This artist's depiction shows a possible Earth-like planet with a rocky surface, oceans and an atmosphere. See more space tourism images.

模拟图描绘了类地行星，有岩石表面、海洋和大气。

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The PHI looks at a different set of factors, such as whether the world has a rocky or frozen surface, whether it has an atmosphere or a magnetic field.

而行星的宜居指数（PHI）则考察另外的一组因素，如该星球是否有岩石或冰冻的表面，是否有大气层或磁场的存在。

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How often the Universe has planets truly like the earth - true earth twins, planets the size of the earth, rocky planets with a surface that supports liquid water - well, we don't have that number yet, but I'm optimistic we'll figure it out.

BBC: From counting to characterising exoplanets

The severity of the laboratory test was equivalent to an oblique collision with the rocky surface of the Earth - a comet coming in at an angle of less than 25 degrees from the horizon, rather than head on (perpendicular to the Earth's surface).

BBC: Comets could have seeded life on Earth

Neutrons ejected from a watery surface move much more slowly than those from a rocky one, because the hydrogen nuclei in water are light enough for an exiting neutron to knock around much like one billiard ball hitting another a process that slows the neutron down.

ECONOMIST: Divining water