Earthlike Look Alike

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 othOn 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 100 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.

er 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 100 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.

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