The Solar System: Why Earth?

HEADS UP!  My blog is moving to Wordpress from now on, at thegreatatuin.wordpress.com, because Blogger is proving to be VERY difficult to work with especially when dealing with my inevitable images and graphs.  To be determined if I continue in both places or switch over there entirely.  I will be putting up notification of this on the home page as well.

So after another entirely too long hiatus, I'm back.  This time even more intense work on my PhD ate my time, along with a number of personal things.  In whatever case, I didn't exactly have much time to write for myself over the last few months.  I'm almost glad though - the last year or so has seen a positive EXPLOSION of amazing origin of life research and I can't wait to pick through it on the record.

But for now before jumping into origin of life research I think I have to instead talk about the Earth itself for a while.  So far I've talked about the position of Earth's biosphere in the extremely large context of the history of the universe, our position in time relative to the formation of stars and planets and our position in space relative to different galaxies.  This has been informative, but we are necessarily working with very little information and coarse scales here.  By comparison within our own solar system there a massive wealth of information that we have nonetheless only barely started to look at.  By looking at the history of Earth and comparing it to other objects in our own solar system, I think some very important principles driving the appearance of biospheres like ours become clear and some very important questions we do not know the answers to fall out.

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Earth is a very special place.  It is a world utterly out of chemical equilibrium.

That is not to say that other worlds in our solar system are in equilibrium - equilibrium is stasis and death, the lack of any energy flow from concentrated sources to diffuse sinks.  The atmospheres of Venus and the gas giants act as vast heat engines, transferring the energy of their daylit sides to the nightside and the poles creating winds and clouds in the process.  Geology driven by radioactive decay and primordial heat of formation deep inside solid worlds turns over rock through volcanoes and drives chemical cycling of elements into the atmosphere and back into rock.  But Earth is a world apart in its disequilibrium.  Our atmosphere and upper geosphere are charged up with energy, all this oxygen mixed with methane just itching to oxidize on decadal timescales.  The ground itself is full of geologically dredged-up iron and other substances constantly sucking oxygen out of the air on rather longer timescales.  And that is to say nothing of all the highly flammable carbon, biomass, ripped out of CO2 and reduced from that highly oxidized form to all kinds of substances and then left laying around or buried.  Anyone with a good enough spectrometer, looking at the Earth across the solar system or even from another star if their instruments were good enough could tell something strange was going on here.  A very dynamic, active process transducing lots of energy and utterly remaking the atmosphere and geochemistry for billions of years, creating an extraordinarily reactive atmosphere and oxidizing the crust to great depth.

It's this complete transformation of Earth and only Earth that I think can be explained much more simply than most people think.

Proponents of the 'rare-Earth hypothesis' like to point to everything unique about Earth in our solar system and suggest that unless every single aspect of our world were recapitulated elsewhere, you wouldn't get a big happy biosphere like ours.  I'm not a big fan of this approach.  Every body in the solar system is the product of a unique history and every body in the universe will be unique if you look close enough.  If you look at enough properties, eventually you will convince yourself that the particular combination is unlikely to be found elsewhere and you will probably be right.  Be it the large moon birthed in a collision that may have temporarily vaporized a significant fraction of the proto-Earth's mantle, plate tectonics smoothing the carbon cycle and delivering fresh nutrients to the surface constantly, or its particular amount of water, Earth has many unique properties.  But with exactly one known biosphere to go on, you run a very real risk of overfitting your models to that one data point.  We really cannot say with any certainty which of these properties might be necessary to create a biosphere in another solar system, or how common such sets of properties could be.

I think we can be more confident when we compare Earth to other objects in our solar system where we have more information.  The conditions in this cluster of worlds are diverse, but there is exactly one known biosphere orbiting the Sun.  Emphasis on 'known' - I will argue in a moment that our knowledge in this area is absurdly incomplete and there could be half a dozen biospheres in our own solar system that we would never have noticed with the science done so far.  Still, ultimately, I would say Earth is unique in exactly one way that matters for any biology and has driven its evolution for at least 3.5 gigayears:

Earth is the only place in the solar system where solvents and clement temperatures meet diverse small-molecule feedstocks and minerals in the presence of large amounts of sunlight.

There's several things going on here.  The first few are what most people think of when they talk about the presence of life on a planet: an environment in which something we would call 'life' is chemically possible.  The question of what exactly 'life' is is a complicated one and one I'll address when talking about its origin at some point, but for now I'm calling it complicated organic chemistry that can carry heredity and dissipate energy in its environment to do its business.

