Its interesting that Alpha has the equation baked in. What would be more interesting would be to see a regularly updated version of this with the latest cosmological data coming in from telescopes like Kepler. Over the next 3-5 years, Kepler is going to be finding Earth-size planets that exist in their star's habitable zones.

For those not familiar with how Kepler is finding planets, I'll explain. Basically, Kepler looks for planets transiting in front of their stars. The telescope watches for dips in the star's light. If the planet has an atmosphere, in some cases it can even get an idea of some of the gases from the light.

The reason that astronomers aren't expecting to find Earth-size worlds in the habitable zones is that they have to wait for several transits to know for sure they've seen a planet. The reason that most of the planets found to date have been gas giants orbiting very close to their stars is that it only takes a few weeks or months for them to transit the star several times. Astronomers can see one transit, but have to wait for a 2nd to hypothesize a planet is found, and have to see a 3rd or more transit to verify the planet is real.

So Kepler has likely already detected suspected Earth-size planets in habitable zones around at least a few stars. However, the telescope has to watch for several years to confirm the transits. Imagine an astronomer in another solar system that has seen Earth transit the Sun. They would have to wait 3-4 years to see Earth transit the Sun several times to confirm our existence.

Edit: Meant to include this earlier. There are multiple ways to detect exoplanets, including the transit method. http://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar...