In 2008, a meteorite fell into Earth's atmosphere and exploded over the Nubian Desert in Sudan.

It was the first time a meteorite had been identified and tracked before it hit the planet, and meteorite hunters rushed to the scene. Many fragments of the Almahata Sitta meteorite, as it became known, were recovered.

It quickly became clear that the rock fragments contained diamonds. That wasn't too surprising: some kinds of meteorite often do. But a new study suggests those diamonds were far larger than any yet seen in a meteorite.

According to the scientists involved, that means these diamonds formed in an unusual way. Large diamonds are most likely to form inside a really big lump of rock – like a planet. If they're right, these diamonds come from a planet that existed when our solar system was forming, and that has since been shattered.

The results are published in the journal Geochimica et Cosmochimica Acta.

Masaaki Miyahara of Hiroshima University in Japan and his colleagues examined samples of the meteorite. The diamonds are mostly tiny, around 40 micrometres across, although one is almost 100 micrometres.

However, some of them appear to have been broken, and the separate crystals are all oriented in the same way.

This suggests that some of the small diamonds used to be part of a larger one.

The standard idea is that diamonds in meteorites form when asteroids collide. The shock of the impact is enough to crush carbon into tiny diamonds.

But these diamonds seem to be too big to be explained that way.

Instead the authors think there are two ways they could have formed.

It could be that the diamonds formed by the slow deposition of single carbon atoms, in the thin gases of outer space. But this seems unlikely.

The more likely possibility is that the diamonds formed inside a "planetesimal": a lump of rock not quite large enough to count as a planet, but far bigger than any asteroid.

This planetesimal must have existed in the early days of the solar system, before the planets had properly formed and settled down into their orbits.

If that's true, it must have long since been blown to bits, and the Almahata Sitta meteorite is a fragment of it.

We can't know for sure if that is true, and it's a big extrapolation from a few diamonds in one meteorite. However, the early system was a turbulent place, with lots of fragments of rock and ice whizzing around and colliding with each other.