The Universe and Galaxies: Extrasolar Planets

Discovery Space: Extrasolar Planets

Our solar system is not the only one of its kind; more and more planets in the vicinity of distant stars are being found all the time.

Even though finding planets similar to Earth is particularly difficult, it is an especially compelling search and remains the goal of many research projects.

In many regions of our galaxy, debris disks can be seen revolving around stars. It is from this debris that planetary systems are formed. However, discovering individual planets in the vicinity of distant stars is significantly difficult because a star and its planet(s) appear extremely close to one another from such vast distances. Further- more, a star like our sun outshines its planets to the extent that it is about a billion times brighter, thus the light reflected from a planet is barely detectable as a separate light source to its star.

A planet only reflects, it does not generate any visible light itself. However, this difference in brightness is somewhat less in the infrared spectrum because a planet does emit some infrared light or thermal radiation, even if it is only very little in comparison with the star it is orbiting.


Extrasolar planets, also called exoplanets, are usually named after the star they revolve around. Letters are also added to their names, indicating the order in which they were discovered, beginning with “b,” for instance, HD 38529 b, HD 38529 c, and so on.

Methods of proof

Most of the exoplanets discovered so far have a mass considerably larger than the Earth and are more comparable in size to the giant planets of our solar system. Furthermore, they revolve around their stars in very close orbits.

Modern methods and technology aid us in demonstrating that these bodies are planets. Under certain conditions even some smaller planets with less mass have been detected. Photo- graphs of planets near stars of normal brightness cannot yet be taken with tele- scopes, so proof that these bodies are planets is more indirect. Of the methods developed for this purpose, the transit method and the Doppler method are those most often applied.

The transit method is based on recording a star’s small drop in brightness when a planet moves in front of it, blocking some of the light. The Doppler-or radial velocity method is based on another effect: strictly speaking, a star and planet revolve around a common gravitational center, even if this center is located within the star itself. Within the orbit interval, the star’s orbit appears to wobble some-what, with heavier planets causing a greater deflection This can be proven with a spectroscopic examination of the star’s light. Scientists
are now developing methods for masking a star’s light.

With these methods, they can investigate even smaller planets that may be similar in composition to the Earth, and will be able to make analysis of their atmospheres. These methods are due to be ready for use from observatories in space within the next decade. Among the related international research projects are Darwin, based in Europe, and Terrestrial Planet Finder, based in the United States.


IN 1995, the first planet in the vicinity of a star similar to the sun, 51 Pegasi, was detected.

THE FIRST PLANETS orbiting other stars were discovered in 1992 and now number in the hundreds. More continue to be found with advances in technology.