The secrets of comets are finally being uncovered. We now know that comets formed at the same time as the rest of the Solar System, according to the latest data from comet 67P/Churyumov-Gerasimenko.

Rosetta, the European Space Agency’s spacecraft, has been analyzing 67P in detail and discovered that most ice found in this object formed from the original protoplanetary nebula from which the Sun and the planets later emerged. The discovery settles a decades-old debate about when comets actually formed.

The structure of the ice molecules tells a lot about the formation of comets. Ice can form in two ways, either as crystal ice or amorphous ice. The latter forms when water molecules cool quickly; they don’t have to organize in a nice precise structure. If the cooling is gradual, the water forms in crystals. There is also another important difference that has an astrophysical consequence: amorphous ice traps a lot more gas when it cools.

Rosina, one of the instruments on board Rosetta, measured the amount of certain gases (nitrogen, argon, and carbon dioxide) in 67P, and detected quantities a hundred times smaller than what’s expected from amorphous ice, indicating a definite crystalline structure.

This is a major discovery because it allow us to determine the age of comets. Amorphous ice forms in interstellar space where there are no heat sources. When stars begin to form, the protoplanetary nebula is warm enough for the interstellar ice to sublimate. As the nebula cools down and the planets begin to form, water freezes into ice crystals, trapping some gas within.

Comets made of crystalline ice then must have formed at the same time as the rest of the Solar System. The research, published in The Astrophysical Journal, indicates that 67P must have formed between -228 and -223°C (-378 to -369°F) to trap the right quantities of gas observed.

These findings could have consequences beyond comets. Many formation scenarios for the gas giant planets and their moons, as well as objects in the Kuiper Belt (like Pluto), require the agglomeration of crystalline ice.