Searching for Extrasolar Planets

It is difficult to find planets around other stars because the light from the star is so bright that the glare drowns out the light reflected from the planet. It's like trying to see a lighted birthday candle placed in front of a search light. So, the only way to detect extrasolar planets at this time is to measure their effects on their parent stars. There are two ways in which planets affect their parent stars: They tug on the star as they orbit it, and they can dim the light from the star if they pass directly between the star and our field of view (eclipsing part of the star's light). The effects of these planetary motions on the star can be detected from Earth by three methods:

• Astrometry - measuring the star's precise position in the heavens
• Doppler spectroscopy - measuring the wavelength spread of light emitted from the star
• Photometry - measuring the intensity or brightness of the light emitted from the star

Astrometry
As a planet tugs on a star with its gravitational pull, it causes the star to wobble in its path across the sky. By making careful, precise measurements of the star's position in the sky, we can detect this extremely slight wobble. When we know the period of the wobble, we can calculate the period of the planet's orbit, the distance or radius of the planet's orbit and the mass of the planet.

Doppler Spectroscopy
As a planet orbits a star, it periodically pulls the star closer to and farther away from Earth (our observation point). This motion has an effect on the spectrum of light coming from the star.

Spectroscopic technique of detecting an extrasolar planet

As the star moves toward the Earth, the light waves coming from it are compressed and shifted toward the blue (shorter-wavelength) end of the spectrum. As the star moves away from us, the light waves are stretched out toward the red (longer-wavelength) end of the spectrum. These shifts in the spectrum of light coming from the star are called Doppler shifts. By making measurements of the star's spectrum over time, we can detect shifts that would indicate the presence of a planet. We can also use Doppler shifts to measure the radial velocity of the star's movement, which is how fast the star moves toward us and away from us.

Photo courtesy European Southern Observatory Radial-velocity measurements of the star Gliese 86 in the constellation Eriadni. The measurements indicate an extrasolar planet with an orbital period of 15.8 days. The calculated mass of the planet is about five times that of Jupiter.

Conceptually, we can deduce the size of the planet from the radial velocity. A massive planet will tug on the star with more gravitational force than a small one, causing the star to have a greater radial velocity. If we graph measured radial velocity versus time, we get a "sine" curve like the one shown above. From the period (peak-to-peak time or trough-to-trough time) and the star's mass, we can get the distance of the planet from the star -- the planet's orbital radius. From the amplitude of the curve, we can calculate its mass (see High precision Doppler measurements: The physics and techniques for finding planets and Doppler-Wobble Tutorial for details).

Photometry
If the orbit of an extrasolar planet is in a straight line of sight with Earth, the planet will pass directly between the star it's orbiting and Earth. When the planet passes in front of the star, it blocks some portion of the star's light, and the star gets slightly dimmer (by about 2 to 5 percent). The planet eclipses the star. As the planet passes behind the star, the star's normal brightness returns. By constantly measuring the star's light intensity over time, we can detect changes in its brightness that might indicate the presence of a planet or planets.

Photometric technique of detecting an extrasolar planet