An artist's impression of an aging star swelling up and starting to swallow its planet (Credit: K. Miller/ R. Hurt - Caltech/I PAC)

Scientists have long theorized that older stars swell up as they age and devour nearby planets. However, the assumption has been derived from observations made before and after the event. Now, for the first time, astronomers have caught a star in the act of engulfing a planet. The star, known as ZTF SLRN-2020, resides approximately 12,000 light-years away in our galaxy near the Aquila constellation.

"For decades, we've been able to see the before and after," says study leader Kishalay De. "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."

In 2020, De was studying data captured by the Zwicky Transient Facility (ZTF) telescope in Southern California. The Massachusetts Institute of Technology (MIT) researcher was hoping to find evidence of two stars merging with each other. This usually results in a nova — bursts of light thousands of times brighter than the stars themselves.

But one luminous star De discovered was much dimmer, only 100 times brighter. Also, the collision of two stars usually produces hot gas. However, this one was surrounded by dust molecules, which could only occur in a cold environment. De wondered if he had detected a star swallowing a planet.

An artist's conception of the Jupiter-sized planet being drawn in by its sun (Credit: K. Miller/ R. Hurt - Caltech/IPAC)

The MIT researcher and his team spent the next two years examining similar data collected by other powerful space telescopes. NASA's NEOWISE telescope provided the final piece of evidence they needed. The infrared images captured by the telescope allowed them to confirm that the molecules were traces of a star eating its planet. The scientists assert that as the planet was being swallowed, it displaced hot gas from the star. The gas then cooled and created dust. Fragments of the planet also blew away from the star, producing more dust.

The researchers, who published their findings in the journal Nature on May 3, 2022, believe the gas planet was about the size of Jupiter. It was large enough to cause a flash. But the light was much fainter than that created by two colliding stars. The team says that ZTF SLRN-2020 has since reverted to its original size. "Almost like the star ate the planet and forgot about it," said De.

Will Earth face a similar fate?

Some scientists believe Earth will also be devoured by the Sun in about five billion years. But Mansi Kasliwal, a professor of astronomy at Caltech, says humans will not be around then. The increasing heat from the expanding Sun will evaporate all the water from Earth long before it gets close enough to swallow it, making it uninhabitable. "We have to find our new home long before this happens," she says.

Smadar Naoz, an astronomer at UCLA, agrees that the Earth's water sources will dry up. However, she believes the Sun will lose mass as it expands. This will push Earth farther away and out of reach for the star.

"Whether or not the Sun will engulf the Earth is quite controversial," she says. "But it wouldn't matter because it will no longer be our beautiful Earth with an atmosphere and oceans. Earth may survive, but not the Earth that we know and love."