Hard SF : Aliens : Where Might Alien Life Develop?
In considering the possibilities of life on other planets, we must keep in mind that we do not yet have a sufficiently advanced understanding of life to create a life-form of terrestrial types in the laboratory. It would be a mistake to think we know enough to anticipate all the various forms life could take when starting from totally independent sources. However, there is one sense in which it is reasonable to use assumptions of life similar to terrestrial forms in SETI and such projects. Any life-forms on other planets which are so different we have trouble imagining them could be different enough that we would not recognize them as living. Certainly, if we could recognize that they were communicating with each other (or with us) we would know they might be alive. If they were communicating, but we were unable to recognize that they would be outside what we could usefully treat as extraterrestrial intelligence.
Such generalities aside, we can list some likely constrains on life developing elsewhere (especially in recognizable and technologically capable forms).
First, it would seem we can exclude any first-generation star systems. These are stars and any orbiting satellites which condensed from more-or-less primordial matter from the big bang. As such, there would be essentially no carbon, oxygen, metals, etc. This would presumably preclude the development of complex life-forms - and even if intelligent life evolved, there would not be resources to develop technology.
Multiple star systems present potential problems for development of intelligent life. The extent and kind of problems would depend on the proximity and type of the stellar masses. When two stars are near enough and there is a sufficient difference in mass, the larger can draw matter from the smaller star. The trail of accreting matter and the radiation released in the process could make complex life unlikely. Depending on the proximity of the masses, gravitational forces could cause eccentricities in planetary orbit around one star or could cause a more complex orbit involving more than one star. Such peculiar orbit or gravitational stresses could cause difficulties. Some multiple star systems include neutron stars or black holes which would be likely to involve especially challenging conditions for evolution of life to a level allowing intelligence. Not the least of the issues would be the supernovas that led to the formation of the neutron star or black hole.
Star classification and stellar evolution would be an important factor. Some stellar types have a relatively short duration, making it unlikely life would have the long period needed to evolve intelligent forms. Some stars will become neutron stars or black holes too soon for life. Others will continue to appear to our telescopes to be visible stars. However, if the star has gone through nova periods, it is likely that any life on its planets was wiped out at that time.
Proximity of stars (not in multiple systems), for instance towards the galactic core, may make regions of the galaxy too highly radiated or otherwise hostile to life. For example, the more closely packed with stars a region is, the more novas there are likely to be near enough to impact life on a planet.
It may also be that some stellar systems form in such a way that asteroid impacts continue to be a greater and more frequent problem than for Earth. This could mean too many mass extinction events for intelligent life to develop. If there was a way to identify which stellar systems are of a type that tends to have too many asteroids, we could consider them as improbable for intelligent life.
On the borderline of stellar and planetary issues is the factor of a planet’s orbit around its star. A planet with an orbit “too far” from its star will be limited in its ability to maintain liquid water, will have less sunlight to assist life and perhaps have other issues if our solar system is an indication. Planets that are “too close” may be too hot for liquid water, get too much radiation and possibly other problems. It is widely thought that there is a “temperate zone” for orbits around a star where complex life is more likely. How far from a star would be advantageous depends on the size and type of the star. In our solar system, Venus is not so close it would have to be as hot as it is, but given the kind of atmosphere it has Venus has heated up to the point complex life is highly unlikely. Mars is thought to be in the acceptable orbit range.
Planet size is an issue. Although Mars may have an orbit that could work for complex life, its mass does not allow it to hold onto as much atmosphere as the Earth. The amount and composition of a planet’s atmosphere will effect how much solar heat is retained and how much solar radiation is filtered out. One way or the other, Mars has come to have too little atmosphere to maintain a temperature suitable for unprotected Earth life. Planet size and density will also determine the gravity life on the planet must be able to function in.
An ongoing difficulty is conceiving the forms ET organisms and environments might take.
In an Earth-type planet which does not have enough regular, stable land mass to have evolved advanced land life forms, there still might be intelligent aquatic life-forms. However, the aquatic environment and resulting form of the organisms could prevent human-type technology which involves fire (metallurgy, glass, ceramics, etc.)
It’s harder to say what might work in less Earth-like settings. Some organisms (or their environment) might be more subject to being harmed by heat and flame, thereby causing even land-based beings to avoid fire. Even if this means a lack of metals, glass, ceramics, etc. we still have to ask whether this means materials needed for a civilization using telecommunications, powered transportation, space travel or such could not be developed. Not knowing what resources might be available makes such questions difficult. We may be able to narrow down the possibilities some by considering the need for technology to evolve. Any intelligent species must rise through the equivalent of our stone age. Using only naturally available materials and progressively using the available tools to create new tools and materials. Even if our modern factories can create plastics and other synthetics which could take the place of metals, glass, ceramics, etc.; we have to ask could plastics be developed without first having metal, glass, ceramic or such tools/materials to make the plastics. Similarly, for any given environment we have to ask what evolutionary course from natural hand-collected/crafted materials to materials capable of telecommunications, space travel, etc. are available.
A related issue could be the general availability of metals and other heavy elements on a specific planet. The presence of heavy elements is believed to depend on the creation of these elements through fusion in stars and the later distribution of these elements when the star goes supernova. Stellar/planetary systems which condense from supernova jetsam can form planets with elements other than the hydrogen and helium that makes up most of the universe. However, we don’t know to what degree these planetary systems might vary in the amount of tin, iron, lead, copper, etc. Further, even planets which include these elements may vary in how much is readily available at or just below the surface where Stone Age peoples can access them. Life forms might develop using lighter elements (such as carbon, hydrogen and oxygen) on a planet which had much less metals, but they could be prevented from taking a technological path similar to ours.
The availability of metal and other heavy elements could depend on the extent the planet has been subject to asteroid impacts and volcanic activity and plate tectonics. Relatively small metallic asteroids could leave a metal deposit near the surface. A larger asteroid might liquefy an area of the planet’s crust and bring up some lower metallic layers. Plate tectonics can move layers of the crust up and down so it is more likely, at one time or another, a layer with metals will be near the surface. Plate tectonic or other forces can cause volcanic activity, bringing up molten material from lower down. A planet that has had less than a certain amount of these kinds of activities might have too few metals near the surface for its people to make the transition from Stone Age to metal age technology.
It has also been suggested whether or not a planet has a substantial moon may be a factor. Especially there has been speculation that in the early development of life in tidal pools or in the process of evolving amphibious forms from aquatic life tides caused by moons may be important.
It seems a species with a technology capable of simple space travel might act on this more quickly if their star system had a more easily habitable second planet/moon. The simplest case would be a second planet that already had oceans and a breathable atmosphere. However, if there’s a parallel to early Earth, the atmosphere of the second planet will not be suitable to those people unless the second planet has its own life that has already increased the oxygen and decreased the CO2 in the air. But suppose our moon or Mars had oceans and a primitive atmosphere. We might already be seeding those oceans with algae to start the process. We might be creating devices to take in that ocean water and output drinking water, and to make oxygen for air and hydrogen for fuel to make bases near the oceans more feasible. The people of such a star system might move into space more decisively.