Kepler is NASA’s first mission that is capable of finding Earth-sized worlds orbiting other stars and Kepler is a space telescope that is named after German astronomer Johannes Kepler. Kepler was launched into space on 7 March 2009 onboard a Delta II rocket. On 12 May 2009, Kepler completed its commissioning phase and started searching for planets around other stars.
Kepler utilizes the transit photometry method to detect extrasolar planets by precisely monitoring the brightness of about 150000 selected stars in its field-of-view. A transit occurs when a planet passes in front of its parent star and causes a slight decrease in the star’s apparent brightness from the blocking of a small fraction of the star’s light by the planet’s opaque disk. Therefore, transits only occur when a planet’s orbit around its parent star happens to be orientated nearly edge-on with respect to an observer’s line of sight. A larger planet will block out a greater fraction of the star’s light compared to a smaller planet and Kepler is sensitive enough to observe the miniscule drop in brightness when an Earth-sized planet transits a Sun-like star.
An Earth-sized planet transiting a Sun-like star will cause an 84 parts-per-million decrease in the star’s apparent brightness while a Jupiter-sized planet transiting a Sun-like star will cause a one percent decrease in the star’s apparent brightness. For the same star, the dip in its apparent brightness from the transit of a Jupiter-sized planet is over 100 times greater than the signal from a transiting Earth-sized planet. Kepler has to observe at least three transits to be sure that the dimming of a star is caused by a planet. Therefore, the discovery of Earth-sized planets in Earth-like orbits around Sun-like stars is expected to take three years or longer. However, Kepler has already turned out a myriad of potential planetary candidates during its first several days of observations.
On 15 June 2010, the Kepler mission team released data on all but 400 of the 706 targets from data collected during the first 43 days of Kepler’s nominal observation phase. This set of data contains viable extrasolar planet candidates with sizes ranging from as small as that of the Earth to larger than that of Jupiter! The paper detailing this is entitled “Characteristics of Kepler Planetary Candidates Based on the First Data Set - The Majority are found to be Neptune-Size and Smaller (2010)” and the appendix of this paper shows a list of 306 viable extrasolar planet candidates.
The publicly released list of 306 targets with viable extrasolar planet candidates shows that most candidate planets are significantly smaller than Jupiter. In fact, most of the candidate planets range in size from being slightly larger than the Earth to Neptune-sized worlds. Interestingly, 5 of the 306 targets are stars with multiple transiting candidate extrasolar planets and a paper entitled “Five Kepler Target Stars that Show Multiple Transiting Exoplanet Candidates (2010)” further describes this.
Data for 400 of the 706 targets with viable planetary candidates were not publicly released because they are bright enough for high-quality Doppler measurements or contain candidate planets with less than 1.5 times the diameter of the Earth, or both. Data for these 400 targets will only be released in February 2011.
Considering that there are approximately 470 known extrasolar planets as of July 2010, follow-up confirmations of the targets in this first set of data from Kepler is expected to dramatically increase the known planet count! Besides the discovery of Earth-sized planets, Kepler is also able to detect interesting objects such as a Saturn-style ring system around a planet or an Earth-sized moon of a gas giant planet. I bet that this initial set of data with 706 targets containing viable extrasolar planet candidates is just a sneak preview of things to come!