Does Life Need a Large Moon?

A large moon has many effects on the planet it orbits. Tides are the one everyone knows about, but there are others.

The major changes in a planet’s orbit, which for fairly stable solar systems like ours, are in three parameters. One is the change in eccentricity. The length of a year of a planet is constant, but the eccentricity varies, meaning this factor which influences seasons varies periodically. Another is the change in the orientation of the major axis of the orbit, which affects when the equinoxes and solstices are. This has an effect on climate on a planet with significant axial tilt, as seasons are more extreme when the projection of the axial tilt is aligned with the semi-major axis of the orbit, and this would vary periodically. The orientation of the axial tilt is also subject to periodic change, and this change, called precession, works in combination with the change of orientation of the major axis. Together they ensure than the length and severity of the seasons changes over tens of millennia.

One of the climate effects of the first two orbital changes are ice ages, and they have been found to be correlated, but not wholly determined, by these effects. Ice ages certainly would affect the evolution of life, and possibly the origination of it. A planet which was on the outside of the temperature-based habitable zone might have the non-frozen area of the planet fluctuating, and the mean temperature of the oceans fluctuating as well. If the origination of life takes a long period of stable conditions, this would interfere with it. Don’t forget that some evolutionary changes come from mutations which only bestow a small change in reproductive rate, and in order for a small effect to overcome the noise of the processes of life, it has to last a long time. If it only provides reproductive success under some conditions, and perhaps negative effects under others, having a fluctuation in the conditions will derail the success of the change. If that evolutionary change is critical to follow-on changes, we have a dead-stop for evolution. Life might get to some early stage, but no further. This is a very early type of plateau world, one which stops at some level in the path to intelligent aliens who can travel to other stars.

The third orbital change is the change in axial tilt. In an earlier post, axial tilt was discussed as a factor in the origination of life, but there it was treated as a constant. Even on Earth it is not a constant, but it varies by a couple of degrees up and down from its average. This variation is not much for changing climate, but it does induce some effects on top of those caused by the first two changes. The interesting thing is that the axial tilt variation of the Earth is affected by the moon. The moon has more angular momentum that the spinning Earth, and tends to prevent the axial tilt from varying much. On planets without large moons, they are more like near-spherical balls, with little resistance to changes in axial tilt from gravitational effects on the non-spherical components their shape by the star of the solar system and the largest planets in the system. We have not yet found the ability to detect axial tilt on exo-planets, and of course not to detect changes in it, but other planets in our solar system exhibit larger changes in axial tilt.

As noted in that earlier post, there is a large difference between the climate and geographic variation of climate on planets which have their rotational axes aligned with their orbital axis and those which have it tilted at right angles to it. The one with an alignment has no seasons. The one with a right angle has extreme seasons. If we concur with the idea that many evolutionary changes require long periods of constant conditions to occur, in order to accumulate effects from a small benefit, the aligned one would be better at evolving life that the tilted one. But if the axial tilt were a varying parameter, and a planet with an aligned axial tilt would be tipped over short periods of evolutionary time into a severely tipped one, the slow selection processes would be short-circuited and we would once again have a plateau planet.

The time scales for changes in orbital parameters are of the order of tens of millennia. Evolutionary changes might take longer than that. There certainly could be evolutionary changes which have instantaneous effects, in that there is a marked difference in reproduction rate in the very first generation to receive the mutation. This type of sudden changes would not be stopped by changes in orbital parameter, or much else, for that matter. But these types of evolutionary mutations might be only a few out of the millions that have to take place.

The slow type of evolutionary change would happen if their were gene combination effects, such as the posts on genius genes discussed. If a significant evolutionary advantage were only realizable if some collection of changes were possible, and the individual changes were neutral in effect, constant conditions would be necessary for a long time until all the changes became common in the gene pool, and then a combination of the right ones happened. Significant evolutionary changes such as the development of new energy sources, new sensors, new skeleton shapes, new chemistry of some cell components, or new organs might all need the fortuitous combination of several otherwise neutral mutations. This would mean that constant conditions were needed for a long time, to allow all the changes to accumulate.

Each mutation in a gene is a random process, caused by some mutagen like a cosmic ray or a transcription error or other source. They might actually have a very slight negative effect on reproductive success, and only if they combined would they give a significant positive effect. This means there would be mutations of each of the required set happening, and then the mutation disappearing. Only after a long period would the probability of all of them having being present at one time be high. Thus it is certainly possible that variations in climate would interfere with the process.

In short, looking for intelligent life may require the ability to detect large moons circling planets in the habitable zone. If only we had better information on the origination and evolution of life, we could decide better on whether even larger telescopes are needed.