We've seen a star devouring a planet for the first time

Astronomers have caught a star in the act of devouring one of its planets for the first time. Someday, our own sun will expand just like this star, enveloping the other inner planets, so this system is a sort of preview of Earth’s fate.

Kishalay De at the Massachusetts Institute of Technology and his colleagues used the Zwicky Transient Facility in California to spot a strange burst of light that was designated ZTF SLRN-2020, coming from a star around 13,000 light years away. Over the course of around 10 days, it brightened by a factor of about 100.

The outburst was similar to a phenomenon called a red nova, caused by a merger between two stars, but it wasn’t quite as bright and didn’t give off quite as much energy. After collecting more observations with other telescopes, the researchers found that the data was consistent with a star devouring not another star, but a gas giant planet at least 30 times the mass of Earth.

We knew that stars eat planets because we have seen the aftermath in stars polluted by chemicals from the worlds they devoured. “In the past, all of the evidence that we’ve had of stars eating planets is from looking at stars that have done that hundreds of thousands of years ago,” says De. “But we have never caught a star red-handed eating a planet.”

This is expected to occur when a sun-like star exhausts its hydrogen fuel and switches to fusing helium. In the process, the star becomes a red giant and its atmosphere expands outwards, engulfing any planets with the misfortune of orbiting too close. In ZTF SLRN-2020’s case, the planet took less than one Earth day to orbit its star.

The sun is due to start its expansion in about 5 billion years. “We are actually seeing the fate of our own planet happen in real time to another unfortunate planet,” says De. “If you were observing our solar system from 10,000 light years away, what you would see is the sun would also brighten in a similar way, but the effect would be nowhere near as dramatic because Earth is so much smaller than [the planet] is.”

Now that we know what a planetary engulfment looks like, it will be much easier to search for them and study them in more detail, De says. The researchers calculated that this should occur around once a year within our galaxy, so we should be able to find more planets being devoured by their stars, as well as continue to observe this one, and work out the details of the process – and of Earth’s future demise.

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