us to systematically start charting the universe” when it was
less than 800 million years old, Ellis says.
Astronomers have begun to pinpoint the era when spiral
galaxies like the Milky Way began taking on their distinctive
appearance. Observers have caught glimpses of some of the
first galaxies with rotating disks — the earmark of a spiral galaxy — that began taking shape when the cosmos was between
2 billion and 3 billion years old.
Using bigger telescopes to study such galaxies in detail “would
allow us to start to really understand what state young galaxies
are in and how we can link them to galaxies today,” says Ellis.
C’est la vie
Where there are galaxies, there are planets — some perhaps
with life. For seekers of life beyond Earth, the solar system
harbors an abundance of possibilities. There’s Saturn’s largest moon, Titan, a frigid world shrouded in an organic haze
with pools of liquid methane on its surface. In 2005, astronomers discovered that a much tinier Saturnian moon, Enceladus, spews geysers of water vapor from its south pole and
have since found hints that the moon’s interior may contain
a reservoir of salty water. The fractured surface of Jupiter’s
icy moon Europa suggests that it may have an underground
ocean that occasionally wells up, heated by the internal flexing that the gravitational tug-of-war between the moon’s siblings and Jupiter generates. And then of course, there’s Mars,
the desiccated reddish world crisscrossed by channels that
might once, at least briefly, have carried water.
“Mars clearly has got to be the top place” to look for life,
“and that’s exactly what NASA is [focusing] on,” says Alan
Boss of the Carnegie Institution for Science in Washington,
D.C. One new wrinkle, he notes, is the seasonal detection of
methane on the planet (SN: 2/14/09, p. 10).
Although plenty of nonbiological processes produce meth-
ane, the gas could be a signature of the decay of biological
material. “NASA’s mantra should not be just ‘follow the water’
but ‘follow the water and follow the methane,’ ” says Boss.
“We would really like to find the locations of the methane; it
really focuses the search.”
But Phil Christensen, a Mars researcher at Arizona State
University in Tempe, says it isn’t clear that the Red Planet was
ever warm and wet long enough for life to gain a foothold. Some
evidence suggests that the era of flowing water lasted for only
hundreds to thousands of years — and was confined to a few
specific places on Mars. Finding out whether that era lasted
long enough to support life could speak volumes about the conditions required to support life elsewhere, says Christensen.
Another planetary scientist, Jonathan Lunine of the
University of Arizona in Tucson, thinks that Saturn’s moon
Titan may offer a more promising venue for life. Pools of liquid methane on Titan, Lunine says, may play the same role
that liquid water does on Earth.
“If I have to identify a place where one might find, right on
the surface, a self-organizing chemical system, even if it’s not
life as we know it, I would say go and look at the hydrocarbon
seas of Titan,” says Lunine. “The most dangerous thing we
can do is to define life so narrowly that the only place we’re
going to find it is Earth.”
Beyond the solar system, researchers have now found more
than 345 planets. While most of these extrasolar orbs are blisteringly hot, hugging their parent stars tightly, some lie in
the habitable zone, where water would be cool enough to be
liquid. NASA’s Kepler mission will soon begin its hunt for
Earth-sized planets around 100,000 sunlike stars.
“I’m a wild-eyed optimist that the Kepler mission is going
to find hundreds and hundreds of Earths,” says Boss. The mission, however, can reveal only the size of the planet and how
far away it lies from its star, not its mass, chemical composition or whether life exists there.
To measure the mass of such a planet
will require a space-based mission that
can monitor stars for telltale wobbles
induced by the planet’s tiny gravitational tug. Such space technology is
already available, and a mission could
be launched in only a few years, if funding were available. Then, another space
mission could examine the starlight filtering through the atmosphere of some
of these Earthlike planets to look for pos-
In the next decade, the search for Earthlike exoplanets, and thus the search for
life, will get a boost from new missions.
A superEarth like the one in this artist’s conception can be twice the size
of Earth, with five to 10 times its mass.