Tonight, Friday March 6, 2009 at 7:49 PM (PDT), a Delta II 7925 is scheduled to launch a space telescope named after Johannes Kepler into solar orbit: it will swing around the sun once every 372.5 days. With a mass of 2290 pounds, the telescope has a 55 inch diameter mirror, making it the largest telescope beyond Earth orbit. With 42 1024×2200 CCDs, it is also the largest camera ever put into space. Its primary mission is designed to last three and a half years, though that could be extended for longer.
And what is the purpose of the Kepler telescope? To discover Earth-like planets around nearby stars.
As of February 2009, three hundred forty-two planets have been found orbiting other stars. Most of these planets are massive gas giants. The majority of these planets were found by means of radial velocity observations, which means they have been detected by the gravitational effect that they have on the stars around which they orbit. That is, just as an Olympic athlete spinning around and around in preparation for releasing the hammer in the hammer throw wobbles back and forth, moved from a center location by the weight of the object he is about to hurl, so a star is pulled back and forth by the planets that orbit it. That wobble, at least for the larger planets the size of Uranus and bigger, can be detected by current instruments.
But finding planets like Earth, until the launch of Kepler, has been impossible. Kepler changes that by giving us an instrument with the ability to locate planets around stars that are like the Earth. A small percentage of the large planets that have been found by our instruments were discovered not by the radial velocity observations, but by the transit method. If a planet crosses in front of a star’s disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet. Our instruments on the ground can detect such drops in brightness if the planet is a large one. The Kepler instrument is sensitive enough to detect such crossings by planets like Earth.
For an Earth-like planet orbiting its star at the same distance as the Earth orbits from the sun, the probability of Kepler catching it passing in front of its parent star is only about 1 in 210. That is, out of 210 stars that have planets, only one of them will have them going around it at an angle that will take it in front of its star from our perspective. Of course, if Kepler catches one planet going around a star, the chances are good it will find some others because planets in a given system tend to orbit in similar planes. For instance, if an alien Kepler-like mission were looking at our solar system and saw the Earth pass in front of the sun, there is a twelve percent chance that such a mission would also detect Venus.
The Kepler Mission is designed to simultaneously observe 100,000 stars like the sun in a small patch of the sky near the constellations Cygnus and Lyra. If you go outside at midnight in late July and look straight overhead, then hold your hand out at arm’s length, your hand would nearly cover the amount of the sky that Kepler will be studying for three and a half years. Kepler will measure variations in the brightness of 100,000 stars in that region every 30 minutes. These stars that Kepler will study are between 600 and 3000 light years away (stars further than 3000 light years away are too faint for Kepler to observe planetary transits). The 1 in 210 probability of finding an Earth-like planet means that if 100% of stars observed in that tiny patch of the sky all had Earth-like terrestrial planets, Kepler would find about 480 of them.
To put this in greater perspective, the Milky Way Galaxy, the galaxy in which we and these 100,000 stars that will be observed all live together, make up only a tiny fraction of the stars that exist in our galaxy. The Milky Way Galaxy has around four hundred billion stars in it. Kepler will be examining but a tiny fraction of one percent of the stars (specifically, about 0.000025 percent) of our own galaxy. There are more than a hundred billion of other galaxies just in the observable universe—and each of those holds multiple hundreds of billions of stars. Chances are, there are a lot of places very much like Earth out there.
Kepler begins the first opportunity in human history to actually find some of them.