2023-05-04 20:00:00
As stars age and reach the end of their lives, they can undergo a series of changes that cause them to increase in size until they become gigantic. For example, when a star similar to our Sun exhausts its fuel, basically hydrogen gas, it begins to cool and contract. However, as the star contracts, the gravity at its core increases, causing the temperature at the center of the star to rise dramatically as well, leading to nuclear fusion within the core of heavier elements such as helium. and carbon, which will gradually cause the star to begin to heat up and grow larger, becoming a red giant.
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In this process, for the bad luck of the planets that are orbiting the star, red giants gobble up everything around them. Thus, for a long time, scientists have found signs of stars that have wiped out the planets in their system. They also know of others, such as Earth itself, that will end up being swallowed up by their host star over time, although for that, in our case, there are still another 4.5 billion years to go. What they had never observed, as stated in a study recently published in the journal Naturewas a star swallowing one of its own planets in situ.
What about the planet that perished?
The team discovered the burst in May 2020, however it took them over a year to find a plausible explanation for it. Reportedly, the planetary disappearance it has taken place in our own galaxy, the Milky Way, some 12,000 light-years away, near the constellation of the Eagle. It was there that astronomers detected the outburst of a star that in just 10 days it became 100 times brighter than usual before quickly fading away.
The initial signal turned up in a search conducted at Caltech’s Palomar Observatory in California as part of a study scanning the sky for stars that change rapidly in brightness. The researchers were particularly looking for the signature of the merger of two binary stars when MIT’s Kavli Institute for Astrophysics and Space Research scientist, Kishalay De, lead author of the paper, noticed how out of nowhere a star brightened by a factor of 100 over the course of a week. “It was unlike any starburst I’d ever seen in my life,” De declares.
“Curiously, this hot flash was followed by a longer lasting colder signal“, he continues. This combination, the scientists deduced, could only have been produced by one event: a star swallowing a nearby planet. “We were witnessing the final stage of swallowing,” adds De. Scientists estimate that the planet was probably a hot Jupiter-sized world that first spiraled toward the dying star and then was dragged, first to its atmosphere and then to its core.
Cold and heat, a thermometer of stellar activity
However, the explosion initially caught investigators off guard, who wondered about the nature of the explosion. What types of explosions generate heat and then emit a much colder and longer lasting signal?
Hoping to get more data, De searched among observations of the same star taken by other telescopes, such as the one at the Keck Observatory in Hawaii. Keck telescopes take spectroscopic measurements of light of stars that scientists can use to discern their chemical composition. What De found confused him even more. While most binary stars emit stellar material such as hydrogen and helium when one engulfs the other, the new source appeared to eject no material at all.
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Instead, what the team observed was a very peculiar trail. “We observe molecules that can only exist at very cold temperatures. When a star lights up, it usually gets hot. Therefore, low temperatures and stars are not two things that usually go hand in hand.
With the possibility that it was a binary star system ruled out, it would take a year for De’s team to look at the star again, this time using the infrared camera at the Palomar Observatory. Within the infrared band, astronomers can see signs of cooler material, in contrast to the fiery optical emissions that arise from binary star mergers and other extreme stellar events. “The infrared data made me fall out of my chair,” De says. “The source was incredibly bright in the near infrared.”
Following the initial explosion, scientists found that the star had been releasing much cooler material into space for a year; probably gas from the star ejected into space that condensed into dust cool enough to be detected at infrared wavelengths. These data at first suggested that the star might be merging with another star, but when the team analyzed them and compared them with measurements taken by NASA’s infrared space telescope, NEOWISE, they came to a much more exciting conclusion.
It was a planet colliding with its star!
Thus, based on the compiled data, they estimated the total amount of energy released by the star since its initial outburst, and found that it was surprisingly smallabout a thousandth of the magnitude of any stellar merger observed in the past.
“That means that whatever is merging with the star has to be 1,000 times smaller than any other star we’ve seen,” says De. “And it’s a happy coincidence that Jupiter’s mass is about one-thousandth of the mass of the Sun. It was then that we realized: it was a planet colliding with its star!
With the pieces in place, scientists were finally able to explain the observed initial stellar outburst. The bright, hot flash was likely related to the last moments of a Jupiter-sized planet being pulled into the atmosphere of a dying star.
A star turned to stone
When the planet fell into the star’s core, the outer layers of the star fell off, settling as cold dust for the next year. “Over the decades we have been able to see the before and after,” says De. “Before, when a planet is orbiting its star very closely, and after, when a planet has already been swallowed and the star becomes gigantic “What we were missing was catching the star at the very moment a planet is experiencing its final fate in real time. And that is precisely what makes this discovery really exciting,” he concludes.
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