Astronomers have gotten a sneak peek at what could be Earth's ultimate fate in about 5 billion years when the sun reaches the end of its life and engulfs the solar system's inner planets – including our own.
That's because, for the first time, they've spotted what appears to be a sun-like star gulping an orbiting planet.
This particular star lies about 15,000 light years away. During a survey of the sky, astronomers saw the star suddenly and briefly brighten, becoming about 100 times more luminous over around 10 days.
Follow-up observations suggest that what they witnessed must have been the star's ingestion of a hot gas giant planet about the size of Jupiter, according to a new report in the journal Nature.
"It's a bit poetic, in that, you know, this is going to be the final fate of the Earth," says Kishalay De, a postdoctoral fellow at the MIT Kavli Institute for Astrophysics and Space Research and the lead author of the new report.
In recent years, scientists have learned that our galaxy is chock-full of planets, and astronomers believe that many of them will get gobbled up at the end of their star's evolution.
But no one had ever caught a star in the act of swallowing a planet.
"We weren't quite looking for this. We were looking for similar things, but not quite this," says De. "Like a lot of discoveries in science, this happened to be an accidental discovery that really opened our eyes to a new type of phenomenon."
For the star, not a big deal
About three years ago, De had been going through observations made by the Zwicky Transient Facility, an instrument near San Diego that routinely scans the skies every night, looking for flashes of cosmic fireworks. De was hoping to find erupting stars called novae.
But one particular stellar outburst looked unusual. Instead of being surrounded by hot gas, it was surrounded by molecules that can only exist at cold temperatures.
And when De started gathering data from infrared telescopes, including archival data, he found something else surprising.
This star had been brightening over time in infrared light, which can indicate the presence of dust. In fact, it turns out that this star started showing a brighter infrared signal months before it made its big outburst of light.
What's more, that infrared brightness continued after the sudden flare-up.
It looked almost like a pair of stars had merged, "but everything was scaled down, from the energy emitted to the mass expelled," says Morgan MacLeod, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics and a member of the research team.
It occurred to them that this star might have merged with something smaller, like a planet.
Mansi Kasliwal, a professor of astronomy at Caltech, says she was skeptical at first, but "when every single clue fell right in place, then I was convinced that what we were seeing here was indeed a star engulfing a planet."
The planet could be anywhere from between a few to maybe ten times Jupiter's mass, she says, "but can't be much more than that. We just don't have enough oomph in that explosion."
The researchers took all the observations they had from various telescopes and created astrophysical simulations that basically let them recreate what must have occurred.
In the beginning, before the outburst, the star looks like our sun will look when it starts to run out of fuel and begins to bloat.
Then, as it puffs up, the star's outer atmosphere comes into contact with the orbiting gas giant planet.
"So it's slamming into the star's atmosphere continuously as it orbits around, it's heating up material, and it's expelling some of that stellar atmosphere gas," says MacLeod.
That gas drifts outward and cools, forming dust, along with bits of the doomed planet that also blow outward.
As the planet goes through the star's atmosphere, the drag makes the planet's orbit tighten. It gets closer and closer to the star, and, as it does, the stellar atmosphere gets denser and denser. That makes its orbit tighten even more.
"And so that process starts slowly, but happens faster and faster," says MacLeod.
When this runaway process reaches its conclusion, the planet plunges into the star, making the star briefly balloon up. Some of the star's outer layers get ejected, creating even more dust.
Although the astronomers can't see the planet at all, their calculations suggest that the final plunge only took a couple of days to a week.
"And so that's the most dramatic moment in this process," says MacLeod. "We see the star brighten as heated up gas is thrown out."
Later, however, the star looked very similar to how it did before the outburst, "almost like the star ate that planet and forgot about it completely," says De.
Will Earth get eaten up too?
The eventual demise of Earth, after the Sun first engulfs Mercury and Venus, will probably proceed much like this, the researchers say.
But the Earth is so much smaller that its engulfment would generate less light and be even less of a perturbation for the aging Sun.
"Truth be told, we won't be around to see this happen. We won't be on planet Earth by then," says Kasliwal. Long before the Earth gets swallowed, the increasing heat output from the Sun will have evaporated all the water and rendered the planet inhospitable.
"We have to find our new home long before this happens," she says.
But some theorists think the Earth won't be a stellar snack.
The Sun could lose a little mass as it expands, which would make the Earth move slightly away and may allow it to avoid engulfment, says Smadar Naoz, an astronomer at UCLA.
"Whether or not the Sun will engulf the Earth is quite controversial," she says. "But it wouldn't matter because it will no longer be our beautiful Earth with an atmosphere and oceans. Earth may survive, but not the Earth that we know and love."
She was thrilled by the new observations of a star eating a planet, saying that theorists have long made predictions about what that process should look like.
"To see that detection, that they caught one, live, in the act, that's very, very exciting," says Naoz. "So I was super excited and happy about that."
She says stars have been seen in the past with spins or compositions that indicated they might have consumed planets, "but we never really saw a star eating a planet, never saw the act."
And Naoz is interested in what this star will be like years from now, post-engulfment.
"I would like to understand the spin of the star. Will it really be spinning fast, as we predict it will?" she asks, adding that even just this one example will help theorists know what they got wrong and what they got right.
Scientists say such planetary endings must be happening all the time, and Kasliwal says there's already instrumentation in the works that should make it easier to detect more of them.
"What about littler planets? What about slightly different stars?" says Kasliwal. "I think there's a whole census to be done of these sorts of events. I mean, what we're seeing is just the first one, but the first of many more to come."
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