A star-devoured planet becomes spectacularly bright

This artist’s impression of an extinct planet skimming the surface of its star. Astronomers using a combination of telescopes have found the first direct evidence of an aging, bloated Sun-like star engulfing its planet. These telescopes include the Zwicky Transient Facility (ZTF) at Caltech’s Palomar Observatory, the WM Keck Observatory, and NASA’s NEOWISE mission. The oldest star depicted here, ZTF SLRN-2020, is about 10 billion years old. As it became a red giant, it began accreting over hundreds of thousands of years, and as a result, became closer to its inner planet. According to astronomers, when the planet almost touched the star’s surface, the increasing frictional forces caused the planet to rapidly spin inward. Eventually, over an indeterminate amount of time, the planet sank into the star’s core. When that happened, the star quadrupled its size and became more than a hundred times brighter. ZTF SLRN-2020 is about 12,000 light-years away in the constellation Aquila. Credit: Image: K. Miller/R. Injury (Caltech/IPAC)

The ZTF and NEOWISE spot evidence is the first case of a star engulfing its planet.

For the first time, astronomers have caught a star in the act of swallowing a planet whole. The Sun-like star, called ZTF SLRN-2020, is about 12,000 light-years away in our galaxy and is thought to have engulfed a hot gas giant about its size.[{” attribute=””>Jupiter. Scientists already knew that older stars will, as they puff up with age, ultimately ingest their inner orbiting planets. Our own sun is predicted to do so in 5 billion years, consuming Mercury, Venus, and likely Earth. But nobody had seen direct evidence for such a remarkable scenario until now.

“The confirmation that sun-like stars engulf inner planets provides us with a missing link in our understanding of the fates of solar systems, including our own,” says Kishalay De (MS ’18, PhD ’21), a postdoctoral scholar at MIT and lead author of a new study about the findings in the journal Nature.

The chubby star was first spotted by the Zwicky Transient Facility, or ZTF, a National Science Foundation-funded instrument that scans the sky every night from Caltech’s Samuel Oshin Telescope at Palomar Observatory near San Diego. ZTF observations show that the star brightened dramatically and then began to fade over a period of about a week. At first, T thought that this variable star might be the result of a nova explosion, which occurs when a dead star is summoned.[{” attribute=””>white dwarf steals matter from its companion star. But follow-up observations with the W. M. Keck Observatory atop Maunakea in Hawai?i revealed something else was going on.

Kishalay De

Kishalay De. Credit: Caltech

“I had been looking for erupting stars called novae,” De says. “But the Keck data indicated that the star was not lighting up hot gas as is expected for novae. I couldn’t make any sense of it.”

De, who was then a graduate student at Caltech, put the object aside to finish his PhD thesis and came back to it about a year later after he had moved to MIT. He and his colleagues then obtained infrared data from a camera at Palomar’s Hale Telescope called WIRC (Wide-field Infrared Camera), “and that’s when things got really interesting,” he says.

Those observations showed that the star was brightening over time in not only optical light as ZTF had observed but also in infrared light, which indicates the presence of dust. The researchers then turned to NASA’s NEOWISE space telescope in search of more clues. NEOWISE, formerly known as WISE (Wide-field Infrared Survey Explorer), has been scanning the skies regularly since shortly after its launch in 2009. NEOWISE detected the star brightening in infrared light about nine months before ZTF caught the extreme rise in optical light. Even now, after the optical light has faded, NEOWISE continues to pick up infrared light from the star.

Viraj Karambelkar

Viraj Karambelkar. Credit: Caltech

“The infrared observations were one of the main clues that we were looking at a star engulfing a planet,” says Viraj Karambelkar (MS ’21), a grad student at Caltech and co-author of the study.

Once the science team put all the evidence together, they realized that the dust they were seeing with NEOWISE was being generated as a planet spiraled into the star’s puffy atmosphere. Like other older stars, the star had begun to expand in size as it aged, bringing it closer to the orbiting planet. As the planet skimmed the surface of the star, it pulled hot gas off the star that then drifted outward and cooled, forming dust. In addition, material from the disintegrating planet blew outward, also forming dust.

What happened next, according to the astronomers, triggered the flare of optical light seen by ZTF.

“The planet plunged into the core of the star and got swallowed whole. As it was doing this, energy was transferred to the star,” De explains. “The star blew off its outer layers to get rid of the energy. It expanded and brightened, and the brightening is what ZTF registered.”

Mansi Kasliwal

Mansi Kasliwal. Credit: Caltech

Some of this expanding stellar material then escaped from the star and traveled outward. Like the boiled-off layers of the star and planet that previously drifted outward, this material also cooled to form dust.

NEOWISE is detecting the infrared glow of all the newly minted dust.

“NEOWISE data are a treasure trove,” says co-author Mansi Kasliwal (MS ’07, PhD ’11), professor of astronomy at Caltech and a co-investigator on the ZTF project. “ZTF caught the event, which is what it excels at, while NEOWISE and other telescopes all helped us figure out what is going on.”

The planetary engulfment is similar to what happens when two stars merge, events called red novae. Stars in our universe often form in pairs. Over time, as one star ages and expands faster than its companion, it can essentially ingest its partner. Twenty of these star mergers have been detected to date by ZTF and other instruments, mostly in galaxies beyond the Milky Way.

“Star mergers are thousands of times brighter than this event,” says Karambelkar, who has observed eight of these eruptions using ZTF as part of his PhD thesis. “This was another clue that we were looking at a planet being eaten by its star. The level of brightening was much fainter due to the small size of the planet.”

“This is just spectacular,” Kasliwal adds. “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. That’s a long time from now, in five billion years, so we don’t have to worry just yet.”

For more on this discovery:

Reference: “An infrared transient from a star engulfing a planet” by Kishalay De, Morgan MacLeod, Viraj Karambelkar, Jacob E. Jencson, Deepto Chakrabarty, Charlie Conroy, Richard Dekany, Anna-Christina Eilers, Matthew J. Graham, Lynne A. Hillenbrand, Erin Kara, Mansi M. Kasliwal, S. R. Kulkarni, Ryan M. Lau, Abraham Loeb, Frank Masci, Michael S. Medford, Aaron M. Meisner, Nimesh Patel, Luis Henry Quiroga-Nuñez, Reed L. Riddle, Ben Rusholme, Robert Simcoe, Loránt O. Sjouwerman, Richard Teague & Andrew Vanderburg, 3 May 2023, Nature.
DOI: 10.1038/s41586-023-05842-x

The Nature study titled “An infrared transient from a star engulfing a planet,” was funded by NASA, the National Science Foundation (NSF), and the Heising-Simons Foundation.
Caltech’s ZTF is funded by the NSF and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by Caltech’s IPAC. NASA supports ZTF’s search for near-Earth objects through the Near-Earth Object Observations program.

Launched in 2009, the WISE spacecraft was placed into hibernation in 2011 after completing its primary mission. In September 2013, NASA reactivated the spacecraft with the primary goal of scanning for near-Earth objects, or NEOs, and the mission and spacecraft were renamed NEOWISE. The mission was selected competitively under NASA’s Explorers Program managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, which is managed by Caltech for NASA, and the University of Arizona and is supported by NASA’s Planetary Defense Coordination Office. Science data processing takes place at IPAC at Caltech.

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