A team of American astronomers have observed for the first time how a star swallows a planet that orbited around it, the same fate that the Earth will suffer in about 5,000 million years, according to research published in the journal Nature.
When a star ends its nuclear fusion cycle and runs out of fuel, it begins to expand to a million times its original size, destroying any celestial object, planets included, that it finds in its path. Scientists have seen hints of stars just before and shortly after the act of consuming entire planets, but never caught sight of destruction in the act until now.
The observation and subsequent study has been signed by scientists from the Massachusetts Institute of Technology (MIT), Harvard University, Caltech and other centers in the United States.
The planetary demise appears to have taken place in our own Milky Way galaxy, only about 12,000 light-years away, near the constellation Aquila. There, astronomers observed a star outburst that grew more than 100 times brighter in just 10 days, before rapidly fading.
Interestingly, this flash of white light was followed by a longer lasting, cooler signal. The scientists deduced that this combination could only be due to one event: a star engulfing a nearby planet.
"We were looking at the final phase of the engulfment," says lead author Kishalay De, a postdoctoral researcher at MIT's Kavli Institute for Astrophysics and Space Research.
Scientists estimate that the planet that vanished was probably a hot Jupiter-sized world that spiraled in, was swept up by the dying star's atmosphere and eventually by its core.
The Earth will suffer the same fate in 5,000 million years, when the Sun is expected to burn out and burn the inner planets of the Solar System. "We are looking at the future of the Earth," says De. "If some other civilization were watching us from 10,000 light-years away as the sun engulfs the Earth, they would see how the sun suddenly shines by ejecting some material, then forms dust around him, before returning to what he was".
The team discovered the burst in May 2020. However, it took astronomers another year to find an explanation for what it could be. The initial signal turned up in a search of data taken by the Zwicky Transient Facility (ZTF), which operates at Caltech's Palomar Observatory in California.
It is an observatory that scans the sky for stars that change rapidly in brightness, the pattern of which could indicate the presence of supernovae, gamma-ray bursts, and other stellar phenomena.
De was looking through the ZTF data for signs of flares in binary stars, systems in which two stars orbit around each other, one of which draws mass from the other every so often and is briefly brightened as a result. "One night, I observed a star that brightened by a factor of 100 over the course of a week, out of nowhere," De recalls. "It didn't look like any starburst I've ever seen in my life."
Hoping to determine the source with more data, De turned to observations of the same star made by the Keck Observatory in Hawaii. Keck telescopes make spectroscopic measurements of starlight, which scientists can use to discern a star's chemical composition.
But what De discovered left him even more perplexed. While most binary stars give off stellar material, such as hydrogen and helium, as one star erodes the other, the new source gave off none of it. Instead, what De saw were signs of "peculiar molecules" that can only exist at very cold temperatures.
"These molecules are only observed in very cold stars -says De-. And when a star lights up, it usually gets hotter. Therefore, the low temperatures and the brightness of the stars do not go hand in hand."
It then became clear that the signal was not from a stellar binary. De decided to wait for more responses to emerge. About a year after its initial discovery, he and his colleagues analyzed observations of the same star, this time taken with an infrared camera at the Palomar Observatory.
Within the infrared band, astronomers can see signs of cooler material, in contrast to the white-hot optical emissions that arise from binaries and other extreme stellar events. "That infrared data made me fall out of my chair," De recalls. "The source was incredibly bright in the near infrared."
Apparently, after its initial hot flash, the star continued to spew out cooler energy for the next year. That icy material was probably gas from the star that shot up into space and condensed into dust, cold enough to be detected at infrared wavelengths. These data suggest that the star could be merging with another rather than shining as a result of a supernova explosion.
But when the team further analyzed the data and combined it with measurements made by NASA's infrared space telescope, Neowise, they came to a much more interesting conclusion.
From the data collected, they calculated the total amount of energy released by the star since its initial outburst and found it to be surprisingly small: about 1/1,000 the magnitude of any observed stellar merger 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 Jupiter's mass is about 1/1,000 the mass of the Sun. That's when we realized: it was a planet crashing into its star."
With the pieces in place, scientists were finally able to explain the initial burst. The bright, hot flash was likely the final moment of a Jupiter-sized planet being swept up in the atmosphere of a dying star. As the planet fell into the star's core, the outer layers of the star fell off and settled as cold dust over the next year.
"For decades we have been able to see the before and after -says De-. Before, when the planets still orbit very close to their star, and after, when a planet has already been swallowed and the star is gigantic. What we were missing was catching the star in the act, when a planet suffers this fate in real time. That's what makes this discovery really exciting."
