The Importance of Gliese 581g

Uploaded by tdarnell on 12.10.2010

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
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