NASA’s Kepler is a space telescope which hunts for planets around other stars by using a method called transit photometry. A transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as it passes in front. Transits only occur when a planet’s orbit around its parent star happens to be orientated almost edge-on with respect to our line of sight. Transit photometry works from observing the amount by which a star dims as a planet passes in front of it to determine the size of the planet. For the same star, a larger planet will occult a greater fraction of the star’s light as compared to a smaller planet. An Earth-size planet transiting a Sun-like star is expected to cause a mere 84 parts-per-million decrease in the star’s brightness and the incredible precision of Kepler’s photometer makes its suited for detecting such planets. Furthermore, by measuring the time between two successive transits, the orbital period and the distance of the planet from its host star can be determined.
The ‘Holy Grail’ for NASA’s Kepler mission is the eventual discovery of Earth-size planets orbiting in the habitable zones of Sun-like stars. However, this will require at least around 3 years of data collection by the space telescope, including a large amount of follow-up observations before these types of planets can emerge from the data. In the mean time, Kepler is already revolutionizing our understanding of extrasolar planets when on 1 February 2011, the Kepler mission team released data for 156453 stars that were observed by the Kepler space telescope from 2 May 2009 to 16 September 2009. During this period of observations, 1235 planetary candidates were detected and these planetary candidates are associated with 997 host stars. These planetary candidates are classified into five classes with 68 Earth-size candidates (less than 1.25 Earth-radii), 288 super-Earth size candidates (between 1.25 to 2 Earth-radii), 662 Neptune-size candidates (between 2 to 6 Earth-radii), 165 Jupiter-size candidates (between 6 to 15 Earth-radii) and 19 super-Jupiter size candidates (between 15 to 22 Earth-radii). For comparison, the radius of Neptune is about 4 Earth-radii and the radius of Jupiter is about 11 Earth-radii.
The planetary candidates in this first 4 months of data released by the Kepler mission team are strictly candidates only as each one of these planetary candidates will require rigorous follow-up observations in order for them to be confirmed as true planets. However, well over 90 percent of the 1235 planetary candidates are expected to be eventually confirmed as true planets due to the extremely meticulous vetting process through which these planetary candidates were extracted from the raw data in order to weed out events that are masquerading as transiting planets. The current set of data is restricted to planetary candidates with orbital periods of less than 125 Earth-days because planets with longer orbital periods will require more time by Kepler for multiple transits to be recorded.
It is interesting to note that out of the 68 Earth-size planetary candidates; around 2 dozen of them are actually somewhat smaller in size than the Earth. Furthermore, 54 of the 1235 planetary candidates are located within the habitable zones of their host stars and they have sizes ranging from Earth-size to larger than that of Jupiter. The habitable zone is basically defined as a region around a star where a rocky planet with an Earth-like atmosphere can have a surface temperature that lies between the freezing point and the boiling point of water. Of the 54 planetary candidates that are located within the habitable zones of their host stars, 5 of them are less than twice the size of the Earth while 2 of them are significantly larger than the size of Jupiter. Additionally, of the 5 approximately Earth-size planetary candidates, one of them is actually smaller than the Earth. Finally, it is also conceivable to expect the presence of potentially habitable Earth-size moons around some of the larger planets.
Of the 997 host stars that contain the 1235 planetary candidates, 170 of the host stars have two or more transiting planetary candidates. Of these 170 host stars, there are 115 stars with 2 transiting planetary candidates, 45 stars with 3 transiting planetary candidates, 8 stars with 4 transiting planetary candidates, one star with 5 transiting planetary candidates and finally, a single star with a staggering 6 transiting planetary candidates! In a recent press conference held on 2 February 2011, the 6 transiting planetary candidates orbiting that single star are no longer just planetary candidates as all 6 of them have been announced to be true planets orbiting the same star which has been named Kepler-11. The 6 planets are named Kepler-11b, 11c, 11d, 11e, 11f and 11g in order of increasing distance from Kepler-11. Since the size of a planet’s orbit is generally much larger than the physical size of its host star, being able to observe 6 planets transiting the same star means that the planetary system of Kepler-11 has to be remarkably flat where the orbital planes of all 6 planets have to be almost perfectly coplanar.
The planetary system of 6 transiting planets around the star Kepler-11 is an extraordinary and unprecedented discovery. The planets Kepler-11b, 11c, 11d, 11e, 11f and 11g have orbital periods of 10.30 days, 13.03 days, 22.69 days, 32.00 days, 46.69 days and 118.38 days respectively, sizes of 1.97 Earth-radii, 3.15 Earth-radii, 3.43 Earth-radii, 4.52 Earth-radii, 2.61 Earth-radii and 3.66 Earth-radii respectively and orbital distances of 13.6 million kilometres, 15.9 million kilometres, 23.8 million kilometres, 29.0 million kilometres, 37.4 million kilometres and 69.1 million kilometres respectively from their parent star Kepler-11. With so many transiting planets for a single star, observing more than one planet transiting the star at the same time is a rather frequent occurrence and on one occasion, the Kepler space telescope was observing three planets transiting the star Kepler-11 at the same time!
All the 6 planets around Kepler-11 have orbits that are almost perfectly circular and one of the most striking features is how close the orbits of the 5 inner planets are to one another. In fact, the inner 5 planets are all closer to their parent star than the planet Mercury is from our Sun and each of the 5 planets are not particularly small either as they have diameters ranging from two to over four times the diameter of the Earth. Dynamically, the inner 5 planets of the star Kepler-11 have one of the most densely packed configurations for any system of planetary orbits ever discovered. The 6th planet orbits significantly further from the star Kepler-11 than the inner 5 planets, but its orbit is still smaller than Venus’ orbit around our Sun!
The transits of a single planet around its host star are strictly periodic. However, for stars with more than one transiting planets, gravitational interactions among planets will cause the orbits of the individual planets to speed up and slow down by small amounts, leading to deviations from perfectly periodic transit timings. Such transit timing variations are strongest for stars with multiple transiting planets whose orbits are particularly close to one another, like in the case for the closely packed inner 5 transiting planets around the star Kepler-11. Hence, analysis of the transit timing variations have allowed the masses of the inner 5 planets around the star Kepler-11 to be estimated and this is the second time transit timing variation measurements have ever been employed to measure the masses of extrasolar planets.
Typically, the mass of an extrasolar planet is determined via radial velocity measurements by observing the Doppler shifts in the star’s spectral lines as the star wobbles back and forth periodically due to the gravitational tug of the orbiting planet. A large number of planets detected by Kepler will be very low mass planets and determining the masses of these planets by radial velocity measurements will not always be possible due to the incredibly small radial velocity amplitudes expected for such planets. Therefore, stars with multiple transiting planets offer a unique opportunity where any low mass transiting planets in such planetary systems can have their masses estimated from observations of transit timing variations which enables much smaller planetary masses to be measured as compared to radial velocity measurements. Knowledge about both the size and the mass of a planet allows the internal composition of the planet to be constrained.
The official paper detailing the release of the 1235 planetary candidates by the Kepler mission team is entitled “Characteristics of planetary candidates observed by Kepler, II - Analysis of the first four months of data” and it can be obtained from http://arxiv.org/abs/1102.0541. Additionally, the official paper on the 170 host stars with multiple transiting planets is entitled “Architecture and Dynamics of Kepler's Candidate Multiple Transiting Planet Systems” and it can be found at http://arxiv.org/abs/1102.0543v1. Finally, the paper confirming the planetary system of 6 transiting planets around the star Kepler-11 is entitled “A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11” and it can be found at http://arxiv.org/abs/1102.0291.