Could Aliens Know We Are Here?

While an alien could not see us without coming here, why pick here as opposed to all the other stars? We see exoplanets and speculate on whether they could hold life, but how many exoplanets could see our planet, if they held life with technology like ours or a little better? When I wrote the first edition of my ebook “Planetary Formation and Biogenesis” I listed a few techniques to find planets. Then, the most had been found through detecting the wobble of stars through the frequency changes of their line spectra to which a Doppler shift was added. The wobble is caused by the gravity of the planets. Earth would be very difficult to see that way because it is too small. This works best with very large planets very close to stars.

While there are several methods for discovering planets that work occasionally, one is particularly productive, and that is to measure the light intensity coming from the star. If a planet crosses our line of sight, the light dims. Maybe not a lot, but it dims. If you have seen an eclipse of the sun you will get the idea, but if you have seen a transit of Venus or of Mercury you will know the effect is not strong. This is very geometry specific because you have to be able to draw a straight line between your eye, the planet and part of the star and the further the planet is from the star, the smaller the necessary angle. To give an idea of the problem, our planetary system was created more or less on the equatorial plane of the accretion disk that formed the sun, so we should at least see transits of our inner planets, right? Well, not exactly because the various orbits do not lie on one plane. My phrase “more or less” indicates the problem – we have to be exactly edge-on to the plane unless the planet is really close to the star, when geometry lends a hand because the star is so big that something small crossing in front can be seen from wider angles.

Nevertheless, the Kepler telescope has seen many such exoplanets. Interestingly, the Kepler telescope, besides finding a number of stars with multiple planets close to the star has also found a number of stars with only one planet at a good distance from the star. That does not mean there are no other planets; it may mean nothing more than that one is accidentally the only one whose orbital plane lies on our line of sight. The others may, like Venus, be on slightly different planes. When I wrote that ebook, it was obvious that suitable stars were not that common, and since we were looking at stars one at a time over an extended period, not many planets would be discovered. The Kepler telescope changed that because when it came into operation, it could view hundreds of thousands of stars simultaneously.

All of which raises the interesting question, how many aliens, if they had good astronomical techniques, could see us by this method, assuming they looked at our sun? Should we try to remain hidden or not? Can we, if we so desired?

In a recent paper from Nature (594, pp505 – 507 2021) it appears that 1,715 stars within 100 parsecs of the sun (i.e. our “nearest neighbours”) would have been in a position to spot us over the last 5,000 years, while an additional 319 stars will have the opportunity over the next 5,000 years. Stars might look as if they are fixed in position, but actually they are speedily moving, and not all in the same direction. 

Among this set of stars are seven known to have exoplanets, including Ross 128, which could have seen us in the past but no longer, and Teegarden’s star and Trappist-1, which will start to have the opportunity in 29 years and 1642 years respectively. Most of these are Red Dwarfs, and if you accept my analysis in my ebook, then they will not have technological life. The reason is the planets with composition suitable to generate biogenesis will be too close to the star so will be far too hot, and yet probably receive insufficient higher frequency light to drive anything like photosynthesis.

Currently, an Earth transit could be seen from 1402 stars, and this includes 128 G-type stars, like our sun. There are 73 K stars, which may also be suitable to house life. There are also 63 F-type stars. These stars are larger than the sun, from 1.07 to 1.4 times the size, and are much hotter than the sun. Accordingly, they turn out more UV, which might be problematical for life, although the smaller ones may be suitable and the Earth-equivalent planet will be a lot further from the star. However, they are also shorter-lived, so the bigger ones may not have had time. About 2/3 of these stars are in a more restricted transit zone, and could, from geometry, observe an Earth transit for ten hours. So there are a number of stars from which we cannot hide. Ten hours would give a dedicated astronomer with the correct equipment plenty of time to work out we have oxygen and an ozone layer, and that means life must be here.

Another option is to record our radio waves. We have been sending them out for about 100 years, and about 75 of our 1402 stars identified above are within that distance that could give visual confirmation via observing a transit. We cannot hide. However, that does not mean any of those stars could do anything about it. Even if planets around them have life, that does not mean it is technological, and even if it were, that does not mean they can travel through interstellar space. After all, we cannot. Nevertheless, it is an interesting matter to speculate about.

Why Plate Tectonics?

How did plate tectonics start? Why has Earth got them and none of the rocky planets have, at least as far as we know? In my ebook “Planetary Formation and Biogenesis” my explanation as to one of the reasons for why plate tectonics are absent on Mars is that the Martian basaltic mantle appears to have about 17% iron oxide whle Earth has 7 – 11%. This means it cannot make eclogite whereas Earth’s basalt can. Eclogite is a particularly dense silicate and it is only made under serious pressure. 

