Around every star is a region of space that could potentially harbor a twin to our own planet. Called the “habitable zone” or the “goldilocks zone,” this is the area around a star in which water can exist in its liquid form. This is important, because life as we know it depends upon nothing so much as liquid water. The size of this zone and distance from its parent star is a function of the size and luminosity of the star itself. Our own planet is snuggled in the middle of the Sun’s habitable zone at 1 AU (between 91 and 95 million miles), and it is brimming with life. Closer in, we find Venus, a veritable hothouse of boiling gases that would be very inhospitable to life. Further out we have Mars, which may have had water in its warmer past, but is now cold and dead.

For a very long time, the very existence of planets around other stars was a mystery. We assumed they were there, because our own solar system has a large number, but we lacked the technical refinement to be able to detect them. Tantalizing clues were available in the wobbles of star light that our increasingly powerful telescopes detected, but it wasn’t until 1992 that Aleksander Wolszczan and Dale Frail announced and confirmed the discovery of planets orbiting around a pulsar, PSR 1257+12. In 1995 Michel Mayor and Didier Queloz at the University of Geneva discovered a planet orbiting a main-sequence star, and advances in technology ushered in an explosive era of planetary discovery. There have now been 677 planets identified and confirmed outside of our own solar system, including one entirely of diamond.

The latest haul of exoplanets made by an European instrument called HARPS brought in 55 newly discovered exoplanets, including 19 so called “super-earths;” planets that are between 1 and 10 times the size of our own world. And to add to the excitement, one of these plants lies in its star’s habitable zone. But don’t pack your bags just yet: the planet is 3.6 times more massive than our own, and the atmosphere would be extremely hot and muggy. “We’re not saying it’s habitable for you and me,” one astronomer at the Max Planck institute was quoted as saying. However, the discovery of this planet is incredible just the same, as only one other planet within the habitable zone has every been discovered (besides earth, of course). Scientists predict that within 3-5 years, the first true Earth analog will be discovered – a planet roughly the same size, orbiting in the habitable zone.

Nasa’s Kepler satellite has so far led the way in exoplanet detection, discovering 1781 potential exoplanets to-date (these all require more study and verification, however). But it is Europe’s HART instrument that will be making the definitive discoveries and ultimate detection of liquid water in the planet’s atmospheres. There are plans to add a 39.2 meter “Extremely Large Telescope” that should be able to image nearby exoplanets directly. It is with this tool that we could potentially detect organic molecules on any potential new Earths.

This is an extremely exciting time to be alive in the field of planet-finding. Indeed, such a field did not even exist 20 years ago, and now it is one of the fastest growing areas in astronomy. In just that short span of time, we have gone from wondering whether or not our Sun was the only star with planets, to wondering just how many other Earths there are out there. Once we find another Earth, however, we still have to solve the problem of getting there.

For more information, there are numerous resources online for exploring and learning about the exoplanets that have been discovered so far. A couple of the best are:
NASA Planet Quest site

Catalog of Exoplanets