Life needs a sun like ours
Ken Tapping, August 29th, 2018
Current thinking is that here on Earth, chemical-based life, like ours, started in bodies of water, muddy waterside places or on slimy rocks. Organic molecules inherited from the Solar System’s birth cloud reacted together to form increasingly complex molecules, until they crossed the boundary between non-living and living material. Laboratory experiments support the idea that this happened. Until we know of other kinds of life, our search for living things on other worlds is directed at finding chemical-based life.
Since the chain of chemical reactions from simple molecules to the complex chemicals driving the processes of life is a long one, we assumed the best planetary candidates for life would be orbiting red dwarf stars. These stars are stable for billions or tens of billions of years, allowing lots of time for the chemistry to proceed, and do not give off harmful levels of ultraviolet radiation, which can break down complicated molecules.
However, we need also to consider the changes that happen on a planet when life gets going. The planet’s original atmosphere, made up of chemicals inherited from the birth cloud, is rich in greenhouse gases. When life got going here on Earth, about 3.5 billion years ago, early plants assimilated those greenhouse gases, and replaced them with oxygen, which is not a greenhouse gas. This would have caused the Earth to gradually cool. However, we know that the Earth was warm enough for liquid water back then and obviously still is today. The reduction in the greenhouse effect has been balanced by the Sun becoming brighter. It is now about 30% brighter than it was when life first appeared. The result is that over 3.5 billion years the Earth’s temperature has not changed enough to endanger the creatures and plants living on it. Consequently, red dwarf stars, with their more or less constant energy output, are probably not the best candidates for having life-bearing planets. If the temperature on a planet was right for life to start, the reduction in the greenhouse effect would lead eventually to the planet freezing solid. This is one reason to look elsewhere for planets bearing life. Now we have another reason to think this is the case.
Laboratory experiments have shown that if there is an appropriate and consistent source of energy, those original chemicals can react, producing more complex ones. We can get as far as aminoacids, the building blocks of proteins, but no further. The rest of the path to life probably needs a lot more time and reactions taking place on a planetary scale rather than in a laboratory. Lightning and electrical discharges have been investigated as possible energy sources for driving the chemical reactions, but an appropriate steady level of ultraviolet light might be better. This would have to come from the new planet’s star. The ultraviolet light would excite the molecules without destroying them, allowing them to form more complex arrangements, moving them along the path to the chemicals that are fundamental to living things.
Red dwarf stars do not produce much ultraviolet, and are not well suited for driving these chemical processes. On the other hand, stars like the Sun produce a good supply of ultraviolet. Hotter, bluer stars produce so much that complex molecules would be destroyed. Since life as we know it is based upon complex molecules, we would not expect to see chemical-based life on planets orbiting hot, blue stars. That is not to say that all chemical-based life has to be like ours, or even that life has to involve chemistry at all. However, for the time being we are concentrating on our familiar chemical forms of life. In this case at least we have some idea of what we should be looking for. Other options can come along later.
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