To see the significance, we have to ask ourselves how plate tectonics works. The core generates hot spots, and hotter mantle material rises and has to push aside other rock, and we get what we call seafloor spreading, although it does not have to be underwater. The African rift valley is an example, in this case a relatively new example where the African plate is dividing, and eventually will have sea between Somalia and the Nubian zone. Similarly, the Icelandic volcanoes are due to “seafloor spreading”. Thus matter coming up pushes the surface plates aside, but then what? On Mars, the cold basalt has nowhere to go so it forms what is called a “stagnant lid”, and heat can only escape through volcanism. On Mars, this resulted in quite significant volcanism about three and a half billion years ago, then this more or less stopped, although not as much as some think because there is evidence of volcanic eruptions around Elysium within the last two million years. The net result is the “lid” gradually gets thicker, and stronger, which means the heat loss of the Martian mantle is actually much less than that of Earth.

On Earth, what happens is that as the basaltic plates get pushed aside, one goes under another, and this is where then eclogite becomes relevant. As the plate goes down, the increased pressure causes the basalt to form eclogite, and because it is denser than its surroundings, gravity makes it go deeper. It is this pull subduction that keeps plate tectonics going.

So, what about Venus? The usual answer is that Venus had a stagnant lid, but at certain intervals the internal heat is so great there is a general overturn and there is a general resurfacing. However, maybe that is not exactly correct. Our problem with Venus is we cannot see the surface thanks to the clouds. The best we can manage is through radar, and recent (June, 2021) information has provided some surprises (Byrne, et al.,   https://doi.org/10.1073/pnas.2025919118).  Basically, what was found was evidence that many of the lowlands had broken into crustal blocks and these blocks are moving relative to each other, in the same way as pack ice moves. The cause would be mantle convection that stresses the crust. The Venusian crust has many landforms, including thin belts where crust has been pushed together to form ridges, or pulled apart to form troughs. However, these ones tend to encompass low-lying regions that are not deformed, but rather appear to be individual blocks that shift, rotate and slide past each other. The authors suggest this what Earth was like before plate tectonics got going.

As to why they started here and not there has no obvious answer. The fact that Earth rotates far more quickly will generate much stronger Coriolis forces. It may be that the absence of water on Venus removes a potential lubricant, but that seems unlikely if blocks of crust are moving. My personal view is that one key point is it needs something to force the crust downwards. Eclogite may pull it down, but something has to push the basalt down to force it to make eclogite. My guess here is that Earth has one thing the other rocky planets do not have: granitic continents. Granite floats on basalt, so if a basaltic mass was pushed against a significant granitic mass, the granite would slide over the top and its weight would push the basalt down. When it made eclogite, the denser basalt would continue its downward motion, pulling a plate with it. Is that right? Who knows, but at least it looks plausible to me.

Where to Look for Alien Life?

One intriguing question is what is the probability of life elsewhere in the Universe? In my ebook, “Planetary Formation and Biogenesis” I argue that if you need the sort of chemistry I outline to form the appropriate precursors, then to get the appropriate planet in the habitable zone your best bet is to have a G-type or heavy K-type star. Our sun is a G-type. While that eliminates most stars such as red dwarfs, there are still plenty of possible candidates and on that criterion alone the universe should be full of life, albeit possibly well spread out, and there may be other issues. Thus, of the close stars to Earth, Alpha Centauri has two of the right stars, but being a double star, we don’t know whether it might have spat out its planets when it was getting rid of giants, as the two stars come as close as Saturn is to our sun. Epsilon Eridani and Tau Ceti are K-type, but it is not known whether the first has rocky planets, and further it is only about 900 million years old so any life would be extremely primitive. Tau Ceti has claims to about 8 planets, but only four have been confirmed, and for two of these, one gets about 1.7 times Earth’s light (Venus get about 1.9 times as much) while the other gets about 29%. They are also “super Earths”. Interestingly, if you apply the relationship I had in my ebook, the planet that gets the most light, is the more likely to be similar geologically to Earth (apart from its size) and is far more likely than Venus to have accreted plenty of water, so just maybe it is possible.

So where do we look for suitable planets? Very specifically how probable are rocky planets? One approach to address this came from Nibauer et al. (Astrophysical Journal, 906: 116, 2021). What they did was to look at the element concentration of stars and picked on 5 elements for which he had data. He then focused on the so-called refractory elements, i.e., those that make rocks, and by means of statistics he separated the stars into two groups: the “regular” stars, which have the proportion of refractory elements expected from the nebular clouds, or a “depleted” category, where the concentrations are less than expected. Our sun is in the “depleted” category, and oddly enough, only between 10 – 30% are “regular”. The concept here is the stars are depleted because these elements have been taken away to make rocky planets. Of course, there may be questions about the actual analysis of the data and the model, but if the data holds up, this might be indicative that rocky planets can form, at least around single stars. 

