A terrestrial planet is a planet that is "earth like" in the sense that it is roughly ten thousand kilometers in diameter and has a density of about five grams per cubic centimeter (about five times that of water) because it is composed of rock and/or metal. Mercury, Venus, Earth and Mars are the terrestrial planets in our solar system, with the Earth being the largest one. But the Earth is no longer the largest known terrestrial planet. Over the past few years, astronomers have detected five "super-earths" -- planets with a masses between one and ten earth-masses -- around other stars. Astronomers working to understand whether the Earth is in any way unique, and how it formed, are excited by the prospect of being able to study super-earths to help sort out possible competing models for the Earth. Since it appears that super-earths are common, our solar system may be unusual only in not having any super-earths among its terrestrial planets.
No information is yet available on the diameters of those five extra-solar planets, which might in the end not really be rocky terrestrial planets after all. Neither are scientists sure about the orbits of these super-earths, although it is certainly possible that one of them, or another as-yet undiscovered super-earth, lies in the "habitable zone" around its star where water can be liquid and temperatures are suitable for life. On the Earth, plate tectonics is active, and it has even been suggested that this activity was required for the development of life on Earth. A team of three scientists including CfA astronomer Dimitar Sasselov is suggesting that plate tectonics is inevitable on super-earths.
The scientists analyzed the structure of the Earth's surface, its material stresses, heat flow, and other parameters affecting plate tectonics. What is unusual in their discussion is that they identify how these various processes depend on the planet's mass. They conclude that plate tectonics is likely to be more vigorous in massive terrestrial planets; conversely, they suggest smaller mass is the reason why Mercury and Mars do not exhibit plate tectonics. If super-earths commonly have plate tectonics, any that happen to reside in a habitable zone would be particularly attractive targets for study.