Astronomy

26 ASTRONOMY • JUNE 2018

our inner solar system: rocky, iron-rich
material that resembled the cores of
busted-up planets, with only a handful
carrying water. But rocky bodies from dis-
tant asteroid belts aren’t the only things
white dwarfs are consuming.

Outer solar system snack
In early 2017, a team led by Siyi Xu at
UCLA found evidence that white dwarfs
also have been consuming material from
their outer solar systems. Xu has been using
the Keck telescope to survey polluted white
dwarfs, and she had worked with UCLA’s
Michael Jura, whom she refers to as “a
pioneer in this kind of work.” (Jura passed
away in 2016.)
One of the objects, WD 1425+540,
didn’t really stand out from the crowd
except for one notable feature. Although it
is a helium white dwarf (more about this
type of object later), it is rich in hydrogen.
When Xu studied the white dwarf with
Hubble, she also discovered it is surpris-
ingly rich in carbon and nitrogen, material
that is rare close to a star, and that only
shows up at distances equivalent to
Saturn’s position in our solar system.
“Nitrogen is a signpost, or an indicator
for low temperatures,” Zuckerman says.
And where nitrogen exists, can water be
far behind?
The high nitrogen content was a signal

for Xu, now at the European

Southern Observatory, who

said that no other white

dwarf had previously

shown signs of accret-

ing the element. The

high quantity of

nitrogen in com-

parison to other

elements sug-

gested that the

destroyed object

came from even

farther out than

a Saturn-like

orbit, perhaps

from an extrasolar

Kuiper Belt. In our

solar system, the

Kuiper Belt is the home

of comets and dwarf plan-

ets. Whatever WD 1425+540

was snacking on was bigger than a

comet, weighing in at about the same

mass as the Kuiper Belt’s most famous

inhabitant, Pluto.

“We really don’t know the bulk compo-

sition [of Pluto],” Xu says. “You don’t know

it until you smash it up and let us measure

it.” So, the distant white dwarf may have

provided the closest look we’ll get at the

inside of one of the outermost worlds in

our solar system.

But while rocky inner worlds are easily

disrupted after a star swells into a red

giant, falling inward if they aren’t

destroyed outright, it can be challenging

to figure out how a more distant object

gets into the maw of a white dwarf. Xu and

her collaborators suspect that the reason

may be the gravity of WD 1425+540’s

companion, a star that orbits more than

2,200 times as far from the white dwarf as

Earth orbits from the Sun. Fellow

researchers are examining if it’s possible

for slight perturbations from this compan-

ion to move a Kuiper Belt object inward to

its doom.

Exo-Kuiper Belts aren’t new — scientists

spotted them around other stars even

before they knew the Sun had a belt of its

own. But never before have they been able

to peer inside of one.

“Now, for the first time we’re actually

able to measure the elemental and chemical

composition of an object that was once in

an extrasolar Kuiper Belt,” Zuckerman says.

If the Sun’s Kuiper Belt tossed comets

and other objects toward Earth, seeding

it with at least some of the water and

elements necessary for life, then an

exo-Kuiper Belt rich in the same ingredi-

ents provides hope for other systems fol-

lowing a similar track.

The mere fact that such objects rich in

volatiles orbit white dwarfs is encouraging.

“Earth-like worlds, if they exist, might also

have a veneer or surface layer that would

be conducive for the origin of life,”

Zuckerman says.

Hydrogen smorgasbord

While Farihi and Xu stalk individual white

dwarfs for signs of water-rich asteroids

and exo-KBOs, Nicola Gentile Fusillo,

postdoctoral associate at the University of

Warwick, decided to take a step back and

survey hundreds of dead stars, focusing

on a smaller class known as helium white

dwarfs. His findings suggest that water-rich

objects are abundant throughout the galaxy.

Helium white dwarfs make up roughly a

In some ways,
it would be easier to
study the composition of a planet like Earth as
a white dwarf star destroys it than by probing it
from above, as scientists living here now do. NASA

One way to find water nearby is to observe a world ejecting it from geysers. Scientists based this

illustration (not to scale) of plumes coming from Saturn’s moon Enceladus on analysis of data from

NASA’s Cassini spacecraft, which passed through the plumes in 2015. The discovery of hydrogen

in the erupting material provides evidence for hydrothermal activity, making the existence of an

underground ocean likely. NASA/JPL-CALTECH