One of the puzzles of planetary formation is exemplified by Tau Ceti. The planet is actually rather short of the heavy elements that make up planets, yet it has so many planets that are so much bigger than Earth. How can this be? My answer in my ebook is that there are three stages of the accretion disk: the first when the star is busily accreting and there are huge inflows of matter; the second a transition when supply of matter declines, and a third period when stellar accretion slows by about four orders of magnitude. At the end of this third period, the star creates huge solar winds that clear out the accretion disk of gas and dust. However, in this third stage, planets continue accreting. This third stage can last from less than 1 million years to up to maybe forty. So, planets starting the same way will end up in a variety of sizes depending on how long the star takes to remove accretable material. The evidence is that our sun spat out its accretion disk very early, so we have smaller than average planets.

So, would the regular stars not have planets? No. If they formed giants, there would be no real selective depletion of specific elements, and a general depletion would register as the star not having as many in the first place. The amount of elements heavier than helium is called metallicity by astronomers, and this can vary by a factor of at least 40, and probably more. There may even be some first-generation stars out there with no heavy elements. It would be possible for a star to have giant planets but show no significant depletion of refractory elements. So while Nibauer’s analysis is interesting, and even encouraging, it does not really eliminate more than a minority of the stars. If you are on a voyage of discovery, it remains something of a guess which stars are of particular interest.

Why We Cannot Get Evidence of Alien Life Yet

We have a curiosity about whether there is life on exoplanets, but how could we tell? Obviously, we have to know that the planet is there, then we have to know something about it. We have discovered the presence of a number of planets through the Doppler effect, in which the star wobbles a bit due to the gravitational force from the planet. The problem, of course, is that all we see is the star, and that tells us nothing other than the mass of the planet and its distance from the star. A shade more is found from observing an eclipse, because we see the size of the star, and in principle we get clues as to what is in an atmosphere, although in practice that information is extremely limited.

If you wish to find evidence of life, you have to first be able to see the planet that is in the habitable zone, and presumably has Earth-like characteristics. Thus the chances of finding evidence of life on a gas giant are negligible because if there were such life it would be totally unlike anything we know. So what are the difficulties? If we have a star with the same mass as our sun, the planet should be approximately 1 AU from the star. Now, take the Alpha Centauri system, the nearest stars, and about 1.3 parsec, or about 4.24 light years. To see something 1 AU away from the star requires an angular separation of about one arc-second, which is achievable with an 8 meter telescope. (For a star x times away, the required angular resolution becomes 1/x arc-seconds, which requires a correspondingly larger telescope. Accordingly, we need close stars.) However, no planets are known around Alpha Centauri A or B, although there are two around Proxima Centauri. Radial velocity studies show there is no habitable planet around A greater than about 53 earth-masses, or about 8.4 earth-masses around B. However, that does not mean no habitable planet because planets at these limits are almost certainly too big to hold life. Their absence, with that method of detection, actually improves the possibility of a habitable planet.

The first requirement for observing whether is life would seem to be that we actually directly observe the planet. Some planets have been directly observed but they are usually super-Jupiters on wide orbits (greater than10 AU) that, being very young, have temperatures greater than 1000 degrees C. The problem of an Earth-like planet is it is too dim in the visible. The peak emission intensity occurs in the mid-infrared for temperate planets, but there are further difficulties. One is the background is higher in the infrared, and another is that as you look at longer wavelengths there is a 2 – 5 times coarser spatial resolution due to the diffraction limit scaling. Apparently the best telescopes now have the resolution to detect planets around roughly the ten nearest stars. Having the sensitivity is another question.

Anyway, this has been attempted, and a candidate for an exoplanet around A has been claimed (Nature Communications, 2021, 12:922 ) at about 1.1 AU from the star. It is claimed to be within 7 times Earth’s size, but this is based on relative light intensity. Coupled with that is the possibility that this may not even be a planet at all. Essentially, more work is required.

Notwithstanding the uncertainty, it appears we are coming closer to being able to directly image rocky planets around the very closest stars. Other possible stars include Epsilon Eridani, Epsilon Indi, and Tau Ceti. But even then, if we see them, because it is at the limit of technology, we will still have no evidence one way or the other relating to life. However, it is a start to look where at least the right sized planet is known to exist. My personal preference is Epsilon Eridani. The reason is, it is a rather young star, and if there are planets there, they will be roughly as old as Earth and Mars were when life started on Earth and the great river flows occurred on Mars. Infrared signals from such atmospheres would tell us what comprised the atmospheres. My prediction is reduced, with a good amount of methane, and ammonia dissolved in water. The reason is these are the gases that could be formed through the original accretion, with no requirements for a bombardment by chondrites or comets, which seemingly, based on other evidence, did not happen here. Older planets will have more oxidized atmospheres that do not give clues, apart possibly if there are signals from ozone. Ozone implies oxygen, and that suggests plants.What should we aim to detect? The overall signal should indicate the temperature if we can resolve it. Water gives a good signal in the infrared, and seeing signals of water vapour in the atmosphere would show that that key material is present. For a young planet, methane and ammonia give good signals, although resolution may be difficult and ammonia will mainly be in water. The problems are obvious: getting sufficient signal intensity, subtracting out background noise from around the planet while realizing the planet will block background, actually resolving lines, and finally, correcting for other factors such as the Doppler effect so the lines can be properly interpreted. Remember phosphine on Venus? Errors are easy to make.

