BOSTON — For astronomers, planets around other stars tend to come in two basic types: rocky worlds and gas giants. Now, scientists have identified a third class of exoplanets, called "gas dwarfs," that fall in between the others. These gas-dwarf alien planets have thick atmospheres like their larger gas-giant cousins but never quite made it to the size of the planetary behemoths found in the Earth's the outer solar system, researchers said. The team studied more than 600 planets discovered by NASA's Kepler space telescope and compared their sizes to the amount of elements other than hydrogen and helium contained in their star — a characteristic known as metallicity.
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