Stephanie White, of Kakanui School, asks :-

How do scientists know what the planets are made of if they haven't been there to take samples?

Pam Kilmartin, an astronomer at the Mt John University Observatory, responded.

There is quite a bit of careful guesswork involved. Here are two examples of how it's done.

If we look at the moon through a telescope we can see that it has a solid surface with craters and mountains. So we know the moon is made of solid stuff: rock or ice? We know that ice would melt in sunshine so the moon must be made of rock.

By measuring the moon's gravity pull we can find its mass or weight. We can then calculate its density: it is the same as rock. So we can be fairly sure that the moon is rock all the way through; it doesn't have a metal core. Spacecraft passing the moon find it has almost no magnetism. This confirms that it doesn't have a metal core. Of course astronauts have now been to the moon and brought back rocks. But the same reasoning can be used with any solid planet or moon.

If we look at Jupiter we see that it has clouds floating in some clear gas. Jupiter is five times further from the sun than we are, so it is very cold. The clouds can't be steam because water would freeze. They must be made of chemicals that stay liquid at that temperature: formaldehyde and similar stuff. The clear 'air' on Jupiter must be gases like hydrogen, helium and methane, that stay gassy when very cold. Some of the gases, methane for one, show up in the spectrum of Jupiter.

Again the density helps. Jupiter is big enough to hold a 1000 earths but it is only 320 times heavier than Earth. So it must be made of much lighter stuff than the earth. Hydrogen is a good guess. In the centre of a giant planet like Jupiter the hydrogen is so squeezed that it turns into solid stuff. Solid hydrogen behaves like a metal. Jupiter has a very strong magnetic field, as though its core is metal. We think the core is solid hydrogen.