[We put this question to Maggie Turnbull as part of the Planets and Cosmology show]

Maggie Turnbull: You can imagine this sphere around the Earth, going out into space, that is filled with radio noise that we have emitted because of our technological activities. If you want to count the very first broadcasts, then the radius of that sphere around us is something like 80 or as much as 100 light years. I don't think in terms of light years, I think in terms of parsecs and 1 parsec is about 3 light years. If we say that this sphere is about 30 parsecs in radius, I know that there are about 3 to 5 thousand stars inside that sphere around us, and of those, about 2 thousand are sun-like stars. These are similar enough to our star that they could live long enough and have planets that could have life on them that are somewhat similar to our own.

This is a lot of stars, but you also have to consider that not every one of those stars is going to have planets and out of those that do, they are not all going to have Earth sized planets that are in the habitable zone.

So I would say that a reasonable number would be about 10% of those, and anywhere between 1 to as many as 5 hundred habitable planets are within that sphere of noise that we've created.

You have to keep in mind that most of the transmissions that are leaking out into space are doing just that, they are leaking, they are not beamed intentionally at any star system, so they are very weak by the time they get there. Also, all the signals that we leak out into space are not going equally in all directions, sometimes you have to be at a certain spot to be able to pick them up.

We think with galaxy formation that everything was very hot after the big bang, so you start off with this enormous cloud of very hot gas that begins to cool. As the gas cools, it starts off radiating x-rays, it cools down more and eventually it condenses down into the stars that form a galaxy.

If you have a large cloud of gas which is quite static, it will condense into an elliptical shaped galaxy.

If it’s rotating even very slightly, the galaxies that condense from it will end up disc-shaped such as the spiral shape our Milky Way is.

It depends on what sort of planet is forming, and it also depends on what your star is made of, how big it is and how much stuff is left behind.

If you have a planet that’s made of gases and volatiles, like Jupiter or Saturn, they’re only going to condense much further out from the sun. They tend to get swept out by the winds from the young star forming in the nebula and condensing down to form the planetary system.

If they’re made of solid, rock material, like Earth, Mercury, Venus or Mars, they tend to coalesce much closer in.

[We put this question to Carolin Crawford as part of the Planets and Cosmology show]

Carolin Crawford: Room temperature where on the Earth?

Chris: They don’t specify, so I guess you have full degrees of freedom, if you’ll pardon the pun.

Carolin: It’s difficult, because as you go up from the surface of the Earth, you get colder. Also, on the side of the Earth facing away from the Sun it will be different from the day side. It’s actually a very difficult question to answer as there’s so many different ways to approach it.

Chris: Recently there was a Swedish space walk, and they were talking about how when people are in space they must wear reflective space suits; because if they’re in the full glare of the Sun their body will quickly heat up to boiling point. In the dark side though, it’s very cold because you radiate the heat straight away again. So it is a difficult question to answer, because if you’re having sunlight hitting you, you soak up a lot of radiation, and if you’re out of direct sunlight, you’re not.

Carolin: Also, if you think about aeroplanes, you can see the condensation freezing on the outside of the aeroplane, and although you’re above the clouds, you’re still nowhere near space.

[We put this question to Carolin Crawford as part of the Planets and Cosmology show]

Carolin: Well certainly all the stars we know about do form in a galaxy but solitary stars would be very difficult to find.

Stars can leave a galaxy. Sometimes two galaxies merge, and in this process they can throw stars out into the vastness of space.

So there may well be lots of orphaned stars out there, but they are very difficult to find. All of the stars we know about are in galaxies.

[We put this question to Carolin Crawford as part of the Planets and Cosmology show]

Carolin Crawford: Anything is possible. It’s unlikely to be a star as we know it, as stars don’t form until thousands of years after the big bang. There’s always an idea that our universe is just part of another universe. This is really a philosophical question, as it’s one that we cannot answer at the moment.

DNA samples are contaminated with bacteria, and other contaminants, but to analyse the sample we amplify human DNA sequences and use a ‘probe’, a little piece of genetic material that only latches on to specific sequences of DNA. We use probes that only recognise human DNA sequences, which are then bound to the sample, so the only patterns you see relate to human DNA in the sample.