COROT

COROT (French: COnvection ROtation et Transits planétaires; English: COnvection ROtation and planetary Transits) is a space mission led by the French Space Agency (CNES) in conjunction with the European Space Agency (ESA) and other international partners. The mission's two objectives are to search for extrasolar planets with short orbital periods, particularly those of large terrestrial size, and to perform asteroseismology by measuring solar-like oscillations in stars.^[1] It was launched at 14:28:00 UTC on 27 December 2006, atop a Soyuz 2.1b carrier rocket,^[2][3][4] reporting first light on 18 January 2007.^[5] Subsequently, the probe started to collect science data on 2 February 2007.^[6] COROT is the first spacecraft dedicated to the detection of transiting extrasolar planets, opening the way for more advanced probes such as Kepler and possibly TESS and PLATO. It detected its first extrasolar planet, COROT-1b, in May 2007,^[7] just 3 months after the start of the observations. Mission flight operations were originally scheduled to end 2.5 years from launch^[8] but operations were extended to 2013.^[9]

Overview

Spacecraft design

The COROT optical design minimizes stray light coming from the Earth and provides a field of view of 2.7° by 3.05°. The COROT optical path consists of a 27 cm (10.6 in) diameter off-axis afocal telescope housed in a two-stage opaque baffle specifically designed to block sunlight reflected by the Earth and a camera consisting of a dioptric objective and a focal box. Inside the focal box is an array of four CCD detectors protected against radiation by aluminum shielding 10mm thick. The asteroseismology CCDs are defocused by 760μm toward the dioptric objective to avoid saturation of the brightest stars. A prism in front of the planet detection CCDs gives a small spectrum designed to disperse more strongly in the blue wavelengths.^[10]

The four CCD detectors are model 4280 CCDs provided by E2V Technologies. These CCDs are frame-transfer, thinned, back-illuminated designs in a 2048 pixel by 2048 pixel array. Each pixel is 13.5 × 13.5μm² in size which corresponds to an angular pixel size of 2.32 arcsec. The CCDs are cooled to −40 °C (233.2 K; −40.0 °F). These detectors are arranged in a square pattern with two each dedicated to the planetary detection and asteroseismology. The data output stream from the CCDs are connected in two chains. Each chain has one planetary detection CCD and one asteroseismology CCD. The field of view for planetary detection is 3.5°.^[10]

The satellite, built in the Cannes Mandelieu Space Center, has a launch mass of 630 kg, is 4.10 m long, 1.984 m in diameter and is powered by two solar panels.^[8]

Mission design

The satellite observes perpendicular to its orbital plane, meaning there will be no Earth occultations, allowing up to 150 days of continuous observation. These observations session, called "Long Runs", allow the detections of smaller and long-period planets. During the remaining 30 days between the two main observation periods, COROT observes other patches of sky for a few weeks long "Short Runs", in order to analyze a larger number of stars for the asteroseismic program. After the loss of half the field of view due to failure of Data Processing Unit #1 in March 2009, the observation strategy changed to 3 months observing runs, in order to optimize the number of observed stars and detection efficiency. In order to avoid the Sun entering in its field of view, during the northern summer COROT will observe in an area around Serpens Cauda, toward the galactic center, and during the winter it will observe in Monoceros, in the Galactic anticenter. Both this "eyes" of COROT have been studied in preliminary observations carried out between 1998 and 2005,^[11] allowing the creation of a database, called COROTSKY,^[12] with data about the stars located in this two patches of sky. This work allow to select the best fields to be observed: the exoplanet research program requires a large number of dwarf stars to be monitored, and to avoid giant stars, for which planetary transits are too shallow to be detectable. The asteroseismic program requires stars brighter than magnitude 9, and to cover as many as possible different type of stars. In addition, in order to optimize the observations, the fields shouldn't be too sparse - fewer targets observed - or too crowded - too many stars overlapping. Several fields have been already observed^[13]:

