This star ate its own planet. Earth may share the same fate

Astronomers got a little taste of what could be The ultimate fate of Earth in about 5 billion years, when the Sun reaches the end of its life and engulfs the inner planets of the Solar System – including our own.

That’s because they’ve discovered for the first time what appears to be a sun-like star engulfing an orbiting planet.

This particular star is about 15,000 light-years away. During a sky survey, astronomers saw the star brighten suddenly and briefly, becoming about 100 times more luminous over about 10 days.

Follow-up observations suggest that what they observed must have been the star’s ingestion of a hot gas giant planet the size of Jupiter, according to a new report in the Journal Nature.

“It’s a bit poetic, you know, that this will be Earth’s ultimate fate,” says Kishalay De, a postdoctoral researcher at MIT’s Kavli Institute for Astrophysics and Space Research and lead author of the new report.

In recent years, scientists have discovered that our galaxy is full of planets, and astronomers believe that many of them will be swallowed up at the end of their star’s evolution.

But no one had ever caught a star swallowing a planet.

“We weren’t looking for exactly that. We were looking for similar things, but not quite like that,” says De. “Like many discoveries in science, this happened to be an accidental discovery that really opened our eyes to a new type of phenomenon.”

No big deal for the star

About three years ago, De reviewed observations made by the Zwicky Transient Facility, an instrument near San Diego that routinely scans the sky each night in search of cosmic firework flashes. De hoped to find erupting stars called novae.

But one particular 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 collecting data from infrared telescopes, including archival data, he found something else that surprised him.

This star had become brighter in infrared light over time, which may indicate the presence of dust. In fact, it turns out that this star began showing a brighter infrared signal months before its big outburst.

Moreover, this infrared brightness persisted after the sudden flare-up.

It almost looked like a pair of stars had merged, “but everything has been scaled down, from the energy emitted to the mass ejected,” 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, professor of astronomy at Caltech, says she was skeptical at first, but “when every single clue came together, I was convinced that what we were seeing here was actually a star devouring a planet.”

The planet could be anywhere from a few to maybe 10 times the mass of Jupiter, she says, “but not much more than that. We just don’t have enough momentum in this explosion.”

The researchers took all the observations they had from different telescopes and created astrophysical simulations that basically allowed them to recreate what must have happened.

In the beginning, before the outburst, the star looks like what our Sun looks like when it runs out of fuel and starts to expand.

Then, as it expands, the star’s outer atmosphere comes into contact with the orbiting gas giant planet.

“So it’s continuously pounding into the star’s atmosphere as it orbits, it’s heating up material, and it’s expelling some of this stellar atmosphere gas,” says MacLeod.

This gas drifts outward, cools, and forms dust, along with bits of the doomed planet also being blown outward.

As the planet passes through the star’s atmosphere, drag narrows the planet’s orbit. It is getting closer and closer to the star and the stellar atmosphere is becoming denser and denser. This makes its orbit even narrower.

“And so this process starts out slow, but it just keeps accelerating,” says MacLeod.

When this runaway process comes to an end, the planet plunges into the star, briefly bloating the star. Some of the star’s outer layers are ejected, creating even more dust.

Although astronomers can’t see the planet at all, their calculations suggest the final fall lasted just a few days to a week.

“And that’s the most dramatic moment in this process,” says MacLeod. “We see the star brighten as heated gas is ejected.”

Later, however, the star looked very similar to how it looked before the outburst, “almost like the star ate that planet and completely forgot about it,” says De.

Will the earth be eaten up too?

Earth’s eventual setting, after the Sun engulfs Mercury and Venus for the first time, will likely be similar, the researchers say.

But the Earth is so much smaller that its entanglement would produce less light and even less disrupt the aging sun.

“To be honest, we won’t see it. Until then, we will not be on planet Earth,” says Kasliwal. Long before the earth is swallowed up, the sun’s increasing heat output will have evaporated all the water and rendered the planet inhospitable.

“We have to find our new home well in advance,” she says.

But some theorists believe Earth will not be a stellar snack.

The sun could lose a bit of mass as it expands, which would cause Earth to move slightly away and could allow it to avoid entanglement, says Smadar Naoz, an astronomer at UCLA.

“Whether or not the sun will swallow the earth is quite debatable,” 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 we know and love.”

She was excited by the new observations of a star eating a planet and said theorists have long been making predictions about what this process should look like.

“To see that recognition that they’ve got you, 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 rotations or compositions that suggested they might have consumed planets, “but we’ve never really seen a star consume a planet, we’ve never seen the act.”

And Naoz is interested in what that star will be like years after the Devour.

“I would like to understand how the star rotates. Will it really spin as fast as we predict?” she asks, adding that even just this one example will help theorists see what they got wrong and what they got right.

Scientists say such planetary ends must happen all the time, and Kasliwal says there are tools already in the works that should make it easier to spot more of them.

“What about smaller planets? What about slightly different stars?” says Kasliwal. “I think there needs to be a whole census of these types of events. I mean, what we’re seeing is just the first, but the first of many more to come.”