Exoplanets: Do They Have Life?

One question that NASA seeks to answer is, is there life somewhere else? That raises the question, how can you tell? The simplest answer is, find something that only life can make. The problem with that is that life uses chemistry, and chemistry occurs anyway, so it is sometimes possible that what you find might be due to life, or it might be due to geophysical or geochemical action. Another problem is, some of the molecules that life makes more readily than simple geology does, say, may be difficult to find. When looking for minor traces, it is possible to find “signals” but misinterpret them. In an earlier post, I suggested the so-called signals for phosphine on Venus fell into that class, and what I have seen since reinforces that view. Many now think it was one of the signals from sulphur dioxide, which is known to be there.

One of the strongest indications we could find would be to find a number of homochiral chemicals. Thus when sugars are made chemically, say by condensing formaldehyde, they are either in the D or L configuration. Chirality can be thought of as “handedness”; your left hand is different from your right hand, and the same thing happens for chemicals used by life – life only makes one sort, the reason being that reproduction from nucleic acids only work if they can make a double helix, and that only works if there is a constant pitch, which in turn requires the linking group, the ribose, to be in one form – left or right handed. Amino acids are similar because enzymes only work in specific configurations, as do many of the other properties of proteins. The problem with chirality as evidence of life is that it is hard to measure. The usual method is to isolate the compound in a pure form in solution, pass polarised light through it, and measure the rotation of the polarization. But that really needs a chemist on the spot. Remote sensing is not really suitable. Forget that for exoplanets.

One approach has been to find a gas in the atmosphere typical of life. If you found an atmosphere with as much oxygen as Earth’s, it would almost certainly have life because oxygen cannot be accreted directly by a planet in the habitable zone. The bulk of Earth’s oxygen has probably come from photosynthesis, or the photolysis of water. The latter occurs in the absence of life, but when it does in the atmosphere, it forms ozone, which stops the reaction because water will be below the ozone. On Mars, some water has been photolysed on the surface, but  it formed peroxides or superoxides with iron oxide, or perchlorates with chlorides. So a lot of oxygen is indicative. Another gas is methane. Methane is given of by anaerobic bacteria, but it is also made geologically by reacting carbon compounds, such as the dioxide, with water and ferrous ions, which are common in the olivine-type minerals, which in turn are very common. Almost any basalt will react, in time. So methane is ambiguous.

Perhaps, we should look for more complicated molecules. There are still traps.  Recent work has shown that the chemicals that are part of the Krebs cycle, which is rather fundamental to life, actually can be made from carbon dioxide, iron, and some metal ions such as zinc. Even these are not characteristic of life, although the work may give further clues as to how life got underway, and why the chemicals used in the Krebs cycle “got involved”.

When NASA sent its Viking rovers to Mars, their approach was to treat soil samples with water and nutrients that microbes could metabolise, and then they looked to see if there were any products. One experiment detected radio-labelled gases from samples treated with carbon-14-labelled nutrients, and the idea was if the 14C got into the gas phase, where its radioactivity could be detected, it would mean life. Maybe not. If the nutrients landed on a superoxide, they would have been converted to gas. It is not easy doing this remotely.The one difference that characterises Earth when seen from space is its colour. However, the blue merely means oceans. It is possible that planets with oceans will also have what is required for life, but we could not guarantee that. If we recognised spectral signals from chlorophyll, that would be a strong indication, but whether such signals can be observed, even if there are plants there, is unclear. Again, this is not easy.

No Phosphine on Venus

Some time previouslyI wrote a blog post suggesting the excitement over the announcement that phosphine had been discovered in the atmosphere of Venus (https://ianmillerblog.wordpress.com/2020/09/23/phosphine-on-venus/) I outlined a number of reasons why I found it difficult to believe it. Well, now we find in a paper submitted to Astronomy and Astrophysics (https://arxiv.org/pdf/2010.09761.pdf) we find the conclusion that the 12th-order polynomial fit to the spectral passband utilised in the published study leads to spurious results. The authors concluded the published 267-GHz ALMA data provide no statistical evidence for phosphine in the atmosphere of Venus.

It will be interesting to see if this denial gets the same press coverage as “There’s maybe life on Venus” did. Anyway, you heard it here, and more to the point, I hope I have showed why it is important when very unexpected results come out that they are carefully examined.

