What It Looks Like When a Dying Star Eats a Planet

Astronomers have witnessed a star gobbling up a planet, offering the first direct glimpse of a gnarly process called planetary engulfment that most likely awaits Earth in the deep future.

Scientists serendipitously spotted a gas planet — like Jupiter but possibly larger — as it was swallowed up by an aging sun-like star about 12,000 light-years from Earth. Tantalizing hints of engulfment events have been spotted in the past, but nobody has ever caught a star in the act of devouring a planet until now.

The discovery “provides a missing link in our understanding of the evolution and final fates of planetary systems,” including the one we inhabit, the astronomers wrote in their study, which was published on Wednesday in the journal Nature.

“This is the eventual fate of the Earth,” said Kishalay De, a NASA Einstein fellow at the Massachusetts Institute of Technology and one of the study’s authors. “We are really seeing what the Earth is going to run into five billion years from now.”

The life cycles of stars are linked to their masses. Small stars, like red dwarfs, may shine for trillions of years, whereas the most massive stars explode just a few million years after their births. As stars like the sun start to die after billions of years, they transform into a class called red giants that expand hundreds of times in size, consuming anything within their advancing borders.

Signs of engulfment events are littered across the Milky Way. The light of some stars is polluted with the chemical signatures of planets, suggesting that whole worlds are being digested before our eyes. Scientists have also spotted hundreds of planets with small orbits that are doomed to fall within the radius of red giants in the future.

But while stars clearly consume the occasional planet, capturing this moment is challenging because the light sparked by these events is faint and ephemeral. In fact, Dr. De was using the Zwicky Transient Facility, a camera on a telescope at the Palomar Observatory in California, in May 2020 to look for something completely different — merging stars, called red novas. It was in those observations that he stumbled across a curious burst of visible light.

What unfolded was like a “detective story,” Dr. De said. To identify the burst, his team obtained visible-light observations of the source captured in November 2020 by the W.M. Keck Observatory in Hawaii. Those images revealed a star chilling at about 5,000 degrees Fahrenheit, about 10 times colder than the searing temperatures expected from red novas.

Puzzled, Dr. De and his colleagues observed the star again, this time in infrared light, using another camera at the Palomar Observatory and NASA’s NEOWISE space telescope. The system turned out to be brilliant in infrared, a band of the light spectrum that is ideal for spotting faint objects that don’t emit much energy. It dawned on the researchers that they were most likely watching a star gulping down a planet in real time.

“My first reaction was disbelief,” Dr. De said. “We see the before and the after” of planetary engulfment, he added, but “these observations give us the first glimpse into how that process plays out.”

The initial burst detected by the Zwicky observatory, which lasted 10 days, occurred at the moment that a dying star at last fully enveloped a gas planet no more than 10 times the mass of Jupiter. For more than a year before its luminous demise, the planet skimmed the outskirts of the star, pulling off chunks of its atmosphere, which explains why the researchers saw cool gas and dust hanging around the system. After the burst, the star eerily glowed for about six months as it swallowed the remains of the planet.

Lorenzo Spina, an astrophysicist at the Astronomical Observatory of Padua in Italy who studies planetary engulfment, called the team’s conclusions “very solid” and described the discovery as “groundbreaking.”

“This is a very important missing piece of the entire story,” Dr. Spina said. “Now, we are going to have a more complete understanding of this entire process.”

Such incredible events can shed light on a host of juicy mysteries, including the odds that life might exist elsewhere in the universe. Starlight that contains chemical hints of planetary morsels can be mined for clues about the interior composition of worlds in other systems. Building an inventory of these ingredients can help to estimate the number of star systems with the right materials to support habitable environments.

Now that scientists have seen a real example of planetary engulfment, they can search the skies for similar patterns that fit the blueprint. The new observations also provide a grisly sneak peek of the literal end of the world. When the sun enters its red-giant phase, our familiar home planet is likely to die within its infernal embrace.

“Finding an event like this really puts all of the theories that have been out there to the most stringent tests possible,” Dr. De said. “It really opens up this entire new field of research.”