The Kepler Space Telescope observed a 115 square degree of space for four years (from May 2009 till May 2013), looking for potential planets around the 150 000 stars in the magnitude range 8-16 within the field. In this time it found a total of 4233 candidate planets, of which 965 have subsequently been confirmed. The conformation of such a planet requires follow up observations from ground-based telescopes in order to eliminate potential sources of false readings, such as previously undetected faint companion or background stars.
In a paper published on the arXiv database at Cornell University Library on 14 November 2014, and in the Astronomical Journal on 15 January 2015, a team of scientists led by Mark Everett of the National Optical Astronomy Observatory present the results of follow up observations on eighteen Kepler candidate stars, several of which are confirmed to host planets.
Follow up observations were made from the Differential SpeckleSurvey Instrument at Gemini North on 25−31 July 2013, although the observations were hampered by light leakage on 25 July and by Tropical Storm Flossie brushing the Hawaiian Islands on 29-30 July, so most of the useful observations were in fact made on 27 July 2013. Further follow up observations of several candidate stars were made with the Lick Observatory Shane 3.5 m, the PalomarObservatory Hale 5 m or the 10m Keck-II Telescopes.
The KOI 115 system (that is ‘Kepler Object of Interest’ 115, the designation given to star systems with unconfirmed planets in the Kepler field), has previously been confirmed to host two planets and redesignated as the Kepler 105 system (i.e. the 105th system in the Kepler Field confirmed to host one or more planets). These planets were reconfirmed by the Gemini North Survey. The star has an effective surface temperature of 6075 K (compared to 5778 K for our Sun) and a mass 1.027 times that of the Sun. The planets were previously found to have orbital periods of 5.4 and 7.1 days, the new data from Gemini North in addition suggests that these planets have radii of 2.54 and 1.44 times that of the Earth, respectively.
The KOI 265 system has a candidate planet with a radius of 1.71 times that of the Earth, with an orbital period of 3.6 days. Everett et al. validate this planet with a 94% certainty level, though this is not considered sufficient to confirm the existence of a planet.
KOI 268 is a dwarf star with an effective surface temperature of 6343 K, with a candidate planet orbiting every 110 days. Everett et al.’s data reveals the presence of two previously undiscovered nearby stars, identified as neighbours B and C. Of these neighbour B is a small star with an effective surface temperature of 4007 Kwhich lies close enough to KIO 268 that the candidate planet is impossible to validate. It could be a planet with a radius of 3.04 times that of the Earth orbiting KOI 268, but it could also be a planet with a radius of 9.33 times that of the Earth orbiting Neighbour B (a larger planet would be needed to reduce the total light produced by the two stars if the planet is occluding the dimmer star); if the latter were the case then it would have a low equilibrium temperature, about 217 K.
KOI 274 has previously been confirmed to host two planets with periods of 15 and 22.8 days, and redesignated as Kepler 128. Everett et al. were able to confirm theses planets, and that the star is a single system. They also estimate that the planets both have about 1.2 times the radius of the Earth.
KOI 284 has previously been confirmed to host three planets, and a possible fourth, and redesignated as Kepler 132. However a nearby star makes it hard to resolve the system further; it was impossible to determine whether this star is a companion (orbiting the main star) or a field star (behind the star and unrelated to it, close only because of the angle from which we are viewing the system), though a companion is thought to be the more likely scenario. If this is the case then all of the planets, including the unconfirmed candidate, will have a radius between 1-2 times that of the Earth.
KOI 369 is a dwarf star with an effective surface temperature of 6157 K has previously been confirmed to host two planets, and redesignated as Kepler 144. Everett et al. are able to reconfirm these planets and estimate that they have radii of 1.78 and 1.69 times that of the Earth, respectively.
KOI 1537 is a dwarf star with an effective surface temperature of 6260 K and a candidate planet with a period of 10 days. A previous study suggested that a second star was close to this star, either as a companion or a field star, however Everett et al. were able to find no evidence for this second star. They were able to validate the planet with an 86.7% certainty (not enough to confirm its existence) and estimate its radius to be 1.35 times that of the Earth.
KOI 1964 is a dwarf star with an effective surface temperature of 5574 K and a candidate planet with an orbital period of 2.2 days. It also has a neighbouring star about 4” (four arc seconds; the sky, imagined as a globe, is divided into 360 degrees, each of which is divided into 60 arcminutes, with each arc minute being further divided into 60 arcseconds) to its north, as seen from Earth. This makes it very hard to confirm the planet is orbiting KOI 1964, though Everett et al. were able to come up with three possible scenarios. Firstly the second star could be a bound companion to KOI 1964, and the planet could also orbit KOI 1964 and have a radius of 0.764 times that of the Earth. Secondly, the second star could be a field star somewhere in the background and the planet could orbit KOI 1964 and have a radius of 0.785 times that of the Earth. Thirdly the second star could be a field star, and the planet could orbit that star, in which case the star would have an effective surface temperature of 3892 K and the planet a radius of 2.03 times that of the Earth.
