Asteroids visited by spacecraft have all turned out to be piles of rubble or chunks broken off of larger bodies, but that's not the case with 21 Lutetia, a 75-mile long, 47-mile wide body circling in the main belt of asteroids between Mars and Jupiter.
Europe's comet-bound Rosetta probe flew by Lutetia last year and gave scientists a big surprise. With its dense body and an interior that seems to have survived intact, the large asteroid appears more like a protoplanet -- a leftover building block from the formation of the solar system.
"It could represent the missing link between smaller asteroids and dwarf-planets, like Ceres or Vesta, or planets," astronomer Fabrizio Capaccioni, with the National Institute for Astrophysics in Rome, Italy, wrote in an email to Discovery News.
For a protoplanet to survive, it needs to be big enough to withstand the heavy impacts of the solar system's early days. Lutetia is about the smallest that could have made it through intact, computer models show. But scientists don't know if it formed where it is found today, or migrated there after being gravitationally slingshot by a larger planet.
"The present day asteroid belt as a whole is a relic and fossil remnant of the original asteroid belt, which, as for the rest of the solar system, was much richer in the number of asteroid in the early phases of development of the solar system," Capaccioni said.
As the planet embryos began to grow, they started to gravitationally interact with bodies in the asteroid belt. Jupiter, in particular, acted as a giant vacuum cleaner to clear out material from the asteroid belt, Capaccioni added.
Protoplanets that remained in the asteroid belt continued to grow, albeit at a slower rate.
Scientists suspect 21 Lutetia is not a special case.
"It's just (by far) the largest visited at that time," astronomer Holger Sierks, with the Max Planck Institute for Solar System Research in German, told Discovery News.
More information about how 21 Lutetia fits into the solar system's puzzle should be coming from NASA's Dawn probe, which is currently in orbit around Vesta, a 330-mile diameter asteroid, also believed to be a remnant protoplanet. Dawn is scheduled to move on to the king of the asteroid belt, the dwarf planet Ceres, next year.
The research appears in this week's Science.