These planets today could harbor life somewhere in interstellar space, according to a planetary scientist at the California Institute of Technology.

In the July 1 issue of the journal Nature, Caltech professor Dave Stevenson says in a new study that such objects could be life-sustaining due especially to the molecular hydrogen they accreted when the solar system formed long ago.

Called "interstellar planets" because they would exist between the stars but no longer in orbit around an original parent star, they have never been directly observed or proved to even exist. But based on what scientists know about the way matter should fall together in forming a solar system, such Earth- like planets could definitely have been formed.

Over a period of several million years, one of two things happened to these planets: either they slammed into Jupiter and made it even bigger, or else they came so close to Jupiter that they were catapulted by gravity completely out of the solar system, never to return.

Because these bodies formed when the solar system was permeated with hydrogen gas, they retained a dense atmosphere of hydrogen, allowing them to have surfaces with temperatures not too different from Earth, and possibly water oceans.

Stevenson writes that in the absence of sunlight, the natural radioactivity inside an Earth-like planet would only be sufficient to raise the radiating temperature of the body to 30 degrees above absolute zero (that's about minus 400 Fahrenheit). But the expected dense hydrogen atmosphere would prevent the surface from radiating effectively -- just like the greenhouse effect on Earth, but more so.

As a result, the surface could have a similar temperature to the current Earth surface, allowing water oceans and a surface pressure similar to that at the bottom of Earth's oceans. For this to happen, the interstellar planet would probably need to be at least half Earth's mass.

Therefore, the energy source would be much the same as that which drives geothermal energy and plate tectonics on Earth.

It is not known whether geothermal heat alone is sufficent to allow life to originate, and the amount of energy is small compared to sunlight, suggesting that the amount of biological activity would also be small. But the existence of life in such an environment would be of great interest even if the mass of living matter were small.

The heat energy, and especially variations in temperature, could potentially allow life to get going, Stevenson says.

"I'm not saying that these objects have life, but everyone agrees that life requires disequilibrium," he says. "So there has to be a way to get free energy, because that's what drives biochemical processes.

"These objects could have weather, variations in clouds, oceans ... even lightning."

If life exists on such objects, an open question is how complex it could be, Stevenson says. "I don't think anyone knows what is required to drive biological evolution from simple to very complex systems."

These interstellar wanderers could also have arisen as a natural outcome of the formation of stars, and not just during the formation of the system we live in. In either case, such planets cannot be seen with present technology because they are so dark and cold-at least from Earth's vantage point.

Although these bodies may have warm surfaces, they would appear to us as very weak emitters of long-wavelength infrared radiation, much below current detection limits.

The best bet for even demonstrating that interstellar planets exist is to have some programmed search for occultations, he says. In other words, the object might pass occasionally in the direct line of sight between Earth and a star, and if instruments were watching, the light of the star might dim or even flicker out for a time.

Programs like this are already advocated for the purpose of looking for planets in orbit around other stars. But looking for interstellar planets would be even harder.

"All I'm saying is that, among the places you might want to consider for sustainable life, you might eventually want to look at these objects. They could be the most common location for life in the universe."

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