Four types of habitable planets proposed

Water-worlds
The fourth kind of habitable planets are made almost entirely of water. These hypothetical worlds would be Mercury to Earth-sized and would feature extensive oceans.  Unlike oceans on Earth, the water on these types of planets would not be in contact with silicates or other rocks.

"These planets can either be completely made of water with high pressure ice at the core, or they can have bodies of liquid water that are separated from a silicate core by a thick layer of high pressure ice," said Bredehöft.

One theory for life's origin on Earth says organic material collected in shallow pools and then became concentrated by clinging to the surface of rocks. Eventually, this early life spread into the wider ocean. Another theory for life's origin is that the necessary chemistry occurred at hydrothermal volcanic vents.  On water worlds, however, these scenarios are impossible.  Therefore, Bredehöft thinks life is not likely to originate on such planets.

"The amount of water on such a planet would be so huge, you would need unbelievable amounts of carbon components concentrated together for a chance of life. It's far too diluted," said Bredehöft.

Considered opinions
After considering all the facts, Bredehöft said the best bet to find extraterrestrial ecosystems is to hunt for Earth-like planets, after all. However, he doesn't think Earth-like worlds will necessarily have advanced life.

"We don't know whether the level of complexity or the size of organisms living on Earth is essentially a logical outcome of evolution or whether it is just some fluke experienced here," said Bredehöft. "Is having talking intelligent beings on the surface of the planet the pinnacle of evolution? We just assume so because we like to see ourselves as something special."

With the rapid pace of development in planet-hunting technology, it is only a matter of time until we learn much more about exotic extrasolar planets and moons, and are able to glean vital information about their properties. Until then though, scientists like Bredehöft will continue to theorise about discoveries.

So in Bredehöft's carefully considered opinion, what kind of organisms are we most likely to find? "Probably something slimy," he said.

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