These shifts in light take place when a star wobbles back and forth. But sometimes they wobble as they spin. That means gravity from another object must be pulling on the star. If we determine that the orbiting object is a planet, we can learn something about the nature of that planet. The way we do this is by studying the way the star wobbles.
More massive planets have more gravity, so they cause more wobble.
The time it takes for one wobble to take place is the period of the orbit. If it takes a week, it means the planet orbits the star in one week. This new method of tracking a star's wobble has led to discoveries of 28 extrasolar planets since 1995. But there's another, more direct way to spot these planets. Click on the next button to find out how one astronomer actually saw an extrasolar planet-and took a picture of it.