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European astronomers confirm a predicted, unseen exoplanet

A team of European astronomers (1) including members of Institut d'astrophysique de Paris and using the SOPHIE (2) spectrograph at the Observatoire de Haute-Provence (France), confirmed the presence of Kepler-88 c, an unseen planet that was previously predicted thanks to the gravitational perturbation it caused on its transiting brother planet, Kepler-88 b. That result is published today in the review Astronomy & Astrophysics.

Searching for periodic transits in hundred of thousand of stars was the primary goal of the Kepler space telescope. More than 3500 of such periodic transits were found during the four years of the mission. However, not all the planets located in the Kepler field-of-view are transiting their host star. Indeed, if their orbital plane is slightly misaligned with the line of sight from the Earth (only a few degrees is enough), the planet is not transiting and thus, is "unseen" from the Kepler spacecraft. Planets that share the same host star gravitationally interact with each other. This interaction between planets can cause perturbations in the predicted times of transit of planets (3) in multi-planetary systems. This is called Transit Timing Variations (TTV). The TTV technique is sensitive to planets in multiple systems down to the mass of the Earth, and can therefore be used to unveil the existence of non-transiting planets, that cause perturbations in the orbital motion of transiting planets. This is the case of the Kepler-88 system, which hosts an inner, transiting planet (Kepler-88 b), discovered by the Kepler space telescope (4) from NASA, that is strongly perturbed by an outer, non-transiting planet (Kepler-88 c). This system presents such strong interactions that it has earned the nickname of the "King of transit variations".

A previous analysis of the dynamical interaction between planets (5) predicted that this system had two planets near a resonance: the orbital period of the unseen, outer planet is two times longer than the the transiting, inner planet. This configuration is similar to the Earth and Mars in the Solar system, with Mars orbiting the Sun in nearly two years. Using the SOPHIE velocimeter, the European team independently measured the mass of Kepler-88 c. SOPHIE is a French instrument capable of measuring the velocity of stars (6) with high precision. The inferred mass and orbital period for the unseen planet are in perfect agreement with the values that were predicted from TTV. This is the first time that an unseen exoplanet inferred based on Transit Timing Variation is independently confirmed by another technique. This result therefore confirms TTV as a valid technique to detect unseen planets and explore multi-planet systems. TTV has already been used to determine the mass of more than 120 exoplanets detected by Kepler, around 47 planetary systems, down to a few Earth-mass planets.

This independent confirmation is an important contribution to the statistical analyses of the Kepler multiple planet systems. It helps to better understand the dynamical interactions and the formation of planetary systems. This also permits to anticipate the future exploration of exoplanetary systems from space as would do the PLATO mission (7). Neptune was the first planet detected based on the gravitational influence it had on another planet (Uranus). The French mathematician Urbain Le Verrier calculated that the anomalies in Uranus orbit were due to a previously unseen planet, in resonance with Uranus. His calculation led Johann Gottfried Galle to find Neptune on September 23rd 1846.

1. The team is composed by S.C.C. Barros (LAM), R.F. Díaz (LAM/Observatoire Genève), A. Santerne (CAUP/LAM), G. Bruno (LAM), M. Deleuil (LAM), J.-M. Almenara (LAM), A.S. Bonomo (INAF), F. Bouchy (LAM), C. Damiani (LAM), G. Hébrard (IAP/OHP), G. Montagnier (IAP/OHP) and C. Moutou (CFHT/LAM). Observations were funded thanks to the French Programme National de Planétologie of INSU / CNRS.

2. SOPHIE is a high resolution spectrograph, with precision to measure radial velocities of around 1 m/s. It is mounted in the 1.93-meter telescope of the Observatoire de Haute-Provence (France).

3. The Planetary Transit method measures the dimming of a stars' brightness, when an exoplanet passes in front of the stellar disk (similar to a micro eclipse). Through transits, it is possible to determine the radius of a planet. It is necessary for the planet and the star to be perfectly aligned with the observers' line of sight.

4. The Kepler space telescope (NASA) was launched on March 5th 2009 to continuously observe 100,000 stars in the region of the constellation Cygnus (the Swan). One of the main mission objectives was to detect transiting exoplanets. Due to technical problems, on August 15th Kepler was put in hibernation mode.

5. Nesvorny et al. : "KOI-142, the King of Transit Variations, is a Pair of Planets near the 2:1 Resonance", The Astrophysical Journal, Volume 777 (2013).

6. The Radial Velocities method detects exoplanets by measuring the small variations in the (radial) velocity of the star, due to the reflex motion the exoplanet imprints in the star. The radial velocity variation the Earth provokes in the Sun is of about 10 cm/s. With this method it is possible to determine the mass of the planet.

7. PLATO is a M3 mission candidate to the Cosmic Vision program of the European Space Agency (ESA), whose goal is to search for transiting Earths around neighbor stars: http://sci.esa.int/plato

The article "SOPHIE velocimetry of Kepler transit candidates X. KOI-142c: first radial velocity confirmation of a non-transiting exoplanet discovered by transit timing" is published today in the Astronomy & Astrophysics journal: http://www.aanda.org/articles/aa/abs/2014/01/aa23067-13/aa23067-13.html (see also http://arxiv.org/abs/1311.4335)

Contact IAP :
Guillaume Hébrard (IAP, Paris)
Institut d'astrophysique de Paris-CNRS-UPMC
hebrard at iap.fr
Tel. : 33-1-4432-8078

Artist impression of the Kepler-88 system

Artist impression of the Kepler-88 system

© Alexandre Santerne (CAUP) / © Background image: ESO / S. Brunier

Picture of the dome of the 1.93-m telescope of Haute-Provence Observatory (France) which hosts the SOPHIE spectrograph, with the Kepler field-of-view

Picture of the dome of the 1.93-m telescope of Haute-Provence Observatory (France)
which hosts the SOPHIE spectrograph, with the Kepler field-of-view

© Alexandre Santerne (CAUP)

Animation of a planetary system, with 2 planets locked in a two-to-one resonance: http://www.astro.up.pt/press/kepler-88/planets.mp4

© Ricardo Cardoso Reis (CAUP)

December 17, 2013

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