Phosphine on Venus

An article was published in Nature Astronomy on 14th September, 2020, that reported the detection of a signal corresponding to the 1 – 0 rotational transition of phosphine, which has a wavelength of 1.123 mm. This was a very weak signal that had to be obtained by mathematical processing to remove artefacts such as spectral “ripple” that originate from reflections. Nevertheless, the data at the end is strongly suggestive that the line is real. Therefore they found phosphine, right? And since phosphine is made from anaerobes and emitted from marsh gas, they found life, right? Er, hold on. Let us consider this in more detail.

First, is the signal real? The analysis detected the HDO signal at 1.126 mm, which is known to be the 2 – 3 rotational transition. That strongly confirms their equipment and analysis was working properly for that species, so this additional signal is likely to be real. The levels of phosphine have been estimated as between 10 – 30 ppb. However, there is a problem because such spectral signals come from changes to the spin rate of molecules. All molecules can only spin at certain quantised energies, but there are a number of options, thus the phosphine was supposed to be from the first excited state to the ground. There are a very large number of possible states, and higher states are more common at higher temperatures. The Venusian atmosphere ranges from about 30 oC near the top to somewhere approaching 500 oC at the bottom. Also, collisions will change spin rates. Most of our data comes from our atmospheric pressure or lower pressures as doing microwave experiments in high-pressure vessels is not easy. The position of the lines depends on the moment of inertia, so different molecules have different energy levels, and there are different ways  of spinning, tumbling, etc, for complicated molecules. Thus it is possible that the signal could be due to something else. However, the authors examined all the alternatives they could think of and only phosphine remained.

This paper rejected sulphur dioxide as a possibility because in the Venusian atmosphere it gets oxidised to sulphuric acid so there  is not enough of it, but phosphine is actually far more easily oxidised. If we look at our atmosphere, there are actually a number of odd looking molecules caused by photochemistry. The Venusian atmosphere would also have photochemistry but since its atmosphere is so different from ours we cannot guess what that is at present. However, for me I think there is a good chance this signal is from a molecule generated photochemically. The reason is the signal is strongest at the equator and fades away at the poles, where the light intensity per unit area is lower. Note that if it were phosphine generated by life and was removed photochemically, you would get the opposite result.

Phosphine is a rather reactive material, and according to the Nature article models predict its lifetime at 80 km altitude as less than a thousand seconds due to photodegradation. They argue its life should be longer lower down because the UV light intensity is weaker, but they overlook chemical reactions. Amongst other things, concentrated sulphuric acid would react instantaneously with it to make a phosphonium salt, and while the phosphine is not initially destroyed, its ability to make this signal is.

Why does this suggest life? Calculations with some fairly generous lifetimes suggest a minimum of about million molecules have to be made every second on every square centimeter of the planet. There is no known chemistry that can do that. Thus life is proposed on the basis of, “What else could it be?” which is a potential logic fallacy in the making, namely concluding from ignorance. On earth anaerobes make phosphine and it comes out as “marsh gas”, where it promptly reacts with oxygen in the air. This is actually rather rare, and is almost certainly an accident caused by phosphate particles  being in the wrong place in the enzyme system. I have been around many swamps and never smelt phosphine. What anaerobes do is take oxidised material and reduce them, taking energy and some carbon and oxygen, and spit out as waste highly reduced compounds, such as methane. There is a rather low probability they will take sulphates and make hydrogen sulphide and phosphine from phosphates. The problem I have is the Venusian atmosphere is full of concentrated sulphuric acid clouds, and enzymes would not work, or last, in that environment. If the life forms were above the sulphuric acid clouds, they would also be above the phosphoric acid, so where would they get their phosphorus? Further, all life needs phosphate: it is the only functional group that has the requirement to link reproductive entities (two to link a polymer, and one to provide the ionic group to solubilize the whole and let the strands separate while reproducing), it is the basis of adenosine tripolyphosphate which is the energy transfer agent for lfe, and the adenosine phosphates are essential solubilizing agents for many enzyme cofactors, in short, no phosphate, no life. Phosphate occurs in rocks so it will be very scarce in the atmosphere, so why would it waste what little that was there to make phosphine?To summarize, I have no idea what caused this signal and I don’t think anyone else has either. I think there is a lot of chemistry associated with the Venusian atmosphere we do not understand, but I think this will be resolved sooner or later, as it has got so much attention.

The Fermi Paradox: Where are the Aliens?

This question, as much as anything, illustrates why people have trouble thinking through problems when they cannot put their own self-importance to one side. Let us look at this problem not from our point of view.

