In a first, astronomers witnessed a star swallowing a Jupiter-sized planet. The Earth could be next

But, fret not, the 'apocalypse' would not happen for another five billion years, when the Sun is expected to burn out and burn up the other planets.

The study was published in Nature by MIT, Harvard University, and Caltech scientists.

The star brightened by a factor of 100 in just a week

The plump and bright star was first spotted by Zwicky Transient Facility(ZTF) in May 2020. In what can be called an act of drama, the star brightened and faded in just a week. 

"One night, I noticed a star that brightened by a factor of 100 over the course of a week, out of nowhere," De said. "It was unlike any stellar outburst I had seen in my life."

De immediately zeroed it on a nova explosion, but follow-up observations with the W. M. Keck Observatory atop Maunakea in Hawaiʻi revealed something bigger. 

Usually, most star binaries produce hydrogen and helium as one erodes the other, but the new incident gave neither. De saw signs of "peculiar molecules" whose presence can only be explained at cold temperatures. 

"These molecules are only seen in stars that are very cold," De said. "And when a star brightens, it usually becomes hotter. So, low temperatures and brightening stars do not go together."

Observing the final moments of a Jupiter-sized world

De concluded that the signal did not arise from a stellar binary. After a year, De and his colleagues examined the same star, taken with an infrared camera. 

"That infrared data made me fall off my chair," De said. "The source was insanely bright in the near-infrared."

It turns out that after the initial hot flash, the star threw out colder energy, probably gas from the star that condensed into dust. This matter, which was cold enough to be detected at infrared wavelengths, suggested that the star could integrate with another star. 

There's more.

The team further analyzed the data and paired it with measurements taken by NASA's infrared space telescope, NEOWISE. They observed that the total amount of energy released by a star was only 1/1,000 the magnitude of any stellar merger observed in the past. 

"That means that whatever merged with the star has to be 1,000 times smaller than any other star we’ve seen," De said. "And it’s a happy coincidence that the mass of Jupiter is about 1/1,000 the mass of the Sun. That’s when we realized: This was a planet, crashing into its star."

Does the puzzle make sense now? "The bright, hot flash was likely the final moments of a Jupiter-sized planet being pulled into a dying star’s ballooning atmosphere. As the planet fell into the star’s core, the star's outer layers blasted away, settling out as cold dust over the next year," the release stated.

"For decades, we’ve been able to see the before and after. Before, when the planets are still orbiting very close to their star, and after, when a planet has already been engulfed, and the star is giant. What we were missing was catching the star in the act, where you have a planet undergoing this fate in real time. That’s what makes this discovery really exciting," De concluded.

Study Abstract:

Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun1,2. Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star3–5. However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae6,7—a class of eruptions now confrmed8 to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 1035 erg s−1) and radiated energy (approximately6.5 × 1041 erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.