An Earth-mass planet in an eight hour orbit.

The Kepler Space Telescope has discovered a large number of planets with orbits of only a few days, mostly in the 'Hot Jupiter' class of objects, which have masses equal to or greater than that of Jupiter and orbit close to their stars, a class of objects that astronomers had not predicted prior to their discovery. As the project has built up more data the presence of smaller planets in short orbits has also been revealed. As part of a project to examine exoplanets in the Kepler Field of View with the shortest orbital periods, a team of scientists led by Roberto Sanchis-Ojeda of the Department of Physics, and Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology, describe an Earth-mass planet in an eight hour orbit about a G-type yellow dwarf star, the shortest known orbit of a confirmed planet, in a paper published on the arXiv online database at Cornell University Library on 17 May 2013.

The planet in question orbits the star KIC 8435766, which has approximately 84% of the Sun's mass, and an effective surface temperature of ~5143 K (compared to 5778 K for our Sun). The planet is named KIC 8435766b, and has approximately 1.1 times the mass of the Earth. It orbits the star once every eight and a half hours, and is estimated to have a surface temperature of 2300-3100 K.

An artists impression of a rocky planet close to a yellow star. Darke Max Macedo.

See also Questioning the possible planet of Alpha Centauri, Planet discovered orbiting the metal-rich K-type star HD77338, The HATNet project announces the discovery of three new Hot Jupiter type planets, Two Hot Jupiters found in the Beehive Cluster and Two new views of τ Boötis b.

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The planets of Gliese 163.

Gliese 163 (or GJ 163) is an M-class Red Dwarf Star approximately 48.6 light years from Earth in the constellation of Dorado, with about 40% of the mass of our Sun and an effective temperature of about 3500 K (compared to 5778 K for our Sun).

Map of Dorado showing the position of Gliese 163 (not naked eye visible). Planetary Habitability Laboratory.

In a paper published in the journal Astronomy & Astrophysics on 10 June 2013, and on the online arXiv database at Cornell University Library on 7 June 2013, Mikko Tuomi of the Centre for Astrophysics Research at the University of Hertfordshire and the Tuorla Observatory at the Department of Physics and Astronomy at the University of Turku and Guillem Anglada-Escudé of the Institut für Astrophysik at Universität Göttingen discuss the results of a study of Gliese 163 using data from the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at La Silla Observatory in Chile.

The study used the Doppler Method to look for planet sized bodies in the system. Planets exert a gravitational pull on the the stars they orbit, which in the case of smaller stars can be enough to make the stars wobble detectably. Astronomers look for variations in the apparent frequency of light produced due to the star moving slightly to-and-fro relative to us, and use these to calculate the presence, period and mass of any planets.

From this data Toumi & Anglada-Escudé conclude that the Gleise 163 system has at least three planets, with the possibility of a fourth.

The first of these, Gliese 163b (when naming bodies in other star systems stars are given a upper case letter, in this case Gliese 163A, and planets lower case letters, in this case Gliese 163b, 163c, 163d and possibly 164e), has a mass at least 9.9 times that of the Earth (larger than an of the inner four planets of our Solar System, but smaller than any of the outer four planets) and an orbital period of 8.63 days, from which Toumi & Anglada-Escudé calculate it orbits at a distance of 0.061 AU (i.e. 6.1% of 1 AU, the distance at which the Earth orbits the Sun).

The second planet, Gliese 163c, has a mass at least 8.7 times that of the Earth and an orbital period of 25.6 days, from which Toumi & Anglada-Escudé calculate it orbits at a distance of 0.126 AU. This is closer to Gliese 163A than Mercury is to our Sun, but as Gliese 163A is a smaller cooler star, potentially within its habitable zone (the zone within which a planet could have liquid water, and therefore life), which Toumi & Anglada-Escudé calculate to extend from 0.123 AU to 0.275 AU. Gliese 163c is is unlikely to be an Earth-like world, due to its high mass, but is clearly interesting and worthy of further study.

Model of the Gliese 163 system showing the orbits of the two innermost planets and the calculated area of the habitable zone. Planetary Habitability Laboratory.

The third confirmed planet, Gliese 163d, has a mass of at least 27 times that of Earth (considerably larger than Uranus or Neptune, but considerably smaller than Saturn or Jupiter), and an orbital period of 572 days, from which Toumi & Anglada-Escudé calculate it orbits at a distance of 1.00 AU, the same distance at which the Earth orbits the Sun (though outside the habitable zone of the smaller Gliese 163A).

Finally, the potential fourth planet, Gliese 163e, is calculated to have a mass 14 times that of the Earth (if it exists) and an orbital period of 125 days, from which Toumi & Anglada-Escudé calculate it orbits at a distance of 0.361 AU (inside the orbit of Gliese 163d).

See also The clear skies of GJ3470b, The atmosphere of GJ 1214b, Kepler 69c; a Super Venus rather than a Super Earth? Questioning the possible planet of Alpha Centauri and The planets of NN Serpentis.

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The clear skies of GJ3470b.

GJ3470b is a roughly Uranus mass planet (14.1 times the Earth's mass, as opposed to Uranus's 14.5 Earth masses), 82 light years from Earth in the constellation of Cancer, orbiting a Red Dwarf star with roughly 54% of the Sun's mass. It orbits its star at approximately 3.6% of the distance at which the Earth orbits the Sun, with a year of only eighty hours, for which reason it has been described as a Hot Uranus.

An artists impression of GJ3470b. National Astronomical Observatory of Japan.

GJ3470b is a transiting exoplanet seen from Earth; it passes in front of its star from out viewpoint, potentially allowing us to calculate its density and study its atmosphere. In a paper published in the Astrophysical Journal on 29 May 2013, and on the online arXiv database at Cornell University Library on 30 May 2013, a team of scientists led by Akihiko Fukui of the Okayama Astrophysical Observatory discuss the results of a study of GJ3470b at optical and near-infrared wavelengths.

Fukui et al. conclude that GJ2470b has an approximate average density of 0.94 g cm¯¹, compared to 1.27 g cm¯¹ for Uranus or 5.515 g cm¯¹ for Earth (or 1.0 for water). This is an average figure, and includes the atmosphere, plus any liquids or solids on the planet, it does not imply the planet is the same throughout. Fukui et al. suggest that the best fit for this density would be a planet with a small rocky core, surrounded by a thick layer of water or ice and a hydrogen rich atmosphere. Furthermore they conclude the atmosphere can be no more than 10% of the total mass (any more and it would boil away) and that the water/ice layer has approximately three times the mass of the rocky part.

They were also able to measure some of the optical properties of the atmosphere. If GJ3470b had a thick cloudy atmosphere it would appear as a solid disk, with little variation in the light it blocked from its parent star, whereas it is possible to detect two dips in stellar output as it crosses the disc of the star, which Fukui et al. conclude represent partial blocking of the star's light by the atmosphere and complete blocking by the planetary core; suggesting that GJ3470b has a reasonably clear atmosphere, lacking a thick cloudy layer. From this they suggest future studies my be able to examine the molecular composition of this atmosphere.

Diagram showing how an observer can detect a non-opaque atmosphere on a transiting planet. National Astronomical Observatory of Japan.

See also The atmosphere of GJ 1214b, Kepler 69c; a Super Venus rather than a Super Earth? The atmosphere of Wasp 12b, Two new views of τ Boötis b and Thermal imaging 55 Cancri e.

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