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The Wobble Method

Our sun is over 300,000 times heavier than Earth. While Earth and the Sun are both attracted to each other gravitationally, the effect on the Earth is much more easily seen because of Earth's lower mass.

However, not all planets and stars are like this. Jupiter, for instance, is only 1000 times lighter than the sun. When we look at some other solar systems we have found, there are planets much heavier than Jupiter that orbit their stars at a closer distance than Mercury orbits our sun.

When the planet's mass becomes significant compared to the star's, it becomes more obvious that the planet and the star are orbiting together. They move together around a single central point. This is true in our solar system as well, but it's easier to see in star systems where the planets are heavier and closer in to their stars.

We could look for the star to move in the sky, but most stars are so far away that we wouldn't be able to see their motion in this way. Luckily, we can use another technique.

The movement of a light source, such as a star, changes the frequencies of light that the star emits. This is the same Doppler effect that makes a fire engine's siren change in pitch as it moves past you (higher pitch as it moves toward you, lower as it moves away). When the star moves towards us, it releases higher-frequency light. When it moves away, the light is of lower frequency. The difference is small, but our instruments are so sensitive to frequency that we can detect a speed of just half a meter per second! That's about the speed of a person walking.

If we see this type of rhythmic change in the light frequency emitted by a star, we know the star is being orbited by a significant body. This approach for detecting extrasolar planets is known as Doppler Spectroscopy, or sometimes the Radial Velocity Method. Because the star moves around in a circle, this is also sometimes called the Wobble Method.

Like all methods, the wobble method has its limits. The most obvious is that the smaller the planet, and the farther away it is from its central star, the smaller the effect on the star. Earth's gravitational effect on the Sun, for example, would be beyond the detection limits of our current spectroscopes. The Doppler Effect is also much more potent when a star is moving toward or away from us in the sky, rather than around in a circle. It is hard or impossible for us to detect solar systems that are tilted at the wrong angle with respect to us.

Even when the wobble method works, it only tells us that a planet is present, and some information about its mass and orbital distance. We do not learn about the size of the planet, which also means we cannot determine its density. The transit method discussed on the last page provides more information about the planet, but it does not give us information about the planet's mass. The wobble method and transit method work best in conjunction with one another.

You can find more about the wobble method on this page, where we will show you how to calculate a planet's mass.

Солнечная система и ее тайны