Conformation of the third planet in the Kepler-51 System.

Kepler-51 is a G-type Yellow Dwarf star 2800 light years from Earth in the constellation of Cygnus. It has a mass 1.04 times that of the Sun, a radius 0.94 times that of the Sun and an effective surface temperature of 6017 K (compared to 5778 K for the Sun). It is a young star, thought to be about 300 million years old. When it was originally designated as a potential planet hosting star in the Kepler Survey it was given numbered KOI-620 (Kepler Object of Interest-620); this was updated to Kepler-51 in 2013, when two transiting planets (planets that pass in front of the star when seen from Earth) in the system were confirmed, Kepler-51b and Kepler-51c; when naming objects in other stellar systems stars are given upper case letters and planets lower case letters, in this instance the star in the Kepler-51 system would be Kepler-51A, though as the only star in the system it is usually referred to simply as Kepler-51. A third probable planet in the system could not be confirmed, and therefore retains the designation KOI-602.02 (planets in the Kepler Survey that have not been confirmed retain a KOI designation, even if the system has been re-designated due to the discovery of other planets).

Kepler-51b (formerly KOI-620.01) was found to have an orbital period of 45 days, a radius equal to 0.07074 times that of Kepler-51A, and a maximum mass of 2.33 times that of Jupiter. Kepler-51c (formerly KOI-620.03) was found to have an orbital period of 85 days, a radius 0.0573 times that of Kepler-51A, and a maximum mass of 2.60 times that of Jupiter. KOI-620.02 was found to have a (probable) orbital period of 130 days and a radius 0.0972 times that of Kepler-51A (it was not possible to estimate the mass of this body). 

The three bodies have orbital periods close to a 1:2:3 orbital resonance. Orbital resonances occur when bodies passing close to one-another exchange some momentum, causing one to slow and the other to accelerate at each pass. Once this starts to happen the two bodies must either settle into a stable resonance or else one will be forced out of its orbit onto a new trajectory. Given that the planets of the Kepler-51 system are all quite close together, it would be expected that they would be in resonant orbits.

In a paper published on the online arXiv database at Cornell University Library on 12 February 2014, Kento Masuda of the Department of Physics at The University of Tokyo presents a new study of the Kepler-51 system, also based upon Kepler Space Telescope data, which confirms the presence of a third planet in the system.

Masuda calculates that Kepler-51b has a mass only 2.1 times that of the Earth, but a radius 7.1 times that of the Earth, and that it orbits Kepler-51A at a distance of 0.25 AU (i.e. 25% of the distance at which the Earth orbits the Sun, and considerably less than the distance at which Mercury orbits the Sun), with an orbital period of 45 days. He also estimates the average equatorial temperature on the planet to be 543 K (270˚C).

He calculates that Kepler-51c has a mass of approximately 4.0 times that of the Earth, a radius 9.0 times that of the Earth, and orbits at a distance of 0.38 AU (38% of the distance at which the Earth orbits the Sun, and slightly less than the distance at which Mercury orbits) with a period of 85 days. He further estimates that the planet has an average equatorial temperature of 439 K (166˚C).

KOI-620.02, now redesignated Kepler-51d, is calculated to have a mass 7.6 times that of the Earth and a radius 9.7 times that of the Earth. It orbits Kepler-51A at a distance of 0.51 AU (51% of the distance at which the Earth orbits the Sun; greater than the distance at which Mercury orbits the Sun, but still considerably less than the distance at which Venus orbits) with a period of 130 days. The planet is estimated to have an average equatorial temperature of 381 K (108˚C), and to lie in the innermost part of the habitable zone of the Kepler-51 system (i.e. the zone in which liquid water might potentially exist).

All of these planets have extremely low densities, which can only be explained if they have substantial hydrogen and/or helium atmospheres. The presence of large, low density planets in orbits close to their host stars has become one of the hardest phenomena to explain in the study of exoplanets. Our understanding of how planetary formation occurs suggests that such planets must be formed beyond the system’s Snow Line; the point beyond which these elements can for ice, which is capable of accumulating into large bodies. In the inner part of a stellar system these elements would be gaseous, due to heating from the star, and could not therefore collect together to form the atmosphere of a planet. Such planets are therefore usually explained by formation in the outer part of a stellar system followed by inward migration, but this is in many cases somewhat hard to explain, particularly in cases such as Kepler-51 where young, hot, low density planets appear to be packed into close, resonant orbits, the evolution of which is difficult to model, particularly where short time periods are available for this evolution to occur.

Masuda also reported the occurrence of a planetary eclipse in the Kepler-51 system; i.e. one of the planets passing in front of another as both transit the star. This is only the second time such an event has been reported, the first having occurred in the Kepler-89 system. This event allows for the measurement of the relative inclinations of the orbits of the planets involved, in this case Kepler-51b and Kepler-51d. Surprisingly this angle of inclination is quite large, with the orbit of Kepler-51d apparently tilted at an angle of 25.3˚ relative to the orbit of Kepler-51b. Such a mismatch between two resonant planets in close orbits is likely to be extremely unstable, unless Kepler-51c orbits at a precisely intermediate angle, which would make the evolution of the system even harder to explain.

Trajectories of the two planets for the best-fit PPE model. This is a snapshot at the time when the two planets are closest in the plane of the sky. Masuda (2014).

See also…

 The planets of HD 141399.

HD 141399 is a K-type orange dwarf star 118 light years from Earth in the constellation of Boötes. It has a mass 1.14 times that of the Sun, a radius 1.46 times the Sun’s, an effective surface temperature of 5360 K (compared to 5778 K for the Sun), and is 1.59 times as luminous as the Sun.





 Kepler 186f: an Earth-sized planet in the habitable zone of a Red Dwarf star.

