Rocky planets, atmospheres and aliens

This week, the second ebook, Dreams Defiled, in my trilogy, First Contact, was published on Amazon. The trilogy is nominally about contact with aliens (at least an alien hologram in A Face on Cydonia) and its consequences. It is also about how civilization might deal with (or perhaps fail to deal with) certain crises that appear to be inevitable. One solution to a crisis that you may or may not like is the proposed solution to the fuels/transport crisis, for no matter what, it is unlikely the whole planet can continue burning energy at the rate some western countries do so now. Check out my solution, and see what you think.

In the meantime, back to the issue of how many planets could have alien life. In previous posts I made an estimate of the likely number of stars that have rocky planets suitable for life. While most stars are not suitable, there are still billions of stars that are, even in this galaxy. The rocky planet then has to be within the right size range. It would have to be somewhat bigger than Mars to ensure it held a significant atmosphere, and there will also be a maximum size, but we do not know what that is. According to my theory, to keep within the right size range, the star has to clear out the accretion disk early, but up to half the stars do this. So, the next question is, will they have water and atmospheric gases? Where do the gases come from?

The usual argument is that the rocky planets get their water and atmospheres through later being bombarded by small asteroids. I don’t believe this either, since, as I show in more detail in Planetary Formation and Biogenesis, since Venus, Earth and Mars have totally different atmospheres, they have to be bombarded selectively by totally different types of asteroids that, as far as we can tell, no longer exist. Thus Venus has about four times as much nitrogen as Earth, but negligible water. Mars has a reasonable amount of water, but almost no nitrogen. How does that come about?

My answer is that the rocky planets form by cement-like dust joining rocks together, and that is where the water comes from. The available cement depends on how hot the solids get during primary stellar accretion, and at what temperature they set during the late cooler accretion disk. Earth happened to set at the optimum temperature – the first stage had been hot enough to get the best cement made, while the second stage was cool enough to let the cement set with the most water. Venus had the same cements, but it was hotter, so it did not set with much water, while Mars had only a limited cement, so while it was cooler, it did not have the means of setting much water. Subsequently, the water reacted with solid sources of carbon and nitrogen and made the atmosphere, and Venus, because it was hotter, had more carbon and nitrogen, so it used up most of its limited water making its very dense atmosphere. If that is true, then most stars that can form rocky planets will have one like Earth in the habitable zone.

That means there are billions of planets in this galaxy capable of forming life. That does not mean that the galaxy is teaming with civilizations. For example, the nearest suitable single star, Epsilon Eridani, is only about 900 million years old. At that age, Earth may or may not have got around to having primitive single-cell life. Of course, in Dreams Defiled I give hints there is a civilization there. How could that be? There is an obvious possibility, but to add to the mystery, I provide evidence that in this fictional story, the food on the rocky planet around Epsilon Eridani and on Earth is each compatible with both life forms, and in general, life forms that evolve separately find that they can only tolerate food that evolved with them.  Now can you guess where this plot is going? As you might guess, I am trying to write stories that also try to impart some scientific knowledge, and which I hope readers will find interesting.

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! 

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?

Planets for alien life

In my novel, “A Face on Cydonia”, an alien message was finally intercepted. That raises the question, what is the probability of alien life? Frank Drake answered that question with the Drake equation, which involved the product of the number of potentially suitable stars, the probability such a star has a suitable planet, the probability that life will evolve on such a planet, and the probability that it will develop to a civilization. (There is a little more to it, relating to communications, but we leave that.)

In my ebook, “Planetary Formation and Biogenesis” I tried to put some numbers on these, or at least the conditions that have to be met. I should add that what I put forward is NOT in accord with most astronomical thinking. Most astronomers and physicists believe that planets form through gravitational attraction of planetesimals (Bodies of the 100 km size) into embryos (bodies about Mars size) then these accrete into planets by gravitational collisions. While this theory has been around for 60 years, nobody has any real idea how planetesimals form. My concept is that the initial bodies accrete through chemistry that differs at different temperatures, and that means you do not get a uniform distribution of planetesimals. Unfortunately, if I am correct, there are a number of different types of solar system that can evolve.

For life to evolve, it is usually considered the planet must be in what is called the “habitable zone”, which is usually defined by a zone in which planets have liquid water. Venus is usually considered to be too hot, and Mars too cold. The distance from the star for the habitable zone depends on the luminosity of the star, which in turn depends on the stellar mass to a power of approximately four. Thus if we require the planet to be in the habitable zone, for very small stars the planet has to be very close to the star. The smaller the star, the more common it is. If the star is very big, it burns so much faster and does not last. For these reasons, it is usually thought that stars have to be roughly the same size as the sun, i.e. G-type stars (our sun is a G-type, but one of the smaller ones) or K-type (the next size range down). The next problem for a planet is whether the star is a single star, and if so, do they come close enough to gravitationally throw the planets away. Double stars are more common than single stars. Further, stars have to have sufficient elements heavier than helium. You cannot have rocky planets without silicon! Finally, for life to evolve very far, the star has to be old enough.

