From Whence Star-burning Planets?

This series started out with the objective of showing how life could have started, and some may be wondering why I have spent so much time talking about the cold giant planets. The answer is simple. To find the answer to a scientific problem we seldom go directly to it. The reason is that when you go directly to what you are trying to explain you will get an explanation, however for any given observation there will be many possible explanations. The real explanation will also explain every connected phenomenon, whereas the false explanations will only explain some. The ones that are seemingly not directed at the specific question you are trying to answer will nevertheless put constraints on what the eventual answer must include. I am trying to make things easier in the understanding department by considering a number of associated things. So, one more post before getting on to rocky planets.

In the previous two posts, I have outlined how I believe planets form, and why the outer parts of our solar system look like they do. An immediate objection might be, most other systems do not look like ours. Why not? One reason is I have outlined so far how the giants form, but these giants are a considerable distance from the star. We actually have rather little information about planets in other systems at these distances. However, some systems have giants very close to the star, with orbits (years) that take days and we do not. How can that be?

It becomes immediately obvious that planets cannot accrete from solids colliding that close to the star because the accretion disk get to over 10,000 degrees C that close, and there are no solids at those temperatures. The possibilities are that either there is some mechanism that so far has not been considered, which raises the question, why did it not operate here, or that the giants started somewhere else and moved there. Neither are very attractive, but the fact these star-burning giants only occur near a few stars suggests that there is no special mechanism. Physical laws are supposedly general, and it is hard to see why these rare exceptions occur. Further, we can see how they might move.

There is one immediate observation that suggests our solar system is expected to be different from many others and that is, if we look again at LkCa 15b, that planet is three times further from the star than Jupiter is from our star, which means the gas and dust there would have more than three times less concentrated, and collisions between dust over nine times rarer, yet it is five times bigger. That star is only 2 – 3 My old, and is about the same size as our star. So the question is, why did Jupiter stop growing so much earlier when it is in a more favourable spot through having denser gas? The obvious answer is Jupiter ran out of gas to accrete much sooner, and it would do that through the loss of the accretion disk. Stars blow away their accretion disks some time between 1 and 30 My after the star essentially finishes accreting. The inevitable conclusion is that our star blew out its disk of gases in the earliest part of the range, hence all the planets in our system will be, on average, somewhat smaller than their counterparts around most other stars of comparable size. Planets around small stars may also be small simply because the system ran out of material.

Given that giants keep growing as long as gas keeps being supplied, we might expect many bigger planets throughout the Universe. There is one system, around the star HR 8799 which has four giants arrayed in a similar pattern to ours, albeit the distances are proportionately scaled up and the four planets are between five and nine times bigger than Jupiter. The main reason we know about them is because they are further from the star and so much larger, hence we an see them. It is also because we do not observe then from reflected light. They are very young planets, and are yellow-white hot from gravitational accretion energy. Thus we can see how planets can get very big: they just have to keep growing, and there are planets that are up to 18 times bigger than Jupiter. If they were bigger, we would probably call them brown dwarfs, i.e. failed stars.

There are some planets that have highly elliptical orbits, so how did that situation arise? As planets grow, they get gravitationally stronger, and if they keep growing, eventually they start tugging on other planets. If they can keep this up, the orbits get more and more elliptical until eventually they start orbiting very close to each other. They do not need to collide, but if they are big enough and come close enough they exchange energy, in which case one gets thrown outwards, possibly completely out of its solar system, and one gets thrown inwards, usually with a highly elliptical orbit. There are a number of systems where planets have elliptical orbits, and it may be that most do, and if they do, they will exchange energy gravitationally with anything else they come close to. This may lead to a sort of gravitational billiards, where the system gets progressively smaller, and of course rocky planets, being smaller are more likely to get thrown out of the system, or to the outer regions, or into the star.

Planets being thrown into the star may seem excessive, nevertheless in the last week it was announced that a relatively new star, RW Aur A, over the preceding year had a 30 fold increase in the amount of iron in its spectrum. The spectrum of a star comes from whatever is on its surface, so the assumption is that something containing a lot of iron, which would be something the size of a reasonably sized asteroid at least, fell into the star. That means something else knocked it out of its orbit, and usually that means the something else was big.

