III.

Locating Extrasolar Planets

Astronomers need sophisticated techniques to locate extrasolar planets. Planets reflect the light of their stars, but a star is millions or billions of times brighter than its planet. The distance between a star and planet is usually so relatively small that the star’s light obscures the planet from view. The most powerful optical telescopes cannot pick out a planet against the glow of its parent star. Sensitive brightness-measuring instruments called photometers, however, can sometimes detect the dimming of a star as its planet passes in front of it. Some warmer planets emit low levels of infrared radiation (light with longer wavelengths than visible light), and astronomers have recently begun detecting planets directly with telescopes sensitive to infrared radiation. See also Photometry; Infrared Astronomy.

Planets are so difficult to observe directly that astronomers usually have to find them indirectly, by observing the behavior of the host star. When planets orbit stars, the gravitational attraction between the star and the planet keeps the planet circling the star. This gravitational attraction also has an effect on the star. Stars are much more massive than planets, though, so the effect the pull has on the star is much smaller than the effect it has on the planet. The pull between the planet and the star is just strong enough to make the star wobble slightly. See also Orbit; Gravitation.

Astronomers detect the telltale wobble either by watching the star very carefully, or by analyzing the star’s light to see how it changes as the star moves slightly toward and away from Earth. The first technique works if the gravitational pull on the star is very strong and the star is relatively close to Earth. If it is, powerful telescopes can directly detect the back-and-forth movement of the star. Even for the largest planets, however, the movement of the star is tiny and difficult to detect.

The second technique—analyzing the star’s light—is much more powerful and successful. This technique uses the Doppler effect, a change in the appearance of a star’s light caused by the star’s movement. When the gravitational pull between a planet and star pulls the star around in a tiny circle, the star moves alternately away from and toward Earth. When the star moves away from Earth, each wave of light leaves the star from slightly farther away than the wave of light before it, making the distance between waves (called the wavelength) longer. When the star is moving toward Earth, each wave of light leaves from slightly closer to Earth than the one before it did, making the wavelength shorter. This change in wavelength, and consequently, in the frequency and color of the light, is called the Doppler effect.

Astronomers detect Doppler effects in starlight by separating the light of a star into the light’s colors in a process called spectroscopy. The elements present in a star emit light especially strongly in particular colors, creating bright lines on a star’s spectrum, or its range of color. The wavelength of light defines its color. Red light has a longer wavelength than green light, which has a longer wavelength than blue light. The movement of the star shifts the star’s spectrum toward the red end (if the star is moving away from Earth) or toward the blue end (if the star is moving toward Earth). Astronomers watch for the regular changes in a star’s spectrum to show the presence of a planet.

Astronomers can also detect the presence of a planet through a phenomenon known as a gravitational lens. This method, known as a microlensing event, is based on a small brightening effect that occurs when an object passes in front of a star from our line of sight on Earth. The gravitational field of the closer object acts as a lens, momentarily amplifying the light from the more distant star. The lensing object is usually a star, but a companion planet can also cause a smaller brightening effect—a microlensing event—and can be detected over a shorter period of time, such as a day or a few weeks, corresponding to the time it takes for the planet to orbit its companion star. A team of astronomers has created a project known as the Optical Gravitational Lensing Experiment (OGLE) to detect extrasolar planets using this method.

When a planet passes in a front of the star it orbits—an event called a transit—it causes a small dip in the brightness of the star. Measuring the slight change in the brightness can be used not only to directly detect a planet, but to determine its size and orbit. However, the planet needs to orbit in a plane that lies in a telescope’s line of sight on the star. Despite long odds, Earth-based telescopes have detected and studied a few exoplanets using this method.

The first space telescope designed to search for extrasolar planets also uses this transit method. Called COROT (COnvection, ROtation and planetary Transits), the mission was developed by the French space agency with the ESA and a group of countries including Brazil. COROT was launched in 2006 and may detect planets the size of Earth or larger that orbit close in to a star.

NASA’s Kepler space telescope looks for planetary transits, as well. Planned for launch in 2008, Kepler has a larger telescopic mirror than COROT. Kepler could find extrasolar planets in orbit at Earth’s distance from the Sun. It may also be able to see light reflected off planets. Kepler is designed to detect planets the size of Earth and smaller.

More sophisticated space telescopes are being planned that could selectively block out the light from a particular star, a method called occultation. This approach could allow extrasolar planets in any orbital plane to be seen directly without the glare of the star they orbit.