NASA's Kepler mission has discovered two new planetary systems that include three super-Earth-size planets in the habitable zone. Courtesy: NASAexplore

EARTH-like planets are a common trope in science fiction, cropping up as frequently as zappy laser guns. From Star Trek to Doctor Who, characters always seem to land on planets which support human life.

But finding Earth-like planets has been a tough ask for astrophysicists. Many exoplanets discovered to date are just desolate chunks of cold rock, or worse - gas giants.

Previously, most of the exoplanets labelled 'super Earths' (as they are slightly larger than our own planet) have also been regarded as uninhabitable, due to their liquid surface.

But new research by Dr Nicolas Cowan of Northwestern University in Illinois suggests that some of these super-Earths will contain land masses as well as oceans, partly because of the pressure the liquid would exert on the planets' tectonic plates.

On his website, Dr Cowan explains that he "constructs maps of these distant worlds by monitoring how their brightness and colour change with time".

"Large terrestrial planets are expected to have muted topography and deep oceans, implying they should be entirely covered in water, so-called waterworlds," Dr Cowan wrote in a recent paper for a meeting of the American Astronomical Society.

By modelling the movement of water between the Earth's "surface reservoirs" (the oceans) and "interior reservoir" (the planet's mantle), and extrapolating that data for larger worlds, Dr Cowan concluded that "tectonically active terrestrial planets" would feature "both oceans and exposed continents".

Having exposed continents is a good thing for the development of a livable climate, he said.

"The temperate climate on Earth is not just because of liquid water, but because of exposed continents," he told the AAS meeting.

But before we start developing plans for a colony on one of these worlds, there will be at least one big problem: hefty gravity.

Gravity that is three times stronger than that on Earth may make human habitation of alien worlds difficult, if not impossible, but Cowan told SPACE.com that it did not rule out the possibility of other life developing.

"I don't think 3-G is a big problem for habitability ... fighter pilots can handle it," he said.

Dr Cowan described his theory as a "shot from the hip" but said it was "an important step in advancing how we think about super-Earths".

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