Planet swallowed by ‘Death Star’ provides insight into Earth’s demise in 5 billion years

A long time ago in the Milky Way galaxy not so far away …

The 10-billion-year-old red giant called ZTF SLRN-2020 continued to expand. Plump with age, the dying star’s puffy atmosphere reached a hot gas giant planet about the size of Jupiter that was orbiting nearby.

The planet spiraled for months around the sun-like star, getting closer with each revolution and skimming the star’s atmosphere.

What happened next was something astronomers on Earth had never before seen.

The planet was swallowed whole, plunging into the red giant’s core and triggering a dramatic flare of optical light that lasted about a week. It was observed by the Zwicky Transient Facility funded by the National Science Foundation.

As the star gobbled up its planetary pupu, its outer layers blew off to expend the influx of energy from devouring the planet, resulting in the brief brightening.

With the help of the W. M. Keck Observatory atop Maunakea on the Big Island, astronomers for the first time saw direct evidence for a scenario they already knew happens throughout the universe. Scientists have seen hints of the process in other stages, before and after a planet is engulfed, but never during.

The discovery also offers insight into what likely will be the fate of Earth, along with Mercury and Venus, about 5 billion years from now — after our own sun becomes a red giant and enters its final stages of existence.

“The confirmation that sun-like stars engulf inner planets provides us with a missing link in our understanding of the fates of solar systems,” said Kishalay De, a postdoctoral scholar at the  Massachusetts Institute of Technology and lead author of a new study about the so-called “Death Star.”

The new study was published on May 3 in the journal Nature, just in time for Star Wars Day that was on May 4.

Keck Observatory Chief Scientist John O’Meara, who was not part of the study, said the discovery in another solar system verifies “our hypothesis” about the fate of inner planets orbiting a star at the end of its life: “This discovery proved that our understanding about how stars in general evolve over time is largely correct.”

Upon first observing the flare up, De thought it was the result of a nova explosion, which happens when a dead star called a white dwarf steals matter from a companion star.

Follow-up observations with the Keck Observatory indicated that ZTF SLRN-2020 was not lighting up hot gas as expected from that type of explosion. Instead, Keck’s low resolution imaging spectrometer confirmed the chemical composition and temperature of the gas meant the outburst from the star was surrounded by cooler material.

About a year later, De came back to the observations after completing his PhD at the California Institute of Technology and moving to MIT. He and his colleagues got infrared data from the wide-field infrared camera at Palomar’s Hale Telescope, which showed the star was brightening in not only optical light but also infrared light, meaning there was dust present.

That dust was created when the spiraling planet pulled hot gas from the star that drifted out and cooled. And as the planet disintegrated, even more material was added to the dust cloud.

The researchers turned to NASA’s near-Earth object wide field infrared survey explorer, a space telescope, to search for more clues. The NASA instrument detected the star brightening in infrared light about nine months before the extreme rise in optical light. Keck’s near-infrared echellette spectrograph confirmed the layer of cool gas and dust surrounding the aftermath of the planet being consumed by the red giant.

“The infrared observations were one of the main clues that we were looking at a star engulfing a planet,” said Viraj Karambelkar, a grad student at Caltech and co-author of the study.

Even now, after the flare up has faded, the NASA telescope continues to detect the infrared glow of all the newly minted dust from the star as stellar material that escaped during and after the planet was swallowed.

A transfer of energy from the planet to the star as it was swallowed caused the red giant to temporarily increase in size and become a few hundred times brighter, according to NASA. Recent observations show ZTF SLRN-2020 has returned to the size and brightness it was before destroying the planet.

The red giant is 15,000 light years away from Earth, so the light being seen is from about 15,000 years ago.

“There were a number of possible reasons to see the star brightening, and the only way one could prove the ‘Death Star’ hypothesis was by understanding what materials were emitting the light as well as the composition of the star and the planet,” O’Meara said,

The stars have different atomic signatures than planets, he added, so the Keck Observatory data was the “slam dunk” needed to confirm the hypothesis.”

“We like to think of stars and planets as being nearly immortal, since we never see dramatic changes over the course of, say, a lifetime,” OʻMeara said. “In this case, with a number of telescopes, including Keck Observatory, we got to see the universe can be a dramatic, sometimes destructive place!”

Study co-author Mansi Kasliwal, professor of astronomy at Caltech and a co-investigator on the ZTF project, called the discovery spectacular.

“We are still amazed that we caught a star in the act of ingesting its planet, something our own sun will do to its inner planets,” Kasliwal said. “Though that’s a long time from now, in five billion years, so we don’t have to worry just yet.”