perfidy

120 extrasolar planets have been discovered over the last decade, orbiting 105 different suns. All of the planets so far discovered are Jupiter sized or larger, due to the limitations of current astronomical instruments, and none are believed capable of supporting life. However, an Open University team has conducted a study of extrasolar planetary systems to determine whether or not earthlike planets could possible exist.

Using computer models of the known characteristics of a sample of the extrasolar systems, they have calculated the possibility of Earth-sized planets orbiting in the habitable zone - that region of a solar system that is neither too warm nor too cold to allow the existence of liquid water.

By launching 'Earths' (with masses between 0.1 and 10 times that of our Earth) into a variety of orbits in the habitable zone and following their progress with the computer model, the small planets have been found to suffer a variety of fates. In some systems the proximity of one or more Jupiter-like planets results in gravitational ejection of the 'Earth' from anywhere in the habitable zone. However, in other cases there are safe havens in parts of the habitable zone, and in the remainder the entire zone is a safe haven.

Nine of the known exoplanetary systems have been investigated in detail using this technique, enabling the team to derive the basic rules that determine the habitability of the remaining ninety or so systems.

The analysis shows that about half of the known exoplanetary systems could have an 'Earth' which is currently orbiting in at least part of the habitable zone, and which has been in this zone for at least one billion years. This period of time has been selected since it is thought to be the minimum required for life to arise and establish itself.

Furthermore, the models show that life could develop at some time in about two thirds of the systems, since the habitable zone moves outwards as the central star ages and becomes more active.

The team also examined the possibility of planet-sized moons of large gas giant planets might also exist in the "Goldilocks Zone" and also be capable of supporting life. A poster setting out the possibilities will be presented during the RAS National Astronomy Meeting.

Most of the planets so far detected have been (in galactic terms) close neighbors. If half of them could harbor earth like worlds, then the possibility for life is certainly much greater than we thought only a decade ago. Which raises again the Fermi Paradox - where are they? If habitable worlds are common, why have they not developed intelligent life? And why has that life not visited Earth?

Perhaps intelligent life is far rarer than we think it should be. Or perhaps the galaxy is a more dangerous place than our imagined in Star Trek's Federation of Planets, and the really intelligent races don't go around shouting at the top of their lungs - because they know that they'll get whacked.