Volatiles on Rocky Planets

If we accept the mechanism I posted before is how the rocky planets formed, we still do not have the chemicals for life. So far, all we have is water and rocks with some planets having an iron core. The mechanism means that until the planet gets gravitationally big enough to attract gas it only accretes solids, together with the water that bonded to the silicates. There re two issues: how the carbon and nitrogen arrived, and if these arrived as solids, which is the only available mechanism, what happened next?

In the outer parts of the solar system the carbon occurs as carbon monoxide, methanol, some carbon dioxide, and “carbon”, which essentially many forms but looks like tar, is partially graphite, and there are even mini diamonds. There are also polyaromatic hydrocarbons, and even alkanes, and some other miscellaneous organic chemicals. Nitrogen occurs as nitrogen gas, ammonia, and some cyanide. As this comes closer to the star, and in the region of the carbonaceous chondrites, it starts getting hot enough for some of this to condense and react on the silicates, which is why these have the aminoacids, etc. However, as you get closer to the star, it gets too hot and seemingly the inner asteroids are mainly just silicates. At this point, the carbon is largely converted to carbon monoxide, and the nitrogenous compounds to nitrogen. However, on some metal oxides or metals, carbon forms carbides, nitrogen nitrides, and some other materials, such as cyanamides are also formed. These are solids, and accordingly these too will be accreted with the dust and be incorporated within the planet.

As the interior of the planet gets hotter, the water gets released from the silicates and they lose their amorphous structure and become rocks. The water reacts with these chemicals and to a first approximation initially produces carbon monoxide, methane and ammonia. Carbon monoxide reacts with water on certain metals and silicates to make hydrocarbons, formaldehyde, which in turn condenses to other aldehydes (on the path to making sugars) ammonia (on the path to make aminoacids) and so on. The chemistry is fairly involved, but basically given the initial mix, temperature and pressure, both in ready supply below the Earth’s surface, what we need for life emerges and will make its way to the surface. Assuming this mechanism is correct, then provided everything is present in an adequate mix, then life should evolve. That leaves open the question, how broad is the “right mix” zone?

Before considering that, it is obvious this mechanism relies on the temperature being correct on at least two times during the planetary evolution. Initially it has to get hot enough to make the cements, and the nitrides and carbides. Superficially, that applies to all rocky planets, but maybe not for the nitrides. The problem here is Mars has very little nitrogen, so either it has gone somewhere, or it was never there. If Mars had ammonia, since it dissolves in ice down to minus 80 degrees C, ammonia on Mars would solve the problem of how could water flow there when it is so cold. However, if that is the case, the nitrogen has to be in some solid form buried below the surface. In my opinion, it was carried there as urea dissolved in water, which is why I would love to see some deep digging there.

The second requirement is that later the temperature has to be cool enough that water can set the cements. The problem with Venus is argued that it was hotter and it only just managed to absorb some water, but not enough. One counter to that is that the hydrogen on Venus has an extremely high deuterium content. The usual explanation for this is that if water gets to the top of the atmosphere, it may be hit with UV which may knock off a hydrogen atom, which is lost to space, and solar wind may take the whole molecule, however water with deuterium is less likely to get there because the heavier molecules are enhanced in the lower atmosphere, or the oceans. If this were true, for Venus to have the deuterium levels it must have started with a huge amount of water, and the mechanism above would be wrong. An embarrassing problem is where is the oxygen from that massive amount of water.

However, the proposed mechanism also predicts a very large deuterium enhancement. The carbon and nitrogen in the atmosphere and in living things has to be liberated from rocks by reaction with water, and what happens is as the water transfers hydrogen to either carbon or nitrogen it also leaves a hydroxyl attached to any metal. Two hydroxyls liberate water and leave an oxide. At this point we recall that chemical bond to deuterium is stronger than that to hydrogen, the reason being that although in theory the two are identical from the electromagnetic interactions, quantum mechanics requires there to be a zero point energy, and somewhat oversimplifying, the amount of such energy is inversely proportional to the square root of the mass of the light atom. Since deuterium is twice the mass of hydrogen, the zero point energy is less, and being less, its bond is stronger. That means there is a preference for the hydrogen to be the one that transfers, and the deuterium eventually turns up in the water. This preferential retaining of deuterium is called the chemical isotope effect. The resultant gases, methane and ammonia as examples, break down with UV radiation and make molecular nitrogen and carbon dioxide, with the hydrogen going to space. The net result of this is the rocky planet’s hydrogen gradually becomes richer in deuterium.