The Fermi paradox is a statement that since there are so many stars, most of which probably have planets, and a reasonable number of them have life, more than half of those are likely to have been around longer than us and so should be more technically advanced, but we have seen no clue as to their presence. Why not? That question begs the obvious counter: why should we? First, while the number of planets is huge, most of them are in other galaxies, and of those in the Milky Way, stars are very well-separated. The nearest, Alpha Centauri, is a three star system: two rather close stars (A G-type star like our sun and a K1 star) and a more distant red dwarf, and these are 4.37 light years away. The two have distances that vary between 35.6 AU to 11.2 AU, i.e. on closest approach they come a little further apart than Saturn and the sun.  That close approach means that planets corresponding to our giants could not exist in stable orbits, and astronomers are fairly confident there are no giants closer to the star. Proxima Centauri has one planet in the habitable zone, but for those familiar with my ebook “Planetary Formation and Biogenesis” will know that in my opinion, the prospect for life originating there, or around most Red Dwarfs, is extremely low. So, could there be Earth-like planets around the two larger stars? Maybe, but our technology cannot find them. As it happens, if there were aliens there, they could not detect Earth with technology at our level either.  Since most stars are immensely further away, rocky planets are difficult to discover. We have found exoplanets, but they are generally giants, planets around M stars, or planets that inadvertently have their orbital planes aligned so we can see eclipses.

This is relevant, because if we are seeking a signal from another civilization, as Seti seeks, then either the signal is deliberate or accidental. An example of accidental is the electromagnetic radiation we send into space through radio and TV signals. According to tvtechnology.com “An average large transmitter transmits about 8kW per multiplex.” That will give “acceptable signal strength” over, say, 50 km. The signal strength attenuates according to the square of the distance, so while the signals will get to Alpha Centauri, they will be extremely weak, and because of bandwidth issues, broadcasts from well separated transmitters will interfere with each other. Weak signals can be amplified, but aliens at Alpha Centauri would get extremely faint noise that might be assignable to technology. 

Suppose you want to send a deliberate signal? Now, you want to boost the power, and the easiest way to get over the inverse square attenuation is to focus the signal. Now, however, you need to know exactly where the intended recipient will be. You might do this for one of your space ships, in which case you would send a slightly broader signal on a very high power level at an agreed frequency but as a short burst. To accidentally detect this, because you have a huge range of frequencies to monitor, you have to accidentally be on that frequency at the time of the burst. There is some chance of Seti detecting such a signal if the space ship was heading to Earth, but then why listen for such a signal, as opposed to waiting for the ship.

The next possible deliberate signal would be aimed at us. To do that, they would need to know we had potential, but let us suppose they did. Suppose it takes something like 4.5 billion years to get technological life, and at that nice round number, they peppered Earth with signals. Oops! We are still in the Cretaceous. Such a move would require a huge power output so as to flood whatever we were using, a guess as to what frequencies we would find of interest, and big costs. Why would they do that, when it may take hundreds or thousands of years for a response? It makes little sense for any “person” to go to all that trouble and know they could never know whether it worked or not. We take the cheap option of listening with telescopes, but if everyone is listening, nobody is sending.

How do they choose a planet? My “Planetary Formation and Biogenesis” concludes you need a rocky planet with major felsic deposits, which is most probable around the G type star (but still much less than 50% of them). So you would need some composition data, and in principle you can get that from spectroscopy (but with much better technology than we have). What could you possibly see? Oxygen is obvious, except it gives poor signals. In the infrared spectra, you might detect ozone, and that would be definitive. You often see statements that methane should be detectable. Yes, but Titan has methane and no life. Very low levels of carbon dioxide is a strong indication, as it suggests large amounts of water to fix it, and plate tectonics to renew it. Obviously, signals from chlorophyll would be proof, but they are not exactly strong. So if they are at anything but the very closest stars they would not know whether we are here, so why waste that expense. The Government accountants would never fund such a project with such a low probability of getting a return on investment. Finally, suppose you decided a planet might have technology, why would you send a signal? As Hawking remarked, an alien species might decide this would be a good planet to eradicate all life and transform it suitable for the aliens to settle. You say that is unlikely, but with all those planets, it only needs one such race. So simple game theory suggests “Don’t do it!” If we assume they are more intelligent than us, they won’t transmit because there is no benefit for those transmitting.

Where to Find Life? Not Europa

Now that we have found so many exoplanets, we might start to wonder whether they have life. It so happens I am going to give a presentation on this at a conference in about three weeks time, hence the temptation to focus attention on the topic. My argument is that whether a place could support life is irrelevant; the question is, could it get started? For the present, I am not considering panspermia, i.e. it came from somewhere else on the grounds that if it did, the necessities to reproduce still had to be present and if they were, life would probably evolve anyway. 

