A few days ago I read an online article about how astronomers had discovered a new planet outside our solar system that seemed to be similar in size and density to Earth. The natural inclination of humans upon discovering a planet roughly the same size as our own is to ask, does it have life on it? Well, in this case, no. It seems that the newly discovered planet orbits so close to its sun it must be molten on the surface. Several thousand degrees (around 5000 F) on the surface. Nothing we know as life is going to develop in that environment.
I’ve always found it interesting that the search for extra-solar planets (and we’ve found over a thousand so far) is always mixed up with the question of life on these planets. Does it have life? we ask, over and over . We don’t look for planets just to see what’s out there, we have to ask about the conditions for life. So far no planet discovered has even come close to having the proper mixture of water, temperature, carbon compounds, oxygen in an atmosphere, and other chemicals such as nitrogen, phosphorus, sulfur, sodium, chloride and so forth to provide the proper incubator for life to develop. (Of course, that’s life as we know it. Other forms of life could exist in wildly other environments.) With all these planets that have so many disadvantages and drawbacks preventing life from developing on their surface, it’s a wonder that we keep asking the question. The large number of known extra-solar planets and the fact that none are even remotely close to having life is telling us something. Telling us not so much to quit asking the question (we’ll never do that) but that the number of planets that actually do have life is going to be small. Very small. In our own solar system, only one moon of one planet (Europa, a moon of Jupiter) is likely to have life outside Earth. Even Mars likely never had living organisms on its surface.
Planets form from the accumulation of the interstellar dust that didn’t get incorporated into a newly born star, and life is not by any means a requirement of planetary existence. Planets take all sorts of different sizes and (perhaps) shapes. They have markedly different temperatures and surface characteristics. Life requires an extremely narrow range of ingredients and we haven’t found it yet anywhere outside Earth. It may be a while before we do, if ever. We’re in for a long search. Get used to it.