Where are the Planets that Might Host Life?

In the previous posts I showed why RNA was necessary for primitive life to reproduce, but the question then is, what sort of planets will have the necessary materials? For the rocky planets, once they reached a certain size they would attract gas gravitationally, but this would be lost after the accretion disk was removed by the extreme UV put out by the new star. Therefore all atmosphere and surface water would be emitted volcanically. (Again, for the purposes of discussion, volcanic emission includes all geothermal emissions, e.g. from fumaroles.) Gas could be adsorbed on dust as it was accreted, but if it were, because heats of adsorption of the gases other than water are very similar, the amount of nitrogen would roughly equal the amount of neon. It doesn’t. (Neon is approximately at the same level as nitrogen in interstellar gas.)

The standard explanation is that since the volatiles could not have been accreted, they were delivered by something else. The candidates: comets and carbonaceous asteroids. Comets are eliminated because their water contains more deuterium than Earth’s water, and if they were the source, there would be twenty thousand times more argon. Oops. Asteroids can also be eliminated. At the beginning of this century it was shown that various isotope ratios of these bodies meant they could not be a significant source. In desperation, it was argued they could, just, if they got subducted through plate tectonics and hence were mixed in the interior. The problem here is that neither the Moon nor Mars have subduction, and there is no sign of these objects there. Also, we find that the planets have different atmospheres. Thus compared to Earth, Venus has 50% more carbon dioxide (if you count what is buried as limestone on Earth), four times more nitrogen, and essentially no water, while Mars has far less volatiles, possibly the same ratio of carbon dioxide and water but it has far too little nitrogen. How do you get the different ratios if they all came from the same source? It is reasonably obvious that no single agent can deliver such a mix, but since it is not obvious what else could have led to this result, people stick with asteroids.

There is a reasonably obvious alternative, and I have discussed the giants, and why there can be no life under-ice on Europa https://wordpress.com/post/ianmillerblog.wordpress.com/855) and reinforced by requirement to join ribose to phosphate. The only mechanism produced so far involves the purine absorbing a photon, and the ribose transmitting the effect. Only furanose sugars work, and ribose is the only sugar with significant furanose form in aqueous solution. There is not sufficient light under the ice. There are other problems for Europa. Ribose is a rather difficult sugar to make, and the only mechanism that could reasonably occur naturally is in the presence of soluble silicic acid. This requires high-temperature water, and really only occurs around fumaroles or other geothermal sites. (The terrace formations are the silica once it comes out of solution on cooling.)

So, where will we find suitable planets? Assuming the model is correct, we definitely need the dust in the accretion disk to get hot enough to form carbides, nitrides, and silicates capable of binding water. Each of those form at about 1500 degrees C, and iron melts at a bit over this temperature, but it can be lower with impurities, thus grey cast is listed as possible at 1127 degrees C. More interesting, and more complicated, are the silicates. The calcium aluminosilicates have a variety of phases that should separate from other silicate phases. They are brittle and can be easily converted to dust in collisions, but their main feature is they absorb water from the gas stream and form cements. If aggregation starts with a rich calcium aluminosilicate and there is plenty of it, it will phase separate out and by cementing other rocks and thus form a planet with plenty of water and granitic material that floats to the surface. Under this scene, Earth is optimal. The problem then is to get this system in the habitable zone, and unfortunately, while both the temperatures of the accretion disk and the habitable zone depend on the mass of the star, they appear to depend on different functions. The net result is the more common red dwarfs have their initial high-temperature zone too close to the star, and the most likely place to look for life are the G- and heavy K-type stars. The function for the accretion disk temperature depends on the rate of stellar accretion, which is unknown for mature stars but is known to vary significantly for stars of the same mass, thus LkCa 15b is three times further away than Jupiter from an equivalent mass star. Further, the star must get rid of its accretion disk very early or the planets get too big. So while the type of star can be identified, the probability of life is still low.

How about Mars? Mars would have been marginal. The current supply of nitrogen, including what would be lost to space, is so low life could not emerge, but equally there may be a lot of nitrogen in the solid state buried under the surface. We do not know if we can make silicic acid from basalt under geochemical conditions and while there are no granitic/felsic continents there, there are extrusions of plagioclase, which might do. My guess is the intermittent periods of fluid flow would have been too short anyway, but it is possible there are chemical fossils there of what the path towards life actually looked like. For me, they would be of more interest than life itself.

To summarise what I have proposed:

  • Planets have compositions dependent on where they form
  • In turn, this depends on the temperatures reached in the accretion disk
  • Chemicals required for reproduction formed at greater than 1200 degrees C in the accretion disk, and possibly greater than 1400 degrees C
  • Nucleic acids can only form, as far as we know, through light
  • Accordingly, we need planets with reduced nitrogen, geothermal processing, and probably felsic/granitic continents that end in the habitable zone.
  • The most probable place is around near-earth-sized planets around a G or heavy K type star
  • Of those stars, only a modest proportion will have planets small enough

Thus life-bearing planets around single stars are likely to be well-separated. Double stars remain unknown quantities regarding planets. This series has given only a very slight look at the issues. For more details, my ebook Planetary Formation and Biogenesis(http://www.amazon.com/dp/B007T0QE6I) has far more details.

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.