An artist's
impression of the trio of super-Earths discovered by
an European team using the HARPS spectrograph on
ESO's 3.6-m telescope at La Silla, Chile, after five
years of monitoring.
Planet hunters say it's just a matter of time
before they lasso Earth's twin, which almost surely
is hiding somewhere in our star-studded galaxy.
Momentum is building: Just last week, astronomers
announced they had discovered three super-Earths —
worlds more massive than ours but small enough to
most likely be rocky — orbiting a single star. And
dozens of other worlds suspected of having masses in
that same range were found around other stars.
"Being able to find three Earth-mass planets
around a single star really makes the point that not
only may many stars have one Earth, but they may
very well have a couple of Earths," said Alan Boss,
a planet formation theorist at the
Carnegie Institution of Washington in
Washington, D.C.�
Since the early 1990s, when the
first
planets outside of
our
solar system were detected orbiting the
pulsar PSR
1257, astronomers have identified nearly 300 such
worlds. However, most of them are gas giants called
hot Jupiters that orbit close to their stars
because, simply, they are easier to find.
"So far we've found Jupiters and Saturns, and now
our technology is becoming good enough to detect
planets smaller, more like the size of Uranus and
Neptune, and even smaller," said one of the top
planet hunters on this world, Geoff Marcy of the
University of California, Berkeley.
Marcy, Boss and other scientists are optimistic
that within the next five or so years headlines will
be splashed with news of a near twin of Earth in
another star system.
"What is amazing to me is that for thousands of
years humans have gazed at the stars, wondering if
there might be another Earth out there somewhere,"
Boss told
SPACE.com. "Now we know enough to say
that Earth-like planets are indeed orbiting many of
those stars, unseen perhaps, but there
nevertheless."
Seeing tiny planets
Two techniques are now standard for spotting
other worlds. Most of the planets noted to date have
been discovered using the radial velocity method, in
which astronomers look for slight wobbles in a
star's motion due to the gravitational tug of an
orbiting planet. This favors detection of
very
massive planets that are very close to
their host stars.
With the transit method, astronomers watch for a
dimming of light when a planet passes in front of
its host star. Though more haphazard, this approach
works when telescopes scan the light from hundreds
or thousands of stars at once.
Both methods are limited by their ability to
block out the overshadowing light of the host star.
For instance, the sun is 100 times larger, 300,000
times more massive and up to 10 billion times
brighter than Earth. "Detecting Earth in reflected
light is like searching for a firefly six feet from
a searchlight that is 2,400 miles distant," writes a
panel of astronomers recently in their final report
of the Exoplanet Task Force.
With upgrades in spectrometers and digital
cameras attached to telescopes, astronomers' eyes
have become more sensitive to relatively tiny
stellar wobbles (measured by changes in certain
wavelengths of light) and dips in starlight
from ever smaller planets.
"I think why astronomers are really excited
[about the super-Earth discovery] is it just shows
that technology has really matured and so they're
able to see these very subtle wobbles due to these
low-mass planets," said David Charbonneau of the
Harvard-Smithsonian Center for
Astrophysics in Massachusetts. "Those were
fairly massive stars. If they were able to get the
same precision on a lower-mass star, they would be
able to look at even lower-mass planets and so those
really would be analogs of the Earth."
The fast track
To eek out even more sensitivity from current
technologies, Charbonneau suggests astronomers look
for worlds around small stars.
He and other astronomers are in fact probing the
universe for transiting planets orbiting M dwarfs,
or red dwarfs, which are about 50 percent dimmer
than the sun and much less massive. Red dwarfs are
also considered the most common star type in the
universe.
"I think the real opportunity there is to study
low-mass stars, and that's because we're looking for
very small planets," Charbonneau said. "The
difficulty is the ratio between the planet's mass
and the star's mass or the planet's size and the
star's size depending on how you want to find it."
The low mass and luminosity means any changes to
the star due to an Earth-mass planet are much more
likely to be detected.
"A late M star is about 10 times smaller than the
sun," said Penn State's James Kasting, who studies
planetary atmospheres and the habitable zones of
exoplanets. "So Earth going in front of an M star
would give a 1 percent signal. That's like
Jupiter going in front of the sun." Kasting
added, "We could conceivably find an Earth analog
planet by this method within the next five or ten
years."
Other teams are gearing up to look for Earth-like
worlds orbiting massive stars like the sun. NASA's
Kepler observatory is scheduled for launch in
February 2009, after which the high-powered
telescope will monitor about 100,000 stars in the
Milky Way looking for periodic dimming of starlight
due to a planet's transit in front of the star.
The French
COROT
mission is already up in space working in
a similar fashion.
Good hunting
The ultimate goal of planet-hunting projects is
to find Earth twins.
"We are looking for twins of the Earth, analogs
that walk and talk and smell like our own Earth,"
Marcy said during a telephone interview. He is
currently looking for super-Earths using the W.M.
Keck Observatory in Hawaii.
Such a twin would be rocky, with a similar
chemical composition to Earth, and would orbit
within the
habitable zone of its star.
The habitable zone defines the distance at which
a planet must orbit from its star for liquid water
to exist on its surface — not too hot like Venus,
not too cold like Neptune or Pluto.
Astronomers have found planets orbiting pretty
close to the habitable zone, but none so far within
it.
"I suspect there are Earth-like planets with
lakes and rivers and waterfalls and deep glacial
gorges and that are spectacularly beautiful," Marcy
said.
Life beyond Earth
Finding a planet in the habitable zone is the
first step toward finding alien life.
"When we say it's a habitable world, all we're
doing is saying it potentially could hold life,"
Boss said. "To go beyond that to say, 'Here's a
habitable world; is it inhabited,' then you need to
start studying the atmosphere of the planet."
The James Webb Space
Telescope (JWST),
scheduled for launch in 2013, could do just that.
"There might be a signal in the atmosphere that
could be a smoking gun and would suggest that
plate
tectonics is there," said earth and planetary
scientist Diana Valencia of Harvard University.
Her computer models have shown that plate
tectonics, the forces that move continents and lift
gigantic mountain ranges, are
key
to life on Earth as we know it, and
possibly to life on other worlds. That's because as
the rocky plates that form the planet's outer shell
move about, they also recycle carbon dioxide. This
greenhouse gas keeps our planet's temperature
balmy, but not too hot. And the telltale signal
would be certain levels of carbon dioxide,
suggesting that just as on Earth, this other world
relies on plate tectonics to cycle carbon.
But first things first. "There's no doubt that
other Earths exist, simply due to the sheer vast
numbers of other stars and galaxies in our
universe," Marcy said. "There's a deeper question —
how common are Earth-like planets? Are Earth-like
planets a dime a dozen, or are they quite rare,
quirky precious planets that are one in a thousand
or one in a million?"
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