To spot a planet
“Planetary peekaboo” (SN: 9/22/12, p. 26) says that to hunt for faraway planets, the Kepler spacecraft “watches for blinks occurring when a planet dims a star’s light by passing in front of it.” For a star to dim when a planet moves in front of it requires us to be in the same plane as the orbits of the remote planets. Evidently it’s expected that this is commonly true. Why is that?
Bryan Mumford, Santa Barbara, Calif.
That’s correct: To see a planet passing between Earth and its star, the system and Earth need to be aligned just right. Astronomers don’t expect this to be common. On the contrary, they’ve calculated that only about 1 percent of planets in an Earthlike orbit would be aligned to produce a transit. So, in general, spacecraft like Kepler can detect only a fraction of the planets that may be out