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.

Planets for alien life (3)

We have a suitable star, but will it have planets? Let me confess at once – I would generally be regarded as being a heretic on this subject, so be warned. The standard theory argues that they form through the gravitational attraction of planetesimals during the second stage of stellar accretion, but it has no mechanism by which planetesimals form, so there isn’t much more to be said about that. In my view, the planets formed in a completely different way, which involves the chemistry that should take place in the accretion disk and the material gradually heats up as it approaches the star.

In my proposal (more details in my ebook, Planetary formation and biogenesis) the four outer planets form the same way snow-balls form: the pressure induced merging of particles that melt-welds the ices into a larger body when collisions occur a little below the melting point of the ice. There are four major ices, with increasing melting points: nitrogen/carbon monoxide; methane/argon; ammonia/methanol/water; water. Bodies will contain the ices that have yet to melt, so all have water as the major component, and the water should hold the more volatile ices in pores. We then have four giants, in order Neptune, Uranus, Saturn and Jupiter. The satellites form the same way, and the internal chemistry of Saturn converts methanol and ammonia into some methane and nitrogen, which is why Titan (a Saturnian satellite) has an atmosphere, and the somewhat larger Jovian satellites do not. In the ebook I show that the planets are at positions that roughly correspond to the expected temperature profile in the disk when they are formed.

You may be skeptical at this point – where are such exoplanets? The reason why hardly any have been found is that they are difficult to find. Remember how long it took to find Neptune? However, one such system has been found: HR 8799. These planets are at 68 A.U. (1 A.U. is the earth-sun distance), 38 A.U, 24 A.U. and 14.5 A.U. and these distances are proportionately similar to those in our solar system, only more spaced out. The greater distances will arise from more energy being converted to heat, through a larger star (more gravitational energy produced per unit mass) or faster accretion (more mass per unit time). So, why is there only one such system discovered? One reason why these planets were detected is that the inner three are about 9 times bigger than Jupiter, and they have only just formed. Their temperature is about 1100 degrees, so they shine, and we can see them! This is rather exceptional. The two main means of finding planets are the Doppler effect, where the planet pulls on the star as it orbits, and its motion has a “wobble” that can be detected, or, with the Kepler telescope, the planet passes in front of the star, giving a transit effect. Both of these favour finding planets close to the star. The Doppler effect is bigger the larger and closer the planet because that gives it a bigger pull, while to observe a transit, the planet has to be on a line between the observer and the star. Close up, there is more angular tolerance because the star is so big, and there may be, say, 2-3 degrees tolerance. If the planet is as far away as Neptune, there is essentially no tolerance, and there is a further problem: a transit cannot happen more often than once the planet’s “year”. For Neptune, that is about once in 165 years. Kepler has been going only a few years and will soon stop.

The giants are hardly likely to have life as we know it, however giants are important because if the giants grow too big and are too close together, their gravitational interactions start to disrupt their orbits, which at first should become more elliptical, and then start moving each other around. The larger the giants, and the closer they are together, the more disruptive they are. Given sufficient time, they may throw one or more of the giants out of the system, while the Jupiter equivalent moves closer to the star, often becoming a star-grazing planet. If it did that, it would most likely totally disrupt rocky planets. So, the number of suitable stars must be reduced by the probability that the giants stay where they are. Since we cannot, in general, see giants in their proposed original positions, it is hard to estimate that probability, but as noted in the last post, the factor will be something less than a half.

There is still one further problem. If, around the Jupiter position, more than one planet started to grow, subsequent gravitational; interactions could lead to one of the bodies being flung inwards, where, if it is big enough, it may continue to grow. This could produce anything from a water world to a small giant. It is rather difficult to guess the probability of that happening. However, if I am correct, all of those with giants in the right position and which only formed one significant Jupiter-type precursor will be likely to have rocky planets in the habitable zone, and of course, a water world does not prohibit life (although there will be no technology – it is hard to invent fire under water!) There are still plenty of stars!