To allow the kinds of organic chemistry needed by life as we know it, you need a few things in the environment.  Firstly, temperature and solvent - it needs to be warm enough for reactions to proceed with some speed and for solvents to be liquid at least some of the time, but not so hot that large complex molecules you need for catalysis and heredity break down into their constituent parts.  Life as we know it seems to have a hard limit of ~120 Celsius on the high side, and rather below freezing in the presence of specialized antifreeze chemicals on the low side even if living things can lay dormant doing nothing when they're much colder.  Secondly, small-molecule feedstocks.  All life on Earth ultimately builds its biomolecules from simple, inorganic molecules - H2O, CO2, and N2 for the vast majority of biomass on Earth.  Anything that eats organic molecules ultimately, if you follow the food webs, will get them from something fixing carbon into biomass from CO2, and very nearly all nitrogen in protein ultimately derives from the pool of N2 in the same air.  Third, minerals.  No living thing on Earth can do without mineral nutrients.  Our genetic material itself contains huge amounts of phosphate, an inorganic ion leached from rocks, and almost every functional energy metabolism on the planet depends completely on electrons hopping along beautifully orchestrated chains of iron, sulfur, copper, and molybdenum ions clutched by proteins that are really just acting as scaffolds for them rather than enzymes.  Even if we didn't need all that calcium in our bones, minerals are needed as catalysts and components of basic biomolecules.

Last comes what I think is far too often overlooked: energy.  All this complex chemistry doesn't just happen.  A reaction that is thermodynamically favorable - releasing energy in the form of heat or introducing entropy by moving something from high concentration to low - will happen spontaneously, but before too long everything that can happen has happened and everything stops.  To drive complex chemistry and the production of biomolecules, there has to be energy flux into the system, a disequilibrium to be tapped.  And this is where Earth shines.  Other places in our solar system have liquid solvents, small molecule feedstocks, minerals, or all three.  But there is nowhere else in the solar system that has all those material requirements, PLUS the flux of over a kilowatt of energy per square meter the sun pours down onto the same surface.

The surface of Mars and the cloudtops of venus get plenty of sunlight, but Mars is nearly vacuum-dessicated and UV-radiation-blasted while Venus's clouds are lacking in anything resembling a mineral surface within 70 kilometers even though sulfuric acid might be a workable biological solvent for something that evolved in it.  The icy moons of the outer solar system have plenty of liquid water underground at clement temperatures full of dissolved minerals and feedstocks, but these clement environments are all under up to kilometers of ice.  Even on Titan, where you might imagine some kind of very low-temperature reactions happening in hydrocarbon solvents, the sunlight at the top of the clouds is something like 1% as bright as here and under the cloud deck it's more like 0.1%.

But on Earth, you have solvent falling from a sky made out of every small molecule feedstock you could want onto mineral surfaces bathed in a kilowatt per square meter of energy.  And photosynthesis capturing this vast energy flux is ultimately what makes the Earth's biosphere what it is, splitting water into oxygen and building biomass by combining the resultant hydrogen with CO2 and N2.  It is what has transformed the planet so completely you could tell it was alive from light-years away if you could get a quick glimpse of its atmosphere.

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I promised when I started this blog to talk about actual observables, what we can know and what we can't with the information actually at hand.  In that spirit, after talking about how Earth is so special when it comes to biology in our solar system I need to insert a vital caveat.

There could be half a dozen active biospheres in this solar system right now and we would never know it with the information at hand.

A distinction has to be made between the presence of life, and the presence of a huge high-energy high-biomass biosphere like that of the Earth that chemically transforms a world.  There are plenty of places on Earth that have every attribute I talked about above except sunlight.  Fifty years ago you might have assumed that without sunlight to drive the primary producers of biomass there would be no life there - and as we have seen recently, you would have been wrong.

Sunlight is a vast, concentrated energy source as biological energy sources go: 170,000 terawatts hitting the Earth at all times is NOT shabby, and even if photosynthesis often only captures a small fraction of that it drives the vast majority of the metabolism on Earth.  But in recent decades, the discovery of organisms creating biomass from CO2 using energy not derived from the sun has shown that photosynthesis is not the only biological energy source on Earth.  Microbes can be found kilometers under the surface of the continents and the ocean, and a large fraction of the biomass of the planet may be living at very low cell densities and metabolic rates kilometers underground where they have no interaction with sunlight or energy derived from it at all.  To quote Jan Amend, a geochemist at the University of Southern California, "We keep digging and digging and digging deeper and have not hit the bottom of the biosphere."

