This artist's concept shows a Super Venus
planet on the left, and a Super Earth on the right. Researchers use a
concept known as the habitable zone to distinguish between these two
types of planets, which exist beyond our solar system. Image credit:
NASA/JPL-Caltech/Ames. › Full image and caption
There is only one planet we know of, so far, that is drenched with
life. That planet is Earth, as you may have guessed, and it has all the
right conditions for critters to thrive on its surface. Do other planets
beyond our solar system, called exoplanets, also host life forms?
Astronomers still don't know the answer, but they search for potentially
habitable planets using a handful of criteria. Ideally, they want to
find planets just like Earth, since we know without a doubt that life
took root here. The hunt is on for planets about the size of Earth that
orbit at just the right distance from their star - in a region termed
the habitable zone.
NASA's Kepler mission is helping scientists in the quest to find these
worlds, sometimes called Goldilocks planets after the fairy tale because
they orbit where conditions are "just right" for life. Kepler and other
telescopes have confirmed a handful so far, all of which are a bit
larger than Earth -- the Super Earths. The search for Earth's twin, a
habitable-zone planet as small as Earth, is ongoing.
An important part of this research is the continuing investigation into exactly where a star's habitable zone starts and stops.
The habitable zone is the belt around a star where temperatures are
ideal for liquid water -- an essential ingredient for life as we know it
-- to pool on a planet's surface. Earth lies within the habitable zone
of our star, the sun. Beyond this zone, a planet would probably be too
cold and frozen for life (though it's possible life could be buried
underneath a moon's surface). A planet lying between a star and the
habitable zone would likely be too hot and steamy.
That perfect Goldilocks planet within the zone wouldn't necessarily be
home to any furry creatures. But it would have the potential for some
type of life to abound, if even microbes.
In one new study, researchers based at NASA's Exoplanet Science
Institute at the California Institute of Technology, in Pasadena,
Calif., carefully analyzed the location of both a planet called
Kepler-69c and its habitable zone. Their analysis shows that this
planet, which is 1.7 times the size of Earth, lies just outside the
inner edge of the zone, making it more of a Super Venus than a Super
Earth, as previous estimates indicated.
"On the way to finding Earths, Kepler is telling us a lot about the
frequency of Venus-like planets in our galaxy," said Stephen Kane, lead
author of the new paper on Kepler-69c appearing in the Astrophysical
Journal Letters.
To determine the location of a star's habitable zone, one must first
learn how much total radiation it emits. Stars more massive than our sun
are hotter, and blaze with radiation, so their habitable zones are
farther out. Similarly, stars that are smaller and cooler sport tighter
belts of habitability than our sun. For example, the Super Earth planet
called Kepler-62f, discovered by Kepler to orbit in the middle of a
habitable zone around a cool star, orbits closer to its star than Earth.
The planet takes just 267 days to complete an orbit, as compared to 365
days for Earth.
Knowing precisely how far away a habitable zone needs to be from a star
also depends on chemistry. For example, molecules in a planet's
atmosphere will absorb a certain amount of energy from starlight and
radiate the rest back out. How much of this energy is trapped can mean
the difference between a turquoise sea and erupting volcanoes.
Researchers led by Ravi kumar Kopparapu of Penn State University,
University Park, Pa., used this type of chemical information to nudge
the habitable zone out a bit farther than previously thought. The team's
2013 Astrophysical Journal study is the current gold standard in
determining how a star's total radiation output relates to the location
of its habitable zone. Kane and his colleagues used this information to
fine-tune the boundaries of Kepler-69c's habitable zone, in addition to
careful measurements of the star's total energy output and the orbit of
the planet.
"Understanding the properties of the star is critical to determining
planetary properties and calculating the extent of the habitable zone in
that system," said Kane.
But before you purchase real estate in a habitable zone, keep in mind
there are other factors that dictate whether a world develops lush
greenery and beaches. Eruptions from the surfaces of stars called
flares, for example, can wreak havoc on planets.
"There are a lot of unanswered questions about habitability," said
Lucianne Walkowicz, a Kepler science team member based at Princeton
University, N.J., who studies flaring stars. "If the planet gets zapped
with radiation all the time by flares from its parent star, the surface
might not be a very pleasant place to live. But on the other hand, if
there's liquid water around, that makes a really good shield from
high-energy radiation, so maybe life could thrive in the oceans."
Flares can also scrape off the atmospheres of planets, complicating the
picture further. This is particularly true for the smaller, cooler
stars, which tend to be more hyperactive than stars like our sun.
Ideally, astronomers would like to know more about the atmosphere of
potentially habitable planets. That way they could look at the planet's
molecular makeup for signs of runaway greenhouse gases that could
indicate an inhospitable Venus-like planet. Or, future space telescopes
might even be able to pick up signatures of oxygen, water, carbon
dioxide and methane -- indicators that the planet might be somebody's
home.
NASA's upcoming James Webb Space Telescope will bring us closer to this
goal, by probing the atmospheres of planets, some of which may lie in
habitable zones. The mission won't be able to examine the atmospheres of
planets as small as Earth, so we'll have to wait for another future
telescope to separate out the Venuses from the Earths.
NASA Ames manages Kepler's ground system development, mission operations
and science data analysis. NASA's Jet Propulsion Laboratory in
Pasadena, Calif., managed Kepler mission development. Ball Aerospace
& Technologies Corp. in Boulder, Colo., developed the Kepler flight
system and supports mission operations with JPL at the Laboratory for
Atmospheric and Space Physics at the University of Colorado in Boulder.
The Space Telescope Science Institute in Baltimore archives, hosts and
distributes the Kepler science data. Kepler is NASA's 10th Discovery
Mission and is funded by NASA's Science Mission Directorate at the
agency's headquarters in Washington. More information about the Kepler
mission is at http://www.nasa.gov/kepler .
More information about exoplanets and NASA's planet-finding program is at http://planetquest.jpl.nasa.gov .
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov
