Earth is one special planet.
It has liquid water, plate tectonics, and an atmosphere that
shelters it from the worst of the sun's rays. But many scientists agree our
planet's most special feature might just be us.
"It's the only planet we know of that has life,"
said Alan Boss, a planet formation theorist at the Carnegie
Institution of Washington in Washington, D.C.
Though other bodies in our solar system, such as Saturn's
moon Titan, seem like they could have once been hospitable to some form of
life, and scientists still have hope of eventually digging
up microbes beneath the surface of Mars, Earth is still the only world
known to support life.
"So far, we haven't found it anywhere else," said Alex
Wolszczan of Pennsylvania State University, who co-discovered the first planets
beyond our solar system. He agreed that life was Earth's single most impressive
characteristic.
None of this is a revelation, but understanding what's
special about Earth is crucial for finding other planets out there and predicting
what they might be like.
The fact that Earth hosts not just life, but intelligent
life, makes it doubly unique. And the planet's intelligent life (humanity) has
even developed rockets that enable travel beyond the planet, said Gregory
Laughlin, astrophysicist and planet hunter at the University of California,
Santa Cruz.
"During the last half century, the planet Earth has
fashioned together tiny pieces of the metal in its crust, and has flung these
delicately constructed objects to all of the other planets in the solar
system," Laughlin said, adding that these achievements should be counted
as an exemplary trait of our planet.
"From our anthropocentric viewpoint, we naturally
separate ourselves from the planet that we live on, but if one adopts the point
of view of an external observer, it is the 'planet' (taken as a whole) that has
done these remarkable things," he told SPACE.com.
Water World
To enable life, this most special of attributes, planet
Earth has a number
of ideal features. It is unique among planets in our solar system for
having water in its liquid form at the surface, in an amount conducive to life
evolving.
"The most impressive attribute of the Earth is the existence
and amount of liquid water on its surface," said Geoffrey Marcy, an
astronomer at the University of California, Berkeley who has helped discover
dozens of extrasolar planets. No one knows why Earth has the exact amount of
water it does, which is relatively small considering that water molecules
outnumber silicate molecules in the galaxy, he said.
"The Earth is remarkable for its precisely-tuned amount
of water, not too much to cover the mountains, and not so little that it's a
dry desert, as are Mars and Venus, our 'sister' planets," he said.
Goldilocks planet
Earth's water is also special in that it has remained liquid
for so long. How has Earth been able to hold on to its oceans while those on
other planets freeze or fry?
"Many details as to why Earth is the only planet
with liquid water in our solar system need to be worked out," said Diana
Valencia, a graduate student in Earth and Planetary Sciences at Harvard
University. "Certainly the distance to the sun has made it possible. A
planet much farther in would receive too much energy from the sun, and a planet
too far out would quickly freeze."
Our planet's Goldilocks-like "just right" location
in the solar system has helped, as has its system of plate tectonics the
slip-sliding movements of Earth's crust that are thought to have created the
planet's towering mountain ranges and plummeting ocean depths.
"The fact that Earth has plate tectonics allows for the
carbon-silicate cycle to operate over geological timescales," Valencia
said. "With the carbon-silicate cycle, the levels of carbon in the
atmosphere get regulated to keep the surface temperature around that of liquid
water."
Plate tectonics and water are inextricably linked. Not only
does plate tectonics enable liquid water to exist by way of regulating the
temperature, but many scientists have argued water enables plate tectonics to
happen.
"Without water the planet would be geologically
dead," said Caltech's Mike Brown, discoverer of the newly reclassified "plutoid"
object named Eris, which lies beyond Pluto in our solar system. "Water is
what lubricates plate tectonics, which is what leads to the extreme difference
between continents and seafloors, the large amount of earthquakes and
volcanoes, fresh mountain-building. Venus has no water, no plate tectonics, no
deep sea floor, no steep mountains, no continents, probably few earthquakes or
volcanoes. A much less geologically interesting place!"
Another "just-right" aspect of Earth is its size:
If it was much smaller, it wouldn't be able to hold on to our precious
atmosphere, but much larger and it might be a gas giant too hot for life.
The presence of our big brother planet, Jupiter, farther out
in the solar system blocking Earth from much of the incoming debris, has also
helped Earth become a safe haven for life. Jupiter acts like a giant broom,
sweeping the solar system of debris rocks as small as cars and as huge as
moons that could snuff out life in one fatal blow. This protective effect was
particularly helpful in the solar system's early years, when Earth still got
pummeled but, scientists say, not nearly as bad as would have been the case
without Jupiter.
A friendly moon
Life
on Earth may also owe a debt to our nearest celestial neighbor, the moon.
Earth's
moon stabilizes our planet's rotation, preventing drastic movements of the
poles that could cause massive changes in climate that some scientists think
could have doomed any chance for budding life to form or evolve.
The
moon also helpfully pulls the ocean's tides, which scientists suggest might
have been the perfect place for early life to begin evolving to survive on
land.
Though Earth has the necessary ingredients for life, it's
unclear whether the development of life here might have been a one-time fluke,
or if it's something that happens pretty much everywhere the conditions are
right.
Rare Earth
All of these features make Earth special among known planets
near and far.
"You hear all the time how Earth-like Mars is, but if
you were taken to Mars you wouldn't feel happy there at all," said
University of Washington astronomer Don Brownlee, author of the book "Rare
Earth" (Springer, 2003). "It's not Earth-like. And Titan, when the [Huygens]
probe landed, there was all this stuff in the media about how Earth-like it is.
Earth-like? It is completely different. It has all this methane on the surface.
Venus has about the same mass [as Earth], almost the same distance from the sun.
But it's a totally different place no oceans, no plate tectonics and it's
not a place you would want to be."
So far, we haven't seen any planet outside the solar system
come very close to Earth either.
Of the nearly 300 new worlds glimpsed elsewhere in the
galaxy, most are "hot Jupiters" large planets that orbit close to
their stars, on which life and liquid water are unlikely to exist.
"I doubt that in our galaxy typical stars have planets
just like Earth around them," Brownlee said. "I'm sure there are lots
of planets in the galaxy that are somewhat similar to Earth, but the idea that
this is a typical planet is nonsensical."
Brownlee's view may be in the minority, however.
Not-so-special Earth
As our planet-hunting technology improves, many planet
hunters expect to find
Earth's twin. The search has led scientists to debate whether Earth is
really as special as we think it is.
"In the past 10 years, everything has been pointing in
the direction of, 'Hey, the solar system, which we thought was unique, is not
unique at all,'" said Alan Boss.
Boss and many other scientists think it's likely that some
form of life exists on some of those countless other planets out there.
"Certainly there will be other planets that support
life," he said. "I think life is actually quite common. I think we're
going to find there are literally billions of them in the galaxy."
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