Gliese 581g

Yesterday, NASA announced the exciting news of a potential habitable exoplanet.

To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one where humans would thrive. Habitability depends on many factors, but having liquid water and an atmosphere are among the most important.

The new findings are based on 11 years of observations of the nearby red dwarf star Gliese 581 using the HIRES spectrometer on the Keck I Telescope in Hawaii. The spectrometer allows precise measurements of a star’s radial velocity (its motion along the line of sight from Earth), which can reveal the presence of planets. The gravitational tug of an orbiting planet causes periodic changes in the radial velocity of the host star. Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to detect planets and determine their orbits and masses.

The new planet, designated Gliese 581g, has a mass three to four times that of Earth and orbits its star in just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and enough gravity to hold on to an atmosphere. Gliese 581, located 20 light years away from Earth in the constellation Libra, has two previously detected planets that lie at the edges of the habitable zone, one on the hot side (planet c) and one on the cold side (planet d). While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are skeptical.

(Those familiar with the Academy's originally produced planetarium show Fragile Planet—which opened in 2008—will be familiar with Gliese 581d, visualized as a potentially habitable planet.)

Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz, who led the Exoplanet Survey, had this to say about the discovery:

Our findings offer a very compelling case for a potentially habitable planet. The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.

One very interesting feature of the planet is that it’s tidally locked to its star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star lies in perpetual darkness. This could stabilize the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the boundary between shadow and light.

Universe Today gives us more details:

The researchers estimate that the average surface temperature of the planet is between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius). Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side…

The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright on the planet, Vogt said.

Vogt goes on to say in Universe Today that:

If these are rare, we shouldn’t have found one so quickly and so nearby… There could be tens of billions of these systems in our galaxy.

Cool!