Looking for a planet outside our solar system is difficult, but not impossible. For years, scientists failed to find any extra-solar planets. However, since the discovery and confirmation of one such planet in 1995, the daunting task has become more possible. Once astronomers found a successful technique, they continued to discover extra-solar planets at an increasing rate.

The discovery of the first extra-solar planet confused the scientific community. A massive planet, roughly the size of Jupiter, seemed to be orbiting its star in just four days. Such a short orbit indicates that the planet is very close to the star, but according to the most accepted theory of solar system formation, such a massive planet should have a fairly large orbit. Our solar system neatly fits the formation theory, with smaller rocky planets near the Sun while larger gaseous planets reside in the cooler distant regions. It seemed impossible to have a Jupiter-sized planet orbiting so closely to its star. As the observations were confirmed, astronomers questioned their understanding of solar system formation. They have since realized that the formation theory may still be correct, but that massive planets may be able to migrate inwards some time after the initial formation.

Most of the discovered extra-solar planets have been massive — ranging from the size of Neptune to ten times the size of Jupiter. Rather than indicating that large planets are more common than small ones, these findings may just be a result of massive planets being easier to find. Our instruments are not yet advanced enough to detect smaller, Earth-like planets, but new technology is being developed to improve the search.

Current research techniques rely on indirect evidence of planets. Indirect searches rely on observations of effects caused by the planet. The majority of planets have been found with the radial velocity, or Doppler, technique, which looks for a back-and-forth shift in the star's spectra due to the gravitational pull of the planet as it orbits the star. Other methods of detection include: the astrometric technique, which looks for a side-to-side wobble against the background stars; the gravitational lensing technique, which looks for a change in the positions of background stars; and the transit technique, which looks for the periodic dimming of the star by the crossing of a planet.

Learn in this NOVA classroom activity how planetary spectra can be used to search for life on other worlds.

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