Two wonderful sentences from Universe Today:
More than 100 planetary systems have already been discovered around distant stars. Unfortunately, the limitations of current technology mean that only giant planets (like Jupiter) have so far been detected, and smaller, rocky planets similar to Earth remain out of sight.
They're wonderful because they're a recognition of the technological limitations to human discovery. Since orbit-wobble planetary detection began, press reports have almost exclusively explained astronomers' findings - enormous planets, brown dwarfs and nothing else - as an absence of small, nickle-iron-core planets like Earth, Mars or Venus. Not so, says UT, and the science-enthusiast public is for the richer:
How many of the known exoplanetary systems might contain habitable Earth-type planets? Perhaps half of them, according to a team from the Open University, led by Professor Barrie Jones, who will be describing their results today at the RAS National Astronomy Meeting in Milton Keynes.By using computer modeling of the known exoplanetary systems, the group has been able to calculate the likelihood of any 'Earths' existing in the so-called habitable zone - the range of distances from each central star where life as we know it could survive. Popularly known as the "Goldilocks" zone, this region would be neither too hot for liquid water, nor too cold.
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.
Interpolation: a lovely thing. Read the article for yourself.