We've come an amazing distance in our ability to detect other planets around distant stars in the last twenty years. As a brief refresher, what happens is that a telescope watches a star for signs of either dimming (from a planet crossing in front of it) or a wobble (indicating an unseen partner is tugging it - imagine holding a child's hand and spinning around with them, and you'll get a picture of what's going on.)

When you have an orbital period and the amount of 'wiggle' in the star, you can make a decent estimate of what the mass of the unseen orbiting body is, by reversing Newton's tidal calculations, and you can also get a rough idea of the orbital velocity and the orbital radius.

To Quote the European Southern Observatory:

At an international ESO/CAUP exoplanet conference in Porto, the team who built the High Accuracy Radial Velocity Planet Searcher, better known as HARPS, the spectrograph for ESO's 3.6-metre telescope, reports on the incredible discovery of some 32 new exoplanets, cementing HARPS's position as the world's foremost exoplanet hunter. This result also increases the number of known low-mass planets by an impressive 30%. Over the past five years HARPS has spotted more than 75 of the roughly 400 or so exoplanets now known.

This discovery was widely reported as having discovered earth like planets, but clearly this is not the case. These techniques have their limits; the first is that we're unable to resolve planets that are lower than about 5 to 6 times the mass of the Earth, which means we're still no closer to finding another habitable planet in the distant solar system around the host star.

The second is that the radial velocity and transit method can only identify planets that have their orbital plane within about 15 degrees of perpendicular to ours. However, we're getting a large catalog of data on extrasolar planets, and are discovering that the Solar System is, in some ways, atypical.

One on usual discovery in exoplanets is that the most commonly found ones are multiple Jovians. That is to say the size of our planet Jupiter and bigger in mass, but orbiting with periods of less than a month - imagine if Jupiter were orbiting at 1/3 the orbital distance of Mercury, if you will.

Indeed, if you did this with Jupiter, you would get a planet that showed enough of a disk in the sky that its phases would be seen with the Naked eye as it traveled around the sun. A quick run on relative diameters shows that Jupiter would show over 850 times the surface area of Mercury at the same distance from either; the difference in reflected light from the greater distance would probably reduce this to about a 700-fold increase in brightness from Mercury. Sky watching in such a system would be pretty spectacular.

However, there's something else we get to see from those Hot Jupiters transiting across the faces of stars hundreds of light years away. They're actually close enough to their stars that they're (slowly) being boiled off by them - and we can do spectroscopic readings of the gases coming off; mostly they've been carbon dioxide and methane, which are two fairly common gasses.

However, techniques for spectroscopy of exoplanets are improving; it's possible that within a decade, with the launch of the James Webb Space Telescope, we could resolve a planet with Terrestrial mass and composition, and be able to look for the telltale signs of life (carbon dioxide, free oxygen and free nitrogen).

So, no other earths out there yet, but we are still looking. Given what we have found so far, it would be surprising if we did not find and Earthlike planet in the near future. Watch this space.