The effects of the two mechanisms are different. For Venus, the first one requires huge oceans; the second one little more than enough water to liberate the gases. If we look at the rocky planets, Earth should have a modest deuterium enhancement with both mechanisms because we know it has retained a very large amount of water. Mars is more tricky, because it started with less water under the proposed accretion of water mechanism, and it has less gravity and we know that all gases there, including carbon dioxide and nitrogen have enhanced heavier isotopes. That its deuterium is enhanced is simply expected from the other enhancements. Venus has about half as much CO2 again as Earth, and three times the amount of nitrogen, little water, and a very high deuterium enhancement. In my mechanism, Venus never had much water in the first place because it was too hot. Most of what it had was used up forming the atmosphere, and then providing the oxygen for the CO2. There was never much on the surface. To start with Venus was only a bit warmer than Earth, but as the CO2 began to build, whereas on Earth much of this would be dissolved in the ocean, where it would react with calcium silicate and also begin weathering the rocks that were more susceptible to weathering, such as dunite and peridotite. (I have discussed this previously: https://wordpress.com/post/ianmillerblog.wordpress.com/833 ), on Venus there were no oceans, and liquid water is needed to form these carbonates.

So, where will life be found? The answer is around any star where rocky planets formed with the two favourable temperature profiles, and ended up in the habitable zone. If more details as found in my ebook “Planetary Formation and Biogenesis” are correct, then this is most likely to occur around a G type star, like our sun, or a heavy K type star. The star also has to be one of the few that ejects it accretion disk remains early. Accordingly life should be fairly well spaced out, which may be why we have yet to run into other life forms.

Star and Planetary Formation: Where and When?

Two posts ago, as a result of questions, I promised to write a post outlining the concept of planetary accretion, based on the current evidence. Before starting that, I should explain something about the terms used. When I say something is observed, I do not mean necessarily with direct eyesight. The large telescopes record the light signals electronically, similarly to how a digital camera works. An observation in physics means that a signal is received that can be interpreted in one only certain way, assuming the laws of physics hold. Thus in the famous two-slit experiment, if you fire one electron through the slits, you get one point impact, which is of too low an energy for the human eye to see. Photomultipliers, however, can record this as a pixel. We have to assume that the “observer” uses laws of physics competently.

The accretion of a star almost certainly starts with the collapse of a cloud of gas. What starts that is unknown, but it is probably some sort of shock wave, such as a cloud of debris from a nearby supernova. Another cause appears to be the collision of galaxies, since we can see the remains of such collisions that are accompanied by a large number of new stars forming. The gas then collapses and forms an accretion disk, and these have been observed many times. The gas has a centre of mass, and this acts as the centre of a gravitational field, and as such, the gas tries to circulate at an orbital velocity, which is where the rate of falling into the star is countered by the material moving sideways, at a rate that takes it away from the star so that the distance from the centre remains the same. If they do this, angular momentum is also conserved, which is a fundamental requirement of physics. (Conservation of angular momentum is why the ice skater spins slowly with arms outstretched; when she tucks her arms in, she spins faster.

The closer to the centre, the strnger gravity requires faster orbital velocity. Thus a stream of gas is moving faster than the stream just further from the centre, and slower than the stream just closer. That leads to turbulence and friction. Friction slows the gas, meaning it starts to fall starwards, while the friction converts kinetic energy to heat. Thus gas drifts towards the centre, getting hotter and hotter, where it forms a star. This has been observed many times, and the rate of stellar accretion is such that a star takes about a million years to form. When it has finished growing, there remains a dust-filled gas cloud of much lower gas density around it that is circulating in roughly orbital velocities. Gas still falls into the star, but the rate of gas falling into the star is at least a thousand times less than during primary stellar accretion. This stage lasts between 1 to 30 million years, at which point the star sends out extreme solar winds, which blow the gas and dust away.

However, the new star cannot spin fast enough to conserve angular momentum. The usual explanation is that gas is thrown out from near the centre, and there is evidence in favour of this in that comets appear to have small grains of silicates that could only be formed in very hot regions. The stellar outburst noted above will also take away some of the star’s angular momentum. However, in our system, the bulk of the angular momentum actually resides in the planets, while the bulk of the mass is in the star. It would seem that somehow, some angular momentum must have been transferred from the gas to the planets.

Planets are usually considered to form by what is called oligarchic growth, which occurs after primary stellar accretion. This involves the dust aggregating into lumps that stick together by some undisclosed mechanism, to make first, stone-sized objects, then these collide to form larger masses, until eventually you get planetesimals (asteroid-sized objects) that are spread throughout the solar system. These then collide to form larger bodies, and so on, at each stage collisions getting bigger until eventually Mars-sized bodies collide to form planets. If the planet gets big enough, it then starts accreting gas from the disk, and provided the heat can be taken away, if left long enough you get a gas giant.

