And researchers say there are three main ways they can form - during a major collision, through the deposit of chemical vapor or through sustained high pressure and temperature, like the way diamonds form here on Earth.
In 2008, an asteroid measuring 13 feet across penetrated Earth's atmosphere and exploded above Sudan, scattering rocky fragments across the Nubian Desert.
The asteroid, called 2008 TC3 or Almahata Sitta, was the size of a auto. The fragments were gathered and organized into the Almahata Sitta collection. It turned out to be a rare type of meteorite called ureilite, which has an unusual composition compared to other stony meteorites - it contains a lot of carbon in the form of nanodiamonds.
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The unanswered question, so far, has been the planetary origin of 2008 TC3 ureilites.
A team of scientists led by Farhang Nabiei from the École Polytechnique Fédérale de Lausanne in Switzerland examined tiny crystals embedded within diamonds in the Almahata Sitta meteorites using a technique known as transmission electron microscopy-which creates an image of a specimen by beaming electrons at it. And the metal deposits found within them would likely have formed in a young planet at least the size of Mercury and maybe as large as Mars, according to the researchers. The impurities are what are known as "inclusions" and are found in Earth's diamonds, too.
The presence of these particular inclusions suggest the diamonds were formed under significant pressure, roughly 20 giga-Pascals. Many of these protoworlds collided, their fragments forming new planets and moons.
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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.
Planetary formation models suggest that in the early solar system, the terrestrial planets-Mercury, Venus, Earth and Mars-were formed after the gradual merger of tens of proto-planets in a series of high-energy impacts.
The authors write "This study provides convincing evidence that the ureilite parent body was one such large "lost" planet before it was destroyed by collisions some 4.5 billion years ago".
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