A star was wrongly accused of a cosmic crime: devouring its own planet

Two years ago, a star on its deathbed was charged with a heinous act — eating a planet — in a system 12,000 light-years away from Earth. 

But new evidence has emerged in the case that astronomers say exonerates this elderly Milky Way star of the crime. Using the James Webb Space Telescope, a collaboration of NASA and its European and Canadian space counterparts, a team observed that while the planet did die in the belly of a stellar beast, it didn’t go down the way they once thought. 

Rather than the star bloating into a red giant that then swallowed the Jupiter-sized world, the planet’s orbit had slowly shrunk, bringing it ever closer to its star. Eventually, the planet collided with the star.

It all boils down to culpability, and it seems, at least in this case, the distant planet essentially jumped down the star's throat.

"So the star actually did eat the planet, just not in the way we initially thought," Ryan Lau, an astronomer at the National Science Foundation's NOIRLab, told Mashable, "and it was maybe more the planet's fault."

In the past, astronomers have found evidence of stars that have consumed planets, sometimes by doing a sort of post-mortem autopsy on what's left of the dead star. But research previously published on this particular event in the journal Nature presented the first direct evidence of a star engulfing a planet as it happened. 

The incident was first seen five years ago as a sudden bright flash of visible light, which scientists named ZTF SLRN-2020. Later, they noticed that the star had already started to glow in infrared a year earlier — a clue that there was dust nearby, possibly in the wake of a destroyed planet.

They thought the star had turned into a red giant, a late stage in a star’s life when it grows much larger and can swallow nearby planets. Scientists have suggested it's likely the fate of the sun and Earth. But the new data from Webb revealed a twist: The star hadn't brightened as it would if it had indeed expanded. 

That meant the star stayed about the same size — and the planet, roughly the size of Jupiter, came to it. Over millions of years, the planet edged closer and closer. Eventually, it grazed the star’s outer atmosphere until it was completely reeled in. The results and new conclusions are published in The Astrophysical Journal. 

The collision caused a giant explosion, creating a swirling disk of gas and dust. By studying the aftermath, Webb detected molecules like carbon monoxide around the star.

"The planet eventually started to graze the star’s atmosphere. Then it was a runaway process of falling in faster from that moment," said Morgan MacLeod of the Harvard-Smithsonian Center for Astrophysics, in a statement. "The planet, as it’s falling in, started to sort of smear around the star."

Unlike giant stars that explode into a supernova and collapse into a black hole, a medium star like the sun suffers a more tortured end by dying slowly. A so-called "planetary nebula" — a confusing misnomer because stars cause them, not planets — is a phenomenon made from the molted layers of an elderly star. Such spectacular clouds of gas and dust occur when a star withers away as it loses nuclear fuel.

Astronomers expect this is the future of the sun in about 5 billion years, though scientists still have a lot to learn about these events. 

It would be impossible to watch a single star go through its entire lifecycle for obvious reasons: That would take billions of years, said Paul Sutter, a professor at Stony Brook University and author of How to Die in Space, in a 2022 interview with Mashable. But experts have been able to predict this kind of death for some planets by studying many stars at different intervals and how they interact with their surroundings at each age.

"It's like taking a snapshot of everyone on the Earth in one moment. You can't capture one person's lifetime, but you can see people being born, you can see people playing soccer in elementary school, and you can see people getting married. You can see people dying, getting sick," said Sutter, who wasn't involved in the new study. "You can reconstruct the life cycle of a person by putting together all these separate pieces, so we have a general picture of how stars evolve and how they live."

Webb's investigation of the gas in the aftermath prompts more questions for researchers about what actually transpired once the star swallowed the planet. Scientists hope to find and study others to collect more data.