In my opinion, there are a number of things wrong with this. The first is, the angular momentum of the planets should correspond roughly to the angular momentum of the dust, which had velocity of the gas around it, but there is at least a hundred thousand times more gas than dust, so why did the planets end up with so much more angular momentum than the star? Then there is timing. Calculations indicate that to get the core of Jupiter, it would take something approaching 10 million years, and that assumes a fairly generous amount of solids, bearing in mind the solid to gas ratio. At that point, it probably accretes gas very quickly. Get twice as far away from the star, and collisions are much slower. Now obviously this depends on how many planetesimals there are, but on any reasonable estimate, something like Neptune should not have formed. Within current theory, this is answered by having Neptune and Uranus being formed somewhere near Saturn, and then moved out. To do that, while conserving angular momentum, they had to throw similar masses back towards the star. I suppose it is possible, but where are the signs of the residues? Further, if every planet is made from the same material, the same sort of planet should have the same composition, but they don’t. The Neptune is about the same size as Uranus, but it is about 70% denser. Of the rocky planets, Earth alone has massive granitic/feldsic continents.

Stronger evidence comes from the star called LkCa 15 that apparently has a gas giant forming that is already about five times bigger than Jupiter and about three times further away. The star is only 3 million years old. There is no time for that to have formed by this current theory, particularly since any solid body forming during the primary stellar accretion is supposed to be swept into the star very quickly.

Is there any way around this? In my opinion, yes. I shall put up my answer in a later post, although for those who cannot wait, it is there in my ebook, “Planetary Formation and Biogenesis”.

Why do we do science?

What is the point of science? In practice, most scientists use their knowledge to try to make something, or solve some sort of problem, or at least help someone else do that. (Like most occupations, most junior ones turn up to work and work on what they are told to work on.) But, you might say, surely, deep down, they are seekers of the truth? Unfortunately, I rather fancy this is not the case. The problem was first noted by Thomas Kuhn, in his book, “The structure of scientific revolutions”. In Kuhn’s view, scientific results are almost always interpreted in terms of the current paradigm, i.e. while the data are reproduced properly, they are interpreted in terms of current thinking, even if that does not fit very well. No other theory gets a look-in. If a result does not conform to the standard theory, the researcher does not question the standard theory. The first effort is to find some way of accommodating it, and if that does not work, it may be listed as a question for further work, in other words the researcher tries to persuade someone else to find a way of fitting it to the standard paradigm rather than taking the effort to find an alternative theory.

According to Kuhn, most science is carried out as “normal science”, wherein researchers create puzzles that should be solved by the standard paradigm, in other words, experiments are set up not to try to find the truth, but rather to confirm what everyone believes to be true. This is not entirely unreasonable. If we stop and think for a moment, an awful lot of such research is carried out by PhD students, or post-doctoral fellows. The lead researcher has submitted his idea as a request for funding, and this is overseen by a panel. If you submit something that would not get anywhere within the current paradigm, you will not get funding because the panel will usually consider this to be a waste of time. On top of that, if you are going to include a PhD student in this work, that student needs a thesis at the end of his work, and that student will not thank the supervisor for coming up with something that does not produce results that can be written up. In other words, the projects are chosen such that the lead researcher has a very good idea as to what will be found, and it will be chosen so that it is unlikely to lead to too great an intellectual challenge. An example of a good project might to make a new chemical compound that might be a useful drug. The project might involve new synthetic work, there will be problems in choosing a route, but the project will not founder on some conceptual problem.

Natually, the standard paradigm clearly must have much going for it to get adopted in the first place. It cannot be just anything, and there will be a lot of truth in it, nevertheless as I mentioned in my first ebook, part 1 of “Elements of Theory”, any moderate subset of data frequently has at least two theories that would explain the data, and when the paradigm is chosen, the subset is moderate. If all that follows it to investigate very similar problems, then a mistake can last. The classic mistake was Claudius Ptolemy’s cosmological theory, which was the “truth” for over 1600 years, even though it was wrong and, as we now recognize, with no physical basis. If you wish to find the truth, you might follow Popper and try to design experiments that would falsify such a theory, but PhD theses cannot be based like that as it is too risky that the student will find nothing and fail to get his degree through no fault of his.

