Four proposed types of habitable worlds: Earth-like, Mars-like, Europa-like, and water worlds

Artist's representation of an exoplanet. Credit:NASA/JPL-CalTech

Interesting read here on space.com about a likely four major types of habitable planets. The first thing I noticed was that no attention was paid to the largely unknown category of super-Earths, but since we know next to nothing about these planets it's almost impossible to speculate at the moment. One of the posters below mentioned the same thing, writing:

They still have a narrow focus. And where does the "Super Earths" category fall into the "Four Types of Habitable Planets"? Or Moons that are large enought, or analogus to "Terrestial Planets"?

Regarding moons, yes, it's technically true that some of these bodies may be moons orbiting other planets, but here the word planet seems to just refer to "planet-sized bodies", not necessarily a body orbiting a star and not a planet.

Bredehöft concludes that Earth-like planets may still be the most likely places to find life, but I'm not sure about that, for a few reasons: 1) Planets smaller than Earth are much more common, and by the time we've refined our abilities to find planets these size they should turn up in much larger numbers than Earth-sized planets; 2) Smaller planets aren't hit by as many meteors as larger planets and this should make it easier for life to continue to thrive; 3) Planets with less mass are easier to leave, and I suspect a civilization on a smaller planet would have a much easier time with taking its first steps into space than we have so far. If Earth's mass was half what it is, for example, we could be sending satellites into orbit with far smaller rockets, and we probably would have been able to reach the edge of space through private companies (a la Virgin Galactic) a few decades ago. Regular airplane flight of course would have been that much easier as well, and could have started much earlier (yes our bodies would have been weaker but the physics involved in creating and using fuel would be the same).

And now, part of the article:

Earth-like words are the first class, and a kind of "control" since we already know such worlds are capable of sustaining complex life. Earth-like worlds feature an appropriate atmosphere, liquid water, moderate temperature ranges, and stable climates.

The second class of planets are those that were once much like Earth, such as Mars and Venus. "For some reason these planets left the classical habitable zone," said Bredehöft. "Mars became too dry, there's very little water left, at least not liquid water. Venus became just so enormously hot due to the greenhouse effect."

Still, Bredehöft believes there is some chance for life to exist on this type of world. He reasons that organisms could have developed when the planet was more hospitable, and this life could maintain a grip even through the hard times. "Once life has established itself it is really hard to kill off," said Bredehöft. "There have been absolutely devastating events in Earth's history that might have wiped out all kinds of life, but usually these served to further enhance biodiversity, rather than destroy it."

...

Bodies that possess liquid water, but under an ice layer rather than on the surface, make up the third class of worlds.

Jupiter's moon Europa is a classic example from our own cosmic neighbourhood. Could there be life in places like this? Bredehöft's ideas here are particularly pertinent as often these worlds do not fit neatly into the conventional view of habitable zones. Europa, for example, lies beyond the solar system's temperature zone where water can remain as a liquid on a planet's surface.

However, there is still potential for life.

The traditional view of habitable zones thinks of a local star as being the prime energy source. But on icy worlds like Europa, other factors come into play, such as the gravitational pull of another planet. Worlds with liquid water hidden beneath icy layers could potentially be inhabited by simple organisms despite being far from the conventional habitable zone, so long as energy is provided in some other way.

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.