Astronomers have yet to find an Earth-size planet beyond our solar system, but that hasn't stopped them from modeling what these worlds might look like.
A new catalog of 14 types of such planets, some fantastical, could help planet hunters spot what has until now remained fictional.
The computer models provide specs for 14 planet types, varying according to mass, diameter, composition and where the worlds could be found in our galaxy. Some are made mostly of pure water ice, carbon, iron, silicate, carbon monoxide or silicon carbide, while others are mixtures of these various compounds.
"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 in Greenbelt, Md.
The work will be detailed in the Oct. 20 issue of the Astrophysical Journal.
Rather than assuming faraway worlds around other stars would be big or small versions of planets in our solar system, the astronomers considered all possible types of planets given what they know about the composition of protoplanetary disks, the wells of raw material that form around young stars.
So far, most of the more than 250 planets discovered beyond our solar system are huge, Jovian-like worlds. So scientists know something about the range of possibilities with giant planets, but they know next to nothing about possible Earth-like and potentially solid or watery planets except what has been learned from our solar system.
"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," Kuchner said.
By modeling how gravity would compress a planet of a certain composition and mass, they predicted each planet's diameter, finding that no matter their composition, the planets followed a similar relationship between mass and diameter.
"All materials compress in a similar way because of the structure of solids," said study team member Sara Seager of MIT. "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."
The team found a pure water planet weighing the equivalent of one Earth will span about 9,500 miles across (15,289 kilometers), while an iron planet of the same mass will be compressed to a diameter of just 3,000 miles (4,828 kilometers). Earth, made up mostly of silicates, is 7,926 miles (12,755 kilometers) across at its equator.
Where in the world ...
The team also provided rough guidelines for possible hideouts of the planet types. "We can make educated guesses about where these different kinds of planets might be found," Kuchner said.
For example, carbon planets and carbon-monoxide planets might take up residence around evolved stars such as white dwarfs and pulsars, or they might form in carbon-rich debris disks like the one around the young star Beta Pictoris.
With a slew of planet-hunting projects and missions launched or in the queue, the researchers hope when astronomers start finding Earth-size planets, these models will yield insights into planet compositions based on size and mass information.
Snags in the model, of course, have already emerged. While the models will work well in distinguishing between a pure water planet and one composed of iron, they could mix up silicate planets with carbon planets, say the researchers. That's because silicate and carbon planets have very similar masses at a given diameter.
Observatories yet to launch, such as NASA's James Webb Space Telescope or Terrestrial Planet Finder, could provide finer details to help decipher the chemical compositions.