My last post gave an estimate of how many stars were suitable for having planets with life, if they had rocky planets in the right place. The answer comes out very roughly as one per every five hundred cubic light years. At first sight, not very common, but galaxies are very big, and we end up with about a hundred billion in this galaxy. The next question is, are there further restrictions? Extrasolar planets are reasonably common, according to recent surveys, however most of these found are giants that are very close to the star, and totally unsuited for life. On the other hand, there is a severe bias: the two methods that have yielded the most discoveries favour the finding of large planets close to the star.
To form stars, a large volume of gas begins to collapse, and as it collapses to form a star, it also forms a spinning disk. Three stages then follow. The first stage involves gas falling into the star from an accretion disk at a rate of a major asteroid’s mass each second. The second involves a much quieter stage, where the star has essentially formed, but it still has a disk, which it is accreting at a much slower rate, about a thousandth as fast. Finally, the star has “indigestion” and in a massive burp, clears out what is left of the disk (technically called a T Tauri event). The standard theory has the planets forming in the second stage or, for rocky planets, even following the T Tauri cleanout.
There are two important issues. As the gas falls into the star, both energy and angular momentum must be conserved. The fate of energy is simple: as the gas falls inwards, it gets hotter, and it is simple gravitation that heats the star initially, until it reaches about 80 million degrees, at which point deuterium starts to fuse and this ignites stellar fusion. However, the issue with angular momentum is more difficult. This is like an ice skater – as she brings her arms closer to herself, she starts spinning faster; put out her arms and the spin slows. As the gas heads into the star, the star should spin faster. The problem is, almost all the mass of the solar system is in the star, but almost all the angular momentum is in the planets. How did this happen?
Either all the mass retained its original angular momentum or it did not. If it did, then the sun should be spinning at a ferocious rate. While it could have lost angular momentum by throwing an immense amount of gas back into space, nobody has ever seen this phenomenon. If the stellar mass did not retain its angular momentum, it had to exchange it with something else. In my opinion, what actually happened is that the forming planets took up the angular momentum from gas that then fell into the star. If that is true, every star with enough heavy elements will form planets of some description because it helps stellar accretion. If so, the number of planet-bearing stars is very close to the number of stars.
There is, however, another problem. In my theory (Planetary Formation and Biogenesis for more details) planets simply keep growing until the stage 3 disk clear-out. If they get big enough, mutual gravitational interactions disrupt their orbits and something like billiards occurs. The planets do not collide, but if they come close enough one will be thrown out of the system (astronomers have already detected planets floating around in space, unattached to any star) and the other will end up as a giant very close to the star. A considerable number of such systems have been found. This would totally disrupt Earth-like planets, so stars with planets suitable for life must have had a shorter stage 2.
How short? Stage 2 can last up to 30 million years, although that is probably an exception, while the shortest stage 2 is less than a million years. The answer is, probably no more than a million years, i.e. our planetary system was formed around a star that had a relatively short secondary accretion. The reason I say that is as follows. The rate of accretion of a gas giant should be proportional to how much gas there is around it, and for how long. The amount of gas decreases as the distance from the star increases, and if you double the distance from the star, the gas density decreases somewhere between a half and a quarter. Now the three million year old star LkCa 15 is slightly smaller than our sun but it still has a second stage gas disk. This star has a planet nearly five times as big as Jupiter about three times further away from the star. This almost certainly means that Jupiter must have stopped growing well within three million years. (As an aside, standard theory requires at least 15 million years to start a gas giant.) Fortunately, it appears that about half the stars have such a short secondary stage. If we then say that about half the stars will be in the wrong part of the galaxy, then the estimate of stars that could be suitable for life reduces to about 25 billion. If we further reduce the total by those that are simply too young, or do not have sufficient metallicity, we could reduce the total to about 10 billion. These numbers are very rough, but the message remains: there are plenty of stars suitable to sustain life-bearing planets in the galaxy. The next question is, how many stars will have rocky planets?