I recently watched a show about alien planets orbiting distant stars. It emphasized the differences between these planets and Earth – too hot, too cold, iron rain, noxious gas – places where humans couldn’t live, but maybe some organisms might.

What I, and everyone else, would like to find is a planet that is like Earth. Some planet orbiting an Earth-like star in the Goldilocks zone – not too hot, not too cold, but just right. A planet with water, and with a breathable atmosphere.

To date, most of the exoplanets found are large gas giants, like Jupiter, since those are more easily detected at a distance. A heavier mass planet causes its star to wobble more, and causes its star to dim more when passing in front of it. Our current telescopes and equipment are just sensitive enough to see a bunch of these types of planets. A catalog of all discovered exoplanets can be found at http://exoplanet.eu/catalog.php , or look at http://exoplanets.org/planets.shtml for a catalog of the closer planets. I scanned through the first catalog, sorting it by distance from our solar system, and looking at planet size and orbital period. Most of the smaller planets have very short orbits, with their “years” being measure in our days. Describing life on such planets might make for a very intriguing science fiction book!

One small planet (OGLE 2005-BLG-390Lb) has an orbit of 3500 days (about 10 years), is about 5 times the size of Earth, and orbits about 3 times as far from its star as Earth does. Interesting, but it’s probably very icy, like a big Pluto, and not too friendly for humans. Plus, it’s 11,415 light years away from Earth.

Another two small planets have orbits from 66 to 98 days – but they orbit a pulsar. A bit too much radiation for comfort.

And then there’s the star Gliese 581. At least 4 planets have been detected around this star, with a few (planets c and d) in the habitable zone, not too much bigger than Earth, and with orbits lasting from around 13 to 67 days. Planet c may have a runaway greenhouse effect, and thus be inhospitable, but planet d looks promising. We humans beamed a radio message toward planet c last year. It should get there in 2029. If someone is there to respond, their reply could get here by 2049. I might still be alive then.

But now there is hope that we will start finding more and more Earth-like planets. The Kepler space telescope was launched on March 6th, and is starting to produce results (see http://kepler.nasa.gov). It recently verified a previously-detected planet, and did so in a high-resolution way, detecting a change in the star’s light when the gaseous, glowing planet traveled behind the star. The planet glows due to its high temperatures and also by reflecting light from its star (just like our moon glows). When it passed behind the star, its light (miniscule compared to its star’s light) disappeared, from our point of view. The light variation is of the magnitude expected for an Earth-sized planet transit in front of a star, so the telescope is looking good for future discoveries. Now it’s time to just sit back and watch the exoplanet catalog grow!