©Centro de Astrofisica da Universidade do Porto
Astronomers used the SOPHIE spectrograph at the Observatoire de Haute-Provence, to confirm the presence of Kepler-88 c, an unseen planet previously predicted thanks to the gravitational perturbation it caused on its transiting brother planet, Kepler-88 b.
Searching for periodic transits in hundreds of thousands of stars was the primary goal of the Kepler space telescope. More than 3,500 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.
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 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 (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 Diaz, 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 Nesvorny (Southwest Research Institute), 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 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 the mass of Kepler-88 c. "SOPHIE is a French instrument capable of measuring the velocity of stars 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 Astrofisica 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 multiplanet 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 analyzes 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" 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 23, 1846.
Laboratoire d'Astrophysique de Marseille (LAM)
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Ricardo Cardoso Reis
Centro de Astrofisica da Universidade do Porto (CAUP)
+351 22 608 98 36
Susana C. C. Barros
+33 (0) 4 91 05 41 97
Rodrigo F. Diaz
Observatoire de Geneve
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+351 22 608 98 08
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 17 December 2013 in Astronomy & Astrophysics [http://dx.doi.org/10.1051/0004-6361/201323067].
The team is composed by S. C. C. Barros (LAM), R. F. Diaz (LAM/Observatoire Geneve), A. Santerne (CAUP), 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. Hebrard (IAP/OHP), G. Montagnier (IAP/OHP) and C. Moutou (CFHT/LAM). Observations were funded thanks to the French Program National de Planetologie of INSU / CNRS.
SOPHIE (Spectrographe pour l'Observation des Phenomenes des Interieurs stellaires et des Exoplanetes, 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.
The 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.
The Kepler Space Telescope (NASA) was launched on March 5, 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.
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's possible to determine the minimum mass of the planet.
PLATO 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.
The Centro de Astrofisica da Universidade do Porto (CAUP, http://www.astro.up.pt) is a private, non-profit, scientific and technical association, recognized as of public utility. It is the largest astronomy research institute in Portugal and since 2000, it has been evaluated as excellent by international panels, organized under the auspices of the national science foundation (FCT). Among its statutory objectives is the support and promotion of astronomy, through research, education at the graduate and undergraduate levels, science outreach and popularization of astronomy. The long-term research strategy of CAUP is the assembly of strong research teams on origin and evolution of stars and planets and galaxies and observational cosmology.
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