Example high resolution imagery of KOI 1964 and its surroundings in 4 filters. The upper two panels are reconstructed images from speckle observations at 692 nm (upper left) and 880 nm (upper right) taken at Gemini North. The lower two panels are adaptive optics images at J (lower left) and Ks (lower right) taken at the Palomar Hale Telescope. Each image is oriented with North at the top and East to the left. The speckle images are 1.8′′ ×1.8′′ and the adaptive optics images are approximately 15′′ ×15′′ as seen by the scales. A faint neighbour star is detected 0.4′′ to the north of the brighter KOI star. Everett et al. (2014).
KOI 2311 is a sunlike dwarf star with two candidate planets, orbiting with periods of 192 and 14 days. There is a faint second star at a separation of 1”, making it hard to establish the true nature of the planets. Everett et al. Calculate that if the inner planet orbits KOI 2311 then it has a radius of 0.932 that of the Earth and if it orbits the second star it has a radius of 4.16 times that of the Earth. The if the outer planet is orbiting KOI 2311 then it is calculated to be cool (337 K) and have a radius of 115 times that of the Earth, while if it orbits the second star then it is colder (117 K) and has a radius of 5.14 times that of the Earth (making it larger than Neptune).
KOI 2365 is a sunlike star with two candidate planets. This was found to be a solitary star, and the planets were validated with a confidence of 99.9%, considered enough to confirm their existence. The system is therefore redesignated as Kepler 430, and the planets as Kepler 430b (when naming objects in other star systems planets are given lower case letters and stars upper case letters), which has a 36 day orbital period, a radius of 3.25 times that of the Earth and a temperature of 667 K, and Kepler 430c, which has an 111 day orbital period, a radius of 175 times that of the Earth and a temperature of 458 K.
KOI 2593 is another isolated star, this time with a single candidate planet. Everett et al. were only able to validate the planet with a 90.6% confidence, though they did establish the star has a surface temperature of 6119 K and the potential planet has a mass of 1.10 times that of the Earth and a temperature of 974 K.
KOI 3097 is an isolated star with three candidate planets. Everett et al. were able to establish that the star has a surface temperature of 6004 K, and validate the planets with a confidence of 99.8%. The system is redesignatedKepler 431, and the planets become Kepler 431b, with a 6.8 day orbital period, a radius of 0.764 times that of the Earth and a temperature of 1032 K, Kepler 431c with an 8.7 day orbital period, a radius of 0.668 times that of the Earth and a temperature of 951 K, and Kepler 431d with an 11.9 day orbital period, a radius of 1.11 times that of the Earth and a temperature of 865 K.
KOI 3204 is a hot dwarf star with a surface temperature of 7338 K and a candidate planet which orbits it every 0.57 days. Everett et al. validated the planet with a confidence of 98.5% (not enough to confirm it), and calculate it has a radius of 1.01 times that of the Earth and a temperature of 3268 K.
KOI 3224 is an isolated dwarf star with a single candidate planet. Everett et al. calculate that the star has a surface temperature of 5382 K, and validate the planet with a 90.5% certainty. They calculate that this candidate planet has a radius of 0.667 times that of the Earth, and a temperature of 1129 K.
KOI 3255 is a faint, cool dwarf star with a surface temperature of 4427 K, a single candidate planet with a 66.7 day orbital period, and two close stars, the relatively bright neighbour B at a separation of 0.18”, and the fainter neighbour C at 3”. The closeness of neighbour B makes it impossible to validate the planet, though Everett et al. calculate that it could be orbiting KOI 3255, in which case it would have a radius of 2.11 times that of the Earth and a temperature of 294 K, or neighbour B, in which case it would have a radius of 2.42 times that of the Earth and a temperature of 276 K, either of which scenarios would make it a habitable zone candidate.
KOI 3284 is the smallest star included in this study. It has a surface temperature of 3688 K (smaller stars are cooler than larger stars) and a candidate planet with a 35 day orbital period. It also has two close stars, neighbour B which has a separation of 0.44” and neighbour C. Because of the closeness of neighbour B it was impossible to validate the planet, nor was it possible to determine if neighbour B was a companion or a field star, though neighbour C did appear to be a background star. Everett et al. calculate that if neighbour B is a background star and the planet orbots KOI 3284 then the planet will have a radius of 0.99 times that of the Earth and a temperature of 272 K, that if it orbits KOI 3284 and neighbour B is a bound companion it will have a radius of 1.00 times that of the Earth and a temperature of 272 K, or if it orbits neighbour B and neighbour B is a bound companion to KOI 3284, then the planet will have a radius of 1.46 times that of the Earth and a temperature of 184 K, scenarios that make it a small habitable zone candidate.
KOI 4407 is a dwarf star with a single candidate planet that has a 1.34 day orbital period, and two close by stars that might be gravitationally bound companion stars or background field stars. It was possible to calculate that the surface temperature of KOI 4407 is 6408 K, but the presence of the neighbour stars makes it difficult to characterise the planet. Everett et al. were able to validate this planet, but only with a confidence of 19.2%. They calculate that if the two neighbour stars are actually components of a triple star system, then the planet probably has a radius of 0.65 times that of the Earth and a temperature of 2121 K, though the radius could possibly be anywhere between 0.64 and 0.75 times the Earth’s.
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