Astronomy

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leftover heat means they’ll spend billions of
years cooling.
Unlike stars, the atmospheres of white
dwarfs are fairly pristine. Astronomers
detect only hydrogen and occasionally
helium, which rise to the top. Other mate-
rial sinks quickly. So when scientists see
something like carbon or nitrogen polluting
the atmosphere, they know something fall-
ing onto the star must have delivered it.
“A white dwarf acts like a blank piece of
paper,” Farihi says. “When stuff falls on
there, we can see what it’s made of.”
And white dwarfs are voracious eaters.
As material orbiting them draws closer, the
object’s intense gravity shreds it. While
Sun-like stars produce winds that drive gas
away, the dead stars are silent, with no
gales that can carry debris to freedom.
“Once you’re trapped in the gravita-
tional field of a white dwarf, it doesn’t mat-
ter what form you’re in — eventually,
you’re going to be gobbled up by that white
dwarf,” Zuckerman said.
That’s when the science starts. Probing
the outer layers of white dwarfs reveals the
guts of their latest meals, consumed any-
where from 10,000 to 100,000 years earlier.
Disks of debris surround white dwarfs.
Recently, astronomers spotted a disinte-
grating Ceres-sized asteroid orbiting a

white dwarf, suggesting that much of the

material in its atmosphere could have come

from the destroyed minor planet.

Because white dwarfs shred objects spi-

raling into them, it can be challenging to

say whether material came from a full

planet or just an asteroid-sized chunk. But

over the past decade, observations of the

last meals of white dwarfs have made it

obvious that water is common in dying

systems, suggesting it’s

an ingredient in plan-

ets as well.

Minor planet

meal

As it became more

obvious that white

dwarfs were snacking

on dying worlds, many scientists wanted

another look. In 2012, Farihi and his col-

leagues captured new images of the white

dwarf GD 61, taking a more in-depth look

with Hubble and the Keck I and II tele-

scopes in Hawaii. After studying the chem-

istry of the white dwarf ’s atmosphere, the

team announced that GD 61 had recently

eaten a water-rich object. For the first time,

water was identified as a major ingredient

in an object outside the solar system.

The chemistry of the Vesta-sized object

suggested that it was once part of an aster-

oid belt when GD 61 was a star. While it’s

impossible to tell if the water arrived as a

solid, liquid, or gas, it was most likely

trapped inside of rocks.

Uri Malamud and Hagai Perets,

researchers at the Israel Institute of

Technology, modeled what might happen

to water both on and in an asteroid-sized

object as its star swells into a red giant.

They found that, for all but the most dis-

tant rocky bodies, any surface water prob-

ably evaporates and is driven off as the star

becomes a giant. But water trapped in

rocks could survive.

Since GD 61 was found consuming an

asteroid-like object, a handful of other

white dwarfs have shown the same eating

habits. According to Boris Gänsicke, pro-

fessor of physics at the University of

Warwick, the white dwarf snacks spotted

before this year all looked like objects from

This artist’s impression shows a massive cometlike object falling into the white dwarf WD 1425+540, which lies in the constellation Boötes some
170 light-years away. NASA/ESA/Z. LEVY (STSCI)

Researchers are unraveling the mystery

of how water got to Earth, and they

assume a similar process worked for

planets around other stars.