Astronomers have observed for the first time how a star swallows a planet, a fate that Earth will suffer in 5,000 million years, according to research published in the journal ‘Nature’.
When a star runs out of fuel, it swells to a million times its original size, inflaming whatever matter it finds in its path, including planets. Scientists have noticed hints of stars before and shortly after the act of eating entire planets, but they haven’t caught any yet.
In the study, scientists from the Massachusetts Institute of Technology (MIT), Harvard University, Caltech, and other centers in the United States report that they have observed for the first time a star swallowing a planet.
The demise of the planets appears to have occurred in our galaxy, about 12,000 light-years away, near the constellation Aquila. There, astronomers observed an explosion of a star that grew more than 100 times brighter in just 10 days, before rapidly fading.
Interestingly, this flash of white light was followed by a cooler, longer-lasting signal. Scientists have concluded that this combination could be due to only one event: a star swallowing a nearby planet.
“We were witnessing the final stage of the engulfment,” lead author Kechalai D, a postdoctoral researcher at MIT’s Kavli Institute for Astrophysics and Space Research, said in a statement.
Scientists estimate that the planet that disappeared was most likely a hot Jupiter-sized world that spiraled, engulfed by the dying star’s atmosphere and eventually its core.
The Earth will suffer the same fate, though not for another 5 billion years, when the Sun is expected to burn out and burn the inner planets of the solar system.
“We’re looking forward to Earth’s future,” Dee says. “If another civilization were watching us from 10,000 light-years away as the sun floods the Earth, they would see the sun come up suddenly while it was ejecting some material, and then dust would form around it before it went back to the way it was.”
The team discovered the explosion in May 2020. However, it took astronomers another year to find an explanation for what that could be.
The initial signal appeared in a search for data captured by the Zwicky Transient Facility (ZTF), based at Caltech’s Palomar Observatory in California. ZTF is an observatory that scans the sky for stars that are rapidly changing in brightness, and whose pattern could indicate the presence of supernovae, gamma-ray bursts, and other stellar phenomena.
De has been looking at the ZTF data for signs of flares in binary stars, systems in which two stars orbit each other, one occasionally pulling mass off the other and brightening briefly as a result.
“One night, I noticed a star that shone by a factor of 100 over the course of a week, out of nowhere. It didn’t look like any starburst I’d ever seen,” Dee recalled.
Hoping to pinpoint the source with more data, Dee turned to observations of the same star made by the Keck Observatory in Hawaii. The Keck telescopes make spectroscopic measurements of starlight, which scientists can use to characterize a star’s chemical composition.
But what he discovered made him even more baffled. While most binary stars produce stellar materials, such as hydrogen and helium, as one star erodes the other, the new source gives none. Instead, what Dee saw were signs of “foreign particles” that can only exist in extremely cold temperatures.
“These molecules are only detected in very cool stars,” Di says. “And when a star gets brighter, it usually gets hotter. So, lower temperatures and star brightness don’t go hand in hand.”
Then it became clear that the signal was not from a binary star. Dee decided to wait for more responses. About a year after its initial discovery, he and his colleagues analyzed observations of the same star, this time taken with the Palomar Observatory’s infrared camera. Within the infrared range, astronomers can see signs of cooler material, in contrast to the white-hot light emissions that arise from binaries and other extreme stellar events.
“The infrared data made me fall out of my seat,” Di recalled. The source was incredibly bright in the near infrared.”
Apparently, after its initial hot flash, the star continued emitting cooler energy for the following year. This icy material was probably gas from the star that shot off into space and condensed into dust, cool enough to be detected at infrared wavelengths. This data indicates that the star could have merged with another star rather than being luminous as a result of a supernova explosion.
But when the team analyzed the data further and combined it with measurements taken by NASA’s infrared space telescope, NEOWISE, they came to an even more interesting conclusion. From the data collected, they calculated the total amount of energy the star has released since its initial eruption and found it to be surprisingly small: about 1/1,000 the size of any observed stellar mergers in the past.
“This means that whatever was combined with the star must be 1,000 times smaller than any other star we’ve seen,” Dee said. “It’s a happy coincidence that Jupiter’s mass is about 1/1,000th that of Earth.” That’s when we realized: This was a planet, colliding with its star.
With the pieces in place, scientists were finally able to explain the initial explosion. The bright flash was likely the final moment for a Jupiter-sized planet swept into the atmosphere of a dying star. As the planet plunged into the star’s core, the star’s outer layers fell off and settled as cool dust over the next year.
“For decades,” Dee points out, “we’ve been able to see the before and the after.” Before that, when the planets were still orbiting very close to their star, and later, when a planet had already been swallowed up and the star had become gigantic. What? What we’ve been missing is catching a star in the act, when a planet suffers that fate in real time. That’s what makes this discovery really exciting.”
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