Searching For Other Earths

We have been finding planets orbiting other stars for a while now and have found well over 300. The first ones were all what are called "hot Jupiters". Hot Jupiters are large gaseous planets that orbit close to the Sun. They were not observed directly, but rather by measuring the movement of the star as the planet orbtited. We measured the Doppler shift and could see the star move toward us and away from us as the planet orbited. This technique works well for large planets close to a star.

We have gotten better at measruing Doppler shifts and have observed for longer periods of time. We have found smaller planets orbiting farther from their stars. We have developed other techniques for finding planets as well.

The transit method measures the brightness of a star continuously. When a planet passes in front of the star, it blocks a little bit of light and the star gets dimmer. This only works for a tiny fraction of the planets since you have to be lined up just right to see a planet cross the face of the star. The advantage is you can find planets that are small much easier and you can even get a measurement of their sizes.

The Kepler mission, scheduled to launch this Friday from Cape Canaveral on a Delta II, will search for Earth like planets through the transit method. Kepler is a .95 meter telescope attached to a sensitive 95 megapixel CCD camera. It will stare at one part of the sky (in the constellation Cygnus) for 3.5 years. It will continually monitor the brightness of about 100,000 stars, looking for the telltale drops in brightness. Kepler will undoubtedly find variable stars and lots of larger planets. The goal is to find planets that are the size of Earth and in similar orbits...orbits where liquid water could exist on the surface. For a planet orbiting a star similar to our Sun at about the same distance, the star will decrease in brightness by about .01%, not an easy measurement.

Yet we can do it. Kepler will not orbit the Earth, but rather orbit the Sun (in an Earth trailing orbit) to eliminate light from the Earth. The part of sky was carefully chosen to have a lot of stars, but not too many that they run together. It was also chosen so Kepler could observe the patch of sky continuously without the Sun getting in the way (and don't forget about maintaining contact with Earth so we can get the data!)

This mission will tell us a lot about our place in the universe. How many Earth-like planets are out there? How many could support life as we know it? Even if we find none, that is still a very important result. It would tell us that we are rare indeed. No one knows for sure how many we will find, but from everything I have read, a few dozen would be a reasonable estimate.

Kepler won't tell us if any of these planets have life. That will have to wait for future missions. We are already testing techniques that we can use to measure characteristics of planetary atmospheres and have had some success with larger planets. Kepler is the an important step on the road to answering the question of how common life is in the universe.

Reprinted with permission from the Half-Astrophysicist Blog.