Star swallowing a planet

Scientists see ‘future of the Earth’ as dying star swallows Jupiter-sized planet

Image credit: R. Hurt/K. Miller/Caltech/IPAC

Astronomers have been able to observe a star swallowing a planet for the first time, allowing them to witness what may well be the Earth's ultimate fate.

In a world-first, Massachusetts Institute of Technology (MIT), Harvard University and the California Institute of Technology (Caltech) scientists have witnessed the death of a star, which caused it to engulf a planet the size of Jupiter.

The planetary demise appears to have taken place in our own galaxy, some 12,000 light-years away, near the eagle-like constellation Aquila.

There, astronomers spotted an outburst from a star that became more than 100 times brighter over just 10 days, before quickly fading away. This white-hot flash was followed by a colder, longer-lasting signal, the researchers reported, concluding that it could only have been produced by one event: a star engulfing a nearby planet. 

“We were seeing the end-stage of the swallowing,” said lead author Kishalay De, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. 

As a star runs out of fuel, it will billow out to a million times its original size, engulfing any matter - including planets and moons - in its wake. In the past, scientists have observed hints of stars just before, and shortly after, the act of consuming entire planets, but this is the first time they have witnessed such an event.

The fate of this planet mirrors what would happen to Earth around five billion years from now, 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 said. “If some other civilisation 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 team discovered the outburst in May 2020, during 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. 

However, it was not until a year later that the astronomers were able to piece together an explanation for what the outburst could be - after De began looking through ZTF data for signs of eruptions in stellar binaries.

“One night, I noticed a star that brightened by a factor of 100 over the course of a week, out of nowhere,” De recalled. “It was unlike any stellar outburst I had seen in my life.”

To better understand the image, 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. 

However, 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.

De and his colleagues compared the image with observations of the same star 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 said. “The source was insanely bright in the near-infrared.” 

After its initial hot flash, the star seemed to have continued to throw out colder energy over the next year, which 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. 

When the team further analysed the data and paired it with measurements taken by Neowise, Nasa’s infrared space telescope, they came to a much more exciting realisation. 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 - approximately 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 said. “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 realised: 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 said. “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.”

This researcher's findings were published in an article in the journal Nature

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