CoRoT-3 b sits on the line between a planet and a brown dwarf, an object too small to shine as a star but much heavier than a typical planet. It is about the size of Jupiter but about 22 times as massive, which makes it extraordinarily dense: about 26 grams per cubic centimetre, more than twice as dense as lead. It circles an F-type star, a little hotter and brighter than the Sun, about 2,500 light-years away, every 4.26 days.
It was found by the CoRoT space telescope, which watched thousands of stars for months at a time and caught 34 of its transits, the dips in starlight as it crossed its star, over 152 days in 2007. A team led by M. Deleuil measured its size from those transits and its mass from the star's wobble, measured from the ground, and reported it in 2008 as the first secure resident of the brown-dwarf desert: a range of masses in which companions on close orbits around Sun-like stars are rare. With the data then available, they could not say whether it formed like a planet or like a brown dwarf, and suggested it might belong to a new class of superplanets.
Its sheer mass makes it visible in other ways. In 2010 a team led by Tsevi Mazeh found two faint effects in CoRoT's measurements of the star's brightness: the star is slightly stretched by CoRoT-3 b's gravity, and its light brightens slightly as the star moves toward us in its orbit, a relativistic effect called beaming. A 2009 study led by Amaury Triaud hinted that the companion's orbit may be tilted relative to the star's spin, but called the result marginal.