Rather sensationalisticly written by NPR, but ---

from
https://www.npr.org/2023/05/03/1173082322/this-star-ate-its-own-planet-earth-may-share-the-same-fate

This star ate its own planet. Earth may share the same fate
May 3, 202311:07 AM ET
Heard on All Things Considered
Nell Greenfieldboyce 2010
Nell Greenfieldboyce

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An artist's impression of an aging star swelling up and beginning to
engulf a planet, much like the Sun will do in about 5 billion years.
K. Miller/R. Hurt (Caltech/IPAC)

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.

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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.

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"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.

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"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."

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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."

nova
star death
stellar evolution
planetary science
astronomy