• IRa01, from 18 January 2007 to 3 April 2007 - 9879 stars observed;
• SRc01, from 3 April 2007 to 9 May 2007 - 6975 stars observed;
• LRc01, from 9 May 2007 to 15 October 2007 - 11408 stars observed;
• LRa01, from 15 October 2007 to 3 March 2008 - 11408 stars observed;
• SRa01, from 3 March 2008 to 31 March 2008 - 8150 stars observed;
• LRc02, from 31 March 2008 to 8 September 2008 - 11408 stars observed;
• SRc02, from 8 September 2008 to 6 October 2008 - 11408 stars observed;
• SRa02, from 6 October 2008 to 12 November 2008 - 10265 stars observed;
• LRa02, from 12 November 2008 to 30 March 2009 - 11408 stars observed;
• LRc03, from 30 March 2009 to 2 July 2009 - 5661 stars observed;
• LRc04, from 2 July 2009 to 30 September 2009 - 5716 stars observed;
• LRa03, from 30 September 2009 to 1 March 2010 - 5289 stars observed;
• SRa03, from 1 March 2010 to 2 April 2010;
• LRc05, from 2 April 2010 to 5 July 2010;
• LRc06, from 5 July 2010 to 27 September 2010;
• LRa04, from 27 September 2010 to 16 December 2010;
• LRa05, from 16 December 2010 to 5 April 2011;
• LRc07, from 5 April 2011 to 30 June 2011;
• SRc03, from 1 July 2011 to 5 July 2011 - a run made to reobserve the transit of COROT-9b;
• LRc08, from 6 July 2011 to 30 September 2011;
• SRa04, from 4 Ocotber 2011, in progress.

The probe monitors the brightness of stars over time, searching for the slight dimming that happens in regular intervals when planets transit their primary sun. In every fields, COROT records the brightness of thousands stars in the V-magnitude range from 11 to 16 for the extrasolar planet study. In fact, stellar targets brighter than 11 will saturate the exoplanets CCD detectors, yielding inaccurate data, whilst stars dimmer than 16 don't deliver enough photons to allow planetary detections. COROT will be sensitive enough to detect rocky planets with a radius two times larger than Earth orbiting stars brighter than 14;^[14] it is also expected to discover new gas giants in the whole magnitude range.^[15]

COROT will also undertake asteroseismology. It can detect luminosity variations associated with acoustic pulsations of stars. This phenomenon allows calculation of a star's precise mass, age and chemical composition and will aid in comparisons between the sun and other stars. For this program, in each field of view there will be one main target star for the asteroseismology as well as up to nine other targets. The number of observed targets have dropped to half after the loss of Data Processing Unit #1.

The mission began on 27 December 2006 when a Russian Soyuz 2-1b rocket lifted the satellite into a circular polar orbit with an altitude of 827 km . The first scientific observation campaign started on 3 February 2007.^[16]

Development

The primary contractor for the construction of the COROT vehicle was CNES,^[17] to which individual components were delivered for vehicle assembly. The COROT equipment bay, which houses the data acquisition and pre-processing electronics, was constructed by the LESIA Laboratory at the Paris Observatory and took 60 person-years to complete.^[17] The COROT camera, also constructed by the LESIA Laboratory, took 25 person-years to complete.^[17]

Potential

Before the beginning of the mission, the team stated with caution that COROT would only be able to detect planets few times larger than Earth or greater, and that it was not specifically designed to detect habitable planets. According to the press release announcing the first results, COROT's instruments are performing with higher precision than had been predicted, and may be able to find planets down to the size of Earth with short orbits around small stars.^[7] The transit method requires the detection of at least two transits, hence the planets detected will mostly have an orbital period under 75 day. Candidates that show only one transit have been found, but uncertainty remains about their exact orbital period.

COROT should be assumed to detect a small percentage of planets within the observed star fields, due to the low percentage of exoplanets that would transit from the angle of observation of our Solar System. The chances of seeing a planet transiting its host star is inversely proportional to the diameter of the planet's orbit, thus close in planets detections will outnumber outer planets ones. The transit method is also biased toward large planets, since their very depth transits are more easily detected than the shallows eclipses induced by terrestrial planets.