If the orbit is sufficiently elliptical to bring it very close to the star one of two things happen. The first is it has its orbit circularized close to the star by tidal interactions, and you get one of the so-called star-burners, where they can orbit their star in days, and their temperatures are hideously hot. Since their orbit is prograde, they continue to orbit, and now tidal interactions with the star will actually slowly push the planet further from the star, in the same way our moon is getting further from us. The alternative is that the orbit can flip, and become retrograde. The same thing happens as with the prograde planets, except that now tidal interactions lead to the planet slowly falling into the star.

The relevance of all this is to the question, how common is life in the Universe? If we want a rocky planet in a circular orbit in the habitable zone, then we can eliminate all systems with giants on highly elliptical orbits, or in systems with star burners. However, there is a further possibility that is not advantageous to life. Suppose there are rocky planets formed but the star has yet to elimiinate its accretion disk. The rocky planet will also keep growing and in principle could also become a giant. This could be the reason why some systems have Neptune-sized planets or “superEarths” in the habitable zone. They probably do not have life, so now we have to limit the number of possible star systems to those that eliminate their accretion disk very early. That probably elimimates about 90% of them. Life on a planet like ours might be rarer than some like to think.

The Formation of the Giant Planets

Before I can discuss how we got the elements required for life delivered to Earth, it is necessary to work out how the planets formed, and why we have what we have. While the giant planets are almost certainly not going to have life, at least not as we would recognise it, they are important because what we have actually gives some important clues as to how planets form, and hence how common life will be, and why so many exoplanetary systems are so different from ours. The standard theory says a core accreted, then when it got sufficiently big, which calculations have at about 10 to 12 times the Earth mass it starts accreting gas in substantial amounts and it grows very slowly, the problem restricting growth being how it can compress its volume and get rid of the heat so generated. After a number of million years, its mass gets big enough, and it accretes everything that comes into range. Apart from the rather slow time the calculations give, that general description is almost certainly essentially correct. The reason we believe the core has to get to about 10 – 12 Earth masses before disk gases get accreted in serious amounts is because the evidence is the Neptune and Uranus have about 2 Earth masses of hydrogen and helium. So far, reasonably good. The fact that there is evidence the calculations are wrong is not damning; the fact that if the mechanism is not properly understood in close detail then the calculations will inevitably be wrong. The original calculations had these stages taking about 10 My to get to a planet the size of Jupiter. The very first calculations had it taking about a billion years to get to Neptune, but that obviously cannot be right because the disk gases had long gone before that. The real problem is how to get to the cores.

The standard theory says they started by the accretion of a distribution of planetesimals that were formed by the accretion of dust, and therefore were distributed according to the dust concentrations through the disk. There are two problems with this for me. First, we see these disks, and we see them because their dust scatters light. Some such disks are 30 My old and still dusty, so the dust itself is not rapidly accreting, altough often there are bands where there seems to be little dust. The second problem is that there is no recognized mechanism by which the dust can accrete and stick together strongly enough not to be disrupted by any other dustball that collides with it. Mathematics indicate that such dustballs, if they reach about 2 cm size, erode from gas motion relative to them.

So, how did the cores form? I think we have evidence from the fact their systems all have different compositions. The theory I outlined in my ebook Planetary Formation and Biogenesis goes like this. The dust that comes from deep space has a lot of very fluffy ice around it, and this has many pores. Within those pores, and around the ice, are the ices of other volatiles. (Such compound ices have been made in the lab, and their behaviour verified.) As the ices come in, the more volatile ones start subliming away at temperatures a little above their melting point, and hydrogen has even been maintained as an ice enclosed in water ice pores up to a little under 15 degrees K, which is well above its boiling point. So, as the ices come in and the disk gets gradually hotter, the ices selectively boil away. The relevant temperatures are: neon (~25 K); nitrogen and carbon monoxide (~65 K); argon and methane (~ 85); ammonia and methanol (~170 K); water (273 K).

