First-Ever Observation: Astronomers Spot a Star Consuming a Planet in Real-Time

This artist’s concept reveals the fascinating image of a gas giant planet plunging into its host star. As the Jupiter-like planet gets closer to its star, it pulls gas away from it, creating dust that is visible to astronomers once it cools down. Credit: R. Hurt & K. Miller (Caltech/IPAC)

A star that is approaching the end of its lifetime expanded and devoured a planet similar in size to Jupiter. In around five billion years, our Sun will be going through this same end-of-life transition.

Astronomers have made a remarkable discovery by observing an older star devouring a planet for the first time. This event offers valuable insights into the future fate of our Sun, which will undergo a similar process in roughly five billion years.

A new study published in the journal Nature documents the first observation of an aging star swallowing a planet. After exhausting all of its fuel in its core, the star began to enlarge, while at the same time approaching the planet that was orbiting it and eventually consuming it completely. Our Sun will undergo a similar aging process in roughly five billion years, possibly expanding to 100 times its current size and transforming into what is called a red giant, eventually absorbing Mercury, Venus, and possibly even Earth in the process.

While astronomers have identified many red giant stars, they have suspected that some of them may be consuming surrounding planets, but this occurrence had never been witnessed before. “We have predicted this type of event for years, but until now, we have never actually observed it,” says Kishalay De, an astronomer at the Massachusetts Institute of Technology in Cambridge and the lead author of the study.

As shown in this video, a planet that orbits a growing host star is gradually disintegrating. As the planet moves closer to the star, it attracts a spray of gas from it. Once the planet is consumed, the star’s size and brightness grow but eventually return to their previous state before the merger. 

Researchers discovered the event, which is formally named ZTF SLRN-2020, with the use of several ground-based observatories and NASA’s Near-Earth Object Wide Field Infrared Survey Explorer (NEOWISE) spacecraft, which is operated by the agency’s Jet Propulsion Laboratory. The planet was likely about the same size as Jupiter, with an even closer orbit to its star than Mercury has to our Sun. The star is at the very beginning of its final lifetime stage, known as the red giant phase, which can last more than 100,000 years.

As the outer atmosphere of the star expanded, it ultimately surrounded the planet. The drag in the atmosphere caused the planet to slow down, reducing its orbit and, ultimately, causing it to descend beneath the visible surface of the star, similar to a meteor burning up in Earth’s atmosphere. The energy transfer caused the star to temporarily enlarge in size and become a few hundred times brighter. Recent observations reveal that the star has returned to the same size and brightness that it possessed before merging with the the planet.

This artist’s concept reveals the Wide-field Infrared Survey Explorer (WISE) spacecraft orbiting around Earth. Its NEOWISE mission discovers and characterizes asteroids. Credit: NASA/JPL-Caltech

The optical light flash (visible to the human eye) after the destruction of the planet was detected in observations made by the Caltech-led Zwicky Transient Facility (ZTF), an instrument located at the Palomar Observatory in Southern California that searches for cosmic phenomena that change brightness quickly, sometimes in just hours. De employed the ZTF to look for events called novae, where a deceased, collapsed star, known as a white dwarf, consumes hot gas from another nearby star. Novae are usually surrounded by flows of hot gas, but follow-up observations of the flash by other ground-based telescopes indicated much cooler gas and dust surrounding the star, implying that it did not resemble a nova or anything else that De had ever seen.

So, he turned to the NEOWISE observatory, which continually scans the entire sky in infrared light (a range of wavelengths longer than visible light) every six months. Launched in 2009 and initially named WISE, the observatory generates all-sky maps that enable astronomers to observe how objects change over time.

This is a mosaic of images that captures the entire sky as seen by the Wide-field Infrared Survey Explorer (WISE) as part of its All-Sky Data Release. 

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 spiraled toward its doom. As the gas drifted out into space, it would have cooled and become dust – like water vapor 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,” said De. “So the fact that NEOWISE saw the star brighten a year before the optical eruption was critical to figuring out what this event was.”

Five billion years from now, when our Sun is expected to become a red giant, swallowing up Mercury, Venus, and possibly Earth, 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 5 billion years from now, I might see the Sun brighten a little, but nothing as dramatic as this, even though it will be the exact same physics at work,” 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.

“This discovery shows that it’s worthwhile to take observations of the entire sky and archive them, because we don’t yet know all of the interesting events we might be capturing,” said Joe Masiero, deputy principal investigator for NEOWISE at IPAC at Caltech. “With the NEOWISE archive, we can look back in time. We can find hidden treasures or learn something about an object that no other observatory can tell us.”

For more on this discovery:

Launched in 2009, the WISE mission surveyed the entire sky in infrared light twice, capturing images of around three-quarters of a billion celestial objects, such as distant galaxies, stars, and asteroids. The mission concluded in 2011, but in 2013, NASA repurposed the spacecraft for tracking asteroids and other near-Earth objects (NEOs), rebranding both the mission and the spacecraft as NEOWISE.

NASA’s Astrophysics Division within the Science Mission Directorate had JPL manage and operate WISE, with Edward Wright from UCLA serving as the principal investigator. The mission was competitively chosen under NASA’s Explorers Program, managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland.

JPL manages and operates the NEOWISE mission on behalf of NASA’s Planetary Defense Coordination Office within the Science Mission Directorate in Washington. The University of Arizona hosts the principal investigator, Amy Mainzer. The Space Dynamics Laboratory in Logan, Utah, constructed the scientific instrument, while Ball Aerospace & Technologies Corp. of Boulder, Colorado, built the spacecraft. IPAC at Caltech in Pasadena handles science data processing, and Caltech manages JPL for NASA.