planets orbiting other stars. How did they do it?
One approach was to examine the gravitational interaction between stars and their planets. Stars are much more massive than planets, but a star’s planets tug on it gravitationally. (Think of a fat man walking a gaggle of small, struggling dogs: He continues on his walk, but the dogs pull him this way and that. In this illustration, gravity is the leash on each dog.) The effect is very subtle, because the star is big and the planet is much, much smaller, but by using high-resolution spectroscopes, astronomers can detect such a small changes via the so-called Doppler shift: When a star is tugged toward us, the lines in its spectrum are shifted slightly toward the blue; when the star is tugged away, the shift is toward the red. Our own Sun moves back and forth because of the pull of Jupiter (its largest planet) by only about 30 miles per hour, roughly the speed with which most of us drive around town, but existing spectroscopes can detect that small a variation on stars hundreds of light years away.
Because this technique works best for planets that are massive and close to their planets, since they tug most strongly on their stars, it preferentially identifies big planets in small orbits. Astronomers call such planets “hot Jupiters.” But while hot Jupiters are the first planets typically discovered, closer examination of the spectra of nearby stars has shown that a number of them have 2 to 4 other, less massive, planets as well. And, as time passes, since the first hot Jupiter was discovered in 1995, planets much farther from their stars are also being found.
Where the Planets Are In the beginning, the Earth was believed to be both flat and at the center of the universe. Then a sailor set sail to prove that the Earth is in fact round. Decades later science proved that the churches were wrong and in fact the Earth orbited around the Sun not the other way around. Now it is commonly known that the solar system that the human race lives in is just a small ...