Astronomy

24 ASTRONOMY • JUNE 2018

Where’s the water from?
Despite our familiarity with our own
planet, scientists still don’t know the source
of Earth’s water. Although some argue the
four rocky inner planets of our solar sys-
tem could have been born wet, the majority
believe the worlds were probably too hot
to hold onto water. Somehow, Earth and
Mars, and possibly even Venus, went from
hot desert worlds to planets with vibrant
oceans. While Venus and Mars lost their
water again, the liquid remained on Earth,
transforming it into a planet rich in life.
But if Earth had formed dry, where did
that water come from? For decades, scien-
tists believed that comets were a strong
contender. The rocky snowballs of the
solar system could have crashed into the
inner planets when everything was collid-
ing in the violent early solar system, bring-
ing not only water, but also other volatile
materials like carbon and nitrogen.
Unfortunately, missions to comets have
revealed that the chemical fingerprint of
their water doesn’t quite match up with
Earth’s oceans, leading most researchers to
shrug them off as a primary water source
— although they may have contributed a
fraction of our current supply.
Today, asteroids remain the strongest
contender for the delivery of water to
Earth. In the asteroid belt, water is locked
up in minerals. If young Jupiter with its
immense gravity stirred up material there,
some may have hurtled inward. The colli-
sions and resulting heat would have
released the water onto the young Earth.
“Asteroids have enough water in them

to give a nice wet surface layer to forming

young rocky planets,” says Ben Zuckerman,

who studies white dwarfs at the University

of California, Los Angeles.

So, researchers are unraveling the mys-

tery of how water got to Earth, and they

assume a similar process worked for plan-

ets around other stars.

The key: white dwarfs

Exoplanets may be shrouded in mystery,

but their remains are providing clues about

their lives. Over the last decade, scientists

have found a way to probe what lies inside

an exoplanet, not from the outside in, but

from the inside out. Such observations are

providing a more detailed look at the com-

position of these bodies than studies of our

closest worlds — including Earth.

“In the solar system, we don’t actually

have a method to see into the interiors of

planets,” says Jay Farihi, an astronomer at

the University College London. “We don’t

know, for example, 70 to 80 percent of

Earth’s composition, even though we are

standing on it.”

That doesn’t mean scientists are blind

about Earth’s makeup. Studying its density

and magnetic field, as well as examining

meteorites, has provided a wealth of

insights. But no one can dig through to

Earth’s core and directly identify the layers

of the planet.

But in a way Farihi may be able to work

around such tactics to study exoplanets.

Rather than looking at the worlds them-

selves, he and his colleagues study white

dwarfs — the remains of Sun-like stars that

have retained much of their mass but are

only the size of Earth. Some of these stars

have consumed the worlds that once

orbited them.

At the end of its life, when it can no

longer fuse helium, a star like the Sun

swells to become a massive red giant before

releasing its outer layers as a planetary neb-

ula. What’s left collapses into a white

dwarf. These stellar corpses no longer fuse

elements, but their high density and

This illustration depicts the extrasolar planet HD 189733b with its parent star peeking above its

top edge. Astronomers used the Hubble Space Telescope to detect methane and water vapor in the

Jupiter-size planet’s atmosphere. They made the finding by studying how light from the host star

filters through the planet’s atmosphere. NASA/ESA/G. BACON (STSCI)

A rocky and water-rich asteroid is being torn apart by the strong gravity of the white dwarf star
GD 61 in this artist’s impression. Similar objects in our solar system likely delivered the bulk of water
on Earth and represent the building blocks of the terrestrial planets. NASA/ESA/M.A. GARLICK (SPACE-ART.CO.UK)/
UNIVERSITY OF WARWICK/UNIVERSITY OF CAMBRIDGE