Astronomy

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How do planets get their water? Scientists

are looking for evidence in the light

from white dwarfs. by Nola Taylor Redd

in the Milky Way

WHEN IT COMES TO EXOPLANETS,

the search for water is paramount, thanks

to its vital role in the evolution of life

as we know it. However, finding the

life-giving liquid on other worlds is an

ongoing challenge.

For nearly a decade, scientists have

probed the composition of planets as the

worlds are shredded and consumed by

white dwarf stars. Because heavy elements

quickly sink below the hydrogen- and

helium-rich stellar surface, any metals

(all elements not hydrogen or helium)

detected in the star must come from plan-

etary debris falling into it. Thanks to this

process, astronomers know more about

the interiors of dead exoplanets than they

do about Earth’s composition.

Uncharted waters

What, then, are scientists looking for?

Water is the key ingredient for life on

Earth. So, when we search for life on

worlds in our solar system, water’s presence

usually dictates our interest. It’s no sur-

prise, then, that astronomers looking for

potentially habitable worlds around other

stars key in on the possibility of water.

Often, the search for exoplanets focuses

on the habitable zone, the region around a

star where water could stay liquid on a

planet’s surface. Unfortunately, it will be a

long time before any of our telescopes can

resolve the surface of a world light-years

away. Instruments like NASA’s Hubble

Space Telescope probe other worlds,

searching for signs of water in their atmo-

spheres. But despite identifying thousands

of planets and planet candidates beyond

the solar system, scientists can glean only

the thinnest of data about them.

Most of the identified planets were orig-

inally found and studied using the transit

method, which examines how an object

blocks light from the star. Unfortunately,

this can provide only the size of the world.

Others were found using the radial velocity

method, which measures how much a

planet tugs on its star, thus revealing its

mass. If scientists follow up on a transit-

ing world with a radial velocity mea-

surement — and they have in many

cases — then they can use the mass

and size to calculate the planet’s

average density, providing a

rough estimate of its composi-

tion and perhaps a clue

whether water is present.

A planet enters the last

phase of its death spiral

into the white dwarf it

has been orbiting. For

some years, the resulting

debris cloud will change

the spectrum of the white

dwarf. MARK GARLICK FOR ASTRONOMY