Detecting Extrasolar Planets
So how do we find planets outside our solar system? We can't actually see these planets the way we can see the planets in our own solar solar system, so we must look for the tell-tale signs of their existence. We do this primarily be detecting the impact of their orbits around a star. There are three basic signs that we can look for:
Wobble in star's position
Doppler shifts in the star's light
Change in star's luminosity
The fist method is probably the easiest to understand, but few planets have been detected by this method, (though that is expected to change in the near future when NASA launches a spacecraft dedicated to finding planets by this method). The second method is the one by which most exta-solar planets have been found so far, and the third method is showing promise with many on-going efforts to detect extra-solar planets by this method. We will consider each in turn.
The first two methods are based on the motions caused by an orbiting planet. The first important point to understand is that when a planet orbits a star, it is really the planet and the star that are orbiting each other. In other words, it is not just the planet that is moving. The star is moving also; the planet and the star orbit around their center of mass. Since the star is much more massive than a planet, it moves only a tiny bit compared to the planet, but it does move, and that is what the first two methods of detection rely on.
To understand why the star's motion gives us two methods of detection, consider the diagram below. Note that as the planet orbits the star, at points A and C the planet is moving directly across the line of site as viewed from Earth. When the planet is at points B and D the planet is moving directly away and directly towards Earth. In between these points, the planet is moving partially across our line of site, and partially towards or away from us. We can actually see the motion of the star across our line of site, (but this motion is very very slight and therefore difficult to detect). We cannot "see" the motion of the star directly toward or away from us, but this radial motion does create a doppler shift which we can see. We will consider each method in more detail in the following sections.
Astrometric Detection Method
The name makes this method sound kind of exotic, but really it is just detecting the tiny "wobble" in a star's position as a star and a planet orbit each other. It is based on the motion of a star across our line of site as caused by the an orbiting planet.
Doppler Shifts
The doppler detection method takes advantage of the fact that as a star moves toward or away from us, its light will be shifted either towards the blue end of the spectrum, (when the star is moving towards us) or towards the red end of the spectrum, (when the star is moving away from us). This shifting of the star's spectrum due to its radial motion relative to us is known as a doppler shift, and we can detect this shift in the star's spectrum, and therefore detect the radial component of the star's motion caused by the orbiting of the star/planet system. The animation below symbolically shows this doppler shift.
Changing Luminosity
The final commonly used method of detecting extrasolar planets does not depend on the motion of the star. Instead it depends on changes in the apparent brightness of the star as an orbiting planet passes between the star and us. Considering that we see a star as just a point of light, and how much smaller a planet is than a star, this detection method seems truly incredible, but it is possible to detect such changes in a star's apparent brightness. The concept is shown in the animation below. As a planet passes in front of a star during its orbit, the amount of light we receive from the star does indeed decrease by a very small amount. The "light curve" is used to illustrate this decrease in the star's brightness.
Next we will consider what types of planets we have found outside our solar system so far.
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