What brought these thoughts on was a recent article in the journal Icarus. The subject was questioning how the Moon was formed. The standard theory of planetary formation goes like this. After the star forms, the accretion disk that remains settles the dust on the central plane, and this gradually congeals into larger bodies, which further join together when they collide, and so on, until you get planetesimals (objects about the size of asteroids) then, apart from the asteroids, eventually embryos (objects about the size of Mars) which gravitationally interact and form very eccentric orbits, and then collide to form planets (except for Mars, which is a remaining embryo). All such collisions once planetesimals form are random, and the underpinning material could have come from a very large region, thus Earth was made from embryos formed from material beyond Mars and Venus. The Moon was formed from the splatter arising from a near glancing collision of a Mars-sized body called Theia with Earth.

If you carefully measure the isotope ratios of samples of meteorites, what you find is that all from the same origin have the same isotope ratios, but those from different parts of the solar system have different ratios. As an example, oxygen has three stable isotopes of atomic weights 16, 17 and 18. We have carbonaceous chondrites from the outer asteroid belt, a number of samples from Vesta, some from Mars, and of course unlimited supplies from here. The isotope ratios of these samples are all the same from one source, but different between sources. We also have a good number of samples from the Moon, thanks to the Apollo program. Now, the unusual fact is, the Moon is made of material that is essentially identical to our rocks, at least in terms of isotope ratios.

This Icarus paper carried out simulations of planetary formation employing the standard theory, and showed that since the Moon is largely Theia, the chances of the Moon and Earth having the same ratio of even oxygen isotopes is less than 5%. So, what conclusion do the authors draw? The obvious one is that the Moon did not form that way; a more subtle one is that planets did not form by the random collision of growing rocky bodies. However, they drew neither. Instead, they really refused to draw a conclusion.

I should add that I have in interest in this debate, as my mechanism outlined in Planetary Formation and Biogenesis has the planets grow from relatively narrow zones, although the disk material is always heading towards the star to provide new feed. The Moon grows at the same distance as Earth (at a Lagrange point) from the star and hence has the same composition. The concept that the Moon formed at either L4 or L5 was originally proposed by Belbruno and Gott in 2005 (Astron. J. 129: 1724–1745) and I regard it as almost dishonest not to have mentioned their work, which predicts their result provided the bodies form from local material. Unfortunately, the citing of scientific work that contradicts the standard theory is not exactly frequent, and in my view, does science no service. The real problem is, how common is this rejection of that which is currently uncomfortable?

You may say, who cares? It may very well be that how the Moon formed is totally irrelevant to modern society. My point is, society is becoming extremely dependent on science, and if science starts to become disinterested in seeking the truth, then eventually the mistakes may become very significant. Of course mistakes will be made. That happens in any human endeavor. But, do we want to restrict them to unavoidable accidents, or are we prepared to put up with avoidable errors?

Planets being formed?

This has been an interesting period for planetary science. In the last post, I mentioned the landing of Philae on a comet. As an update, unfortunately all has not gone well. The comet landed well, it bounced, and ended up in a shady spot, do most of what it has managed has relied on batteries. We do not know yet what data it has sent back, so we have no real idea on how successful the venture was, but from my point of view, the news is less good. In my last post, I mentioned that I would like to see what was encased in the ice. What happened was that Philae left it to the last to drill down (because they were afraid that the action of the drill might launch Philae back off the comet, as its gravity is very weak) and they wanted to do as much as they could before that risk was taken. They drilled, but apparently the drill hit something very hard, and when they withdrew the drill and tried to analyze its core, it appeared that there was no sample inside the drill. This is one of the curses of this sort of work. When designing some form of robot, you have to guess exactly what conditions you will meet.

However, a most interesting image has also been released by the European Space Agency. The star, HL Tauri has been found with an accretion disk around it. The star is about 1 million years old, and the disk has rings in it, with dark gaps between them. The most obvious cause for such rings would be the formation of planets, although that does not mean there is a planet in every gap, because while a planet will clear out dust on its path, gravitational resonance will also clear out material. Gravitational resonance is a term for when the orbital period at a given position is an exact multiple/fraction of another. Thus if the planet had a period, say, 12 years (roughly Jupiter’s “year”) there would be 2:1 resonances at a distance where the orbital period was 24 years, or at a distance where the orbital period was 6 years. Where this happens, over a period of time the various gravitational effects, instead of cancelling and circularizing, tend to reinforce and the bigger object causes the very much smaller one to change orbit.