I consider the ability to reproduce to be critical because, from the chemistry point of view, it is the hardest to get right. One critical problem is reproduction itself is not enough; it is no use using all resources to make something that reproduces a brown sludge. It has to guess right, and the only way to do that is to make lots of guesses. The only way to do that is to tear to bits that which is a wrong guess and try again and re-use the bits. But then, when you get something useful that might eventually work, you have to keep the good bits. So reproduction and evolution have opposite requirements, but they have to go through the same entity. Reproduction requires the faithful transmission of information; evolution requires the information to change on transmission, but eventually not by much. Keep what is necessary, reject that which is bad. But how?

Information transfer requires a choice of entities to be attached to some polymer, and which can form specific links with either the same entity only (positive reproduction) or through a specific complementary entity (to make a negative copy). To be specific they have to have a strongly preferred attachment, but to separate them later, the attachment has to be able to be converted to near zero energy. This can be done with hydrogen bonds, because solvent water can make up the energy during separation. One hydrogen bond is insufficient; there are too many other things that could get in the road. Adenine forms two hydrogen bonds with uracil, guanine three with cytosine, and most importantly, guanine and uracil both have N-H bonds while adenine and cytosine have none; the wrong pairing either leads to a steric clash that pushes them apart or ends up with only one hydrogen bond that is not strong enough. Accordingly we have the condition for reliable information transfer. Further good news is these bases form themselves from ammonium cyanide, urea and cyanoacetylene, all of which are expected on an earth-like planet from my concept of planetary formation.

The next problem is to form two polymer strands that can separate in water. First, to link them something must have two links. For evolution to work, these have to be strong, but breakable under the right conditions. To separate, they need to have a solubilizing agent, which means an ionic bond. In turn, this means three functional valence electrons. Phosphate alone can do this. The next task is to link the phosphate to the bases that carries the information code. That something must also determine a fixed shape for the strands, and for this nature chose ribose. If we link adenine, ribose and phosphate at the 5 position of ribose we get adenosine monophosphate (AMP); if we do he same for uracil we get uridine monophosphate (UMP). If we put dilute solutions of AMP and UMP into vesicles (made by a long chain hydrocarbon-based surfactant) and let them lie on a flat rock in the sun and splash them from time to time with water, we end with what is effectively random “RNA” strands with over eighty units in a few hours. At this point, useful information is unlikely, but we are on the way.

Why ribose? Because the only laboratory synthesis of AMP from only the three constituents involves shining ultraviolet light on the mixture, and to me, this shows why ribose was chosen, even though ribose is one of the least likely sugars to be formed. As I see it, the reason is we have to form a phosphate ester specifically on the 5-hydroxyl. That means there has to be something unique about the 5-hydroxyl of ribose compared with all other sugar hydroxyl groups. To form such an ester, a hydroxyl has to hit the phosphate with an energy equivalent to the vibrations it would have at about 200 degrees C. Also, if any water is around at that temperature, it would immediately destroy the ester, so black smokers are out. The point about a furanose is it is a flexible molecule and when it receives energy (indirectly) from the UV light it will vibrate vigorously, and UV light has energy to spare for this task. Those vibrations will, from geometry, focus on the 5-hydroxyl. Ribose is the only sugar that has a reasonable amount of furanose; the rest are all in the rigid pyranose form. Now, an interesting point about ribose is that while it is usually only present in microscopic amounts in a non-specific sugar synthesis, it is much more common if the sugar synthesis occurs in the presence of soluble silica/silicic acid. That suggests life actually started at geothermal vents.

Now, back to evolution. RNA has a rather unique property amongst polymers in that the strands, when they get to a certain length and can be bent into a certain configuration and presumably held there with magnesium ions, they can catalyse the hydrolysis of other strands. It does that seemingly by first attacking the O2 of ribose, which breaks the polymer by hydrolysing the adjacent phosphate ester. The next interesting point is that if the RNA can form a double helix, the O2 is more protected. DNA is, of course, much better protected because it has no O2. So the RNA can build itself, and it can reorganise itself.

If the above is correct, then it places strong restrictions on where life can form. There will be no life in under-ice oceans on Europa (if they exist) for several reasons. First, Europa seemingly has no (or extremely small amounts of) nitrogen or carbon. In the very thin atmosphere of Europa (lower pressures than most vacuum pumps can get on Earth) the major gas is the hydroxyl radical, which is made by sunlight acting on ice. It is extremely reactive, which is why there is not much of it. There is 100,000 times less sodium it the atmosphere. Nitrogen was undetected. The next reason is the formation of the nucleic acid appears to require sunlight, and the ice will stop that. The next reason is that there is no geothermal activity that will make the surfactants, and no agitation to convert them to the vesicles needed to contain the condensation products, the ice effectively preventing that. There is no sign of hydrocarbon residues on the surface. Next, phosphates are essentially insoluble in water and would sink to the bottom of an ocean. (The phosphate for life in oceans on Earth tends to come from water washed down from erosion.) Finally, there is no obvious way to make ribose if there is no silicic acid to orient the formation of the sugar.