What I suggest happened is the same mechanism that forms snowballs started planetary accretion. When snow is squeezed at a temperature a little below its triple point, the pressure causes localised melt fusion, and the particles stick together. In this case we have several ices entrained in the ice/dust, and I suggest the same happens for each ice. This has consequences. The temperature profile in these disks is observed to be where the temperature T is proportional to r^-0.75, r the distance from the star, with a significant variation, which is expected because the faster the gas comes in, or the warmer it was to start with, the further out a specific temperature will be found, while the denser the gas flow, the greater the temperature gradient. Now, because Jupiter is the biggest planet, and water ice is the most common single material, assume (like everyone else) that Jupiter is more or less where the water ice so fuses. If we assume the average disk temperature profile (actually r^-0.82 is better for what follows for our solar system) then the remaining giants are quite close to where they are supposed to be. So the mechanism is that ices come together, they hit, the collisional energy melts an ice in the impact zone such that they rapidly refreeze, and the particles stick together. To predict where the planets should be I put Jupiter at 5.2 A.U. as a water-ice core sets the constant of proportionality. The next ice is ammonia/methanol/water, which could melt between 164 – 195 oK, which suggests that Saturn should be between 7.8 – 9.6 A.U. Saturn has a semimajor axis of 9.5 A.U. The next ice out is methane/argon, with melting between 84 – 90 oK. The calculated position of Uranus is between 20-21.7 A.U., while the observed position is 19.2 A.U. The next ice, carbon monoxide/nitrogen melts between 63 – 68 oK, which predicts Neptune to be between 28.1 – 30.7 A.U., and Neptune has a semimajor axis of 30 A.U. Note that as they form, we excpect some movement through gravitational interactions and the effects of the gas.

This means the Jovian system is both nitrogen and carbon deficient, apart from Jupiter itself which accreted gas from the disk, and the very tenuous atmosphere of Europa is reported to actually have more sodium in it than nitrogen. Sorry, but no life under the ice at Europa because there is nothing much with which to have organic chemistry. The reason for the lack of atmosphere is the satellites have nothing in them that could form a gas at those temperatures. The major component is hydroxyl, from the photochemiclo deomposition of water, but this is extremely reactive and does not build up.

The Saturnian system has water, plus methanol and ammonia. The ammonia has been seen at Enceladus, and its decay product during UV radiation, nitrogen, is the main gas of Titan. The methane there will come from reactions of methanol and rocks. The Uranian system has methane and argon. Unfortunately the satellites are too small to have atmospheres, and Neptune’s satellites are similar, as while Triton has nitrogen volcanoes, it is probably a captured Kuiper Belt object, as it orbits Neptune the wrong way. However the atmosphere of Neptune has more nitrogen than expected from the accretion disk, whereas Uranus does not. More specific details are in the ebook, but in my opinion, the above describes reasonably well how these systems formed, and why they have the chemical composition we see. The Kuiper belt objects are the same as the core of Neptune, and are essentially a “tail” of the accretion process.

Finally, the perceptive will notice the possibility of two further zones of accretion. Further out, there will be a zone where neon trapped in ice might accrete, and even further out, because hydrogen can be trapped in ice even up to about 15 K, a hydrogen accretion zone where the liquid hydrogen dissolves neon, which then refreezes. The latter is not impossible. There are exoplanets a few hundred A.U. from the star, or, say, over ten times further than Neptune. On the other hand, there is a further mechanism that could form them, namely collapse of the disk, which presumably starts the star. So I should be able to predict where this planet 9 is? That is not so easy because the temperature of the disk follows the above relationship only approximately, and when we get down to these low temperatures, any deviation, including the initial temperature of the gas (which in the above relation is taken as zero, but it isn’t) suddenly becomes important. My published estimate for a neon-based planet is at a hundred A.U., with a possible minus 30 and a plus fifty A.U. Not exactly helpful. If I knew thie initial temperature, and the rate of heat loss from the disk by radiation at those distances I could be far more precise.