Wavelength (microns)Brightness Debris
cloudWhite
dwarf110110WWW.ASTRONOMY.COM 27third of all white dwarfs. Unlike their more
plentiful cousins, they have atmospheres
rich in helium rather than hydrogen. In
fact, their source of hydrogen is something
of a mystery. Some researchers contend
that these white dwarfs formed with a res-
ervoir of hydrogen that was gradually
diluted by the helium atmosphere. Others
wonder if the stars might have picked up
hydrogen on their surfaces as they passed
through interstellar material.
Fusillo and his colleagues recently dis-
covered a new helium-rich white dwarf,
GD 17, whose composition strongly resem-
bled GD 61. Both are heavy in hydrogen
and rich in other elements. Wondering if
the two characteristics might be connected,
Fusillo surveyed 729 helium white dwarfs.
He found that hydrogen was nearly twice
as common in polluted white dwarfs as in
their counterparts.
What if the hydrogen in these rich
white dwarfs was the only surviving sign of
water-rich objects? As with GD 61, an
asteroid or KBO may have crashed into the
dying star. But while the oxygen, carbon,
nitrogen, and everything else would even-
tually sink out of the atmosphere, the
hydrogen would linger. Over time, it would
pile up, leaving white dwarfs that had con-
sumed water with an exceptionally thick
hydrogen atmosphere.
Consuming planetary debris isn’t the
only source of hydrogen in helium white
dwarfs. Fusillo still thinks that a lot of
white dwarfs probably retain traces of a
primordial hydrogen atmosphere. But the
debris definitely makes an important con-
tribution. “A significant amount of them
must have undergone this accretion event,”
he says.
With no debris disk to provide addi-
tional clues, it’s impossible to tell if unpol-
luted hydrogen-rich helium white dwarfs
devoured a few large planetlike objects or a
wealth of tiny asteroids over their billion-
year lifetime. “Hydrogen can look back in
history, but that information is lost,”
Fusillo says. “It could be separate events
over time, each carrying a tiny amount of
water over long scales of time.”
Farihi cautions against the possibility of
overstating the link between water and
hydrogen-rich atmo-
spheres. With polluted
objects like GD 61 and
GD 17, it’s easier to
make the case for
water by matching up
the signatures of the
elements present. Once
the elements have sunk into the star, how-
ever, all that’s left is water.
Still, Fusillo’s co-author and adviser
Gänsicke thinks the research reveals that
water-rich planetesimals — big or small
— are frequent in other planetary systems.
“It’s exciting in a sense, but maybe actually
natural, because we know in the solar sys-
tem that water occurs in many places, some
of them unexpected,” he says. After all,
water shows up in the shadowed craters of
Mercury, and in oceans deep inside the
moons of Saturn and Jupiter, and maybe
even beneath the icy surface of Pluto.Testing the water
So while understanding living worldsremains a challenge, dead planets are slow-
ly giving up their secrets. And it looks like
their secrets could be very wet, indeed.
“There is evidence that water seems to
be a general ingredient of planetary sys-
tems, even ones that have evolved to the
very end of the lifetime of their host stars,”
Gänsicke says.
Fusillo agrees. “Water is not rare,” he
says. “Whenever a white dwarf is accreting
rocks, it’s also accreting water. It’s a small
amount, but very commonly present.”
If water is abundant not only in dead
planets but also in living ones, that could begood news for those hunting potentially
habitable worlds. Planets around living stars
may also have received water, either from
asteroids or comets, and may hold onto that
water until the end of their lifetimes.
“If rocky planets form in the habitable
zone, there are a sufficient number of
water-carrying bodies that deliver material
and make them habitable, even if they were
not habitable in the first place,” Gänsicke
says. “The kind of story that happened in
the solar system is quite likely to happen in
other planetary systems as well.”Left: An asteroid heads for its destruction at the hands of white dwarf Giclas 29–38. Right: The Spitzer Space Telescope acquired this spectrum of
G 29–38. A normal white dwarf shows a blue-dominated spectral signature like the one on the left side of the chart. G 29–38, however, has another,
reddish component scientists think comes from a disk of dust surrounding the star. They believe the debris is the remains of an asteroid that was part
of the solar system that existed when G 29–38 was still a Sun-like star. NASA/JPL-CALTECH/W. REACH (SSC/CALTECH)
Nola Taylor Redd is a freelance science writer
who writes about space and astronomy while
home-schooling her four kids.“The kind of story that happened in the
solar system is quite likely to happen in
other planetary systems as well.”
G 29–38 spectrum