A couple of Nova programs on PBS last night got me to thinking. Always a dangerous concept. But these two programs were all about the possibility of life on other planets, and in this discussion I’m including planets in our solar system as well as planets circling other stars. The prevailing wisdom to now has been that in order for life to develop on a planet, it has to be within what’s called the “habitable zone,” (sometimes called the “Goldilocks zone”) which is a region around a star where the temperature is neither too hot nor too cold so that life, at least carbon-based life (which is what we are) can develop. In our solar system, only Earth and Mars lie inside that zone, so the assumption has been that life could never develop on any other planet. Mercury and Venus are way too hot (think of melting lead), and anything farther out is too cold. At first glance that seems reasonable.
But a new concept of the development of life has taken hold in the past several years which overturns that old paradigm, and the possibility of life in our solar system has been extended to moons of the larger planets Jupiter and Saturn. Carbon based life couldn’t exist on those two giant planets themselves because they don’t have any carbon, the gravity is crushing, and their strong magnetic fields would probably prevent life from gaining a foothold. But each of those have a large selection of moons on which life could develop. A good example is Io, one of Jupiter’s moons.
Io gets pushed and pulled by the tremendous attraction of Jupiter’s strong gravitational field, and over the billions of years Io has been circling the planet, it’s been heated from the inside, and the liquified interior has made its way to the surface in the form of volcanoes of molten sulfur. Now, molten sulfur isn’t exactly compatible with life, but the important point here is that there’s heat on Io, heat that doesn’t come from the sun and heat that could conceivably power the development of life forms. But that’s not all.
Other moons of the two largest planets in our solar system are warm, not from tidal heating, but from internal heat left over from when they were formed. Like the molten core of Earth. Europa, another moon of Jupiter, is covered with ice, and liquid water may exist down below that ice. Liquid water. Warm, liquid water. A perfect breeding ground for living organisms.
But our solar system has only four planets on or near which life could exist. Billions of stars exist in our galaxy, and the chance that the same or similar conditions exist on planets around them is virtually a guaranteed proposition. Life exists out there right now, though that doesn’t mean it’s nearby nor that it has visited us here. (Little green men? Put your press release away.) But somewhere there’s life, in one form or another. All we have to do is find it. With the requirements for life expanded away from the old habitable zone, that just increases the chance. Anyone want to take a trip to the nearest star?