stars that have been searched, simply because they shouldn't have planets,” says Dr Montet.
“But we've been surprised before and we'll probably be surprised again.”