Astronomers finally saw a star eating a planet

While Earthlings went about their daily tasks in May 2020, another planet in our galaxy was meeting its demise, a star swallowing it for galactic supper. Researchers spotted the event as it happened, marking the first time anyone has caught a star engulfing a planet in real time.

In a study released Wednesday in Nature, a team at MIT, Harvard University, Caltech and other institutions reported that they observed a planet, probably a hot Jupiter-size world, spiraling close to a dying star that was 1,000 times its size, until it was finally ingested into the star’s core. The scientists say the star grew bigger and more than 100 times brighter in just 10 days, quickly faded and then eventually turned to normal as if it finished digesting the planet.

The novel observation helps us understand more about Earth’s own final bow. Many astronomers believe Earth will suffer a similar fate billions of years down the line, when our own evolving Sun will run out of fuel, balloon and consume its closest planetary neighbors. Humans probably won’t be around for this event, however, as our growing Sun will probably fry Earth first, making it inhospitable for life.

“It’s somewhat poetic in that all of that we see around us, all the stuff that we’ve built around us, this will all be burned in a flash when the Sun decides to evolve and become puffy in 5 billion years,” Kishalay De, the study’s lead author and postdoctoral student at MIT, said in a news conference.

The sobering future of Earth aside, scientists are impressed with the groundbreaking observation.

“Wow! That is my first reaction,” Amanda Karakas, an astrophysicist at Monash University in Australia who was not involved in the study, said in an email. “It certainly provides clues as to what will happen to the planets in our solar system and the Earth, many years from now.”

Discovery by accident

The team stumbled upon this discovery by accident. Mr. De was initially searching for signs of eruptions of binary star systems, in which two stars orbit one another and one periodically brightens as it pulls mass from the other. He started looking at data from the Zwicky Transient Facility at Caltech’s Palomar Observatory, which takes images of the sky every night and allows researchers to detect stars that rapidly change in brightness. Mr. De noticed one star grew more than 100 times brighter in a little over a week some 12,000 light-years away, near the eagle-shaped constellation Aquila in our galaxy.

To further pinpoint the source, he studied the star’s chemical composition using observations from the Keck Observatory in Hawaii, but the results were perplexing. The binary star system that he was initially looking for is usually surrounded by a lot of hot gas, but he saw signs of molecules that can only exist at cold temperatures.

Cold objects tend to shine more vividly at infrared wavelengths, so the team obtained infrared observations of the star nearly a year after the initial observation.

That data astounded the team. The star shone very brightly in the infrared bands, even though the star no longer appeared bright in the visible light spectrum seen with our naked eyes. Mr. De said the star must have ejected gas into space, which condensed into dust cold enough to be observed in the infrared – suggesting the star could be merging with something else and brightening as a result.

Using additional infrared data from NASA’s Neowise space telescope, the team estimated the total amount of energy released by the star to be very small – about 1,000 times smaller than past observations of stars merging with one another. That means whatever the star merged with must have been comparatively small.

“You ask, ‘What is a thousand times less massive than a star?’ ” Mr. De said. “What you have is the star that engulfed the planet.”

Co-author Mansi Kasliwal said planet engulfments are fairly common but are also dim and “wimpy,” which make them hard to find. The infrared data helped illuminate these processes hidden against much brighter stellar eruptions.

“When it’s just a planet merging into a star, it’s intrinsically very weak, so it’s harder to find them,” said Ms. Kasliwal. “When something is harder to find, all we need is a more powerful camera.”

Other study authors modeled the observations and reconstructed what might be happening. The team said the bright flash that they initially saw was probably the final moments of the planet getting eaten by the star. Then when the Jupiter-size planet fell into the star’s core, the star’s outer layers blasted away and settled as cold dust over the next year. The star has since returned to more or less its state before the planet engulfment, except it seems to be surrounded by leftover dust. It now continues its normal evolution.

It is natural for a planet to be devoured by its star

It’s natural to see this Jupiter-size planet get devoured by a sun-like star and think about our planet’s own future. But researchers say Earth’s demise probably won’t be as dramatic due to our size.

“I think the distinction is pretty crisply around whether the planet is a gas giant planet, then it will be a major disturbance to a star if it’s in a close in orbit,” said Morgan MacLeod, co-author and postdoctoral student at Harvard, at the news conference. “If it’s a rocky planet, it will be too small to cause a dramatic episode.”

There’s still a few billion years before the Sun is expected to grow large enough to encompass Earth. Mr. MacLeod said, our host star would first swallow Mercury and Venus before eventually getting to Earth. But he estimates it would still take tens of thousands of years for the aged Sun to expand from Mercury to Earth. The Sun’s radius will probably not expand so far as to reach the outer, larger planets in our solar system though.

Astrophysicist Enrico Ramirez-Ruiz, who was not involved in the study, called the study’s findings a “groundbreaking result” but thinks it’s probably not relevant to understanding Earth’s fate and whether it would be ultimately engulfed by our Sun.

“It continues to be an open question as to whether the Earth could be engulfed or not based on its current orbital position,” said Dr. Ramirez-Ruiz, a professor at the University of California Santa Cruz, in an email. “I think this technique could help us understand the demographics of planetary engulfment and the ultimate fate of planets around other solar systems.”

Mr. De said planet engulfment probably occurs in our galaxy about once every few years, which would now be easier to detect given they know what to look for in regards to chemical composition, temperature and infrared signatures.

“The hope is that we would actually be able to use this entire new suite of instruments to try to find every single planet being engulfed in our galaxy in real time,” said Mr. De. “That’s only going to become possible now because of this discovery and together with the availability of instrumentation.”

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