Kepler is a planet-hunting telescope that searches for planets by precisely measuring the dip in a star’s brightness when a planet happens to pass in front of its host star in an event known as a transit. The primary goal of Kepler is to determine the frequency of Earth-like planets around Sun-like stars. To date, over 2000 planet candidates have been found by Kepler with a trend towards an increasing number of Earth-size planet candidates at larger distances from their host stars as such planet candidates are harder and take longer to sniff out. An independent project to reanalyse the existing public Kepler dataset revealed 84 new transit signals on 64 star systems. Each transit signal represents a planet candidate and I shall describe some of the more notable ones.
Image Credit: NASA/JPL-Caltech
This system was known to contain two transit signatures with the first having a period of 20.55 days and the other yet to be determined. The four new transit signatures found in this reanalysis have periods of 3.93 days, 33.04 days, 66.30 days and 9.92 days. Previously, the only know star with six transiting planets was the Kepler-11 system.
A transiting planet candidate with a period of 114.74 days is known to exist around KOI 1574. In this reanalysis, a second signal with a period of 191.51 days was detected. The outer planet candidate seems to be in a 3:5 orbital resonance with the inner one. What is more interesting is that the outer planet candidate is about twice the Earth’s diameter and its equilibrium temperature is estimated to be 281 degrees Kelvin. Given that the average surface temperature of the Earth is 287 degrees Kelvin, the outer planet candidate of KOI 1574 is a promising Earth-like planet.
KOI 277 (Kepler-36)
Previously known to have a single transiting planet candidate with a period of 13.84 days, this reanalysis found a second transit signal with a period of 16.24 days. The two planet candidates around KOI 277 are remarkable in the sense that their orbits bring them within 5 Earth-Moon distances from one another. On the last day of work on the paper detailing this reanalysis of the Kepler dataset, the Kepler team announced their discovery of the two planets around KOI 277 and named the system Kepler-36. The inner planet (Kepler-36b) is a super-Earth and the outer planet (Kepler-36c) is a mini-Neptune. These planets are twenty times more closely spaced than any adjacent pair of planets in the Solar System. At closest approach, an observer on Kepler-36b will see Kepler-36c appear 2.5 times larger than the Moon as seen from Earth.
With two planet candidates already known with periods of 4.19 days and 6.36 days, a third transit signal with an incredibly short period of 0.176 days (4.25 hours) was revealed in this reanalysis. For each day on Earth, 5.65 years would have elapsed on this planet candidate. KOI 1843 is a cool and small star with an effective temperature of 3673 degrees Kelvin and 52 percent the Sun’s diameter. The third transit signal has a very small transit depth of 120 parts-per-million which means that this planet candidate is just 68 percent the diameter of the Earth, making it one of the smallest exoplanet candidates known to date.
Reference: Aviv Ofir and Stefan Dreizler (2012), “An Independent Planet Search in the Kepler Dataset. I. A hundred new candidates and revised KOIs”, arXiv:1206.5347v1 [astro-ph.EP]