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Eccentric Habitable Zones
While Earth and the other planets in our solar system travel
around the sun in near-circular orbits, planets in other systems can have more
comet-like orbits in which the distance from the planet to star varies. Such
orbits, termed eccentric, would cause the planet to move in and out of the
habitable zone. A habitable zone, shown in green here, is defined as the region
around a star where liquid water, an essential ingredient for life as we know
it, could potentially be present. Earth always remains in its habitable zone.
The hypothetical planet is depicted here moving through the
habitable zone and then further out into a long, cold winter. During this phase
of the orbit, any liquid water on the planet will freeze at the surface;
however, the possibility remains that life could, in theory, hibernate beneath
the surface.
Image credit: NASA/JPL-Caltech
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Astronomers have discovered a veritable rogues' gallery of
odd exoplanets -- from scorching hot worlds with molten surfaces to frigid ice
balls.
And while the hunt continues for the elusive "blue
dot" -- a planet with roughly the same characteristics as Earth -- new
research reveals that life might actually be able to survive on some of the
many exoplanetary oddballs that exist.
"When we're talking about a habitable planet, we're
talking about a world where liquid water can exist," said Stephen Kane, a
scientist with the NASA Exoplanet Science Institute at the California Institute
of Technology in Pasadena. "A planet needs to be the right distance from
its star -- not too hot and not too cold." Determined by the size and heat
of the star, this temperature range is commonly referred to as the
"habitable zone" around a star.
Kane and fellow Exoplanet Science Institute scientist Dawn
Gelino have created a resource called the "Habitable Zone Gallery."
It calculates the size and distance of the habitable zone for each exoplanetary
system that has been discovered and shows which exoplanets orbit in this
so-called "goldilocks" zone. The Habitable Zone Gallery can be found
at www.hzgallery.org . The study describing the research appears in the
Astrobiology journal and is available at http://arxiv.org/abs/1205.2429 .
But not all exoplanets have Earth-like orbits that remain at
a fairly constant distance from their stars. One of the unexpected revelations
of planet hunting has been that many planets travel in very oblong, eccentric
orbits that vary greatly in distance from their stars.
"Planets like these may spend some, but not all of
their time in the habitable zone," Kane said. "You might have a world
that heats up for brief periods in between long, cold winters, or you might
have brief spikes of very hot conditions."
Though planets like these would be very different from
Earth, this might not preclude them from being able to support alien life.
"Scientists have found microscopic life forms on Earth that can survive
all kinds of extreme conditions," Kane said. "Some organisms can
basically drop their metabolism to zero to survive very long-lasting, cold
conditions. We know that others can withstand very extreme heat conditions if
they have a protective layer of rock or water. There have even been studies
performed on Earth-based spores, bacteria and lichens, which show they can
survive in both harsh environments on Earth and the extreme conditions of
space."
Kane and Gelino's research suggests that habitable zone around
stars might be larger than once thought, and that planets that might be hostile
to human life might be the perfect place for extremophiles, like lichens and
bacteria, to survive. "Life evolved on Earth at a very early stage in the
planet's development, under conditions much harsher than they are today,"
Kane said.
Kane explained that many life-harboring worlds might not be
planets at all, but rather moons of larger, gas-giant planets like Jupiter in
our own solar system. "There are lots of giant planets out there, and all
of them may have moons, if they are like the giant planets in the solar
system," Kane says. "A moon of a planet that is in or spends time in
a habitable zone can be habitable itself."
As an example, Kane mentioned Titan, the largest moon of
Saturn, which, despite its thick atmosphere, is far too distant from the sun
and too cold for life as we know it to exist on its surface. "If you moved
Titan closer in to the sun, it would have lots of water vapor and very
favorable conditions for life."
Kane is quick to point out that there are limits to what
scientists can presently determine about habitability on already-discovered
exoplanets. "It's difficult to really know about a planet when you don't
have any knowledge about its atmosphere," he said. For example, both Earth
and Venus experience an atmospheric "greenhouse effect" -- but the
runaway effect on Venus makes it the hottest place in the solar system.
"Without analogues in our own solar system, it's difficult to know
precisely what a habitable moon or eccentric planet orbit would look
like."
Still, the research suggests that habitability might exist
in many forms in the galaxy -- not just on planets that look like our own. Kane
and Gelino are hard at work determining which already-discovered exoplanets
might be candidates for extremophile life or habitable moons. "There are
lots of eccentric and gas giant planet discoveries," Kane says. "We
may find some surprises out there as we start to determine exactly what we consider
habitable."
NASA's Exoplanet Science Institute at Caltech manages time
allocation on the Keck Telescope for NASA. NASA's Jet Propulsion Laboratory in
Pasadena, Calif., manages NASA's Exoplanet Exploration program office. Caltech
manages JPL for NASA. More information about exoplanets and NASA's
planet-finding program is at http://planetquest.jpl.nasa.gov .
Source: http://www.nasa.gov
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