Failure of Data Processing Unit #1

On March 8, 2009 the satellite suffered a loss of communication with Data Processing Unit #1, processing data from one of the two photo-detector chains on the spacecraft. Science operations resumed early April with Data Processing Unit #1 offline while Data Processing Unit #2 operating normally. The loss of photo-detector chain number 1 results in the loss of one CCD dedicated to asteroseismology and one CCD dedicated to planet detection. The field of view of the satellite is thus reduced by 50%, but without any degradation of the quality of the observations. The loss of channel 1 appears to be permanent.^[18]

Follow-up program

The rate of discoveries of transiting planets is dictated by the need of ground based follow-up observations, needed to verify the planetary nature of the transit candidates. Candidate detections have been obtained for about 2,3% of all COROT targets, but finding periodic transit events isn't enough to claim a planet discovery, since several configurations could mimic a transiting planet, such as stellar binaries, or an eclipsing fainter star very close to the target star, whose light, blended in the light curve, can reproduce transit-like events. A first screening is executed on the light curves, searching hints of secondary eclipses or a rather V-shaped transit, indicative of a stellar nature of the transits. For the brighter targets, the prism in front of the exoplanets CCDs provides photometry in 3 different colors, enabling to reject planet candidates that have different transit depths in the three channels, a behaviour typical of binary stars. These tests allow to discard 83% of the candidate detections,^[19] whilst the remaining 17% are screened with photometric and radial velocity follow-up from a network of telescopes around the world. Photometric observations, required to rule out a possible contamination by a diluted eclipsing binary in close vicinity of the target,^[20] is performed on several 1 m-class instruments, but also employs the 2 m Tautenburg telescope in Germany and the 3,6 m CFHT/Megacam in Hawaii. The radial velocity follow-up allows to discard binaries or even multiple star system and, given enough observations, provide the mass of the exoplanets found. Radial velocity follow-up is performed high-precision spectrographs, namely SOPHIE, HARPS and HIRES^[21]. Once the planetary nature of the candidate is established, high-resolution spectroscopy is performed on the host star, in order to accurately determine the stellar parameters, form which further derive the exoplanet characteristics. Such work is done with large aperture telescopes, as the UVES spectrograph or HIRES.

Interesting transiting planets could be further followed-up with the infrared Spitzer Space Telescope, to give an independent confirmation at a different wavelength and possibly detect reflected light from the planet or the atmospheric compositions. COROT-7b and COROT-9b have already been observed by Spitzer.

Papers presenting the results of follow-up operations of planetary candidates in the IRa01^[22], LRc01^[23], LRa01^[24], SRa01 and SRa02^[25] fields have been published. Sometimes the faintness of the target star or its characteristics, such as a high rotational velocity or strong stellar activity, do not allow to determine unambiguously the nature or the mass of the planetary candidate.

Discoveries

Asteroseismology

By revealing their microvariability, measuring their oscillations at the ppm level, COROT provides a new vision at stars, never reached before by any ground-based observation^[26]. More than one hundred star have already been observed in the asteroseismic channels. A review of the results is presented:

Solar-like pulsators

COROT measured Solar-like oscillations and granulation in stars hotter than the Sun^[27]. With the scientist surprise, the granulation signature suggest time scales 30% larger than the Sun and granule size 4 times bigger than the Sun. Oscillations amplitudes were found 25% larger than the ones predicted from current theoretical models, which suggest the need to reformulate current star models.

Red Giants

COROT provided evidence that both radial e non radial modes in Red Giants are excited, confriming the existence of modes with lifetimes in order of a month. It seems that the distribution of frequencies at maximum power among all the observed red giants, present a rather narrow peak, which scientists think has to bear the print of the evolution of our galaxy.

Chimera, a new type of pulsators

The study of the star HD 180872 revealed, at a very low amplitude, the existence of higher frequency modes, due to stochastic oscillations. This discovery opens new perspectives in the study of these objects, where low frequency oscillations and higher frequency ones could be used in a complementary way to probe the centre and outer layers of the stars.

