Two planets transiting their star
This illustration shows the planets of the Kepler-9 system, passing in front of the system’s central star.
Credit: NASA/Ames/JPL-Caltech

Planets orbiting other stars are termed exoplanets or extrasolar planets. The first exoplanet was discovered in 1992. There are now approximately 500 exoplanets known. NASA recently announced that its Kepler spacecraft has discovered two new exoplanets, passing in front of, or transiting, their parent star, termed Kepler-9.
The planets orbiting Kepler-9 were designated Kepler-9b and 9c. The discovery is the result of seven months of observations of more than 156,000 stars, as part of an ongoing search for Earth-sized exoplanets.

Kepler's ultra-precise camera measures tiny dips in the stars' light that occur when a planet transits them. The size of the planet can be derived from these temporary brightness drops. The distance of the planet from its star can be calculated by measuring the time between successive dips, as the planet circles the star. Small variations in the regularity of these dips can be used to determine the masses of planets and discover other non-transiting planets in the system.

In addition to the Kepler-9a and Kepler-9b, Kepler scientists have also identified what seems to be a third, much smaller transit signature in the observations of Kepler-9. That signature is consistent with the transits of an exoplanet, about 1.5 times the radius of Earth, in a very tight orbit around its star.

Most of the known exoplanets have been discovered by indirect methods, rather than imaging. Methods of detection include observing the gravitational influence of an exoplanet on the path of its star. The majority of the known exoplanets are giant, Jupiter-like planets. However, several exoplanets have been shown to be comparable to Earth in size.

References

NASA Press Release
http://www.nasa.gov/centers/ames/news/releases/2010/10-73AR.html

Kepler Mission
http://www.nasa.gov/mission_pages/kepler/main/index.html

Wikipedia

Aymen Mohamed Ibrahem
Senior Astronomy Specialist