A group of French researchers with the Laboratoire de Métrologie Dynamique (CNRS/UPMC/ENS/Ecole Polytechnique) at the Institut Pierre Simon Laplace in Paris have posited Gliese 581d as the first exoplanet possibly able to sustain life.
The team utilized a new atmospheric model, similar to the climate models currently employed to study the Earth’s atmosphere. The team’s findings have indicated that due to its location in the “Goldilocks zone” – that distance from a star where temperatures on a planet allow for the presence of liquid water – the planet may be habitable.
Gliese 581d’s atmosphere, believed to consist primarily of carbon dioxide, is thought to allow for features and weather phenomenon akin to those seen on Earth, such as liquid oceans, clouds and rain.
Discovered in 2007, the rocky Gliese 581d is one of six planets thought to orbit the star Gliese 581. Located approximately 20.3 light years away in the constellation Libra, Gliese 581 is a red dwarf star. Although the most common type of stars in the galaxy, red dwarfs can be more volatile than their larger cousins – because of the variance created by star spots and flares, their energy outputs can vary from almost half their normal average to twice their normal average, sometimes for months at a time. Gliese 581 is a typical red dwarf star, with a mass one third that of our Sun. Unlike the Sun, however, the light emitted by Gliese 581 is generally in the infrared spectrum.
Gliese 581d orbits its star outside its planetary neighbor, Gliese 581g, which was originally considered a stronger candidate for possible habitation. Also thought to be a rocky world, Gliese 581g was discovered in September of 2010, and was believed to be three times larger than the Earth. Gliese 581g’s discovery caused a great deal of excitement, as it orbits in the middle of Gliese 581’s “Goldilocks zone”. However, as other astronomers have had difficulty independently verifying the planet’s existence, there is now some debate regarding the methodology used to find Gliese 581g, raising questions as to whether the planet exists at all.
Physically, although Gliese 581d is twice the Earth’s size, its mass is seven times larger than that of our planet. Gravity’s effect on the face of Gliese 581d would therefore be double that of Earth. Also, although Gliese 581d is closer to its star, it receives only one third of the energy from its star that the Earth receives from the Sun.
Scientists had previously discounted Gliese 581d for a number reasons. First, the planet was considered too far from its red dwarf star, and therefore its temperatures too cold to sustain liquid water. Secondly, Gliese 581d may be tidally locked to its star, meaning it’s always day on one side of the planet and always night on the other. The possibility of the planet being tidally locked seemed to preclude the existence of a moderate climate – any atmosphere significant enough to warm the planet would probably freeze off completely on the night side.
The new model, however, posits that a greenhouse effect – the circulating of warm temperatures on the sunlight side to the dark side by the atmosphere – combined with a thick, carbon atmosphere could allow for temperatures warm enough to maintain liquid water.
Despite being in close cosmic proximity to Earth, however, Gliese 581d’s distance renders it too far for human visitation in the near future. Utilizing current Earth technology, it would take almost a third of a millennium to reach the planet.
Even studying the rocky world has proven difficult, as current telescopes are not powerful enough to observe the planet in sufficient depth. However, as telescope technology improves, Gliese 581d will remain a prime candidate in our quest for exoplanets capable of sustaining life.
Photo Credit: Lynette Cook nasa.gov/topics/universe/features/Gliese_581.html