None of the closest stars to Earth seem particularly promising. The most promising is Alpha Centauri, which also happens to be the closest, at a little over 4 light years, and has two stars that approach about as close as the Sun-Saturn distance. One star is slightly bigger than Sol, and the other is a smaller star. Neither star could hold a gas giant, but rocky planets might be possible, and the smaller star appears to have a small planet. A star like Sirius or Procyon is simply too big and will not last long enough to let animal-type life evolve. The two closest single stars that seem big enough have their problems. Epsilon Eridani is known to have a Jupiter-type planet, but is only 900 million years old, so any planets will not have had time to evolve advanced life. Tau ceti is probably old enough, but it has a low fraction of heavy elements, and may not be able to form rocky planets.

There are only 2 G-type stars (our sun is a G-type star) within ten light years, and about 18 within thirty light years, however K-type stars might also be adequate, and there are about 38 of them within 30 light years. Unfortunately, the heavier G-type and the lighter K-type are probably not suitable, so we may have a lot of space to ourselves. On the other hand, our galaxy is huge, and by my count it probably contains something like a hundred billion suitably sized stars. Those near the centre of the galaxy probably have to be discounted (the region is too violent) and we may have to eliminate about half of the rest for various reasons, nevertheless, it is almost certain that there are plenty of suitable stars. It is just that they are rather far away both from us and from each other. How many will have planets? That is for a later post.

Terraforming Mars

In the 1990s, there was much speculation about terraforming planets, particularly Mars. The idea was that the planet could be converted into something like Earth. To make Mars roughly like Earth, the temperature has to be raised by about ninety Centigrade degrees, atmospheric pressure has to be raised by something approaching a hundred times present pressure, and a lot of water must be found. That presumably comes from buried ice, so besides uncovering it, an enormous amount of heat is required to melt it. The reason Mars is colder is that the sun delivers half the power to Mars than Earth, due to Mars being further away. The gas pressure depends on two things. The first is there has to be enough material, and the second is we have to get it into the gas phase. The most obvious gas is carbon dioxide, because as dry ice, it could be in the solid state, but would be amenable to heating. The problem is, if carbon dioxide is present with a lot of water, it will be absorbed by the water, particularly cold water, and slowly turned into material like dolomite. Nitrogen is the major gas in our atmosphere, but that would be a gas on Mars, and there is very little in the Martian atmosphere.

Why did anyone ever think Terraforming was possible? One reason may be that about 3.6 Gy ago (a gigayear is a thousand million years) it was thought that there were huge rivers on Mars. The Viking images found a huge number of massive river valleys, and so it was thought there had to be sufficient temperatures to melt the water. Subsequent information has suggested that these rivers did not persist over a prolonged wet period, but rather there were intermittent periods where significant flows occurred.  Such rivers probably never flowed for more than a million years or so, and while a million years might seem to be an extremely long period to us, it is trivial in the life of the solar system. Nevertheless the rivers meandered for that period, which is at least suggestive that they were relatively stable for that time, so what went wrong?

When I wrote Red Gold, I needed the major protagonist to make an unexpected discovery to expose a fraud, and it was then that I had an idea. The average temperature on Mars now is -80 degrees C, and while we could imagine some sort of greenhouse effect warming the early Mars, the sun only emitted about two-thirds the energy it does now, so temperature would have been a more severe problem. To me, it was inconceivable that the temperature could get sufficiently above the melting point of ice to give significant flows, but there is one way to make water liquid at -80 degrees C, and that is to have ammonia present. If the volcanoes gave off ammonia as well as water, that would give some greenhouse gas, and the carbon would be present as methane, this being what is called a reducing atmosphere. Sunlight tends to act with water to oxidize things, giving off hydrogen that escapes to space. This has happened extensively on Mars, indeed at many sites where chloride has been deposited on the surface, it has been converted to perchlorate. So methane would oxidize to carbon dioxide, and carbon dioxide would react with ammonia to make first, ammonium carbonate, then, given heat or time, urea. So my “unexpected discovery” was the fertilizer that would make the settlement of Mars possible. I had something that I thought would make my plot plausible.

Funnily enough, this thought took on a life of its own; the more I thought about it, the more I liked it, because it helps to explain, amongst other things, how life began. (The reduced form of nitrogen is a set of compound called nitrides. Water on nitrides, plus heat, makes ammonia, and also cyanide, which is effectively carbon nitride.) Standard theory, of course, assumes that nitrogen was always emitted as the nitrogen gas we have in our atmosphere. Of course you might think that all the scientists are right and I am wrong. Amongst others, Carl Sagan calculated that if ammonia was emitted into the atmosphere, it would be removed by sunlight in a matter of a decade or so, and he had to be right, surely? Well, no. Anyone can be wrong. (Of course you may say some, such as me, are more likely to be wrong than others!) However, in this case I maintain that Sagan was wrong because he overlooked something: ammonia dissolves in water at a very fast rate, and in water it will be protected to some extent. To justify that, we have found rocks on Earth that are 3.2 billion years old and that have samples of seawater enclosed, and these drops of seawater have very high levels of ammonia. These levels are sufficiently high that about 10% of Earth’s nitrogen must have been dissolved in the sea as ammonia at the time, and that is after the Earth had been around for about 500 million years after the water flowed on Mars.

If anyone is interested in why I think this occurred, Red Gold has an appendix where my first explanation is given in simple language. For those who want something a bit more detailed, together with a review of several hundred scientific papers, you could try my ebook, Planetary Formation and Biogenesis.