So, are there planets there? One answer is, we don’t know because we have not seen them. Up to a point, this is a bit of a negative in this case. At first sight it may seem obvious that we would not see planets because they are too dull, but that is not the case with very newly formed giants. Thus there is a star HR 8799 and we can see four giant planets around it. The reason we can see them is that they are newly-formed giants, and when they take up the gas, the gravitational energy of the gas falling onto the planet heats it to a yellow-white heat, and the planets glow relatively brightly. Given we cannot see planets here, but we can see the disk, what does that mean?

One obvious thing that it can mean is that planets have yet to get big enough to glow brightly. In my theory of planetary formation (Planetary Formation and Biogenesis) our star had to have formed its planets by about 1 million years. The reason for this assessment is that there is a star LkCa 15 that is 3 million years old, and it has a planet much bigger than Jupiter, and significantly further from the star. Planetary growth should be faster, the closer to the star, at least for the same sort of planet, because the density of matter increases as it falls into the star. (The circumferences of the orbits decrease, and if the same amount of matter is presence, there much be more per unit volume.) Incidentally, we know about the planet around LkCa 15 because we “see” it, at least in images obtained by powerful telescopes, so it is glowing. Since we only see one giant, my theory requires there to be three other giants we cannot see, presumably because they are yet of insufficient size to glow sufficiently brightly for us to image them. So, if I am right, 1 My gets you giants of the size we have, and the longer the disk lasts, the bigger the giants get.

All of which shows there is still a lot of interest in planetary research

Theory and planets: what is right?

In general, I reserve this blog to support my science fiction writing, but since I try to put some real science in my writing, I thought just once I would venture into the slightly more scientific. As mentioned in previous posts, I have a completely different view of how planets, so the question is, why? Surely everyone else cannot be wrong? The answer to that depends on whether everyone goes back to first principles and satisfies themselves, and how many lazily accept what is put in front of them. That does not mean that it is wrong, however. Just because people are lazy merely makes them irrelevant. After all, what is wrong with the standard theory?

My answer to that is, in the standard theory, computations start with a uniform distribution of planetesimals formed in the disk of gas from which the star forms. From then on, gravity requires the planetesimals to collide, and it is assumed that from these collisions, planets form. I believe there are two things wrong with that picture. The first is, there is no known mechanism to get to planetesimals. The second is that while gravity may be the mechanism by which planets complete their growth, it is not the mechanism by which it starts. The reader may immediately protest and say that even if we have no idea how planetesimals form, something had to start small and accrete, otherwise there would be no planets. That is true, but just because something had to start small does not mean there is a uniform distribution throughout the accretion disk.

My theory is that it is chemistry that causes everything to start, and different chemistries occur at different temperatures. This leads to the different planets having different properties and somewhat different compositions.

The questions then are: am I right? does it matter? To the first, if I am wrong it should be possible to falsify it. So far, nobody has, so my theory is still alive. Whether it matters depends on whether you believe in science or fairy stories. If you believe that any story will do as long as you like it, well, that is certainly not science, at least in the sense that I signed up to in my youth.

So, if I am correct, what is the probability of finding suitable planets for life? Accretion disks last between 1 to even as much as 30 My. The longer the disk lasts, the longer planets pick up material, which means the bigger they are. For me, an important observation was the detection of a planet of about six times Jupiter’s mass that was about three times further from its star (with the name LkCa 15) than Jupiter. The star is approximately 2 My old. Now, the further from the star, the less dense the material, and this star is slightly smaller than our sun. The original computations required about 15 My or more to get Jupiter around our star, so they cannot be quite correct, although that is irrelevant to this question. No matter what the mechanism of accretion, Jupiter had to start accreting faster than this planet because the density of starting material must be seriously greater, which means that we can only get our solar system if the disk was cleared out very much sooner than 2 My. People ask, is there anything special regarding our solar system? I believe this very rapid cleanout of the disk will eliminate the great bulk of the planetary systems. Does it matter if they get bigger? Unfortunately, yes, because the bigger the planets get, the bigger the gravitational interactions between them, so the more likely they are to interact. If they do, orbits become chaotic, and planets can be eliminated from the system as other orbits become highly elliptical.

If anyone is interested in this theory, Planetary Formation and Biogenesis (http://www.amazon.com/dp/B007T0QE6I )

will be available for 99 cents  as a special promo on Amazon.com (and 99p on Amazon.co.uk) on Friday 13, and it will gradually increase in price over the next few days. Similarly priced on Friday 13 is my novel Red Gold, (http://www.amazon.com/dp/B009U0458Y  ) which is about fraud during the settlement of Mars, and as noted in my previous post, is one of the very few examples of a novel in which a genuine theory got started.

Planets for alien life (2)

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?