Download Press Release KOI-142 (english version)x

First detection of a predicted unseen exoplanet
A team of European astronomers1, using the SOPHIE2 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.
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 planets3 in multiplanetary 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 telescope4 (NASA) , that is strongly perturbed by a nontransiting 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 measured5 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 mission7” 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.
NOTES:
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 – 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.
2. SOPHIE (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.
3. 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.
4. The 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.
5. 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
(see
also
http://arxiv.org/abs/1311.4335).
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’s possible
to determine the minimum 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 other Earths around neighbor stars.
Science contacts:
 Susana C. C. Barros (LAM)
[email protected] / +33 (0) 4 91 05 41 97
 Rodrigo F. Díaz (Observatoire de Genève)
[email protected] / +41 (0) 22 379 22 64
 Alexandre Santerne (CAUP)
[email protected] / +351 226 089 808
PIO contact:
 Thierry Botti (Head communication at LAM)
[email protected] / +33 (0) 4 95 04 41 06 / +33 (0) 6 72 53 79 46
IMAGES:
ANIMATION:
can be found here: http://www.astro.up.pt/press/kepler-88/planets.mp4
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)