In a groundbreaking discovery, scientists have observed a star devouring a planet for the very first time. The event, which had been theorized but never directly witnessed until now, occurred in our own galaxy, roughly 12,000 light-years away in the constellation Aquila.
Researchers from several esteemed institutions, including MIT, Harvard University, and Caltech, collaborated on the study, which is set to be published in the journal Nature. The team observed a sudden outburst from a nearby star, which rapidly became over 100 times brighter in just ten days before fading away.
What followed was a cooler, longer-lasting signal that led the scientists to the conclusion that the event could only have been caused by the star devouring a nearby planet. Lead author Kishalay De, a postdoctoral researcher at MIT's Kavli Institute for Astrophysics and Space Research, explained that they were witnessing the final stage of the planet's consumption.
The scientists estimate that the lost planet was a large, Jupiter-sized world that had orbited too closely to the dying star and ultimately spiraled into its core. While stars consuming planets as they run out of fuel is a well-established phenomenon, this marks the first direct observation of such an event, providing new insights into the life cycles of stars and planets.
A similar fate will befall the Earth, though not for another 5 billion years, when the sun is expected to burn out, and burn up the solar system's inner planets.
"We are seeing the future of the Earth," De says. "If some other civilization was observing us from 10,000 light-years away while the sun was engulfing the Earth, they would see the sun suddenly brighten as it ejects some material, then form dust around it, before settling back to what it was."
The study's MIT co-authors include Deepto Chakrabarty, Anna-Christina Eilers, Erin Kara, Robert Simcoe, Richard Teague, and Andrew Vanderburg, along with colleagues from Caltech, the Harvard and Smithsonian Center for Astrophysics, and multiple other institutions.
Hot and cold
The team discovered the outburst in May 2020. But it took another year for the astronomers to piece together an explanation for what the outburst could be.
The initial signal showed up in a search of data taken by the Zwicky Transient Facility (ZTF), run at Caltech's Palomar Observatory in California. The ZTF is a survey that scans the sky for stars that rapidly change in brightness, the pattern of which could be signatures of supernovae, gamma-ray bursts, and other stellar phenomena.
De was looking through ZTF data for signs of eruptions in stellar binaries -- systems in which two stars orbit each other, with one pulling mass from the other every so often and brightening briefly as a result.
"One night, I noticed a star that brightened by a factor of 100 over the course of a week, out of nowhere," De recalls. "It was unlike any stellar outburst I had seen in my life."
Hoping to nail down the source with more data, De looked to observations of the same star taken by the Keck Observatory in Hawaii. The Keck telescopes take spectroscopic measurements of starlight, which scientists can use to discern a star's chemical composition.
But what De found further befuddled him. While most binaries give off stellar material such as hydrogen and helium as one star erodes the other, the new source gave off neither. Instead, what De saw were signs of "peculiar molecules" that can only exist at very cold temperatures.
"These molecules are only seen in stars that are very cold," De says. "And when a star brightens, it usually becomes hotter. So, low temperatures and brightening stars do not go together."
"A happy coincidence"
It was then clear that the signal was not of a stellar binary. De decided to wait for more answers to emerge. About a year after his initial discovery, he and his colleagues analyzed observations of the same star, this time taken with an infrared camera at the Palomar Observatory. Within the infrared band, astronomers can see signals of colder material, in contrast to the white-hot, optical emissions that arise from binaries and other extreme stellar events.
"That infrared data made me fall off my chair," De says. "The source was insanely bright in the near-infrared."
It seemed that, after its initial hot flash, the star continued to throw out colder energy over the next year. That frigid material was likely gas from the star that shot into space and condensed into dust, cold enough to be detected at infrared wavelengths. This data suggested that the star could be merging with another star rather than brightening as a result of a supernovae explosion.
But when the team further analyzed the data and paired it with measurements taken by NASA's infrared space telescope, NEOWISE, they came to a much more exciting realization. From the compiled data, they estimated the total amount of energy released by the star since its initial outburst, and found it to be surprisingly small -- about 1/1,000 the magnitude of any stellar merger observed in the past.
"That means that whatever merged with the star has to be 1,000 times smaller than any other star we've seen," De says. "And it's a happy coincidence that the mass of Jupiter is about 1/1,000 the mass of the sun. That's when we realized: This was a planet, crashing into its star."
With the pieces in place, the scientists were finally able to explain the initial outburst. The bright, hot flash was likely the final moments of a Jupiter-sized planet being pulled into a dying star's ballooning atmosphere. As the planet fell into the star's core, the outer layers of the star blasted away, settling out as cold dust over the next year.
"For decades, we've been able to see the before and after," De says. "Before, when the planets are still orbiting very close to their star, and after, when a planet has already been engulfed, and the star is giant. What we were missing was catching the star in the act, where you have a planet undergoing this fate in real-time. That's what makes this discovery really exciting."
