Earth-Size Planets Come in Two Flavors: Rocky or Gassy

Earth-size planets circling nearby stars come in two flavors, either rocky or gassy, astronomers reported on Monday. And more than three-quarters of stars likely host at least one of these alien Earths. (See also: "Earth-Like Worlds 'Very Common.'")

"We are talking about worlds barely larger than our own," says astronomer Geoff Marcy of the University of California, Berkeley, speaking at the American Astronomical Society meeting in Washington, D.C. "That's how far we have come." (See "MacArthur Genius Searching for Signs of Life on Exoplanets.")

When astronomers began reporting the discovery of planets orbiting nearby stars in 1995, the few worlds they detected were as large or larger than Jupiter. Now measurements from NASA's Kepler space telescope—which has discovered 237 of the more than 1,000 planets detected, according to Michele Johnson of NASA's Ames Research Center in Mountain View, California—are helping us learn what worlds in Earth's weight class are made of.

The findings narrow the range of planets on which we might expect to see alien life, Marcy says, to the smaller ones nearest to Earth in size. However, none of the planets reported in the new Kepler data orbited well enough inside the "habitable zone" of their stars to be amenable to oceans and life, he noted. (Related: "Newfound Earth-Size Exoplanet Doomed.")

On the plus side, that still leaves a lot of planets for future alien hunters to investigate, as roughly three-quarters of the 3,538 still-unconfirmed candidate planets detected by Kepler appear to be Earth-size, Marcy reported at the meeting. And roughly one in five stars are orbited in their habitable zone by a planet one to two times as wide as Earth.

Planetary Dividing Line

"We are finding a dividing line between two classes of Earth-size planets," says astronomer Yoram Lithwick of Northwestern University in Chicago, who presented a study of 60 "Super-Earth" worlds, ones roughly one to four times as wide as Earth. "Many are even fluffier than Neptune and Uranus."

In the study presented by Lithwick and a separate study of 42 planets presented by Marcy, the cutoff is between planets more or less than two times as wide as Earth.

Those less than two times as wide as Earth are either rocky or are draped with an outermost layer of cloudy hydrogen and helium gas haze, while those more than two times as wide as our planet all have densities that suggest they are gassy worlds. These "mini-Neptunes" likely look more like Uranus and Neptune in our solar system.

Planetary Chemistry

What causes the dividing line between rocky Super-Earths and mini-Neptunes? Basically, rocky planets can't get much wider than twice the size of Earth, says Marcy. Once they reach that size, added rock is just compressed further, making the planet more dense but leaving it at the same width.

Gassy worlds, in contrast, grow wider as more gas is added, because the thin gas allows them to balloon out.

"It's a reasonable conclusion, supported by theory, and the observations seem solid," says astronomer Stephen Maran, author of Astronomy for Dummies. "The thing that really stands out is that we live in an oddball solar system that doesn't have one of these mini-Neptunes."