Proxima b

The news this week is that a planet has been found around Proxima Centauri, the nearest star to our solar system. Near, of course is relative. It would take over four years for a radio message to get there, or 40 years if you travel at 0.1 times light speed. Since we can never get anywhere near that speed with our current technology, it is not exactly a find critical to our current society. The planet is apparently a little larger than Earth, and it is in the so-called habitable zone, where the star gives off enough heat to permit liquid water to flow, assuming it has sufficient atmospheric pressure of a suitable composition. That last part is important. Thus Mars and Venus might permit water to flow if their atmospheres were different. In Venus’ case, far too much carbon dioxide; in Mars’ case, insufficient atmosphere, although it too might need something with a better greenhouse effect than carbon dioxide.

Proxima Centauri is what is called a red dwarf. Its mass is about 1/8 of the sun’s, so it gives off a lot less energy, however, Proxima b is only 0.0485 times as far from the star as Earth is, so being closer, it gets more of what little heat is available. The question then is, how like Earth is it?

Being so close to the star, standard wisdom argues that it will be tidally locked to the star, i.e. it always keeps the same face towards the star. So half of the planet is unaware of the star’s presence; one side is warm, the other very cold and dark. However, maybe here standard theory does not give the correct outcome. That it should be tidally locked is valid as long as gravitational interactions are the only ones applicable, but are they? According to Leconte et al. (Science 346: 632 – 635) there is another effect that over-rides that. Assuming the planet does not start tidally locked, and if it has an atmosphere, then there is asymmetric heating through the day, and because the highest temperature is at about 1500 hrs, the heat causes the air there to rise, and air from the cold side to replace it, which leads to retrograde rotation through conservation of angular m omentum. (Prograde rotation is as if the planet went around its orbit as if rolling on something.) Venus is the only planet in our system that rotates retrogradely, and Leconte argues it is for this reason. The effect on Venus is small because it is quite far away from the star, and it has a very thick atmosphere.

Currently, everyone seems to believe that because the planet is in the habitable zone then it will be a rocky planet like Earth. This raises the question, how do planets form? In previous posts I have outlined how I believe rocky planets form, ( and ). These posts omit the cores of the giants, which in my theory accrete like snowballs. There are four cores leading to giants (Jupiter, Saturn, Uranus and Neptune) and there are four sets of ices to form them. (There are actually potentially more that would lead to bodies much further out.) The spacing of those four planets is very close to the projected ice points, assuming Jupiter formed where water ice would snowball.

Standard theory has it that we start with a distribution of planetesimals about the star, and these attract each other gravitationally, and larger bodies accrete until we get protoplanets, then there are massive collisions. The core of Jupiter, for example, would take about 10 My to form, then it would rapidly accrete gas.

There are, in my opinion, several things wrong with this picture. The first is, nobody has any idea how the planetesimals form. These are bodies as large as a major asteroid. The models assume a distribution of them, and this is based on the assumption all mass is evenly distributed throughout the accretion disk, except that the density drops off inversely proportional to rx. The index x is a variable that has to be assumed, which is fine, and it is then assumed that apart from particle density, all regions of space have equal probability of forming planetesimals. It is the particle density that is the problem. Once beyond the distance of Saturn, collision probability becomes too low to form planets, although the distribution of planetesimals permits Uranus and Neptune to migrate out of the planet-forming zone. Now, for me a major problem comes from the system LkCa 15, where there is a planet about five times bigger than Jupiter about three times further away from a star slightly smaller than our sun that is only 3 My old. There has simply been insufficient time to form that. In my ebook Planetary Formation and Biogenesis I proposed that bodies start accreting in the outer regions similar to snowballs. As the ices are swept towards the star, when they reach a certain temperature the ices start sticking together, and such a body can grow very quickly because as it grows, it starts orbiting faster than the gas. Accordingly, what you get depends on the temperatures in the accretion disk. Unfortunately, for any star we have no idea of that distribution because the disk is long gone. However, to a first approximation, the temperature is dependent on the heat generated less the heat lost. The heat generated at a point would depend on the gravitational potential at that point, which is dependent on stellar mass M, and the rate of flow past that point, which to a very rough approximation, depends on M2. That is by observation, and is + over 100%. So if we assume that all disks radiate equally (they don’t) and we neglect accretion rate variability, the position of a planet depends on M3. That gives a rough prediction of where planets might be, within a factor of about 3, which is arguably not very good.

However, if we use that relationship on a red dwarf of Proxima’s mass, the Jupiter core would be about 0.01 A.U. from the star. In short, Proxima b is at the very centre of where the Saturn equivalent should be, although I think it is far more likely to be the Jupiter core. The relationship is very rough, as the rate of accretion varies considerably from that relationship, and also, as the distances collapse, the size of the star now becomes significant, and back-heating will push the ice point further out. However, what is important here is that this “ice point” is relevant to any accretion theory. If ice is a solid at a given distance, then the ice should be accreted alongside any other solid.

So my opinion is that Proxima b would be a water world, with no land on the surface at all. Further, if it is the Jovian equivalent, it probably will not have an atmosphere, or if it does, it will be mainly oxygen from photolysed water. So I am very interested in seeing the future James Webb telescope aimed at that planet, as water gives a very clear infrared spectrum.


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