Only about 20 light years away ? practically in our backyard ? an Earth-like planet was recently discovered
by Stéphane Udry of Geneva and collaborators
, using a technique that measures a star's tiny shifts in velocity towards and away from us
Those tiny shifts are caused by a planet, and their timing tells us the period of the planet's orbit. For the Earth, that period would be one year. For this planet, that period is about 13 days. With such a short period, it's possible to collect data for many repeating orbits, and build confidence in the signal over time. If astronomers know the mass of the central star (usually from the spectral type of the star), then the amplitude of the velocity shift reveals the minimum mass of the planet. In this case, that mass is about 5 times the Earth's mass.
Then, if you assume what the planet is made out of (and therefore guess its average density), you can estimate its volume, and therefore its size. So if it's made out of rock, the planet is about 1.5 times the size of Earth; if it is made out of water ? which is not as dense as rock ? it would be bigger.
With such a short period, you might guess that the planet is very close to its central star and you'd be right. But this central star is not like the Sun. It is much cooler and it is less massive than the Sun. So it's possible this planet might be habitable.
You might be wondering how anybody knows the temperature of the planet if they can't see the planet at all. It's easy to estimate the temperature of an airless rock of some size orbiting a star at a constant distance. The main assumption there is a guess at how reflective the rock is. From the size of the rock, its distance from the star, the radiation emitted by the star, and the reflectivity of the rock, scientists can estimate the equilibrium temperature.
The equilibrium temperature is that temperature that the rock settles on, once the radiation that is absorbed by the rock equals the radiation that is emitted by it. The calculation gets a little bit more complicated once you include an atmosphere with greenhouse gases. Such gas allows optical light to pass through to the ground, but absorbs the infrared light emitted by the planet. That gas thus slows the speed with which the planet can radiate energy back into space, and thereby increases the equilibrium temperature of the planet.
It's this greenhouse effect
that allows the Earth to have a quite lovely average equilibrium temperature as opposed to below freezing; it's this same greenhouse effect ("set to 11" as a certain pseudo-heavy metal band
would say) that transforms Venus's surface into an unsurvivable vision of hell
So the crucial question of whether this new planet has the right conditions for liquid water hinges on its atmosphere. If it utterly lacks an atmosphere, the surface can't maintain liquid water no matter what temperature it is. Like the surface of the Moon, the lack of vapour pressure would cause any liquid water to quickly disperse. If it has an atmosphere, the question is whether the greenhouse gases are just enough to allow a hospitably warm surface like Earth. Excessive greenhouse gases result in a Venus.
The main excitement is that this planet is the smallest planet discovered to date, and that this planet, because of its distance from its central star, has the potential to have the right conditions for liquid water. But I wouldn't hop on my interstellar spaceship just yet. One hundred trillion miles is a long way to go just to find out there's nothing there to drink.Megan Donahue, Michigan State University astrophysicist (Illustration: ESO)
Labels: atmosphere, exoplanet, extrasolar, Gliese 581, greenhouse effect, radial velocity