One of the key objectives in the search for planets orbiting other stars has been to locate planets in the habitable zones of such stars, i.e. planets on which liquid water could potentially exist. A number of...



 The Kepler 210 planetary system.

Since it began its survey of the section of the sky dubbed the Kepler Deep Field in 2009, the Kepler Space Telescope has discovered 2321 probable planets around 1790 stars, and it is thought that the vast majority of these probable planets will turn out to be true...

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Citizen scientists discover a sub-Jovian planet in a quaternary star system.

The Planet Hunters is a citizen science project (i.e. a science project carried out largely by members of the public) which asks visitors to a website to look at randomly selected 30 light-curves from the Kepler Space Telescope, and look for patterns. Volunteers who find patterns that may indicate planets can flag these up for investigation by astronomers.

In a paper published on the online arXiv database at Cornell University Library on 12 October 2012, a team of scientists led by Megan Schwamb of the Yale Center for Astronomy and Astrophysics and the Department of Physics at Yale University describe the first planet discovered by the Planet Hunters project, a sub-Jovian planet in the KIC 4862625 star system (Kepler Input Catalogue 4862625), roughly 5000 light years from Earth in the constellation of Cygnus.

The planet, named PH1, has a maximum possible mass 169 times that of the Earth, slightly over half the mass of Jupiter but considerably larger than Saturn. It orbits a binary pair of stars, named Aa and Ab, every 138 days at a distance of 0.634 AU (63.4% of the distance at which the Earth orbits the Sun). The larger of these two stars Aa, is an F-type yellow-white star, with a mass of 1.5 times the mass of the Sun. This is orbited by a smaller, M-type red dwarf star with a mass 0.4 times that of the Sun every 20 days at a distance of 0.17 AU. All of these components eclipse (pass in front of) one-another when seen from our Solar System.

Diagram showing the relative positions of PH1, Aa and Ab in the inner part of the KIC 4862625 system. Shwamb et al. (2012).

Orbiting this system at a distance of roughly 1000 AU is a second pair of binary stars, detected by their gravitational influence upon the primary binary. These are named Ba and Bb, Ba probably being a G-type yellow dwarf star with 99% of the mass of our Sun and Bb probably being a M-type red dwarf star with a mass 51% if the Sun's.

The whole system is thought to be about 2 billion years old, compared to about 4.5 billion years for our Solar System.

See also A fourth body in the KOI-13 system, Two new more planets discovered in the Gliese 676 system, Two new views of τ Boötis b, Thermal imaging 55 Cancri e and Exoplanets on Sciency Thoughts YouTube.

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A new study of the Kepler 11 planetary system.

The Kepler 11 system was the eleventh planetary system discovered by the Kepler Space Telescope; its discovery being announced in a paper in the journal Nature in February 2011. The star (Kepler 11A) is a G-type star with 95% of our Sun's mass approximately 2000 light years from Earth in the constellation Cygnus. This is surrounded by a system of 6 known transiting planets (i.e. planets which pass across the face of the star when seen from Earth), all of which orbit closer to the Star than Venus does to our Sun.

The orbits of the planets of the Kepler 11 System, with the orbit of Mercury given for scale. The Visual Exoplanet Catalogue.

In a paper published on the arXiv database at Cornell University Library on 3 May 2012, and also due to be published in the Monthly Notices of the Royal Astronomical Society, Cezary Migaszewski, Mariusz Słonina and Krzysztof Gózdziewski of the Torún Centre for Astronomy at Nicolaus Copernicus University present the results of a longer term study of the Kepler 11 system, updating our understanding of its planets. This includes revised measurements of the masses and orbital parameters of the planets, plus some analysis of the atmospheres of those planets, made by spectrographic analysis as those planets passed in front of the star.

Kepler 11b is the innermost of the planets; orbiting at a distance of 0.091 AU (i.e. 9.1% of the distance between the Earth and the Sun) every 10.3 days. It has a mass between 1.0 and 6.6 times that of the Earth, and appears to be quite dense, containing a high proportion of heavy elements; this is not surprising, a planet this close to its parent star would be expected to lose lighter elements to evaporation (although it should be noted that Kepler 11b still appears to be less dense than the Earth).

Kepler 11c is the next planet from the star, orbiting at 0.106 AU every 13 days. It has a mass between 1.5 and 13.0 times that of the Earth. It appears to have a density similar to that of Uranus or Neptune, which is surprising as these planets lie in the cool, outer reaches of our Solar system. Kepler 11 is slightly cooler than our Sun, with an effective surface temperature of 5680 K (compared to 5778 K for the Sun), but not enough so for Ice Giant type planets to exist this far in in the system. It is likely to be comprised of heavier elements than Uranus or Neptune, with these elements in a less dense state due to the extreme heat from the star.

The next planet is Kepler 11d, orbiting at 0.159 AU every 22.7 days. This has a mass of between 5.5 and 12.7 times that of the Earth, and a density similar to that of Uranus.

Kepler 11e orbits at 0.194 AU, every 32 days. It has a mass of between 8.5 and 15.7 times that of the Earth, and appears to have a density close to that of Saturn or Jupiter.

Kepler 11f orbits at 0.25 AU every 46.7 days. It has a mass of between 1.0 and 10.1 times that of the Earth, and a density close to that of Uranus.

Kepler 11g orbits at 0.462 AU (the only known planet in the Kepler 11 system with an orbital distance greater than that of Mercury. It is the least well constrained of the planets with a mass less than 42 times that of the Earth (probably less than 28 times), and may be less dense than Saturn or Jupiter.

See also Thermal imaging 55 Cancri e, Planets discovered orbiting the ancient, second generation star HIP 11952, Cooking the planets if CoRoT-7, Planets in the NY Virginis system and Exoplanets on Sciency Thoughts YouTube.

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