Saturday, September 24, 2016

Life Around Cool Stars

Can life originate around a red dwarf? A few elementary observations have been pointed out already. There is a liquid water zone around red dwarfs, and if an Earth-sized planet were located within it, that would be a good start. An origin planet, one that originates life but not necessarily an alien civilization, has to have certain requirements. In one sense, a red dwarf is a easier home for life in that the lifetime of a red dwarf is immense, greater that the age of the universe, and therefore there is no bother about having to maintain the liquid water zone requirement with some combination of greenhouse gases during a long enough period to get life started. Earth’s star started out too cold for us, and Earth stayed warm enough due to a combination of planetary heat and greenhouse effects during the first part of its life. This isn’t necessary on a red dwarf’s planet.

If the opposite is the case, that planetary formation heat boots the planet out of the liquid water zone for the first billion or two years of its existence, it wouldn’t be a problem as the star-generated liquid water zone is going to be available for a lot longer than that. What is possibly a problem in this case is the amount of water that is going to be held onto in the atmosphere. If the heating is not too extreme, the usual mesosphere effect might limit the loss of water molecules into space, and when the planet cools enough to have condensed water, it might have enough for the necessary liquid ocean.

A different problem entirely is the other planet problem. If there are gas giants in the system, located at more distant orbits, they will affect the eccentricity of the target planet. With the ratio between the orbital periods high, the target planet will see almost the same gravitational pull over many of its orbits, and eccentricity can mount and the argument of periapsis (the direction of the furthest point of the orbit) can vary to exaggerate the effect. This means that eccentricity can get large, and then grow smaller again, and so on. How would this affect the origination of life?

This is a question of the rate of change of conditions on the planet and how that compares with what is needed to provide an environment for life to originate and evolve, first chemically and then biologically. Later, the question becomes a comparison of this rate of change of environmental conditions and the rate at which some primitive form of life might adapt to it, once life already has been originated and the question is whether it can survive for long. Perhaps the use of the term origin planet should not be devoted to planets where chemical evolution goes on for a while and then everything dissolves back into chaos, but only for planets where biological cells have come into existence.

Mercury is our hometown example of a planet whose eccentricity varies greatly, from zero up to 0.40, as a result of the gravitational influence of Jupiter and Saturn. The same type of planetary dynamics could affect a planet around a red dwarf. High levels of eccentricity denote extreme levels of seasonality on the planet. This does not cast it out of the liquid water zone, which is, possibly vaguely, defined as an orbital range where liquid water can exist during part of the orbit. Unless there was only a small amount of water on the planet, it should not all freeze during the short winter that such a planet would have. However, if life origination occurs as in the organic oceans theory, an ice cover would freeze the upper depths of the water ocean where the meniscus was and where the first membranes formed. Would that doom a continuation of chemical evolution?

If the organics froze at different temperatures than the water did, there might be disruptive motions at the surface which would certainly disrupt a membrane, held together only by intermolecular forces. So, here is an obstacle to life formation around a red dwarf star.

If there are no other planets, especially large ones, around this star, tidal effects would eliminate the eccentricity, circularizing the orbit, and so this extreme seasonality would not exist and the planet would not have this obstacle to the origination of life. The initial planetary heat obstacle is mitigated by the long lifetime of this class of star, but that raises its own problems. As the planet’s orbit becomes circularized, it is also likely to become phase-locked, such as our moon, at a 1:1 ratio of rotation period to orbital period, or like Mercury, at a 3:2 ratio. This slowing of rotation does not only affect the crust, but also the mantle and core. And when the core stops rotating, there is no magnetic dynamo effect to produce a magnetic shell to protect the planet’s surface from charged high-energy solar wind particles.

In this situation, there is only the atmosphere to absorb the high-energy particles, and the upper reaches of the atmosphere will be stripped. Over time the atmosphere would be thinned. Red dwarfs do not have either the quantity nor the temperature to produce as much solar wind as a G2 star, but they do produce some, and thus the numerical question needs to be asked: Can the atmosphere stay heavy enough for long enough to allow this planetary heating period to pass and still have sufficient atmosphere to let life originate and evolve? Solar wind might preferentially break up water molecules in the upper atmosphere into hydrogen and hydroxyl ions, and then the hydrogen would escape from the atmosphere, essentially depleting the water supply.

Climate on a phase-locked planet would be very different that what it is on a rapidly rotating one. With concentrated solar heating on one area, either permanently or for a good fraction of the planetary year, there would be strong convection winds generated. These winds should extend both to the surface and up into the troposphere in a toroidal fashion, although topography, if it exists, might disrupt this. Strong persistent winds over an ocean surface creates great waves, and if an organic ocean existed above a liquid ocean, it might be totally broken up by waves which were deep enough to involve both the upper organic layer and the lower deep water layer. This would mean no meniscus and no origination of life, by the theory used here. There might be some craters where wind played a lesser role and origination could take place. Remember, however, that evolution, both chemical and biological, is proportional to the numbers of potential candidates for mutation, and if only a crater of them existed, life origination might need tens of billions of years instead of only a couple of billion.

This would mean that even though red dwarfs, formed near the beginning of the Milky Way, might be in the process of originating life in a crater on some phase-locked planet in the liquid water zone, the atmosphere might be lost before the process completed. Life origination involves a complex system of very diverse elements, and figuring out just how it might happen around a red dwarf star, when we have only Earth as an example, is extremely taxing mentally.

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