When studying the Earth, the sample size (i.e. one) is a serious limitation. The study of other Solar system objects can give us some insight. However, the sample size is still small and unavoidably the Earth is unique in the Solar system in many ways. For example it is the only body with currently active plate tectonics. The lithospheres of Mars and Venus, are not thought to be undergoing any active plate tectonics. The study of exoplanets, i.e. planets orbiting other stars, would definitely solve the sample size issue. When the class of 2012 geology graduates were born, hardly any exoplanets were known. As of yesterday Wikipedia listed 687 of them, and many more are being identified by the Kepler mission. The problem is, only a few characteristics of these planets can be deduced, and plate tectonics is not one of them. Of course, we can use models, initially analytical ones and then run computer simulations based on them.
A particularly interesting class of planets are Super-Earths. These are terrestrial planets with a mass up to ten times greater than the Earth's. Planets more massive than this are thought not to be terrestrial. Models of Super-Earths have produced mixed results on the plate tectonics question. A new paper by van Heck and Tackley, published at Earth and Planetary Science Letters and made available online yesterday, suggests that plate tectonics on Super-Earths, are either equally or even more probable than on Earth, depending on how the planet's mantle is heated. If this result stands, then we are one small step closer to figuring out plate tectonics on the Earth and one small step further from a rare Earth. However, whichever way it turns out, it will be cool.
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