New observations of the Lutetia asteroid have revealed that it's a planet that never properly formed.
Analysis of data gathered during a flyby of ESA's Rosetta spacecraft in July suggest that it may even have a molten metal heart.
Closeup images from Rosetta's Osiris camera show that parts of Lutetia's surface are around 3.6 billion years old. It's covered with craters, and its irregular shape is now believed th have been caused by other impacts.
Astronomers now believe it's what's called a planetesimal - a remnanat from the earliest stages of the solar system that never quite made it to planethood.
"We don't think Lutetia was born looking like this," says Holger Sierks of Germany's Max-Planck-Institut für Sonnensystemforschung. "It was probably round when it formed."
Lutetia's composition appears to be remarkably uniform.
"It is striking that an object of this size can bear scars of events so different in age across its surface while not showing any sign of surface compositional variation," says Fabrizio Capaccioni of Italy's INAF.
Beneath the surface, Lutetia turns out to have one of the highest densities of any known asteroid - 3,400 kg per cubic metre. This implies that it contains significant quantities of iron, although not necessarily in a fully-formed core.
To form an iron core, Lutetia would have had to melt because of radioactive isotopes in its rocks, with the dense iron then sinking to the centre and rocks remaining at the surface.
However, no such rocky material has been observed, indicating that Lutetia was subjected to some internal heating early in its history but didn't melt completely, so that it lacks a well-defined iron core.
Rosetta has now left Lutetia far behind, and is in hibernation as it travels to a 2014 rendezvous with comet Churyumov–Gerasimenko.