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!