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What Kinds of Planets Are Out There?
09.24.07
 
In the Star Wars movies, we see fictional planets covered with forests, oceans, deserts, and volcanoes. But what kinds of planets are actually out there?

This is an artist's concept of an Earthlike planet around another star. Image right: This is an artist's concept of an Earthlike planet around another star. Credit: NASA JPL

Until the 1990s, astronomers didn’t know about a single planet outside our solar system. They were forced to base their ideas largely on the planets they already knew about — the ones orbiting our Sun. The planets in our solar system exhibit a wide range of diversity, from giant gas-dominated worlds like Jupiter and Saturn to small, barren balls of rock like Mercury. Still, astronomers’ knowledge was limited, and envisioning new types of worlds fell mostly in the domain of science fiction.

But all that has changed over the past decade. Astronomers have found more than 230 planets outside the solar system, and many of them are wildly different from those in our solar system. But most of these worlds are giant planets. What about worlds the size of Earth? The lesson of the new planets is that astronomers need to broaden their imaginations.

A team of four astronomers has done just that. They developed computer models to describe 14 different types of solid planets that might be orbiting other stars.

This image shows the relative sizes of six different kinds of planets with different compositions, and depending on whether they have the same mass as Earth, or five times the mass of Earth. Image left: Astronomers have calculated the diameters of various types of planets given certain compositions and masses. This image shows the relative sizes of six different kinds of planets with different compositions, and depending on whether they have the same mass as Earth, or five times the mass of Earth. Note that the 5-Earth-mass planets are larger than their 1-Earth-mass counterparts, but they are not five times larger due to the gravitational compression that occurs when a planet's mass is increased. The planets are shown silhouetted against the Sun, as if they are transiting planets seen from afar. Credit: Marc Kuchner/NASA GSFC.

"We’re thinking seriously about the different kinds of roughly Earth-size planets that might be out there, like George Lucas, but for real," explains team member Marc Kuchner of NASA’s Goddard Space Flight Center in Greenbelt, Md. "We let our imaginations run wild and tried to cover all the bases with our models of smaller planets."

The team studied 14 different types of planets, with compositions ranging from pure water to pure iron. Other types are made of pure carbon or silicates, to mixtures of various compounds. The four astronomers were interested in finding out how big a planet would be for a certain amount of mass. Mass is a measure of the total amount of material an object contains. A 5-Earth-mass planet, for example, contains five times more total material than Earth.

The team calculated how the inward pull of gravity would compress planets of varying compositions. The resulting computer models show that a planet with the exact same mass as Earth, but made of pure water, will be about 9,500 miles across. An iron planet with the same mass will be only about 3,000 miles in diameter. For comparison, Earth, which is made mostly of silicate rock, is 7,926 miles across at its equator.

These theoretical models plot a planet's size and mass given a certain composition. Image right: These theoretical models plot a planet's size and mass given a certain composition. Future observations might be able to distinguish a pure water planet from a pure iron planet, but might have difficulty distinguishing a carbon planet from a silicate planet, for example. Click here to download an unlabeled version of this image. Credit: Marc Kuchner/NASA GSFC.

The team hopes that these models will yield insights into planet compositions when astronomers start finding Earth-sized planets around other stars. Missions such as the French Corot satellite, which launched on December 27, 2006, and NASA’s Kepler spacecraft, scheduled to launch in 2009, can find planets not much larger than Earth by watching them pass in front of their host stars, where they block a portion of the star’s light.

Astronomers can follow up the discovery to measure the planet’s size and mass. By comparing a planet's size and mass, astronomers can use these new computer models to help determine whether the planet is mostly water ice or mostly iron, for example. As Kuchner points out, "Ultimately, we need observations to give us the answers."

The other team members are Sara Seager of the Massachusetts Institute of Technology in Cambridge, Mass., Catherine Hier-Majumder of the Carnegie Institution of Washington, (deceased), and Burkhard Militzer, also at Carnegie. The paper describing the group’s research is scheduled to appear in the October 20 issue of the Astrophysical Journal.

 
 
Bob Naeye
Goddard Space Flight Center