A Star Devours Its Planet: JWST Catches a Cosmic Meal in Action

In a paper led by Ryan M. Lau, astronomers report groundbreaking observations of a star that appears to have swallowed a planet—offering the most direct evidence yet for what’s known as a “planetary engulfment event.” The subject of the study is ZTF SLRN-2020, an unusual cosmic outburst originally discovered by the Zwicky Transient Facility (ZTF). The team used the James Webb Space Telescope (JWST) to follow up more than two years after the event, revealing a dusty and glowing aftermath that supports the idea of a planet being consumed by its host star.

A Red Nova Unlike Any Other

The story begins with ZTF SLRN-2020, an optical outburst that didn’t look quite like a typical nova. Normally, nova-like outbursts show strong atomic emission lines, but this one showed a smooth, red glow with few features. Follow-up observations soon suggested that the explosion came from a small object, like a giant planet, merging with its star. The event was classified as a subluminous red nova (SLRN), and the low brightness and short duration hinted that a planet—not another star—was the star’s merger partner. This made ZTF SLRN-2020 a rare opportunity to study a real-time planetary engulfment.

Peering into the Aftermath with JWST

To dig deeper into this event, the team turned to JWST’s powerful infrared vision. Using the NIRSpec and MIRI instruments, they observed light from ZTF SLRN-2020 across a range of infrared wavelengths. The resulting spectra showed a wealth of features, including emission from molecules like carbon monoxide (CO) and possibly phosphine (PH₃), a gas found in the atmospheres of gas giants like Jupiter. The data also included a hydrogen emission line known as Brackett-α, pointing to hot gas near the star. These features together confirmed the presence of warm, dusty material surrounding the star—likely debris from the disrupted planet.

Modeling the Dusty Scene

Using a computer model called DUSTY, the researchers examined how dust near the star was heated and emitted light. They found that the current infrared glow could be explained by a warm dust shell about 720 K (around 450°C) in temperature, located about 50 times the Sun’s radius from the star. The amount of dust was tiny—only about 10⁻¹¹ times the mass of the Sun—but enough to glow brightly in infrared. A re-analysis of earlier data (+320 days after the peak) showed the presence of cooler and more massive dust, possibly from the initial outburst. The authors suggest that the warm dust seen later may be “fallback” material—ejecta that didn’t escape the system and instead returned to orbit the star.

A Star That’s Still on the Main Sequence

A major question was whether the star had evolved in a way that caused it to expand and engulf the planet. But the JWST data point to a K-type star, similar in mass to the Sun but cooler and smaller, that is still on the main sequence (the main stage of a star’s life). This suggests that the star wasn’t naturally expanding when the engulfment occurred. Instead, the planet’s orbit may have decayed over time due to tidal interactions, eventually spiraling into the star. The authors also considered whether leftover dust could be hiding some of the star’s light, but even correcting for that, the star still appears too young for stellar evolution to be the cause.

Chemical Clues from Molecules

One of the most intriguing findings is the possible detection of phosphine, a molecule rarely seen in space but known from gas giants in our Solar System. If confirmed, this could be a direct fingerprint of the planetary material, now floating around the star as gas. The observed abundance of PH₃ was similar to levels seen in the dusty envelopes of aging stars, hinting that the engulfment might create conditions similar to those found around red giants.

A New Way to Watch Planetary Death

This paper provides a unique glimpse into the fate of planets orbiting too close to their stars. Many “hot Jupiters”—gas giants in tight orbits—may one day meet the same fate as the planet devoured in ZTF SLRN-2020. By catching this event in the act and studying it in detail, Lau and colleagues have opened a new window into how planetary systems evolve and sometimes meet a violent end. As JWST and other observatories continue watching the skies, more of these cosmic meals may soon be on the menu.

Source: Lau