Exiled Planets Might Support Life

WASHINGTON (Reuters) - Earth-like planets that were ejected from our infant solar system might just be warm and wet enough to support life, scientists reported Wednesday.

Even without a sun-like star to heat them up, these exiled interstellar bodies could be wrapped in dense hydrogen atmospheres that would trap the warmth generated by the planets' natural radioactivity, David Stevenson wrote in the current edition of the journal Nature.

``They don't so much warm up as fail to cool down completely,'' Stevenson said in a telephone interview from the California Institute of Technology in Pasadena. ``They may have started hot by processes of formation of the solar system.''

Astronomers have theorized for decades that such exiled bodies could exist in interstellar space as the natural by-product of the creation of the solar system.

Bodies with about the same mass as Earth's were thought to follow two paths in those early times: either they would slam into a large planet like Jupiter and add to its mass, or come within range of a large planet's gravitational pull and be catapulted out into the void.

Stevenson concentrated on the ones that got flung into space a few million years -- an instant in cosmological terms -- after the solar system coalesced some 4.5 billion years ago.

Because hydrogen would have been abundant at that stage of solar system development, any ejected planets would have been swathed in hydrogen, presumably allowing them to retain surface temperatures comparable to Earth, and possibly water oceans.

Without sunlight, the radioactivity inside an Earth-like interstellar planet would only raise the temperature to a bit above absolute zero (about minus 400 degrees F.). But a thick blanket of hydrogen would prevent much of that internally generated heat from escaping, keeping the exiled planet comfortably cozy, Stevenson said.

Liquid water is seen as a requirement but not a guarantee of Earth-type life. But Stevenson said these bodies may also have volcanoes and lightning, causing temperature variations that might foster life and maintain it.

Still, the available energy would be about 5,000 times less than that on Earth, he said, which might mean less development of life and less complexity in any life-forms that developed.

What might it look like on the surface of such a planet?

``If you happened to be standing near a volcanic eruption, instead of pitch-darkness, you would see a landscape lit up with a dull-red glow and the sky would most likely be cloudy, with water, ammonia and methane clouds in layers,'' he said. ``You might not actually see a beautiful starry sky.''

In any event, Stevenson acknowledged that interstellar planets are virtually impossible to detect from Earth with current technology, making the theory of their existence very hard to test.

``To be frank, I debated whether to submit this (for publication),'' Stevenson said. ``In the end I decided that ideas play an important role in science, even when they don't have an immediately testable aspect.''

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