In the Star Wars movies, fictional planets are covered with forests, oceans, deserts and volcanoes. But new models from a team of MIT, NASA and Carnegie Institution scientists begin to describe an even wider range of Earth-size planets that astronomers might actually be able to find in the near future.
Sara Seager, the Ellen Swallow Richards Associate Professor in the Department of Earth, Atmospheric and Planetary Sciences, and colleagues have created models for 14 different types of solid planets that might exist in our galaxy. The 14 types have various compositions, and the team calculated how large each planet would be for a given mass. Some are pure water ice, carbon, iron, silicate, carbon monoxide and silicon carbide; others are mixtures of these various compounds.
A paper on the work appeared in the Oct. 20 issue of the Astrophysical Journal.
"We're thinking seriously about the different kinds of roughly Earth-size planets that might be out there, like George Lucas, but for real," said Marc Kuchner of NASA's Goddard Space Flight Center.
The team took a different approach from previous studies. Rather than assume that planets around other stars are scaled-up or scaled-down versions of the planets in our solar system, they considered all types of planets that might be possible, given what astronomers know about the composition of protoplanetary disks around young stars.
"We have learned that extrasolar giant planets often differ tremendously from the worlds in our solar system, so we let our imaginations run wild and tried to cover all the bases with our models of smaller planets," said Kuchner. "We can make educated guesses about where these different kinds of planets might be found. For example, carbon planets and carbon-monoxide planets might favor evolved stars such as white dwarfs and pulsars, or they might form in carbon-rich disks like the one around the star Beta Pictoris. But ultimately, we need observations to give us the answers."
The team calculated how gravity would compress planets of varying compositions. The resulting computer models predict a planet's diameter for a given composition and mass. For example, a 1-Earth-mass planet made of pure water will be about 9,500 miles across, whereas an iron planet with the same mass will be only about 3,000 miles in diameter. For comparison, Earth, which is made mostly of silicates, is 7,926 miles across at its equator.
Some of the results were expected, such as the fact that pure water planets (similar to the moons of the outer planets in our solar system, which consist mostly of water ice) were the least dense of the solid planets, while pure iron planets were the densest. But there were some surprises. The team discovered that no matter what material a planet is made of, the mass-diameter relationship follows a similar pattern.
"All materials compress in a similar way because of the structure of solids," explained Seager. "If you squeeze a rock, nothing much happens until you reach some critical pressure, then it crushes. Planets behave the same way, but they react at different pressures depending on the composition. This is a big step forward in our fundamental understanding of planets."
For more information, please visit www.nasa.gov/centers/goddard/news/topstory/2007/earthsized_planets.html.
A version of this article appeared in MIT Tech Talk on October 31, 2007 (download PDF).