An international team of astrophysicists - including Tel Aviv University Prof. Tsevi Mazeh - has completed measurements that have firmly established the nature of the smallest and fastest-orbiting exoplanet known, CoRoT-7b. The rocky exoplanet, which revolves around a distant star 500 light years from our Sun, has a mass five times that of Earth's but a density similar to that of our planet, it was revealed on Wednesday. The planet, discovered last February by the French spaceship CoRoT, is so far away from Earth that it can't be viewed from here. It was revealed as a small black "stain" projected on its "rather unremarkable" mother star, clumsily called TYC 4799-1733-1 at the outset; the "stain" reappears on the star's surface every 20.4 hours. The star is now referred to as CoRoT-7 in honor of the French spaceship. Measurements conducted by astronomers resulted in calculations of the planet's radius as twice that of Earth's. To measure the planet's mass, the international team - comprised of scientists from Switzerland, France, Germany, Spain, Israel and other countries - had to find out exactly the star's velocity in space. The initial set of measurements, however, could not provide the mass of the exoplanet. Such a result requires extremely precise measurements of the velocity of the star, which is pulled slightly by the gravitational tug of the orbiting exoplanet. The problem with CoRoT-7b is that these tiny signals are blurred by stellar activity in the form of "starspots" (just like sunspots on our Sun), which are cooler regions on the surface of the star. Therefore, the main signal is linked to the rotation of the star, with makes one complete revolution in about 23 days. To get an answer, astronomers had to call upon the best exoplanet-hunting device in the world, the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph attached to the ESO 3.6-meter telescope at the La Silla Observatory in Chile. Combining CoRoT-7b's known radius, which is less than twice that of our Earth, the scientists learned that the exoplanet's density is quite similar to the Earth's, suggesting a solid, rocky world. The extensive dataset also reveals the presence of another so-called super-Earth in this alien solar system. "This is science at its thrilling and amazing best," says Didier Queloz, leader of the team that made the observations. "We did everything we could to learn what the object discovered by the CoRoT satellite looks like, and we found a unique system." The discovery of the exoplanet facing the constellation of Monoceros (the Unicorn) was announced a year after its detection and after several months of painstaking measurements with many telescopes on the ground. Cooler than our Sun, CoRoT-7 is also thought to be younger, estimated at 1.5 billion years. Mazeh, director of TAU's Raymond and Beverly Sackler Institute of Astronomy and the only Israeli in the international team, is an expert in binary stars, brown dwarfs and the search for substellar objects and extrasolar planets. He said that new knowledge about exoplanets far from Earth has boomed in recent years. "It's amazing that 25 years ago, when I started to become interested in this field, not a single exoplanet outside our solar system was known. Now, we have succeeded in discovering solar systems very similar to ours. This new achievement is especially exciting, as were were able to estimate the plant's mass and radius, which made it possible to calculate its density." Mazeh noted that even though the densities of CoRoT-7b and Earth were alike, they each have very different conditions on their surfaces. The scientists in the project believe, however, that the rocks on the surface of the exoplanet are similar to those on Earth. It may be, continues the TAU scientist, that billions of planets very similar to Earth exist throughout the galaxy, which contain about one hundred billion suns. "They are just waiting for us to come and discover them," he added.