Expert: Courtney Seligman - 5/17/2011

Question
We would like to ask a question.
If there is no air in space, how come there is air on earth?
Thank you for your time.
From Room 4
Sandspit Road School
New Zealand

Answer
When the planets formed, there was a lot of gas moving around the Sun, in what is called the Solar Nebula. There was probably a hundred times more gas than all the gas and rocks and metals in all the planets combined. Some of that gas ended up inside the planets, or was attracted to them by their gravity. That is where the atmospheres (= the air) of the planets came from.

We know this is correct partly from theories of how the Solar System formed, and partly from observations of stars which are now forming in places like the Orion Nebula, and the Tarantula Nebula. In such regions, about half the stars are surrounded by clouds of dusty material which absorb the heat of the forming stars, then give off infrared radiation. Although what we observe is the infrared radiation given off by the dust, studies of clouds of gas and dust in interstellar space show that there is far more gas than dust, so the presence of dust suggests that there is a lot of gas, whether we can see it or not. (In some places, where there is still gas very close to very hot, very bright stars, we can also see radiation given off by the hot gas; but in the dusty regions that planets are made in, the heat of the dust is easier to observe.)

Of course, this leaves the question, where did all the gas go, if there was so much of it around early on? We aren't entirely sure, but we know that although stars that are still forming are often surrounded by clouds of gas and dust, stars that have already formed usually have very little gas or dust nearby. Somehow, the gas and dust has been swept away.

For the dust, the process is simple. It runs into the planets while they are forming, and they run into it after they form. Eventually, almost all the dust is inside one planet or another, and very little is left over, between the planets. For the gas, things are more complicated, but there is no doubt that in one way or another the star pushes the gas into space. You can see something like this in the tails of comets, which are partly composed of gases released by the evaporation of ices in the nucleus. For a little while the gas spreads outward in all directions, but as it does so it absorbs radiation from the Sun which ionizes the gas. That allows the solar wind to drag the gas outward, into interstellar space. Whether that is the exact way in which the Sun got rid of the gas surrounding it is not known, because there are several different ways in which stars can push the gas around them into interstellar space. But however it is done, after a while you have only planets, and almost completely empty space between them and their star.

Although the gas in the space between the planets is lost, gases which were trapped inside or at the surface of the planets can be held onto for very long periods of time, provided the planets have enough gravity. Really big planets like Jupiter can hold onto any gas, forever and ever. Smaller ones like the Earth can hold onto heavy gases like nitrogen and oxygen, but gradually lose their lighter gases, like hydrogen and helium. Very small planets like Mercury can't hold onto any gases, so they end up without any air, even though they must have started with some. So in our case, being a middling size planet, we can hold onto an atmosphere of heavy gases. Venus has done the same, although the heavy gases involved are different (mostly carbon dioxide). But the gas that was all around us is gone, so there is no air in the space between the planets, just at their surfaces.

I hope this answers all of your question, without being too detailed and technical.