page_title: Kepler-47
section_title: Planetary system
hierarchical_section_title: Kepler-47 / Planetary system
caption_attribution_description: English: Artist's depiction of the relative sizes of the three planets in the Kepler-47 system. The large middle planet is the newly discovered planet, Kepler-47d.
context_page_description: Kepler-47 is a binary star system with three exoplanets in orbit around the pair of stars located about 1055 parsecs away from Earth. The first two planets announced are designated Kepler-47b, and Kepler-47c. Kepler-47 is the first circumbinary multi-planet system discovered by the Kepler mission. The outermost of the planets is a gas giant orbiting within the habitable zone of the stars. Because most stars are binary, the discovery that multi-planet systems can form in such a system has impacted previous theories of planetary formation.
  A group of astronomers led by Jerome Orosz at San Diego State University, including astronomers from Tel-Aviv University in Israel, discovered the planetary system via NASA's Kepler space telescope in 2012. In November 2013, evidence of a third planet orbiting between the planets b and c, Kepler-47d, was announced. Later analyses of transit data from the Kepler space telescope confirmed the existence of Kepler-47d.
context_section_description: Prior to the discovery of the Kepler-47 planetary system by Jerome Orosz, his colleagues, as well as astronomers from Tel-Aviv University in 2012, it was thought that binary stars with multiple planets could not exist. It was believed that gravitational perturbations caused by the orbiting parent stars would cause any circumbinary planets to collide with each other or be ejected out of orbit, either into one of the parent stars or away from the system. However, this discovery demonstrates that multiple planets can form around binary stars, even in their habitable zones; and while the planets in the Kepler-47 system are unlikely to harbor life, other planets orbiting around binary star systems may be habitable and could support life. Because most stars are binary, the discovery that multi-planet systems can form in such a system has impacted previous theories of planetary formation, and could provide more opportunities for finding potentially habitable exoplanets.
  The binary system is known to host three planets, all orbiting close to each other and larger than Earth, with no solid surface. All three of the planets in the Kepler-47 system have a very low density, less than that of Saturn. The densities of the planets are estimated to be around 0.26 g/cm³ to 0.68 g/cm³. The low densities of the planets are unusual for their relatively mild temperatures; planets with such low densities are typically hot jupiters that orbit close to their host stars, being known as so-called puffy planets. Low density planets with such mild temperatures are thought to be uncommon.
  Kepler-47b is a super-Earth and the innermost planet of the Kepler-47 system. It resides close to its parent stars, at a distance of 0.2956 AU. It completes one full orbit around its parent stars in less than 50 days. The equilibrium temperature of Kepler-47b is 442 K, therefore being inhospitable to life. Due to the high equilibrium temperature of Kepler-47b, methane gas in its atmosphere would be broken into other compounds, leading to a thick haze that would cover the planet's atmosphere. It is the smallest planet of the Kepler-47 system, being 3.1 times the size of Earth.
  The second planet discovered, Kepler-47c, is a Neptune class planet and the outermost planet, orbiting its parent stars from a distance of 0.989 AU, nearly the distance from Earth to the Sun. It completes one full orbit around its parent stars in about 300 days. Kepler-47c is situated within the habitable zone, with an equilibrium temperature of 241 K. The radius of Kepler-47c is 4.7 times that of Earth, comparable in size to Neptune. Although it is assumed Kepler-47c is not capable of harboring life, it could possibly have a dense atmosphere of water vapor.
  The most recently discovered planet in the system, Kepler-47d, was announced as being discovered by astronomer Jerome Orosz and his colleagues at San Diego State University in November 2013. From transit data of the Kepler-47 system from the Kepler space telescope, Orosz's team had noticed one orphan transit signal that lasted for 4.15 hours, and was not attributed to the two previously known planets. Due to the weak transit signals of Kepler-47d, it was not detected earlier in 2012. Only one noticeable transit of Kepler-47d has been detected, thus an additional transit of the planet was needed to confirm its existence. From dynamical simulations, the orbit of Kepler-47d was shown to precess over time, resulting in a four-year period without transits from Kepler-47d. Later studies of the Kepler-47 system led to the confirmation of Kepler-47d, which was announced in April 2019. The discovery of Kepler-47d was unexpected for Orosz's team, as they had expected to find additional planets with more distant orbits. Kepler-47d is the largest planet of the Kepler-47 system, with a radius at least 7 times the radius of Earth. It orbits between the planets Kepler-47b and c at a distance of about 0.7 AU, completing an orbit every 187.35 days. Its equilibrium temperature is around 283 K.
caption: Artist's depiction of the relative sizes of the three planets in the Kepler-47 system. From the left: Kepler-47b; Kepler-47d; and Kepler-47c.
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