“This increases by 10% the number of known transiting exoplanets”, says Claire Moutou enthusiastically, researcher at the LAM (Laboratory of Astrophysics of Marseille) and head of CoRoT’s exoplanet programme. The diversity of these planets, so different from those of our Solar System, will shed new light on the mechanisms behind the formation and evolution of these strange worlds.
RE = planet radiux (measured in Earth radius)
d = rotation period of the planet around its star
The diversity of the 2010 harvest strikes
The planets are presented following the chronological order of their discovery.
CoRoT-8 b: the smallest of the batch
Among known transiting exoplanets, CoRoT-8 b is quite a small one, being 0.7 times the size of Saturn and weighing 0.7 times its weight. Its internal structure should be similar to that of giant ice planets like Neptune and Uranus. It is the almost the smallest planet discovered so far by the CoRoT team, second only to the first transiting super Earth, CoroT-7 b.
CoRoT-9 b : a temperate exo-Jupiter
The discovery of CoRoT-9 b, a very strange planet, was announced in March 2010.
CoRoT-10 b: a giant with a strongly elongated orbit
During its “year” which lasts 13 days, this planet closes in then moves away from its parent star so much that the energy it receives varies by a factor 10 depending on its position. The surface temperature of the planet could then vary between 250°C and 600°C in a matter of a few days.
CoRoT-11 b: a giant orbiting a quickly rotating star
The star around which CoRoT-11 b orbits rotates quickly on its axis, in only 40 hours. This is even less than the orbiting period of the planet, which takes three days to complete its year. In comparison, our Sun takes 26 days to fully rotate on its axis. The high velocity of CoRoT-11 makes it hard to detect the planet with the HARPS instrument. It is the second known planet to possess these characteristics.
CoRoT-12 b: a real gas giant
Bigger than Jupiter, and with a radius 16 times that of Earth, its orbits near its star and thus receives powerful radiation which delay its contraction, which explains its abnormal size. Scientists call these planets bloated hot Jupiters.
CoRoT-13 b: a dense gas giant
This gas giant is smaller than Jupiter, but it is twice as dense (it’s another bloated hot Jupiter), which leads scientists to believe it hosts a massive rocky core. Its host star is abnormally rich in Lithium but the link between this peculiarity and the existence of the planet has not yet been established.
CoRoT-14 b: yet another bloated hot Jupiter
Paradoxically, CoRoT-14 b has a similar size to Jupiter, even though it is much closer to its host star. Its mass is 7.5 times that of Jupiter, making it 6 times denser. This rare combination makes it the second such planet (very massive and very close to its star) discovered so far. It is 15 times bigger than Earth.
CoRoT-15 b: a very particular brown dwarf
This star/planet has a mass 60 times bigger than that of Jupiter, while its radius is only slightly bigger. Its density thus reaches 40 times the density of Jupiter. Researchers thus define it as a brown dwarf: an object between a giant planet and a star. Brown dwarfs are much rarer than planets, which makes its discovery a very interesting one. It rotates around its star in only three days.
Detecting exoplanets using the transit method: a precise process
Since February 2007, the CoRoT satellite has been observing around 80,000 stars every year. The variation of the star’s intensity as a function of time, more commonly called “light curve” (LC) by astronomers, is recorded during 20 to 150 days. A scientific team then looks for micro-eclipses (also called transits) which could result from the periodic transit of a planet in front of the star. “We decided to work simultaneously, with up to eight researchers who analyse the data independently and afterwards compare their results; it takes more time, but it helps discover more planets!” says Pascal Bordé of IAS (Institute of Space Astrophysics), who is in charge of this team, responsible for the analysis of the LCs recorded by CoRoT.
Every year, the team isolates up to a thousand LCs containing transits, of which more than a hundred may result from the passing of a planet… But once these “potential planets” have been identified, there is still a lot of work to do as out of a hundred, less than five are actually planets. The others can be binary stars or artefacts.
The necessary complementarity between the ground and space
The planets are only clearly identified as such when all other possible scenarios have been discarded. Between the detection of a transit by CoRoT and the official announcement of the discovery of a new planet hide a series of complementary observations done by telescopes on the ground. “Realizing and analyzing these observations requires meticulous work that can last up to two years!” explains Claire Moutou from LAM.
Researchers participating in the CoRoT programme must thus put to the test -of their ground telescopes- the hundred or so candidates detected each year. Some fifteen telescopes around the world are used to do that. The first step is to confirm the position of the host star, then prove that the body which causes the transits is truly a planet and not another star; this can be done by measuring the mass of the body. High precision radial velocity spectrographs such as HARPS1, HIRES1 and SOPHIE3 are used for this. Finally, the VLT of ESO in Chile is used to point towards the targets and determine with a high precision the mass and radius of the star; which enables to establish the mass and radius of the planet.
It is a long process as stars are only visible five months a year, but the end result is worth the wait! Indeed, even though researchers cannot directly see these far away planets, they can measure their density (for those that transit) and start to understand their characteristics. Among the 440 exoplanets discovered so far (of which 15 thanks to CoRoT), only about 82 transits in front of their star. The transits provide invaluable information for scientists, as they give out the diameter of the planet, the relative inclination of the system as seen by the observer and the exact period of the planet around its parent star.
Ground measurements give the mass of the planet and the eccentricity of its period.
Together, ground and satellite data give the density of the planet and provide clues on its internal structure (rocky, gaseous, icy, with or without core, etc.). The scientists call this “characterizing” the planet.
References of articles
This discovery was announced just before the publication of articles in several refereed magazines. The references will be completed at a later stage.
- Scientific contact: Claire Moutou, Laboratoire d'Astrophysique de Marseille (LAM) 38 rue Frédéric Joliot-Curie 13388 MARSEILLE CEDEX (France)
- Astrophysics program scientist: Olivier La Marle