Transiting planets teach us a lot about properties of extrasolar systems, since we measure both the radius and the mass, and thus the mean density of the planet. This population displays a puzzling diversity, from inflated Jupiter-like to rocky or watery telluric planets. The main issues in this topic are now:
- to characterize the population of transiting planets at longer orbital periods;
- to explore the domain of close-in planets towards the small size end ;
- to enlarge the planet detection to parent stars with a much wider range of properties (young, massive or evolved stars).
As its first published results nicely illustrate, Corot has the capacity to address these and bring significant breakthroughs in our understanding of the planetary systems.
Corot photometric lightcurves hide hundreds of transit series
Some portraits of the Corot transit family.
Each plot shows the folded lightcurve of the planetary transit,
for some of the most interesting systems that were discovered during
the first three years of Corot.
In the exoplanet field, Corot observes several thousands stars at a time, during runs that last from 20 to 150 days. About 200 transiting events are detected in each sample. Most of them are binary stars, and some are transiting planets.
The first Corot exoplanets that have been discovered and fully characterised include:
- several hot jupiters, gas giant planets with periods less than 4 days, CoRoT-1 b, CoRoT-2 b, CoRoT-5 b, CoRoT-11 b, CoRoT-12 b, CoRoT-13 b
- the first transiting brown dwarfs, CoRoT-3 b, CoRoT-15 b
- the first transiting super Earth, CoRoT-7 b
- a hot and dense sub-Saturn, CoRoT-8 b
- several warm jupiters, gas giants with periods longer than 5 days, CoRoT-4 b, CoRoT-6 b, CoRoT-10 b
- the most temperate transiting exoplanet with 95-day orbital period, CoRoT-9 b
Corot detects transit-like features
The raw lightcurve of a star (left), as collected by CoRoT over months, is affected by high-amplitude variations due to the star or the observing conditions. These variations are filtered out (middle), to enhance the weak periodic signal due to planet crossing the stellar disc. All transits are then folded to measure the planet parameters (right). Detection teams are dealing with thousands such lightcurves at a time.
Exoplanet transits appear as periodic U-shape dips in stellar light curves. They typically last a few hours and repeat with a period equal to the exoplanet orbital period. After proper data processing and removal of low-frequency variations, stellar light curves can then be searched for transits by matched-filtering, i.e., cross-correlation with a set of square-shape signals with continuously varying parameters. Within the CoRoT detection group, several independent algorithms have been developped and are routinely applied to the ligthcurve. In addition to the pure detection of transit-like features, this group searches for more subtle signatures:
- out-of-transit sinusoidal oscillations at the same period than the transit, indicating tidal effects in close-in binaries; also phase modulations can be detected that way;
- secondary transits at the same period than the primary ones, that can be signatures of binary stars or star-planet systems, depending on the amplitude;
- chromatic effects: by comparing the transit depth in the blue, green and red chanels of CoRoT, one can investigate the likelihood of the planetary nature of the transiting candidate;
- finally, the transit shape and the duration of each phase (egress, flat part, total) are examined and again, planet-likelihood is estimated.
Ground-based support of Corot candidates distinguishes planets among binaries
The radial-velocity curve of the faint planet-host star CoRoT-10, obtained by HARPS.
Its shows that 1) the transiting companion is a planet, with a mass of 2.75 Mjup
and 2) that it orbits the star with a strongly elongated (or eccentric) orbit.
Both characteristics of mass and orbital eccentricity are unconstrained by CoRoT
and need additional observations from the ground.
In order to establish the planetary nature of Corot transits, one has to perform a series of measurements with ground-based telescopes:
- Ground-based photometry, to confirm that the main target is transited, and not a nearby faint eclisping star. Several 1-m class telescopes are routinely used for this verification test.
- Radial-velocity measurements allow to measure the mass ratio of the two bodies; small mass ratios indicate grazing stellar binaries and are discarded. Then, intensive measurements over the full orbit are performed to characterize planet's mass and eccentricity at the highest accuracy, and a thorough characterization of the star parameters is obtained. OHP/SOPHIE and ESO/HARPS are used.
Corot unveils the diversity of planetary systems
The planets found by Corot are extremely diverse: their radii range from 1.6 Earth radius to 1.5 Jupiter radius, they have masses from 5 to 20000 Earth masses, periods from 20 hours to 95 days! Corot has hence discovered the first transiting super-Earth, but it has also discovered what is probably the youngest transiting planet, a system that is key to understanding the formation of planetary systems, the first transiting brown dwarfs, and finally, the most temperate gas giant outside Solar System.
Extreme photometric precision allows ultra-deep planetary studies
The lightcurve of CoRoT-3 folded to the orbital period of the transiting 22-Mjup companion (the transit has been removed). Two modulations are detected simultaneously: the ellispoidal variation, due to the deformation of the star by the massive companion, and the relativistic beaming effect (or Doppler boosting). The detection of this last effect is reported for the first time as produced by a substellar object (Mazeh & Faigler, 2010)
Apart from transit features, the Corot lightcurves of planetary systems may reveal secondary signatures, when the conditions are favorable: the phase modulation along the orbit, the tiny occultation of the planet by the star, the ellipsoidal modulation, the relativistic beaming effect, or the periodical variation of transit timings due to other planets in the system.
The Rossiter effect on CoRoT 2 b indicates that the planet's orbit and the stellar spin planes are identical
Observations by Spitzer of the flux emitted by the young and hot planet CoRoT-2b at 4.5 and 8 microns (dots) are compared to models of the atmosphere's spectrum. They show for instance that the heat from the day side of the planet is only poorly redistributed to its night side (Gillon et al , 2010).
As new transiting planets are discovered, additional measurements are performed, that allow a refined analysis of the system's configuration. For instance, from the measurement of the spectroscopic transit with a spectrograph like ESO/HARPS or Keck/HIRES we may derive the angle between the orbital plane of the planet and the rotational plane of the star (perpendicular to its spin axis). Occultations of the planet by the star can be measured from the ground or from space, that gives access to the temperature of the day side of the planet, and some indication of temperature inversions in its atmosphere (Gillon et al 2009, 2010).
Occultations of the planet by the star can be measured from the ground (VLT/HAWKI) or from space (Spitzer), that gives access to the temperature of the day side of the planet, and some indication of temperature inversions in its atmosphere (Gillon et al 2009, 2010)
A thorough knowledge of the host stars is required: first, to derive the planet parameters when only values relative to the star are measured, second, to draw statistical properties of planet and star populations, and third, to study specific features on the star like the rotation, or the spot distribution.
The distibution of metallicity (amount of heavy elements in the stellar atmosphere)
of a sub-sample of stars observed by Corot towards the center and the anticenter directions,
separated in dwarfs (right) and giants (left) (Gazzano et al 2010).
Planet-host stars also are of great variety in the solar-like family, with masses ranging from 0.9 to 1.4 solar mass, and ages from 0.5 to 7 Gyrs. The rotation of their surface spots usually shows imprints on the photometric lightcurve, with periods of a few days to several tens of days. Spot modeling can then be used to determine active longitudes and see how they relate with the planet footprint on the star.
A single data base for stellar and planetary content from Corot
EXODAT is the data base gathering stellar and planetary data
available to the public. It can be queried at URL
EXODAT gathers both measurements and derived parameters, for the targets, transit detections, and planetary systems unveiled by Corot. Once Corot lightcurves are released to the public on the IAS Data Center, corresponding data for the same targets are released on EXODAT. Keep tuned!