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The Transit Method

Our current best method for finding and understanding extrasolar planets is the Transit Method. This method is difficult to execute, but simple to understand.

When an object passes between our viewpoint and a star, this is called a "transit." You may have heard about the recent transit of Venus, where we could see the planet Venus moving between us and the sun. There are also frequent transits of Mercury.

The most dramatic example of a transit would be a solar eclipse, when the moon passes between the sun and Earth. The sunlight we receive dims dramatically because the moon blocks our view of the sun. When a planet like Mercury or Venus transits, the sun dims by a lesser amount. That's not because those planets are smaller than the moon, it's because they're farther away from us so they appear smaller. You can see a picture of the transit of Venus below, as taken from a ground-based telescope.

This looped animation from NASA shows an exoplanet transit: Kepler 10c crossing in front of its home star. You can click on the link below the animation to get the original page (requires Flash).

You can see some "noise" in the light curve on this video. This may be variability in the star, or gas and dust moving between us and the star, or some other effect. Some observations have a lot of noise, some have very little.

When we look at distant stars, we can't actually see planets transiting their stars. They're too far away for us to see such small features, even with our best telescopes. However, we can tell that the amount of light coming from the star decreases. The actual data recorded by telescopes appears in the middle right-hand side of this image as tiny dots of light. Everything else, from the size of the planet to the radius of its orbit and many other factors, are things that we calculate from our data.

The larger the planet, the dimmer the star appears during the transit. In fact, the light from the star during a transit follows a characteristic curve from which we can learn much information.

The image above shows a simplified light curve, with the data smoothed out. You can see the percentage of light that is blocked out, which tells us how large the planet is in comparison with its home star. You can also see how long the transit lasted, which gives us information about the planet's orbit. We can obtain even more information if we can see the exact shape of the curve.

You'll learn more about transits on this page, where we teach you how to estimate the size of a planet based on facts about the light curve.

Optional Topic: Secondary Transits.

A Secondary Transit is when a planet passes behind its star rather than in front. There is a very small decrease in the amount of light from the star, because the light that would normally be reflected off the planet is now blocked by the star itself.

Below you can see two examples of actual data from planet Kepler 47-b. Pay close attention to the vertical scale on these graphs - they are different! You can see that the secondary transit (on bottom) is a much, much weaker effect than the primary transit (on top).

Secondary transits give us additional information about the planet: specifically, the amount of light that it reflects. Looking at this transit in different spectra of light will let us guess what materials are on the surface of the planet.

Солнечная система и ее тайны