The Hunt for Extrasolar Planets
What if the Universe contains other planets like Earth outside the Solar System? After all, the Sun is only one of 100 billion stars in the Milky Way Galaxy alone. Why shouldn't similar extrasolar planets exist around other stars, perhaps in vast numbers?
Planet Hunting Strategies
For centuries, scientists have pondered the possible existence of extrasolar planets, but only in recent years have such planets actually been discovered. In 1995, University of Geneva astronomer Michel Mayor and his graduate student Didier Queloz detected the first planet orbiting a star other than the Sun. Since then, this exciting new field of astronomy has exploded. As of February 2007 more than 212 extrasolar planets have been discovered. Most of them are very close to their mother star, and are Jupiter-like: massive and gaseous.
Still, scientists have yet to see an extrasolar planet directly or even take a picture of one. Thus far, their presence has only been inferred from either their effect on the orbit of their host stars or by measuring the amount of infrared light they emit. The Spitzer Space Telescope is the only instrument that has successfully used the latter strategy. (To learn more about the Spitzer Space telescope, read "Seeing the Invisible.")
The job of finding and imaging extrasolar planets is extraordinarily difficult. The biggest challenge is the planets' feeble luminosity. A star, fueled by nuclear reactions, is typically a billion times brighter than any orbiting planet, whose light is primarily reflected from the host star. This overwhelming brightness renders nearby extrasolar planets invisible to astronomers. Invisible does not mean undetectable, however.
The Telltale Wobble
One technique scientists use to detect extrasolar planets is to look for a slight wobble in the motion of the host stars. The wobble is caused by the gravitational tug of the planet as it orbits the star. Astronomers predicted this effect decades ago, but the movement is miniscule and tricky to detect.
The key to detecting the wobble is a phenomenon called the Doppler effect. When you hear a police car approaching, the Doppler effect causes the sound of its siren to increase in pitch. Likewise, as the siren recedes, its pitch drops. The change in pitch occurs because the sound waves are being compressed or stretched. The same thing happens with light waves. When a star wobbles slightly in the direction of an observer on Earth, the light it emits appears to shift towards the blue end of the spectrum, which has shorter wavelengths. When the star wobbles away from Earth, the wavelengths stretch slightly and the light appears redder.
The shift in wavelength when a star wobbles is very tiny, so this technique has thus far detected only very massive planets. That's because the more massive the planet, and the closer it is to the host star, the larger and quicker the wobble. These instances produce relatively larger shifts in wavelength. For example, the Sun wobbles at about 12 meters (40 feet) per second in response to the gravity of Jupiter, the most massive planet in our Solar System.
Seeing Secondary Eclipses
The Spitzer Space telescope has pioneered a newer planet-finding technique called the "secondary eclipse method." It has added several more extrasolar planets to the roster since 2005. The method is possible only because Spitzer detects infrared wavelengths of light, not visible wavelengths. In visible light, the glare of a star overwhelms the light reflected by any planetary companions. But planets emit their own infrared radiation. When Spitzer observes a solar system, the star still outshines the planet, but much less than it would in visible light. This makes it easier to detect the planet itself.
Finding extrasolar planets with Spitzer is still hardly as simple as taking a snapshot. Instead, Spitzer relies on the fact that some planets periodically eclipse, or block, the light from their host stars. Imagine Spitzer has its infrared eye trained on a nearby star with a planet in orbit around it. When the planet passes in front of the star, it blocks a portion of the starlight. When the planet continues to circle around the back of the star, it no longer blocks the starlight. Spitzer can measure the amount of infrared light from the star plus planet vs. the star alone, subtracting to determine the amount of infrared light emitted by just the planet.
The secondary eclipse method does not image extrasolar planets directly. But it has allowed astronomers to measure the amount of infrared light emitted from extrasolar planets for the first time. More recently, Spitzer has focused its spectrometer instrument on two such planets, HD 209458b and HD 189733b, using the same secondary eclipse method. Spectrometers measure spectra—more detailed graphs of the specific wavelengths of light the planets emit and absorb. Spectra can be used to identify molecules in the atmospheres of the planets. That's because specific molecules absorb and emit specific wavelengths of infrared radiation as they vibrate. Using this method, scientists found no evidence of water around HD 209458b and HD 189733b. But they did find evidence of clouds of silicates, or tiny sand grains, in HD 209458b's atmosphere.
Several ground- and space-based missions are now in the works to search for extrasolar planets. Driving this quest is a curiosity that a world may be found with a composition similar to that of Earth, or a world with life forms on it. "Is Earth's environment unique?" asks American Museum of Natural History astrophysicist Rebecca Oppenheimer, "Or could others be spread across the Universe?" Like many of her colleagues, she suspects the answer is yes.