Rousko Slavov, of Kirkwood Intermediate School, asks :-

Is there a planet with air on it?

John Hearnshaw, an astronomer at the University of Canterbury, responded.

The atmosphere of the Earth does, of course, consist of air, which is a mixture of gases, mainly nitrogen, but with some oxygen. There are nine planets in orbit around the Sun, but the Earth is the only one to have air.

Mercury, the innermost planet, has a rocky surface, but essentially no atmosphere. Venus and Mars have carbon dioxide atmospheres, while the next four planets have dense atmospheres consisting of a mixture of hydrogen, helium, ammonia and methane. The outermost planet, Pluto, may well have a thin atmosphere, but it is almost certainly not air. We can therefore state that none of the planets in the Solar System, except for the Earth, has air. Since air is essential for life, it would be very difficult for humans or other animals living on Earth, to live on other planets, unless they were to take oxygen or air with them together with breathing apparatus. Possibly the Earth's atmosphere once, a long time ago, contained carbon dioxide like that of Venus, but somehow this was modified by plants that take in carbon dioxide and give out oxygen. However we are not sure if this idea is really correct.

At the present time the Sun is the only star known to have planets going around it. However there are billions of stars in our Milky Way system similar to the Sun. Astronomers are at present trying to find planets around other stars. Indeed here in New Zealand, at Mt John Observatory, there has been research done to try and find planets around other stars. So far no-one has definitely proved they exist, but it seems very possible that there could be planets around many other stars, but they are too faint to see directly. If this turns out to be correct, then quite possibly some of them could have atmospheres consisting of air like here on Earth. It is too early to guess what may be found, but the question of whether other planets exist around other stars, and what they may be like, even whether they support life, is, to my mind, one of the most exciting and challenging problems in astronomy today.