Goodbye Kepler: March 7, 2009 to May 11, 2013

The Kepler spacecraft appears to be dead, done in by wheel bearing failures in two of the devices that aim it toward target stars. NASA has put the spacecraft in a "safe mode" that allows constant communication with the earth, in case the bearings decide to unfreeze or somebody thinks up a brilliant solution. But the team engineers seem very pessimistic.

So farewell to one of the most extraordinary explorations in human history. In its four-year career Kepler has discovered more than 3,000 planets, eight times as many as had been discovered in all the millennia before it was launched. The official count of extra-solar planets is much less than that, but that is only because it is taking astronomers a long time to work through the mountain of data Kepler has beamed home.

The first discovery of an extra-solar planet was not made until 1988. Until that time, worlds around other stars remained only a dream, and we had no idea how many there were. Based on what we have now learned about extra-solar planets, astronomers think there are at least 100 billion planets in the Milky Way Galaxy, perhaps 400 billion, at least 17 billion of them roughly the same size as Earth. Spectrographic analysis has already produced evidence of water and carbon dioxide in the air of a world 50 light years away; if we find a planet with a large amount of oxygen, we may well have found a world with life something like our own.

Whatever else is wrong with the age we live in, it is the greatest age of scientific discovery, a time when wonders from the molecular machinery of the cell to planets around other stars have been mapped for the first time.