Additional programmes

Albeit with a lower Signal to noise ratio, interesting science on stars is obtained also on the exoplanets channel data, where the probe records several thousands of light curves from every observed field. Stellar activity, rotation period, star spots evolution, differential rotation, pulsations are nice extras in addition to the main asteroseismic program. Star-planets interactions have also been studied for the first time, and statistics about binary and multiple star system could be defined by the large sample of star observed.

In October 2009 the COROT mission was the subject of a special issue of the Astronomy and Astrophysics Journal, dedicated to the early results of the probe.^[28] Several papers with results on the asteroseismic field were published.

Exoplanets

The COROT exoplanet science team has decided to publish confirmed and fully characterized planets only and not simple candidate lists. This strategy, different from the one pursued by the Kepler mission, where the candidates are regularly updated and made available to the public, is quite lengthy. On the other hand, the approach also increases the scientific return of the mission, as the set of published COROT discoveries constitute some of the best exoplanetary studies carried out so far.

Timelines of Planets discoveries

COROT discovered its first two planets in 2007: the hot Jupiters COROT-1b and COROT-2b.^[7][29] Results on asteroseismology were published in the same year.^[30] By further analysis, COROT-1b became the first exoplanets to have its secondary eclipse detected in the optical,^[31] thanks to the high precision lightcurve delivered by COROT.

In May 2008, two new exoplanets of Jupiter size, COROT-4b and COROT-5b, as well as an unknown celestial object, COROT-3b, were announced by ESA. COROT-3b, for its mass, appears to be "something between a brown dwarf and a planet." According to the definition of planet proposed by the owners of the exoplanet.eu database^[32] three years later, COROT-3b, being less massive than 25 Jupiter masses, is classified as an exoplanet.

In February 2009, during the First Corot Symposium, the super-earth COROT-7b was announced, which at the time was the smallest exoplanet to have its diameter confirmed, at 1.58 Earth diameters. The discoveries of a second non transiting planet in the same system, COROT-7c, and of a new Hot Jupiter, COROT-6b, were also announced at the Symposium.

In March 2010 COROT-9b was announced. With 80% of Jupiter mass, and an orbit similar to the Mercury one, this is the first transiting temperate planet found known to be similar to those within our own Solar System.^[33] At the time of the discovery, it was the second longest period exoplanet found in transit, after HD80606 b.

In June 2010 the COROT team announced^[34] six new planets, COROT-8b, COROT-10b, COROT-11b, COROT-12b, COROT-13b, COROT-14b, and a brown dwarf, COROT-15b.^[35] All the planets announced are Jupiter sized, except COROT-8b, which appears to be somewhat between Saturn and Neptune.

In an August 2010 paper, COROT detected the ellipsoidal and the relativistic beaming effects in the COROT-3 lightcurve.^[36] The probe was also able to tentatively detect the reflected light at optical wavelengths of HD46375 b, a non transiting planet.^[37]

In June 2011, during the Second Corot Symposium, the probe added ten new objects to the Exoplanet catalogue^[38]: COROT-16b, COROT-17b, COROT-18b, COROT-19b, COROT-20b, COROT-21b, COROT-22b, COROT-23b, COROT-24b, COROT-24c. The last two planets are of Neptune size, and orbit the same star, thus representing the first multiple transiting system detected by COROT. COROT-22b is also notable for its small size, having less than half the mass of Saturn.

As of June 2011, 401 further planet candidates are being screened for confirmation.

List of Planets discovered

The following transiting planets have been announced by the mission.

Other discoveries

The following table illustrates brown dwarf detected by COROT as well as non transiting planets detected in the follow-up program:

See also

• List of planetary systems
• NASA Star and Exoplanet Database
• Kepler (spacecraft)
• Terrestrial Planet Finder
• High Accuracy Radial Velocity Planet Searcher
• Space Interferometry Mission
• Darwin (spacecraft)
• Automated Planet Finder

References

External links

• COROT on the CNES site
• CoRoT N2 Public archive