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).
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