On Tuesday, astronomers announced the discovery of Gliese 581 C, an "Earthlike" planet 20 light-years outside our solar system. The New York Times, among other news organizations, repeated the word Earthlike in headlines. What does it mean to say that a planet is "Earthlike"?
It's small, rocky, not too hot, not too cold, and it might be able to support life. In general, an Earthlike planet should be no more than 10 times as massive as the Earth, but big enough that its atmosphere doesn't drift off. (The newly discovered planet is roughly five Earth masses.) For comparison, Jupiter is 318 * times Earth's mass. The planet also needs to have a rocky core. Big balls of gas like Jupiter aren't likely to be conducive to living organisms. But most importantly, to be capable of supporting life a planet must have a surface temperature that can sustain water in liquid form. (We can't know for sure, but it's safe to assume that alien life-forms would need water to survive.)
A planet's temperature is determined by a planet's distance from the star around which it orbits: It should be close enough that water doesn't freeze, but far enough away that it doesn't boil off. (The star in question, Gliese 581, is dim compared to our sun, but the planet orbits close enough to it that water can probably maintain liquid form.) Planets such as Neptune, a giant ball of ice and rock surrounded by a thick envelope of gas, could never support life, since so little heat penetrates its atmosphere. In the case of Gliese 581 C, scientists have a good idea of the planet's temperature and mass but can only speculate about its composition.
So, how do they know if a planet fits these guidelines? Scientists first determined the mass of Gliese 581 C by observing the light emitted by its star. As the planet spins around, it tugs the star in different directions, creating a subtle variation in its wavelength, or color. (This is referred to as the star's "wobble.") The exact pattern of changes depends on the mass of the planet.
Once the astronomers had figured out the mass of Gliese 581 C, they then used one of Kepler's laws to figure out its distance from the star. That gave them a rough estimate of the planet's temperature—anywhere from 0 to 40 degrees Celsius. Where the true temperature lies along that spectrum depends on how much light the planet reflects, which in turn depends on its composition. But you can't tell what a planet's made of unless you know how dense it is. To get the density, you have to figure out the volume, which usually requires watching the planet "transit" across its star—a partial eclipse that lets you measure its size. Astronomers can then compare the measured density to the known values for substances like rock, iron, and hydrogen. A good sense of the planet's composition helps them make a better guess about its temperature, and the likelihood that it can support life. However, it's very unlikely that we'll ever see Gliese 581 C transiting across its star, so scientists may not be able to determine its composition.
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Explainer thanks David Charbonneau and Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics and Dr. Jo Pitesky of NASA.