All of which suggests that life essentially requires an earth-like planet. To get the silicic acid you need geothermal activity, and that may mean you need felsic continents. Can you get silica deposits from volcanism/geothermal activity when the land is solely basalt? I don’t know, but if you cannot, this proposed mechanism makes it somewhat unlikely there was ever life on Mars because there would be no way to form nucleic acids.

Space News

There were two pieces of news relating to space recently. Thirty years ago we knew there were stars. Now we know there are exoplanets and over 4,000 of them have been found. Many of these are much larger than Jupiter, but that may be because the bigger they are, the easier it is to find them. There are a number of planets very close to small stars for the same reason. Around one giant planet there are claims for an exomoon, that is a satellite of a giant planet, and since the moon is about the size of Neptune, i.e.the Moon is a small giant in its own right, it too might have its satellite: an exomoonmoon. However, one piece of news is going to the other extreme: we are to be visited by an exocomet. Comet Borisov will pass by within 2 A.U. of Earth in December. It is travelling well over the escape velocity of the sun, so if you miss it in December, you miss it. This is of some interest to me because in my ebook “Planetary Formation and Biogenesis” I outlined the major means I believe were involved in the formation of our solar system, but also listed some that did not leave clear evidence in our system. One was exo-seeding, where something come in from space. As this comet will be the second “visitor” we have recorded recently, perhaps they are more common than I suspected.

What will we see? So far it is not clear because it is still too far away but it appears to be developing a coma. 2 A.U. is still not particularly close (twice the distance from the sun), so it may be difficult to see anyway, at least without a telescope. Since it is its first visit, we have no real idea how active it will be. It may be that comets become better for viewing after they have had a couple of closer encounters because from our space probes to comets in recent times it appears that most of the gas and dust that forms the tail comes from below the surface, through the equivalent of fumaroles. This comet may not have had time to form these. On the other hand, there may be a lot of relatively active material quite loosely bound to the surface. We shall have to wait and see.

The second piece of news was the discovery of water vapour in the atmosphere of K2-18b, a super-Earth that is orbiting an M3 class red dwarf that is a little under half the size of our sun. The planet is about eight times the mass of earth, and has about 2.7 times the radius. There is much speculation about whether this could mean life. If it has, with the additional gravity, it is unlikely that, if it did develop technology, it would be that interested in space exploration. So far, we know there is probably another planet in the system, but that is a star-burner. K2-18b orbits its star in 33 days, so birthdays would come round frequently, and it would receive about five per cent more solar radiation than Earth does, although coming from a red dwarf, there will be a higher fraction of infra-red light and less visible.

The determination of the water could be made because first, the star is reasonably bright so good signals can be received, second, the planet transits across the star, and third, the planet is not shrouded with clouds. What has to happen is that as the planet transits, electromagnetic radiation from the star is absorbed by any molecule at the frequency determined by the bond stretching or bending energies. The size of the planet compared with its mass is suggestive of a large atmosphere, i.e.it has probably retained some of the hydrogen and helium of the accretion disk. This conclusion does have risks because if it were primarily a water or ice world (water under sufficient pressure forms ice stable at quite high temperatures) then it would be expected to have an even greater size for the mass.

The signal was not strong, in part, from what I can make out, it was recorded in the overtone region of the water stretching frequency, which is of low intensity. Accordingly, it was not possible to look for other gases, but the hope is, when the James Webb telescope becomes available and we can look for signals in the primary thermal infrared spectrum, this planet will be a good candidate.So, what does this mean for the possibilities of life? At this stage, it is too early to tell. The mechanism for forming life as outlined in my ebook, “Planetary Formation and Biogenesis” suggests that the chances of forming life do not depend on planetary size, as long as there is sufficient size to maintain conditions suitable for life, such as an adequate atmospheric pressure, liquid water, and the right components, and it is expected that there will be an upper size, but we do not know what that will be, except again, water must be liquid at temperatures similar to ours. That would eliminate giants. However, more precise limits are more a matter of guess-work. The composition of the planet may be more important. It must be able to support fumaroles and I suspect it should have pre-separated felsic material so that it can rapidly form continents, with silica-rich water emitted, i.e.the type of water that forms silica terraces. That is because the silica acts as a template to make ribose. Ribose is important for biogenesis because something has to link the nucleobases to the phosphate chain. The nucleobases are required because they alone are the materials that form with the chemicals likely to be around, and they alone form multiple hydrogen bonds that can form selectively and add as a template for copying, which is necessary for retaining useful information. Phosphate is important because it alone has three functional sites – two to form a polymer, and one to convey solubility. Only the furanose form of the sugar seems to manage the linkage, at least under conditions likely to have been around at the time and ribose is the only sugar with significant amounts of the furanose form. I believe the absence of ribose means the absence of reproduction, which means the absence of life. But whether these necessary components are there is more difficult to answer.