NASA, MIT and Carnegie scientists have begun to describe a wide range of Earth-sized planets in the expectation that models will yield insights into planet compositions when astronomers start finding Earth-sized planets around other stars.
The team of scientists have created models for 14 different types of solid planets that have varying compositions and might actually exist in our galaxy. Calculations of how large each planet would be were made for various given composition masses. Some compositions modeled are pure water ice, carbon, iron, silicate, carbon monoxide, and silicon carbide; others are mixtures of these various compounds.
Marc Kuchner of NASA Goddard Space Flight Center said: “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…We can make educated guesses about where these different kinds of planets might be found…such as white dwarfs and pulsars, or… carbon-rich disks like the one around the star Beta Pictoris.”
Calculations were made of how gravity would compress planets of varying compositions. The computer models that resulted from the calculations predict a planet’s diameter for a given composition and mass. For example, Earth has a diameter of 7,926 miles across at its equator and is composed mostly of silicates.
Some of the results from the models were expected 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.
Sara Seager of Massachusetts Institute of Technology explained that “All materials compress in a similar way because of the structure of solids… 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.”
The hope is that these models will help to yield insights into planet compositions when astronomers start finding Earth-sized planets around other stars. By employing these models and comparing a discovered planet’s size and mass, astronomers might be able to determine its composition, whether water ice, iron, carbon or silicate, etc.
“To make this [fine a] distinction, we will need some help from NASA’s James Webb Space Telescope or Terrestrial Planet Finder,” says Kuchner. “With these instruments, we could take spectra of Earth-mass planets which will tell us about their chemistries.”
Research team members are: Sara Seager, Massachusetts Institute of Technology, Cambridge, Mass.; Marc Kuchner, NASA Goddard Space Flight Center, Greenbelt, Md.; Catherine Hier-Majumder, Carnegie Institution of Washington, (deceased); and Burkhard Militzer, Carnegie.
The team’s paper is currently scheduled to appear in the October 20, 2007 issue of the Astrophysical Journal.
“Scientists Model a Cornucopia of Earth-sized Planets,” NASA.