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.
The discovery
of super-Earths announced last week reflects this technological leap.
"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 eke 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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