Most asteroids are known to be fragments of parent asteroids or rubble piles, but the asteroid known as 21 Lutetia appears to be neither of those things.
Three articles in this week's Science come to similar conclusions: Data from the European Space Agency's Rosetta craft show that 21 Lutetia is more like a planetesimal, or a planet-precursor, than an asteroid. Planetesimals are important because they can provide information about the formation of the Solar System.
What makes 21 Lutetia unique? Researchers using OSIRIS, the Optical, Spectroscopic and Infrared Remote Imaging System, studied the asteroid's surface and determined that it's covered in a thick layer of regolith, or loose surface material. The regolith flows across the surface of the asteroid in landslides.
Another team of scientists used radio-tracking to calculate the mass of 21 Lutetia, and determined that the density of the asteroid is about 212 pounds per cubic foot - one of the highest densities for an asteroid.
A third team of researchers used VIRTIS, the Visible, Infrared and Thermal Imaging Spectrometer, carried by Rosetta to measure the temperature and composition of the asteroid. 21 Lutetia appears to have a max surface temperature of 254 degrees Kelvin, without any evidence of space weathering or aqueous processes.
These three articles are likely just the beginning, as the uniqueness of the asteroid means it'll probably be studied further.