FINDING planets with a mass similar to Earth’s in habitable zones around other stars is a top priority for astronomers on the lookout for extraterrestrial life. But Marc Kuchner of Princeton University Observatory warns that not all such planets will be rocky worlds like Earth or Mars.
In the cold outer regions of a planetary system, planets that are up to 10 times the mass of Earth and made mainly of water ice and frozen ammonia can form. Even if such a body migrated inwards to a warm spot near their star like Earth’s in relation to the sun, it…
