When NASA launched Stardust in February 1999 to investigate the comet Wild-2, it did not expect the findings to overthrow the theories about the differences between asteroids and comets. The probe returned to Earth in January 2006 after collecting cosmic dust samples from the comet, ending a successful mission and giving researchers something to talk about. The team of scientists concluded that the matter contained in the comet resembles the one found in asteroids, which means the definition of a comet could be redefined.

The Wild -2 was discovered in 1978 by Swiss astronomer Paul Wild is believed to be 4.5 billion year-old. Scientists believed it to be “a reservoir of pre-solar material, including stardust cryogenically preserved since the accretion of the planets,” according to the report published in the journal Science. Previous theories said asteroids were affected by Sun’s heat before ending up in the Asteroid Belt, while comets were believed to be unaltered by the Sun’s heat, containing only dust from other stars, ice and gas particles.

The investigation of the samples taken from the Wild-2 concluded something different though. The materials were not as primitive as scientists would have expected. The calcium aluminum inclusions found led to the idea that the comet was submitted to some sort of heating, as this particular compound could only be the result of high-temperature processes in the solar system, as Hope Ishii from the Institute of Geophysics and Planetary Physics said.

Up to this point, the conventional theories clearly separated the origins of asteroids and comets, as the first came from the inner, warmer areas of the solar system, while the latter came from more distant regions. So far, scientists do not doubt that Wild-2 is truly a comet, as it is known to have spent most of its life beyond the frost-line of the asteroid belt, until 1974, when a close encounter with Jupiter altered its orbit and sent it closer to Earth.

So, what is the explanation for the unusual particles found in Wild-2? Apparently, this is an asteroid-like comet, not as primitive as other comets out there, possibly made up of particles that formed in the inner solar system, which were later transported outward by solar winds or gravitational forces, which might explain why an object coming all the way from Kuiper Belt contains inner solar system materials.