In the first, astronomers saw a star swallowing a planet
(Nanowerk News) When a star runs out of fuel, it balloons to millions of times its original size, swallowing up any material — and planets — behind it. Scientists had observed star signs before, and shortly after, the act of eating entire planets, but they had never caught one until now.
In a study appearing in Natural (“Infrared transients from stars swallowing planets”), scientists at MIT, Harvard University, Caltech, and elsewhere report that they have observed a star swallowing a planet, for the first time.
The planetary crash appears to have taken place in our own galaxy, some 12,000 light years away, near the eagle-like constellation Aquila. There, astronomers saw a burst from a star that became more than 100 times brighter in just 10 days, before rapidly disappearing. Surprisingly, this white-hot flash was followed by a longer lasting colder signal. This combination, the scientists concluded, could have been produced by only one event: a star engulfing a nearby planet.
“We’re looking at the final stages of the swallowing process,” said lead author Kishalay De, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research.
What about an extinct planet? Scientists estimate that it may be a hot Jupiter-sized world that is spiraling closer, then being pulled into the dying star’s atmosphere, and finally into its core.
A similar fate will befall Earth, though not for another 5 billion years, when the sun is expected to burn up, and burn the planets in the solar system.
“We see the future of Earth,” said De. “If any other civilization had observed us from 10,000 light years away as the sun engulfed the Earth, they would have seen the sun suddenly brighten as it ejected some material, then formed dust around it, before returning to normal.”
The MIT study’s 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 Centers for Astrophysics, and several other institutions.
The team discovered the burst in May 2020. But it took astronomers another year to piece together an explanation for the burst.
The initial signals emerged in search 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 rapidly changing brightness stars, whose patterns can be signs of supernovae, gamma-ray bursts, and other stellar phenomena.
De was searching through ZTF data for signs of eruptions in stellar binaries — systems in which two stars orbit each other, with one mass pulling from the other every so often and briefly brightening as a result.
“One night, I saw a star that was bright by a factor of 100 for a week out of nowhere,” recalls De. “It was unlike any star explosion I’ve ever seen in my life.”
Hoping to find a source with more data, De observed the same star observations taken by the Keck Observatory in Hawaii. The Keck Telescope takes spectroscopic measurements of starlight, which scientists can use to distinguish the chemical composition of stars.
But what De found confused him even more. While most binaries eject stellar material like hydrogen and helium as one star erodes another, the new source doesn’t release either. Instead, what De saw were signs of “strange molecules” that could only exist at extremely cold temperatures.
“These molecules are only seen in very cold stars,” said De. “And when a star shines, it usually gets hotter. So, low temperatures and bright stars don’t go together.”
It was then clear that the signal was not a binary star. De decided to wait for more answers to appear. About a year after his initial discovery, he and his colleagues analyzed the same observations of the star, this time taken with an infrared camera at the Palomar Observatory. In the infrared band, astronomers can see signals from cooler material, as opposed to the hot, white optical emission that emerges from binaries and other extreme stellar events.
“The infrared data knocked me out of my chair,” says De. “The source is very bright in the near-infrared.”
It appears that, after the initial hot flash, the star continued to emit cooler energy over the next year. The cold material is likely gas from a star that shot out into space and condensed into dust, cold enough to be detected at infrared wavelengths. These data suggest that the star may have merged with other stars rather than brightened by a supernova explosion.
But when the team analyzed the data further and paired it with measurements taken by NASA’s infrared space telescope, NEOWISE, they came to a much more interesting realization. From the data gathered, they estimated the total amount of energy released by the star since its initial outburst, and found it to be minuscule — about 1/1,000 the magnitude of stellar mergers observed in the past.
“That means anything joining the star has to be 1,000 times smaller than any other star we’ve ever seen,” De said. “And it is a happy coincidence that Jupiter’s mass is about 1/1,000th the mass of the sun. That’s when we realized: This is a planet, crashing into its star.”
With the pieces in place, scientists can finally explain the initial explosion. The bright hot flashes are likely the last moments of a planet the size of Jupiter being pulled into the ballooning atmosphere of a dying star. As the planet plunges into the star’s core, the star’s outer layers explode, settling as cold dust over the next year.
“For decades, we can see befores and afters,” says De. “Before, when the planets were still orbiting very close to their star, and after, when a planet had been swallowed up, and the star was gigantic. What we’re missing is catching a star in action, where you own a planet experiencing this fate in real time. That’s what makes this discovery so exciting.”