When a star dies, what happens to its planets? Well, if that star is a
certain white dwarf 86 light-years away, those planets are currently in the
process of being torn apart and eaten by the star, like some grotesque
cosmic performance of Kronos devouring his children.
This is not entirely unusual for white dwarfs. But this particular star,
named G238-44, is a glutton: for the first time, astronomers have seen one
of these stars swallowing material from both the inner and outer reaches of
its planetary system at the same time, in the most far-reaching display of
stellar filial cannibalism observed to date.
In G238-44's atmosphere, astronomers have detected traces of elements that
suggest the dead star has recently accreted material that is metallic and
rocky, like inner Solar System asteroids, as well as material that is icy,
like the frozen bodies that can be found in the outer Solar System's Kuiper
Belt.
"We have never seen both of these kinds of objects accreting onto a white
dwarf at the same time," said physicist and astronomer Ted Johnson of the
University of California Los Angeles. "By studying these white dwarfs, we
hope to gain a better understanding of planetary systems that are still
intact."
White dwarfs are what happens when a regular star up to eight times the mass
of the Sun reaches the end of its life. Once such a star runs out of
material to fuse, it puffs up to red giant size before ejecting its outer
material, and the stellar core collapses under gravity to form a dense
object, shining brightly with the light of residual heat. That's the white
dwarf.
Although this process seems like it would be pretty rough on the planets
orbiting the star – the Sun might puff up big enough to engulf Mars when it
hits red giant o'clock in a few billion years – but recently, astronomers
have been finding evidence to suggest that some parts of planetary systems
can actually survive it.
Exoplanets have been spotted orbiting white dwarfs. And then there's
necroplanetology – the study of the remains of white dwarf exoplanets based
on traces of the heavy elements they contained "polluting" white dwarf
atmospheres.
Because white dwarfs are so dense (think of something the mass of the Sun,
packed into a sphere the size of Earth), heavy elements should sink out of
view pretty rapidly, which means any heavy element pollution in a white
dwarf atmosphere needs to have been deposited recently.
This is exciting, because it means that we have an indirect probe into
exoplanetary interiors. We know what Earth is made of, and we're pretty sure
we understand other Solar System planets' composition to some degree, but
exoplanets orbiting distant stars are impossible to probe the way we can
Earth, or even other planets in the Solar System.
Because other planetary systems detected to date seem to be very unlike the
Solar System in many ways, probing the guts of exoplanets munched by white
dwarfs can help scientists determine whether exoplanetary interiors are
different, too. Which brings us back to G238-44.
The pollution in this white dwarf's atmosphere is unlike any seen to date,
Johnson and his colleagues found. Ten elements heavier than helium were
detected: carbon, nitrogen, oxygen, magnesium, aluminum, silicon,
phosphorus, sulfur, calcium, and iron.
The iron and nitrogen abundances were particularly high; the former, the
team said, is suggestive of a body with a differentiated iron core, while
the latter suggests the presence of icy bodies.
"The best fit for our data was a nearly two-to-one mix of Mercury-like
material and comet-like material, which is made up of ice and dust," Johnson
said. "Iron metal and nitrogen ice each suggest wildly different conditions
of planetary formation. There is no known solar system object with so much
of both."
The results also suggest that the ingredients for making a habitable world
might not be so rare in the Milky Way galaxy. Earth is a rocky world, and is
thought to have been seeded with the elements vital for life, such as water,
by asteroid bombardment. The detection of nitrogen-rich material could mean
that frozen reservoirs of these elements might be common.
"Life as we know it requires a rocky planet covered with a variety of
volatile elements like carbon, nitrogen and oxygen," said physicist and
astronomer Benjamin Zuckerman of UCLA.
"The abundances of the elements we see on this white dwarf appear to have
come from both a rocky parent body and a volatile-rich parent body – the
first example we've found among studies of hundreds of white dwarfs."
In fact, aliens peering at the Sun from afar, once it has evolved into a
white dwarf in about 5 billion years, might expect to see something similar.
Although the inner Solar System objects might be vaporized by the expanding
white dwarfs, the asteroid belt between Mars and Jupiter could survive, to
be perturbed by a destabilized Jupiter and rain onto the dead star.
The team's research was presented at the 240th meeting of the American
Astronomical Society.
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Space & Astrophysics