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First detection of a predicted unseen exoplanet

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A team of European astronomers [1], 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.

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 4 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 (only a few degrees is enough) with the line of sight from the Earth, the planet is not transiting and thus, is “unseen” from the Kepler spacecraft.

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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.

Crédit : Alexandre Santerne (CAUP)
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Artist impression of the Kepler-88 system. Background image : ESO / S. Brunier

Crédit : Alexandre Santerne (CAUP)

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)” explains the leading author of the paper, Susana Barros, a researcher at the Laboratoire d’Astrophysique de Marseille (LAM).

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 a transiting planet (Kepler-88 b), discovered by the Kepler space telescope [4] (NASA) , that is strongly perturbed by a non-transiting planet (Kepler-88 c). “This system presents such strong interactions that it has earned the nickname of the king of transit variations” adds Rodrigo Díaz, a researcher working at the Geneva Observatory (OAUG).

A careful analysis of the dynamical interaction between planets, previously performed by a team led by David Nesvorný (Boulder University), predicted that this system had two planets near a two-to-one resonance (the orbital period of the unseen outer-planet is exactly 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 2 years.

Using the SOPHIE velocimeter, the team independently measured [5] the mass of Kepler-88 c. “SOPHIE is a French instrument capable of measuring the velocity of stars [6] with a precision equivalent to the speed of a bicycle. It has been used to characterize nearly 20 Kepler planets so far” adds Alexandre Santerne a researcher at Centro de Astrofísica da Universidade do Porto (CAUP) and responsible of the observations of Kepler targets with SOPHIE.

The inferred mass for the unseen planet is in perfect agreement with the value that was predicted from TTV. “This is the first time that the mass of an unseen exoplanet inferred based on Transit Timing Variation is independently confirmed by another technique” comments Susana Barros. 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 a very 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]" concludes Magali Deleuil, professor at Aix-Marseille University and leader of LAM’s exoplanet research.

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 two-to-one resonance from a previously unseen planet. His calculation led Johann Gottfried Galle to find Neptune on September 23rd 1846.

First detection of a predicted unseen exoplanet on INSU website, in french.

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Animation : Can be found here :

Fig1 : Artist impression of the Kepler-88 system
Credit : Alexandre Santerne (CAUP) / Background image : ESO / S. Brunier

Fig2 : 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. Credit : Alexandre Santerne (CAUP)

Animation1 : Animation of a planetary system, with 2 planets locked in a two-to-one resonance. Credit : Ricardo Cardoso Reis (CAUP)

Voir en ligne : High resolution images and videos available for download here


[1The 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 – Osservatorio Astronomico di Torino), 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.

[2SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes, or Spectrograph for Observation of phenomena of stellar interiors and Exoplanets) is a high resolution spectrograph, with precision to measure radial velocities of around 1 m/s. It’s mounted in the 1.93 meter telescope of the Observatoire de Haute-Provence (France), the same telescope where, in 1995, Michel Mayor and Didier Queloz detected the first exoplanet orbiting a Sun-like star.

[3The Planetary Transit method measures the dimming of a stars’ brightness, when an exoplanet passes in front of the stellar disk (something similar to a micro eclipse). Through transits, it’s possible to determine the radius of a planet, but not the mass. This method is complicated to use, because it’s necessary for the planet and the star to be perfectly aligned with the observers’ line of sight.

[4The Kepler Space Telescope (NASA) was launched on March 5th 2009, to continuously observe 100 thousand 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.

[5The 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 (see also .

[6The 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’s possible to determine the minimum mass of the planet.

[7PLATO is a M3 mission candidate to the Cosmic Vision program of the European Space Agency (ESA), whose goal is to search for other Earths around neighbor stars.