What is the evidence for planetary systems around other stars?

For hundreds of years, philosophers have speculated about the possibility of planetary systems around stars other than our Sun. These exoplanets — extrasolar planets — are too distant, small, and dim to be seen directly, even with a large telescope such as the Hubble Space Telescope. (Recall that planets shine by reflected light from a star, not of their own light.) Nonetheless, astronomers have employed several different methods to search for exoplanets.

The planetary transit method is the most "direct" detection method available. Astronomers look for the drop in brightness of a star as an orbiting planet transits or passes in front of it in our line of sight. The mass and radius of an orbiting planet may be determined from the transit data.

This site shows a transit lightcurve.

Credit: NASA

Other methods try to detect the "wobble" as the star orbits around the center of mass of the stellar system. Even in our solar system, where the Sun is so much more massive than even the biggest planets, the center of mass is just outside the Sun, and the sun orbits around this center of mass point.

The "astrometric wobble" method — detecting the back-and-forth wobble of a star against a backdrop of background stars — is very difficult. It will not have much chance of success until the launch of planned space-based telescopes designed for this.

The radial velocity or Doppler wobble method is successful. This method detects the star's wobble through subtle red- and blue-shifts in its spectral lines.

Gravitational lensing is another method. Just as a lens bends light rays, the gravity of the star with the planetary system can bend light from a background star that happens to be in the line of sight. The gravitational lens formed by the star under consideration makes the background star appear brighter. The lensing happens by chance as the foreground star moves through space. If there is a planet around the star, it will contribute to the brightening effect.

Exoplanets have been discovered.

The first planet to be found around a Sun-like star was detected using the Doppler wobble method. In 1995, Michel Mayor and Didier Queloz at the Geneva Observatory in Switzerland found a wobble in 51 Pegasi, a star about 40 light-years away. The data indicate a Jupiter-like planet orbiting extremely close to the star.

Hundreds of exoplanets have been found to date. Most have been found using the Doppler wobble method. In some cases, the star is discovered using the Doppler wobble method, and then a transit of the planet is observed. Many systems have Jupiter-sized planets relatively close to their parent stars.

In 1995, a Swiss team discovered 51 Pegasi, 48 light-years from Earth. Credit: NASA

The search for exoplanets is connected to some of the most profound questions we can ask: are we alone in the universe? Has intelligent life arisen elsewhere? Whether the answer is yes or no, the implications are profound.

Good places to seek updates on discoveries: