Wednesday, May 4, 2016

Life on a Phase-Locked Planet

Phase-locking of a liquid water zone (LWZ) can happen when the parent star is a red dwarf. The phase locking is of the rotation of the planet, where the tidal interactions on the planet itself couple its spin into its orbit. The planet’s orbit is not phase-locked into the parent star’s spin.

What this means is that the dipole moment of the planet’s mass distribution is permanently pointed toward the star. There is a subsolar point where the center of the star is directly overhead at all times. Someone standing there any time would get sunburn on the top of their head. More accurately, because red dwarfs put out little UV, it would be likely that the person would find their scalp getting sweaty. It also means that as far as solar illumination goes, there is at any point on the lit side a direction pointing toward the subsolar point. The star is always there. Everything is quite constant on a planet of this type. Unlike the Earth, where illumination varies a lot over a day and over a year, it doesn’t vary at all there, assuming there are no variable clouds. If the atmosphere can absorb enough stellar energy to heat it up as hot as the surface, there would be no convection driving some sort of axisymmetric circulation. If the atmosphere is sufficiently transparent and the albedo of the surface is not too high, surface heating of the lower layer of air would cause air to rise in some cylindrical area around the subsolar point, and spread uniformly outward, before descending into the wind heading toward the subsolar point. This circulation might spread a bit into the dark side, but the constancy of temperature there would not allow it to penetrate all the way to the opposite side of the subsolar point.

Thus, someone on the planet, with circulation going, who faced the wind, would always have the sun at his back. This very symmetric pattern could be upset by tectonics.

If the planet did not cool into a fairly smooth surface, but had gone through a period of tectonic activity, there could be mountains obstructing the circulation’s flow near the surface. This could either simply modulate the flow toward the subsolar point or induce some rotary motion to the air as it approached the subsolar point. The important part of the wind is that it should be constant. There is nothing on the planet to induce seasons or anything else similar. Perhaps there could be some inherent instability in the air layers, but there is no obvious reason why there should be.

Since it was initially postulated that this planet is in the LWZ, there should be some place where the water sits. If it is on the outer edge of the LWZ, the subsolar point should be the area where water is present in liquid form, and then at some distance from the subsolar point, there would be ice. There is a boundary in the LWZ where the subsolar zone gets too hot to support liquid water. It would be a hot desert. Approaching the inner edge of the LWZ finds the planet with desert on the illuminated side, with some liquid water on the dark side.

How much relief would there be in such a planet? The processes by which a planet segregates its interior into a solid core and a liquid mantle do not much depend on the type of star or the type of orbit. Instead it depends on the materials forming the planet, and how much heat was generated within it by the condensation process. The gas cloud that formed the solar system would likely have been smaller in total mass, so that it only formed a M-class star, perhaps 10% of Earth’s sun’s mass. The planetary disk may have been proportionally similar in mass, so that only some smaller planets formed. But to concentrate on an Earth-sized planet in a less dense disk could easily mean that the mass forming the Earth-size planet had to fall in from a larger radius. If instead the disk was as dense as other planetary disks, but simply smaller in outer radius, then the mass inflow would come from shorter distances. Larger distances would mean more heat to be dissipated, and vice versa. So there easily could be more heat initially that Earth or Venus had, and then the same segregation would happen, and the same variation in crustal thickness in different parts of the planet’s surface.

This implies that some red dwarf planets could have a high plateau near the subsolar point, which in turn means that there could be dry areas surrounded by a circular sea. The inrushing circulation of air would pick up moisture from the sea, and when the air rose in the circulation, it might get cold enough to rain. Thus, there could be land regions of constant rain near the subsolar point. Otherwise the rain would fall upon the sea.

Where on any of the planetary types could life originate? If the Early Life/ Organic Oceans hypothesis is true and is the only path for life to form, then without a possibility of impact of a large planetoid, there would be no life on a red dwarf planet any more than there would be for any other planet. If other pathways to life hold, such as the Late Life/ Sea Vent Hypothesis, the constancy of the oceans would assist a bit in the formation of life. A lack of tidal forcing on the planet’s mantle might make a sea vent exist for much longer than on a more ordinary planet. One aspect of the Late Life/ Sea Vent hypothesis is that the formation of the initial membranes for life is a very low-probability process, and more stability, if there is much, could assist in this.

This does not mean that life would ever advance far on a red dwarf planet. Perhaps the biggest barrier would be photosynthesis, which occurs because of the low level of blue photons in the spectrum of the star. Without such photons, finding a way to produce energy would be much more difficult. Advanced life is all about replicating organic forms finding energy to power their life. On such a planet, it would simply not be there. Thus, the conclusion is that for most red dwarf planets phase-locked to the star, there are no special conditions promoting the formation of life, except for one, and in that case, only simple life is likely. No aliens will be coming from red dwarf origin planets.

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