Kepler In Space

NASA successfully launched its Kepler planet hunting probe Friday night from California. Kepler, if all goes well, will spend at least three years focused on a small, star-rich area of the Milky Way, searching for planets similar to Earth.

Kepler will do that by looking for transits-- catching a planet moving across the disk of its parent star. To see that requires that faraway star system to be perfectly aligned with Kepler, which seems wildly unlikely. Astronomers argue, however, that the sheer number of stars in the area being studied will allow them to find many new worlds.

Transits are actually extremely useful events. Timing the transits can yield the speed of the planet, which-- using Johannes Kepler's Laws of Planetary Motion-- gives the distance of the planet from the star. That distance can tell us whether the planet is in the "habitable zone" of the star-- where life could exist. The magnitude of the dip in starlight which signals a transit is occuring tells us the size of the planet.