Searching for extra-solar planets (or other low mass companions such as brown dwarfs) is far from an easy task. An analogy that has been used is that it is like trying to find the light from a small flashlight when placed next to a large spotlight. However, there are ways to detect the flashlight in all that glare. Each has its benefits and its downfalls. Besides detecting transits, the two most commonly used means to search for extra-solar planets are the radial velocity method and the astrometric method. We describe radial velocity and astrometric planet searches below.

Radial Velocity Searches
The radial velocity method has been the most successful method used to date. As a planet revolves around a star, the gravity from the planet pulls slightly on that star.

For this method, researchers use an instrument called a spectrograph to study a star. This instrument takes the light of a star and breaks it down into a spectrum, or rainbow. Superimposed on this "rainbow" are many fine dark lines called absorption lines. Each of these lines represents a different element or compound in the atmosphere of the star. One can use the spectrum to accurately measure the wavelengths of the these lines and see if they change with time.

In the case of an extra-solar planet tugging on the parent star, these lines will shift in wavelength. As an object moves towards or away from us, the position of these lines shift back and forth. This Doppler shift is similar to the way the pitch of a train whistle changes as the train passes by. If the star is moving towards us, the spectrum is blueshifted. If it is moving away from us, it is redshifted.

By carefully monitoring a star's spectrum, we can see the small back and forth wobbles of a star as the planet goes around it. We can then tell roughly how massive the planet is (by how much it pulls on the star) and determine its orbital period.

However, this search technique requires observing one star at a time and a large allocation of observing time on a large telescope with a high-precision spectrograph. The mass of any planet found is also uncertain because the orientation of its orbit is generally not known except in cases where the planet transits.

Astrometric Searches
We can also see this wobble in a different way. If we look very closely at a star, we can see it trace out a small circle in the sky as the planet pulls on it.

The pull on a star as caused by an orbiting planet. As the planet revolves around the star, it pulls slightly on that star, actually causing it to move. However, the star is so massive that it moves very little. Imagine a long teeter-totter with ten children on one end and one at the other. To get the system to balance, the balance point must be closer to the large group of children. The center of balance is known as the barycenter in planetary systems. The small cross marks the position of the barycenter. Here the position has been exaggerated. Actually, the barycenter usually lies inside the star, explaining why the circle it traces is so difficult to see.

This technique proves to be rather difficult. Imagine a small circle the size of a dime that is placed 1200 miles (2000 km) away. This is the size of circle that needs to be measured to see a planet like Jupiter orbiting around a star like the Sun. The closer to the Earth the star being studied, the bigger the circle is and the farther away the star is the smaller the circle. The "dime" example represents the average expected circle for a detection by this process.

Despite the obvious difficulty in measuring this little circle in the sky, there is one planet candidate that may have been seen by this method. Despite this potential success, the promise for detecting many planetary systems with this method is poor. Seeing this small wobble from the surface of the Earth is quite difficult, and there are very few stars close enough to the Earth on which to use this technique.

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