The Kepler launch is coming up on March 5, marking the first time we will have the ability to find a true Earth analogue around another star; i.e., a planet of about Earth’s mass in the habitable zone where water can exist in liquid form on the surface. Which is not to say that COROT may not come close, though Kepler’s enormous star-field (100,000 targets in the Cygnus-Lyra region) and incredibly sensitive camera — a 95-megapixel array of charged coupled devices (CCDs) — is optimized for planets down to Earth size rather than larger ‘super-Earths.’


Image (click to enlarge): Kepler’s target region, the Milky Way ni the Cygnus region, with the instrument’s field of view superimposed. Each rectangle indicates the specific region of the sky covered by each CCD element of the Kepler photometer. There are a total of 42 CCD elements in pairs, each pair comprising a square. Credit: NASA/Carter Roberts (1946-2008).

We just looked at Alan Boss’ remarkable statement that there could be 100 billion trillion Earth-like planets in the visible universe. It’s startling to think that a mission to be launched within weeks could so quickly give us a chance to size up the idea. The thinking is that dozens of planets like ours in the habitable zone should be visible to Kepler if such worlds are common, but if it comes up short, with few or none, we’re going to quickly re-evaluate how unusual a world we live on. A null result would be striking indeed.

William Borucki, who is science principal investigator on the mission, has this to say:

“Finding that most stars have Earths implies that the conditions that support the development of life could be common throughout our galaxy. Finding few or no Earths indicates that we might be alone.”

But let’s get more specific about terrestrial, as opposed to gas giant planets. The Kepler site posts its own numbers, assuming only orbits with four transits in 3.5 years (the mission duration), and assuming that such planets are common around other stars. Kepler should find fifty terrestrial planets in one-year orbits if most are roughly the size of the Earth, and about 185 planets if most are roughly 1.3 the size of Earth’s radius. The number goes up to 640 if most such planets have a size of 2.2 Earth radii, and goes substantially higher still if orbits ranging from a few days to more than a year are considered.

Kepler is looking for transits in those systems where planets cross in front of their stars as seen from Earth, using an instrument that can detect brightness changes of twenty parts per million. After launch, the spacecraft will undergo a two-month checkout as it achieves its Earth-trailing orbit around the Sun. Eventually, Kepler’s science data will be sent to NASA’s Deep Space Network on a monthly basis, with data analysis performed at Ames Research Center in California. We’re obviously going to have much to say about Kepler in coming months.