New ‘Gas Dwarf’ Class of Alien Planets Revealed

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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“Determining the composition of a planet — and whether it is a gas giant or an enormous rocky world — requires knowing its mass. But mass is especially difficult to calculate with smaller worlds, such as the rocky bodies with the potential to host life.”

So, how did they determine the mass accurately? Doppler shift of star? Gravitational effect on other planets in orbit? Last I heard, the Doppler shift of Kepler stars was too small to measure accurately enough.