What other planets and moons are habitable? What kinds of stars support habitable planets? What about moons?
Scientists’ answers to these questions are shifting. Partly, this is due to what’s been learned about extremophiles, organisms found on earth that can survive “scalding waters, subzero temperatures, bone-crushing pressures, corrosive acid, extreme salt and arid conditions…(and) withstand massive doses of radiation, breath rust, eat sulfur, belch methane and live without oxygen or sunlight.”
“Finding extremophiles on Earth has just been mind-blowing,” said Carol Tang, a researcher from the California Academy of Sciences who studies extremophiles. “If you think about how there’s very few places on Earth where there isn’t life, you can’t think about the solar system and the universe in a very limited way anymore.”
(Ever-politically correct, Wikipedia hastens to point out that “the definition of ‘extreme’ in this context is anthropocentric; from the point of view of the organism, its environment is completely normal.” Yeah, we don’t want to insult the one-cell community!)
Scientists from the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, CA, are entertaining a debate about whether there is a chance for intelligent life to exist on planets that circle around the dimmer red dwarf stars that comprise 85 percent of all stars. It’s a resource question for SETI; they’re going to soon deploy its Allen radio telescope array to try to pick up radio signals from intelligent extraterrestrial life. They need to decide if the red dwarfs are worth listening to.
One of the main objections was that the habitable zones of red dwarfs would be very narrow and very close to the stars. For a planet orbiting a red dwarf to be warm enough to have liquid water, it would need to be located closer to the star than Mercury is to our own Sun. At such a close distance, the planet would become tidally locked to the red dwarf the way our Moon is to Earth. Any water existing on such a planet would be boiled away on the side facing the star and frozen solid on the other.
In recent years, however, new computer models have suggested that the situation isn’t as impossible as it might seem. The models predict that if an orbiting planet had a thick enough atmosphere, heat could be redistributed from the lit side of the planet to the side that was dark.
As for the criticism that a red dwarf’s habitable zone is very narrow, Todd Henry, an astronomer at Georgia State University, has an interesting view. Because there are so many more red dwarfs than stars like our Sun, Henry has performed calculations suggesting that if the narrow habitable zones of all the red dwarfs in our galaxy were combined, they would equal the habitable zone of the all the Milky Way’s Sun-like stars.
Jupiter’s moon Europa presents another possibility — a moon far away from the sun, but with an internal heat source that keeps some of its waters in a fluid state.
Scientists think Europa stays warm by a process called tidal heating. All moons, including our own, are stretched and pulled by the planet they orbit. Jupiter is so massive and its gravity so strong that it actually causes Europa’s surface to bulge and shrink as it circles around in its orbit. This constant motion generates friction and heat.
Saturn’s cloud-covered moon, Titan, is thought to be warmed by the same process. Other moons generate heat through different means. Scientists recently discovered that Saturn’s moon Enceladus, for example, contains a mysterious hot spot in its southern hemisphere that might be caused by radioactive material left over from the moon’s formation billions of years ago.
This is not to say that Europa, Titan or Enceladus have intelligent life. But the existence of such processes in our solar system greatly increase the number of possible configurations that could hypothetically support life, even intelligent life, in the Universe.