Red giant caught devouring a planet for the first time

US astronomers have recorded the first known observation of an aging star swallowing a planet. After running out of fuel in its core, the star began to grow in size until it turned into a red giant, shrinking the gap with its neighbouring planet and eventually consuming it entirely. The event has been documented in the journal Nature.

Astronomers have previously identified many red giant stars and suspected that in some cases they consume nearby planets, but the phenomenon had never been directly observed before. As noted by MIT astronomer Kishalay De, “This type of event has been predicted for decades, but until now we have never actually observed how this process plays out.”

Researchers discovered the event — formally called ZTF SLRN-2020 — using multiple ground-based observatories and NASA’s NEOWISE spacecraft, which is managed by the agency’s Jet Propulsion Laboratory. The planet was likely about the size of Jupiter, with an orbit even closer to its star than Mercury’s is to our Sun. The star was at the beginning of the final phase of its life — its red giant phase, which can last more than 100,000 years.

As the star expanded, its outer atmosphere eventually surrounded the planet. Drag from the atmosphere slowed the planet down, shrinking its orbit and eventually sending it below the star’s visible surface, like a meteor burning up in Earth’s atmosphere. The transfer of energy caused the star to temporarily increase in size and become a few hundred times brighter. The star has since returned to the size and brightness it was before merging with the planet.

The flash of optical light after the planet’s demise showed up in observations by the Caltech-led Zwicky Transient Facility (ZTF), an instrument based at Palomar Observatory in California that looks for cosmic events that change in brightness rapidly, sometimes in a matter of hours. De was using ZTF to search for events called novae, but follow-up observations of the flash by other ground-based telescopes confirmed that it didn’t look like a nova or anything else De had ever seen. So he turned to the NEOWISE observatory, which scans the entire sky in infrared light every six months in order to produce all-sky maps that enable astronomers to see how objects change over time.

Looking at the NEOWISE data, De saw that the star brightened almost a year before ZTF spotted the flash. That brightening was evidence of dust (which emits infrared light) forming around the star. De and his colleagues think the dust indicates that the planet didn’t go down without a fight and that it pulled hot gas away from the puffy star’s surface as it spiralled toward its doom. As the gas drifted out into space, it would have cooled and become dust — like water vapour becoming snow. Even more gas was then flung into space during the collision of the star and the planet, producing more dust visible to both the ground-based infrared observatories and NEOWISE.

“Very few things in the universe brighten in infrared light and then brighten in optical light at different times,” De said. “So the fact that NEOWISE saw the star brighten a year before the optical eruption was critical to figuring out what this event was.”

In about five billion years, our Sun is expected to go through a similar aging process — expanding up to 100 times its current diameter and in the process absorbing Mercury, Venus and possibly Earth. But in this case the light show should be much more subdued, according to De, since those planets are many times smaller than the Jupiter-size planet in the ZTF-captured event.

“If I were an observer looking at the solar system five billion years from now, I might see the Sun brighten a little, but nothing as dramatic as this,” he said.

Most mid-size stars will eventually become red giants, and theorists think that a handful of them consume nearby planets each year in our galaxy. The new observations provide astronomers with a template for what those events should look like, opening up the possibility of finding more.

Image caption: This artist’s impression shows a doomed planet skimming the surface of its star. Image credit: K Miller/R Hurt (Caltech/IPAC).