Potentially Habitable Planets Are Common, Study Says

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"What we thought is, our outer solar system is actually our middle solar system," Stern said.

It would be a vindication for Pluto, which was recently "demoted" from full planet status by astronomers after a lively and controversial debate.

Pluto might be the best known representative of a third major class of planets, the dwarfs, "which could be far more common than either the terrestrial or gas giant planets," Stern said.

The initial solar system was quite cluttered with small bodies, he explained, but these were swept out as the four gas giant planets—Saturn, Jupiter, Uranus, and Neptune—finished forming.

Evidence for that can be seen in Uranus, which is lying almost on its side compared to the other planets, Stern said.

It must have been struck by a massive object several times the mass of Earth—an extraordinary coincidence if there were only a few such bodies around.

Exploring the Kuiper belt will be a slow process, Stern pointed out, as the objects in it are extremely difficult to find because of their distance from Earth.

These worlds would mostly be rocky bodies with icy surfaces, though larger ones might be able to harbor gassy envelopes.

But there is also the possibility that some could have "warm, wet interiors," Stern said.

Some scientists think it is "likely Pluto has an ocean in its interior, as does [Jupiter's moon] Europa and many of the other satellites of other planets," Stern said.

In the future, we might focus the search for life on such worlds, which could be far more common than planets like Earth with liquid water on their surface, he added.

Slow Search for Life

But directly detecting the kinds of planets that could harbor life remains a huge challenge, said Debra Fischer of San Francisco State University.

Such planets fall into an "anti-sweet spot," she said—far too small to detect using any of the common planet detection methods, which have so far found about 250 or more extrasolar planets.

But scientists have reasons to remain optimistic, she said.

If a planet with the right mass is found at the right distance from a star—in the so-called Goldilocks zone, where it is neither too hot nor too cold—most of the work is done.

"The raw materials for life are common," she said. "Water is probably the most common molecule in the universe."

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