Astronomers Spot First Direct Evidence of a Star Engulfing Its Planet
Maunakea, Hawaiʻi – 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, lies about 15,000 light-years away in our Milky Way galaxy and is thought to have engulfed a hot gas giant planet about the size of Jupiter or smaller.
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, a postdoctoral scholar at MIT and lead author of a new study about this so-called “Death Star” publishing May 4, Star Wars Day, in the journal Nature.
The plump star was first spotted by Zwicky Transient Facility, or ZTF, a National Science Foundation-funded instrument that scans the skies every night from Caltech’s Samuel Oschin Telescope at Palomar Observatory near San Diego. ZTF observations showed the star had dramatically brightened and begun to fade in a period of about a week.
At first, De thought this variable star might have resulted from a nova explosion, which occurs when a dead star called a white dwarf steals matter from its companion star. But follow-up observations with the W. M. Keck Observatory atop Maunakea on Hawaiʻi Island revealed something else was going on.
“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.”
Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) confirmed the chemical composition and temperature of the gas, which indicated the outburst was surrounded by cooler material.
De, who was then a graduate student at Caltech, said he 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 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.
“The infrared observations were one of the main clues that we were looking at a star engulfing a planet,” says Viraj Karambelkar, a grad student at Caltech and co-author of the study.
Keck Observatory’s Near-Infrared Echellette Spectrograph (NIRES) confirmed that the aftermath was indeed surrounded by a layer of cool gas and dust.
Once the science team put all the evidence together, they realized the dust they were seeing with NEOWISE was being generated as the 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.
For the first time, astronomers have caught a star in the act of engulfing its planet, an ill-fated encounter that will play out in our own solar system in 5 billion years. This artist’s animation shows the gas giant meeting its demise as it spiraled into its parent star. Ultimately, the planet plunged into the core of the star, which triggered the star to expand and brighten. The aging star depicted here, called ZTF SLRN-2020, is roughly 10 billion years old. It had begun to inflate over a period of hundreds of thousands of years as it transformed into a red giant and, as a result, inched closer to its inner planet. As the planet came close to touching the surface of the star, the increasing frictional forces caused the planet to rapidly spiral inward. Eventually, on timescales that are not certain, the planet plunged into the core of the star. When that happened, the star inflated to four times its size and brightened by a factor of more than a hundred. Animation credit: R. Hurt/K. Miller (Caltech/IPAC)
“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.”
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.
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. 20 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,” says co-author Mansi Kasliwal, professor of astronomy at Caltech and a co-investigator on the ZTF project. “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. Though that’s a long time from now, in five billion years, so we don’t have to worry just yet.”
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.
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.
The Near-Infrared Echellette Spectrograph (NIRES) is a prism cross-dispersed near-infrared spectrograph built at the California Institute of Technology by a team led by Chief Instrument Scientist Keith Matthews and Prof. Tom Soifer. Commissioned in 2018, NIRES covers a large wavelength range at moderate spectral resolution for use on the Keck II telescope and observes extremely faint red objects found with the Spitzer and WISE infrared space telescopes, as well as brown dwarfs, high-redshift galaxies, and quasars. Support for this technology was generously provided by the Mt. Cuba Astronomical Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.