Tomorrow evening (US eastern time) is the earliest window in which NASA’s Kepler mission may launch. This is very exciting because Kepler’s main mission is to locate planets that are similar, and in similar positions, to our own. Planets like Earth are the ones where we’d be most likely to find life as we know it.
Nearly all of the planets we’ve spotted that are located outside our own Solar System have so far been gas giants like Jupiter and Saturn. They’re easy to spot, though, because they’re big and hot. Kepler will find smaller, rockier, Earth-like planets.
There’s a huge amount of really fascinating science, from the general to the detailed, on the mission page. Here are some excerpts I really like.
How will Kepler look for extrasolar planets? By looking at stars, and watching for signs that something has moved across the front of them:
The Kepler spacecraft…will orbit our own Sun, trailing behind Earth in its orbit, and stay pointed at Cygnus starfield for 3.5 years to watch for drops in brightness that happen when an orbiting planet crosses (transits) in front of the star. Cygnus was chosen because it has a very rich starfield and is in an area of sky where the Sun will not get in the way of the spacecraft’s view for its entire orbit.
How does a transit tell us that there’s a planet there?
Transits by terrestrial planets produce a small change in a star’s brightness of about 1/10,000 (100 parts per million, ppm), lasting for 2 to 16 hours. This change must be absolutely periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.
Once detected, the planet’s orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler’s Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet’s characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
What else will Kepler do?
The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:
- Determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars;
- Determine the distribution of sizes and shapes of the orbits of these planets;
- Estimate how many planets there are in multiple-star systems;
- Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets;
- Identify additional members of each discovered planetary system using other techniques; and
- Determine the properties of those stars that harbor planetary systems.
This is a really exciting mission to undertake during the International Year of Astronomy.