# How long does it take for a comet to form?

23 February 2016

## Question

I'm just wondering how long it takes for a comet to form?

Kat quizzed Stuart with this spacey question...

Stuart - So a comet is this mixture of frozen gases and frozen ice and dust compacted together into an object, and it's different to a meteor so it's different to just the rock that we see. Because comets, particularly, have this phenomena that as they come into our solar system, as they get near to the sun, they get bathed in sunlight, that some of these gases will start to melt and vaporise and that's what give comets it's atmosphere and sometimes it's tail that travels across the universe. And how long do they take to form - you kinda need to ask where do they come from? So it goes all the way back to the start to the start of our solar system. So 4.6 billion years ago, where the solar system basically consisted of a large cloud of dust and particles in a large field and, over time, these dust particles start to aggregate, start to clump together and form combinations of material that got larger and larger. Now some of those went on to be very big and become planets but some of them remained quite small and developed into comets. And it's very difficult to say exactly how long they take to form because it's a very long time ago. So things that are a very long time back we have quite large uncertainty, we have quite large error bars on our calculations, but some of the estimates put it around 10,000 to 100,000 years for a comet to form in the form it's in. And surround our solar system is something called the oort cloud, which is a giant cloud of comets just sitting there waiting there and, occasionally, one of them gets bashed into and starts its journey into our solar system and either does an orbit around the sun, or goes crashing into a planet, or crashing in and out again.

Kat - I guess that's like the 67P - I'm going to try and say it. Come on Chris

Chris - 67P/Churyumov-Gerasimenko.

Kat - Gerasimenko. That was a very impressive comet in our solar system that we actually managed to land something on.

Stuart - Incredibly impressive. And just to show how recent and how our understanding of comets is still developing, in the last few weeks the Rosetta mission has actually announced some new results, saying that their spacecraft Rosella, which is orbiting this comet, has been doing some measurements to understand the composition of the comet. And actually, we thought what's a comet like inside, has it got caverns, has it got holes but actually, they're very uniform. This dust and frozen mix is very uniform throughout the entire comet.

Kat - I do love the idea - they're like the stuff at the back of the freezer of the universe, aren't they really? All this stuff is there and has always been there.

Stuart - And that's why it's so exciting, so it can tell us something about the very origins of our solar system.