Detecting other Solar Systems

Although astronomers have long assumed that many other stars have planets, they have been unable to detect these other solar systems until recently. Planets orbiting around stars other than the Sun are called extrasolar planets. Planets are small and dim compared to stars, so they are lost in the glare of their parent stars and are invisible to direct observation with telescopes.

Astronomers have tried to detect other solar systems by searching for the way a planet affects the movement of its parent star. The gravitational attraction between a planet and its star pulls the star slightly toward the planet, so the star wobbles slightly as the planet orbits it. Throughout the mid- and late 1900s, several observatories tried to detect wobbles in the nearest stars by watching the stars’ movement across the sky. Wobbles were reported in several stars, but later observations showed that the results were false.

In the early 1990s, studies of a pulsar revealed at least two planets orbiting it. Pulsars are compact stars that give off pulses of radio waves at very regular intervals. The pulsar, designated PSR 1257+12, is about 1,000 light-years from Earth. This pulsar's pulses sometimes came a little early and sometimes a little late in a periodic pattern, revealing that an unseen object was pulling the pulsar toward and away from Earth. The environment of a pulsar, which emits X rays and other strong radiation that would be harmful to life on Earth, is so extreme that these objects would have little resemblance to planets in our solar system.

The wobbling of a star changes the star’s light that reaches Earth. When the star moves away from Earth, even slightly, each wave of light must travel farther to Earth than the wave before it. This increases the distance between waves (called the wavelength) as the waves reach Earth. When a star’s planet pulls the star closer to Earth, each successive wavefront has less distance to travel to reach Earth. This shortens the wavelength of the light that reaches Earth. This effect is called the Doppler effect. No star moves fast enough for the change in wavelength to result in a noticeable change in color, which depends on wavelength, but the changes in wavelength can be measured with precise instruments. Because the planet’s effect on the star is very small, astronomers must analyze the starlight carefully to detect a shift in wavelength. They do this by first using a technique called spectroscopy to separate the white starlight into its component colors, as water vapor does to sunlight in a rainbow. Stars emit light in a continuous range. The range of wavelengths a star emits is called the star’s spectrum. This spectrum has dark lines, called absorption lines, at wavelengths at which atoms in the outermost layers of the star absorb light.

Astronomers know what the exact wavelength of each absorption line is for a star that is not moving. By seeing how far the movement of a star shifts the absorption lines in its spectrum, astronomers can calculate how fast the star is moving. If the motion fits the model of the effect of a planet, astronomers can calculate the mass of the planet and how close it is to the star. These calculations can only provide the lower limit to the planet’s mass, because it is impossible for astronomers to tell at what angle the planet orbits the star. Astronomers need to know the angle at which the planet orbits the star to calculate the planet’s mass accurately. Because of this uncertainty, some of the giant extrasolar planets may actually be a type of failed star called a brown dwarf instead of planets. Most astronomers believe that many of the suspected planets are true planets.

Since 1995 astronomers have discovered more than 160 extrasolar planets. Astronomers now know of far more planets outside our solar system than inside our solar system. Most of these planets, surprisingly, are more massive than Jupiter and are orbiting so close to their parent stars that some of them have years (the time it takes to orbit the parent star once) as long as only a few days on Earth. These solar systems are so different from our solar system that astronomers are still trying to reconcile them with the current theory of solar system formation. Some astronomers suggest that the giant extrasolar planets formed much farther away from their stars and were later thrown into the inner solar systems by some gravitational interaction.