Most planetesimals stay that size.
This reinforces the theory that the current planets of the solar system were formed from the remains of dozens of large protoplanets, the researchers add.
"We are probably looking at an object that was one of the first planets to circle the sun before they collided with each other to create the actual planets we have today", said Gillet.
And the asteroids still floating around the Solar System, astronomers believe, are the leftovers from those days - from the repeated collisions that blasted material back into space. Over time, the fragments were gathered and catalogued for study into a collection named Almahata Sitta (Arabic for "Station Six", after a nearby train station between Wadi Halfa and Khartoum).
These meteorites-referred to as Almahata Sitta-belong to a rare type, known as ureilites-rocky meteorites with a unique mineralogical composition that contain diamonds.More news: Andres Iniesta's exit from Barcelona a 'done deal'
More news: Netflix shares up 7% in premarket on subscriber growth beat
More news: Four baboons escape research facility in Texas
Orality Almahata Sitta formed in the explosion of an asteroid EC3 and crashed into Northern Sudan in the Nubian desert ten years ago. Finding diamonds in that meteorite.
The other fascinating thing about diamonds is that, as they are forming, they often trap minerals present in their formation environment.
They discovered chromite, phosphate and iron-nickel sulfide embedded in the diamond, with compositions and morphologies that could only have occurred under greater pressure than 20 gigapascals - almost 200,000 times that of sea level atmospheric pressure. Called "inclusions", these particles are only found in diamonds formed under sustained high pressures of more than 20 gigapascals.
Gillet, a planetary scientist at the Federal Institute of Technology in Lausanne, said researchers calculated a pressure of 200,000 bar (2.9 million psi) would be needed to form such diamonds, suggesting the mystery planet was as least as big as Mercury, possibly even Mars. There are hundreds more ureilites that could offer clues to the nature of the early solar system.
Models have predicted that a number of protoplanets existed during the early years of our solar system and that through accretions and collisions helped form the existing terrestrial planets and some of their moons.