To date, astronomers have discovered nearly
500 exoplanets. These are planets outside of our solar system, orbiting other stars.
So far, most of them have been very large, Jupiter-sized worlds - gas giants with diameters
and masses more closely resembling a star than a planet. As our techniques for finding exoplanets improve,
we've been able to detect smaller and smaller worlds. Recently, astronomers have found several
planets that are less than 10 Earth-masses. These have been commonly called super-earths,
and while they are candidates for finding life elsewhere in the galaxy, they are not
ideal. The holy grail of exoplanet research has been
the detection of Earth-sized planets orbiting in the habitable zone of stars. Astronomers
define a habitable planet as one that is close to the size of the earth, and that lies within
a distance that allows for the presence of liquid water. In late September 2010, astronomers announced
the discovery of just such a planet: Gliese 581g, a rocky planet three times the mass
of the Earth orbiting just 17 million miles, 27 million kilometers, from it's home star,
revolving in an orbit with a year of only 37 days. Gliese 581, the parent star of this new planet,
has been under survey at the Keck Observatory for over a decade. It is an M3V dwarf star
lying a distance of 20 light years away from us. It is a cool star, its photons bathing
all of its planets in a red glow. It is much, much smaller than our Sun, containing only
0.3 solar masses, 30% that of our star and it is only 1% as bright. Because of its cool temperatures and small
mass, the habitable zone of Gliese 581 lies much closer than ours does from our sun. The
Earth lies 93 million miles - 150 million kilometers - from our Sun. Orbiting our Sun
at this distance, liquid water is possible. At Gliese 581, that distance is only 14 million
miles, or 22 million kilometers - much, much closer. Gliese 581 is home now to at least six worlds,
each orbiting the star at 3.2, 5.4, 12.9, 37, 67 and 433 days respectively. Of these,
the 37 day planet is the most intriguing. It is measured to be 3.1 times more massive
than the earth, with a surface gravity between 1.1 and 1.7 times that of our home planet.
We would weigh up to 70 percent more on Gleise 581g. What's more, the orbit of the star matches
its rotation rate, meaning that one side of the planet is always facing the star, while
the other has a perpetual view of the night sky. While this may have an effect on it's
habitability, it is still possible for life to exist in such a place. There are many factors that affect habitablity,
the distance from the star is but one of these. A planet may not have formed or retained any
water, gravity may be too weak to hold an atmosphere or a planet may have accreted a
massive hydrogen or helium envelope keeping the surface pressure too high to prevent water
in liquid form. None of these factors appear to be in play
on Gleise 581g, at least with what we've learned so far. The planet might have an active geological
cycle to replenish CO2, the mass of the planet appears to satisfy the gravity conditions,
so an atmosphere is possible. And finally, an equally important consideration is surface
temperature. The earth has a surface temperature of 255 Kelvin with the greenhouse effect warming
the surface to a globally-averaged 288 Kelvin, that's15 celsius and 59 degreesFahrenheit. Another important factor of habitability is
surface temperature. The temperature on Gleise 581g, if we assume a greenhouse effect similar
to that on Earth, is a cooler 245 Kelvin, -19 fahrenheit, -28 celsius. Gliese 581 g easily meets the requirements
for habitability. With the surveys done so far, we now know of two habitable systems
in our stellar neighborhood, Gliese 581 g and us. It is extremely remarkable that we have found
such a planet so early in our search. If confirmed by independent observations, Gleise 581g would
be the first world ever found, where life could actually exist besides Earth. Finding
a habitable planet this soon among the few hundreds of stars in our local neighborhood,
has important implications for the number of habitable systems in our galaxy. It implies
that either we were incredibly lucky, or that there are many more habitable systems than
we ever thought. Of the 100 billion stars in our galaxy, up
to 20-30 percent may harbor at least one habitable planet.
Awesome.
Chills.