All across the Milky Way, dying stars are swallowing their planets. Even the Earth is likely to be destroyed in about five billion years from now, when the Sun expands and swallows its planets.
But Halla, a giant planet orbiting a star 520 light-years from Earth, seems to have narrowly escaped such a doomsday scenario. According to a study published Wednesday in Nature, a new explanation for its survival conditions suggests that there may be a hidden population of death-defying planets elsewhere in the galaxy.
Most current research efforts are aimed at identifying Earth-like planets suitable for life. These efforts focus on stars with so-called ‘anacole’ alignments, just like our Sun. Anacole stars get their power from the fusion of hydrogen into helium at their core. And so far, more than 90 percent of all known exoplanets have been detected around main-sequence stars.
When stars like the sun reach the end of their lives, they turn into red giants that grow exponentially in size and incinerate planets that enter their boundaries. But Halla, which was first discovered in 2015 and resembles Jupiter, has added a new dimension to this story of planetary ‘swallowing’.
With a narrow 93-day orbit, Halla was supposed to be in the belly of its star Baekdu. But during observations with ground-based telescopes in Hawaii, scientists found that Halla is still there, perfectly intact and defying all predictions.
Marc Hon, a NASA Hubble researcher at the University of Hawaii at Manoa and one of the study’s authors, emphasized that Halla’s survival around an inhospitable host star, and at such a close distance, could indicate a very unusual history.
“In our research, published in Nature, we address the riddle of this planet’s existence and put forward some possible answers,” said astronomer Daniel Huber of the University of Sydney, who is part of the international team.
Huber recently summarized the possible scenarios in an article published in Conversation. Here is his account of what follows.
A glimpse into our future: Red giant stars
This NASA animation shows a red giant swallowing a planet.
Just like humans, stars change as they age. As a star uses up all the hydrogen in its core and cools, its core shrinks and its outer layer expands.
During this ‘red giant’ phase of their evolution, stars can grow to more than 100 times their original size. When this happens to our Sun in about 5 billion years, we estimate that it will be big enough to swallow Mercury, Venus and possibly Earth.
We know of hundreds of planets orbiting red giant stars. One of them, 8 Ursae Minoris b, is a Jupiter-mass planet. It was discovered in 2015 by a team of Korean astronomers. In 2019, the International Astronomical Union named the star Baekdu and the planet Halla after the highest mountains on the Korean peninsula.
A planet that shouldn’t be there
Analysis of new data collected about Baekdu by NASA’s Transiting Exoplanet Survey Satellite (TESS) space telescope has revealed a surprising discovery. Unlike other red giants, Baekdu had already begun helium fusion at its core. Using asteroseismology techniques, which study the waves inside stars, we can determine what material a star is burning. In the case of Baekdu, the frequencies of the waves made it clear that it had already started burning helium in its core.
This discovery was puzzling: Because if Baekdu was burning helium, it should have been much bigger in the past. So big, in fact, that it should have swallowed the planet Halla. So how was it possible for Halla to survive?
As is often the case in scientific research, the first thing to do is to eliminate the least important explanation: Halla never existed.
However, follow-up observations ruled out this scenario. The Doppler signal from Baekdu had remained constant over the last 13 years. Close examination of other indicators revealed no other possible explanation. Halla was real, which brings us back to the question of how she survived death.
A possible survival scenario: Two stars merge
After confirming the planet’s existence, we arrived at two scenarios that could explain Baekdu and Halla’s relationship.
At least half of the stars in our galaxy are part of binary systems, not single stars like our Sun. If Baekdu was once a binary star, Halla may never have been in danger of being swallowed up.
It is possible that these two stars merged, preventing one of them from expanding enough to swallow Halla. If one of the stars were to become a red giant on its own, it could swallow Halla, but if it merged with a companion, it could jump straight to the helium-burning stage without growing large enough to reach the planet.
Another possibility is that Halla could be a relatively newborn planet. The violent collision between the two stars may have produced a cloud of gas and dust from which the planet could have formed. In other words, Halla is a recently born ‘second generation’ planet.
So what do scientists think about this planet that managed to escape a fiery end?
In addition to explaining Halla’s existence, the team’s merger hypothesis could also explain Baekdu’s high concentrations of lithium, an element not normally found in red giants, but which could be produced by the merger of two stars.
“The planet is really hard to explain, but theirs is the best explanation I’ve heard,” says Andrew Vanderburg, a professor of physics at M.I.T. who studies exoplanets and reviewed the study for Nature.
Melinda Soares-Furtado, a NASA Hubble researcher at the University of Wisconsin-Madison, called the study an exciting example of the “unexpected” that occurs in star-planet interactions. She recommends that future research on the system should include experts studying blue stragglers, a class of bright stars thought to be formed by stellar mergers.
Whatever the correct explanation, the discovery of a nearby planet orbiting a helium-burning red giant star shows that planets can find ways to appear where we least expect them.