How old are the planets?

Izzie Clarke found this question from Hans on The Naked Scientists Forum. Physicist Ben McAllister, from the University of Western Australia, was able to shed some light on the matter...

Ben - This is another great question and the answer is yep, pretty much within a narrow window within the degree of confidence we have in saying a date for any of these things. We're pretty sure that the entire solar system, the sun, the earth, all the planets are something like 4.6 billion years old, again, give or take a few million years. This is a very large large number type problem that we're dealing with here.

The easiest one, the first thing that we can do is just basically date rocks on earth. We can use geological processes and radioactive decays in order to figure out how old rocks are on earth, and that's easy to do because we have rocks on earth kicking around all over the place. So how you might do that is if you pick up a rock that you think is one of the older rocks on earth, maybe it's deep down in the core. I'm not entirely sure where the oldest rocks on earth actually come from, but if you were to find one of these older rocks on earth you could look at the relative populations of these different what are called radioactive isotopes inside them. There are these natural chemicals that undergo this process called radioactive decay; we were hearing a little bit about it before turning one element into another element by stripping off a few protons. There are things like uranium which break down into smaller things and they do this with a characteristic time associated with them. There's a bit of variation, it's a bit random, but for a very large population of say uranium there's going to be a certain amount of time with which about half of that uranium breaks down.

So if you look at a bit of rock, and you can figure out how much uranium is in there. And also look at, for example, how much of the stuff that uranium breaks down into is in the rock, you can figure out how long ago that decay started happening by knowing how much uranium there was originally and how long it takes to break down, and then you can date the rock.

We can do a similar process with rocks from the Moon because been there. We can do a similar process with bits of rock that have landed on Earth that we are pretty sure have come from Mars at some point in the past.
So basically, we've got Earth, the Moon, Mars that we can, very directly, access and then you can look at the other planets and you have to do a slightly different process to figure out how old they are because we don't have any rocks from Jupiter for anything.

Izzie - So how does that work?

Ben - One of the best methods that people use to figure out how old something like Jupiter is, is basically looking at the surface of Jupiter and counting how many craters there are. Because one thing that we are pretty sure about is that the number of stray bodies moving through our solar system that collide with the planets has been constant over a really long period of time - billions of years. So if we know how many craters there are on something like the Earth or the Moon that we're pretty confident in how old it is, and we can compare that with how many craters there are and Jupiter just because we can be sure that the relative rate with which their being struck by stuff is the same, we can infer the age something like Jupiter or one of the other planets in the solar system.

Izzie - Hans there you have it. I guess it also depends on what you are using as a reference of a year because someone who is 30 in earth terms is also 120 years on Mercury or 2 1/2 on Jupiter, so we’ll say earth years. 

Ben - Yeah. Also if you use the kind of uncertainty that we have in dating planets they might be a million years old or they might not have been born.