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For white dwarfs,
Earths for dinner
By Nadia Drake
Astronomers studying the atmospheres
of planet-munching white dwarf stars
have found that some stellar meals
include the same ingredients as Earth.
Remains of rocky bodies that once
circled the white dwarfs pepper the gas
envelopes around the dead stars. The
ratios of elements in the remains — called
“pollution,” since it mars a star’s pristine
atmosphere — tell astronomers what the
digested bodies were made of.
“We think that most of these systems
that show pollution must in some way
approximate ours,” says astronomer
John Debes of NASA’s Goddard Space
Flight Center in Greenbelt, Md. “This is
the first hint that despite all the oddball
planetary systems we see, some of them
must be more like our own.”
Using the Keck I telescope in Hawaii,
Ben Zuckerman of UCLA and colleagues
found that each of two polluted white
Roughly Earth-sized (right) —but with the mass of the sun (left) —a white dwarf
(center) shreds and munches anything that gets too close. Astronomers have identi-
fied chemical signatures of Earthlike worlds smearing the tiny stars’ atmospheres.
dwarf stars snarfed at least 10 quadrillion
metric tons of rocky dust. White dwarf
PG1225-079 has a mix of magnesium,
iron and nickel in ratios resembling
Earth’s; white dwarf HS2253+8023
munched material containing more than
85 percent oxygen, magnesium, silicon
and iron — very much like Earth, the
team reports online August 7 at arXiv.
org and in an upcoming Astrophysical
Journal.
“This means that planetlike rocky
material is forming at Earthlike distances or temperatures from these
stars,” says Zuckerman. He notes that
it’s still unclear whether the material
is from a planet, planetlike bodies or an
asteroid.
For years, astronomers thought the
dwarfs were simply catching dust during
their interstellar travels. Now, scientists
think the debris signals ancient orbiting
planetary systems. Zuckerman says that
25 to 30 percent of white dwarfs have
orbital systems containing both large
planets and smaller rocky bodies. After
a dwarf forms, Jupiter-mass planets can
perturb the orbits of smaller bodies and
bounce them toward the star.
White dwarfs are about the size of
Earth but as massive as the sun. They
mark the final stage of stellar evolution
for most stars in the Milky Way. But
before reaching that stage, stars puff up
into red giants, a process that can rearrange an orbiting planetary system.
Galactic bull’s-eye
hoag’s object, shown at left, is a galaxy made up of a golden central
sphere of stars surrounded by a much bigger star-studded hula hoop.
When Arthur hoag discovered the object in 1950, he thought the ring
was the image of a distant galaxy smeared into a circular halo by the
gravity of the dense central orb. not until 1987 did researchers confirm
that the orb and ring were part of the same galaxy. now, using ground-and space-based observations, Israeli and russian astronomers propose that the object’s core formed first —at least 10 billion years ago.
soon after, the core skirted itself with a disk of hydrogen gas that it
pulled from surrounding material. the disk’s spiral pattern could be
caused by rotation of the core if the central cluster isn’t quite spherical. that setup would also explain ongoing star formation that dots the
ring with young, massive stars. the object could be a good test bed
for understanding how important gas-snatching is in already formed
galaxies, the astronomers report in an upcoming Monthly Notices of the
Royal Astronomical Society. — Camille M. Carlisle
