Whether habitable or abominable, planets are inescapable. 'You make astar, you’re probably going to get planets,' said Seth Shostak, a senior scientist at the SETI Institute in Mountain View, Calif. 'They’re like those knives that get thrown in for free when you order a Veg-o-matic.'
The idea that there might be other planets like ours orbiting distant stars is hardly new - exo-planet hunters have cataloged over 300 in the past dozen years - but the notion that Earth-sized planets might be fairly common is a more recent development. As one planetary theorist says in the article above, it's very suggestive that just as soon as astronomers were able to find low-mass planets, they found them. And as observational techniques are further refined, the detection of an exo-planet very similar in mass to ours now seems almost a matter of time.
Wikipedia: planetary formation