CoRoT unveils a rich assortment of new exoplanets

One of these exoplanets, designated CoRoT-11b, has twice the mass of Jupiter and orbits a rapidly rotating star; this type of star is an extremely difficult target for exoplanet searches and its detection marks a significant achievement for the CoRoT team.

In order to detect planets orbiting other suns, the CoRoT satellite, which is operated by CNES (the French space agency), observes a large number of stars over a significant period of time, trying to spot a subtle decrease in their luminosity: this 'dimming' could be a signature that the star hosts a planet, which is transiting in front of it and partially obscuring its light. This transit technique is one of several methods used to search for exoplanets but is the only one that allows astronomers to determine the radius of the planet - by measuring the depth of the transit.

Other geometrical configurations of a stellar system, for instance the presence of one or more companion stars can, however, mimic the presence of a planet. For this reason follow-up observations are needed to confirm the planetary nature of the transiting body. Alerted by CoRoT's detection of a candidate planet-hosting star, some of the foremost ground-based observatories collect high-resolution images and spectra, yielding a wealth of additional information.

In particular, astronomers look for a Doppler shift in the stellar spectrum, highlighting the periodic 'wobble' of the star in the two-body system. From the amplitude of this wobble, it is possible to estimate the mass of the transiting body and, consequently, to determine whether or not it is indeed a planet. Once the mass and the radius are known, the mean density of the planet can be derived – a key factor in distinguishing between gaseous giant planets and rocky terrestrial ones. The discovery of these six new exoplanets adds variety to the large number of exoplanets that have been detected to date.

"With the addition of this new batch, the number of exoplanets discovered by CoRoT has risen to 15," says Magali Deleuil from Laboratoire d'Astrophysique de Marseille, Head of the CoRoT exoplanet programme. "The increasing size of the census, which includes objects with very diverse characteristics, is of vital importance for a better understanding of planetary systems other than our own," she adds.

The new discoveries exhibit a wide variety of physical properties, spanning a broad range of sizes and masses: the smallest of the sample, CoRoT-8b, is about 70% of Saturn's size and mass, while CoRoT-10b, CoRoT-11b, CoRoT-12b, CoRoT-13b and CoRoT-14b are larger, belonging to the class known as 'hot Jupiters'. CoRoT-15b, being 60 times as massive as Jupiter, is a brown dwarf, an intermediate object between a planet and a star. In addition, other peculiarities are exhibited in this very heterogeneous set of exoplanets: CoRoT-10b has an extremely eccentric orbit, resulting in large variations in its surface temperature over the course of its year, and CoRoT-11b's parent star spins around its axis at an extraordinarily fast rate.

"The rich diversity emerging from this sample is a very interesting result, showing CoRoT's ability to detect exoplanets which are rather different from each other", comments Malcolm Fridlund, ESA's Project Scientist for CoRoT. "Being able to study a wide variety of planets will provide important insights into the formation and evolution of planetary systems", he adds.

As of 14 June 2010, the CoRoT family of exoplanets comprises 15 members - the most recent additions being the following six:

CoRoT-8b: At about 70% of the size and mass of Saturn, CoRoT-8b is moderately small compared to the previously known transiting exoplanets. Its internal structure should be similar to that of ice giants, like Uranus and Neptune in our Solar System. It is the smallest planet discovered by the CoRoT team so far after CoRoT-7b, the first transiting Super-Earth.

CoRoT-10b: The orbit of this planet is so elongated that the planet passes both very close to, and very far away from, its parent star. The amount of radiation it receives from the star varies tenfold in intensity, and scientists estimate that its surface temperature may increase from 250 to 600°C, all in the space of 13 Earth-days (the length of the year on CoRoT-10b).

CoRoT-11b: CoRoT-11, the host star of this planet, rotates around its axis in less than two days. For comparison, the Sun's rotation period is 26 days. It is particularly difficult to confirm planets around rapidly rotating stars, so this detection marks a significant achievement for the CoRoT team.

CoRoT-12b, 13b and 14b: These three planets all orbit close to their host star but have very different properties. Although CoRoT-13b is smaller than Jupiter, it is twice as dense. This suggests the presence of a massive rocky core inside the planet. With a radius that is 16 times larger than the Earth, CoRoT-12b belongs to the family of 'bloated hot Jupiters', whose anomalously large sizes are due to the intense stellar radiation they receive. Amazingly, CoRoT-14b which is even closer to its parent star has a size similar to that of Jupiter. Its mass is 7.5 times the mass of Jupiter making the planet 6 times denser. Such a combination, very massive and very hot, is rare and CoRoT-14b is the second such planet discovered so far.

CoRoT-15b: the brown dwarf. CoRoT-15b's mass is about 60 times that of Jupiter. This makes it incredibly dense, about 40 times more so than Jupiter. For that reason, it is classified as a brown dwarf, intermediate in nature between planets and stars. Brown dwarfs are much rarer than planets, which makes this discovery all the more exciting.

The figure above depicts the relative sizes and distances from parent stars of the CoRoT exoplanets.

Credit: CNES

One of the planets, CoRoT-11b, stands out from the set of six because of the rotation velocity of CoRot-11, its parent star, which spins around its axis in less than 2 days - an exceptionally high speed, as compared to the Sun's rotation period of about 26 days.

"This is the third exoplanet discovered around such a rapidly rotating star", notes Davide Gandolfi, the ESA Research Fellow who led the study of CoRoT-11b. "Because of the fast rotation of its host star, such a planet could only have been discovered because it transits in front of it, thus only a transit-hunter, such as CoRoT, could have spotted it", he adds.

The search for Doppler shifts in the spectra of stars, which represents another prolific method for detecting exoplanets, is in fact biased against planets orbiting fast rotators, as the high rotational velocity of the star makes it extremely hard to achieve high-precision Doppler measurements and hence to detect the tiny signature of the presence of a planet. "If it had been included as a possible exoplanet candidate during such a campaign, CoRoT-11b would have been rejected because of the intensive observational effort needed to achieve the required accuracy", explains Gandolfi.

Instead, the object was first noticed by CoRoT, and then became the subject of extensive photometric and spectroscopic follow-up observations across the world, using the Swiss Leonhard Euler 1.2 m telescope at ESO's La Silla Observatory and the TEST 30 cm telescope at the Thüringer Landessternwarte Tautenburg, as well as a number of world-class spectrographs (HARPS at ESO's La Silla Observatory, SOPHIE at the Haute-Provence Observatory, UVES at ESO's Very Large Telescope and HIRES at the Keck Observatory), and the high- and low-resolution spectrograph also at Tautenburg, in Germany. Thanks to the combination of these exceptional data, it was possible to estimate the mass of CoRoT-11b, which is about twice as massive as Jupiter, and its radius, which is about 1.4 times that of Jupiter, thus confirming its planetary nature.

"This result anticipates what may be achieved by future space-based missions searching for exoplanets", says Fridlund. CoRoT is in fact a precursor for PLATO, a Cosmic Vision candidate mission that will seek planetary transits over a much larger sample of stars - the size of the sample is an important factor determining the number of planets that may be discovered. This significant increase in the sample size is possible because of PLATO's very wide field of view, which in turn relies on the combined use of 34 small telescopes. In addition, PLATO will study brighter stars than those that can be observed with CoRoT, making it possible to determine the age of the planet-hosting stars through asteroseismology measurements. This, combined with the tremendous improvement in the accuracy on the estimate of exoplanet masses and sizes that is expected from PLATO, will provide an important step in the quest to understand the conditions that favour the formation of Earth-like planets.

Source: ESA Science & Technology
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