# scifi Astrophysics thus provides two important constraints on imagined planets.  First, planets capable of supporting life are very unlikely to be orbiting anything other than a main sequence star.  The other stages of stellar life are too short and/or unstable.  Second, the elements that make up planets are hydrogen plus those produced by the nuclear processes that provide energy to the star and by the energy which ultimately tears large stars apart.

The most abundant element in the universe by far is hydrogen.  For each million hydrogen atoms, there are roughly sixty thousand helium atoms, seven hundred oxygen atoms, four hundred carbon atoms, and ninety nitrogen atoms.  Hardly surprising that these same elements make up most of living matter!  Beyond these relatively common elements, we expect fifty atoms of silicon, forty each of neon and magnesium, thirty of iron and twenty of sulfur.  Five other elements have one to ten atoms per million atoms of hydrogen: sodium, aluminum, argon, calcium and nickel.

Helium, neon and argon are “noble” gasses that rarely take part in chemical reactions, and which thus remain gaseous and unlikely to be of much importance in planet formation.   The remainder can combine with hydrogen and with each other, and the resulting chemical compounds are the building blocks from which planets are made.  It is extremely unlikely that a planet would form which was very rich in lithium, boron or beryllium, these being extremely uncommon elements in the universe as a whole.  Oxygen, on the other hand, will certainly be present in rocks both as part of their chemical structure (quartz, for instance, is an oxide of silicon) and usually as water of crystallization.  If a substantial atmosphere or oceans are present, oxygen is again likely to be represented, but more likely as water or carbon dioxide than as free oxygen.

In fact, a reactive gas such as oxygen or chlorine is not expected to occur uncombined in an atmosphere unless some process – such as photosynthesis – is constantly producing it.  A chemically unstable atmosphere is probably possible only in the presence of life.  (This was actually used to check for life on Mars.)  In the case of earth, it is thought that the development of an atmosphere with free oxygen (a very reactive waste product of photosynthesis which was also extremely toxic to the life forms then existing) took close to two billion years after the first development of life, and concentrations of oxygen sufficient to support land animals probably took another billion and a half years after that.