Photosynthesis and related activities (i.e. us) on the surface of the Earth definitely represents the majority of the metabolic activity of the planet.  But it is NOT all there is, and perhaps more importantly, given how complicated a process photosynthesis in all its forms is it can not have been how the first life on Earth got its energy.  Something else has to have been the energy source for the earliest life.

Chemolithotrophs, organisms that fix CO2 into biomass like plants but using energy from small chemical disequilibria within rock to do it, have been found kilometers deep under the continents.  Methanogens and acetogens, organisms that take H2 and oxidize it with CO2 into methane and acetate for their energy (and may have a key role to play in the origin of life on Earth - more on that in a future post) are found anywhere biological processes create H2 but also deep on the bottom of the ocean, living off  geological processes that make H2.  The total wattage available to these organisms is constrained.  I am unsure how to quantify it precisely, but given sheer thermodynamics it probably has to be comfortably less than the estimated geological heat flux of the Earth (about 47 terawatts, ~0.03% of what the planet gets from the Sun) plus a little extra for chemical reactions that occur at the mouths of certain types of hydrothermal vents, but the discoveries in this new sector of the biosphere just keep coming.

Pictured above is Desulforudis audaxviator, image taken from the Microbe Wiki (https://microbewiki.kenyon.edu/index.php/Desulforudis_audaxviator).  This bacterium was discovered kilometers underground in an anoxic aquifer at a pH of 9.3 and a temperature of 60 C.  It dies from the tiniest whiff of oxygen, and carries out all the reactions necessary to take rock and N2 and CO2 and turn it into biomass, functions that can be spread out across a dozen species in a surface ecosystem.  It is believed to live off the energy of radioactive decay in the rocks around it that split water and create the tiniest of chemical disequlibria for it to insert itself into as a middleman in their dissipation.

This planet has been crawling with scientists for a few hundred years, and swarming with pretty-damn-smart people for two hundred thousand years before that.  D. audaxviator was discovered in 2005.  These non-solar-driven living things are very easy to miss.  They can be detected with the latest molecular biology techniques because we know exactly what we are looking for, chemicals that are shared by all terrestrial living things that we have learned to detect at the single-molecule level, and because of recent advances in telling apart geochemistry from biochemistry which is not always as straightforward as it appears.

What have we done on other worlds in our own solar system compared to the massive, systematic exploration that was needed to find these things on  Earth?  It's very hard to do science somewhere by dropping a few kilograms of automated scientific instruments from the sky every few years that cost well over their weight in gold and have severe bandwidth constraints.  Big things are being missed all the time.  It took until 2008 to figure out a basic, extremely important fact about the Martian soil - the fact that it contains up to 0.5% extremely reactive (and toxic) perchlorate created by the interaction of radiation and ancient salt deposits.  Every result from every instrument that probed and sampled the Martian soil from the Viking landers in the 1970s onwards had to be reinterpreted in the light of this finding and it was only conclusively discovered in the first place because one of the Phoenix lander's instruments basically malfunctioned in the presence of the unexpected substance.  The geysers of Enceladus, discovered in 2004 when the Cassini probe reached Saturn, spew the ocean of that tiny geologically active world into space where it can be sampled.  The instruments on Cassini are not sensitive enough to resolve the composition in all but the most general of terms - I have been to a talk in which one of the head scientists of the mission shrugged while pointing at a graph from the mass spec and said "there are molecules with at least three carbons in there somewhere."

Would we ever detect the presence of D. audaxviator deep underground at Mars, or microbes living in hydrothermal vents within Europa or Enceladus from this kind of data?  At this point, I really don't think so. By my count, there are at least 6 places in our solar system with everything necessary for a low-biomass biosphere and the origin of life (again more on this later) that we simply couldn't detect with the data at hand - underground at Mars, the oceans of Europa, the oceans of Ganymede, the deep underground water oceans of Titan, the surface hydrocarbon lakes of Titan, and the oceans of Enceledus.  Count me in  halfway for more speculative ideas about the cloudtops of Venus (where the limiting factor would be availability of minerals and not getting plunged down into the sterilizing depths of the atmosphere) and potential oceans within other icy moons and Kuiper belt objects.  We don't know if there are biospheres in these places, and with current data we can't know.  The answer is just insufficiently constrained.

We have only just gotten to the good part of solar system exploration.  It wasn't that many decades ago that the planets were only small discs through telescopes to us with rudimentary maps if any.  Now they are worlds, and we finally know enough about them and about Earth itself to actually start asking the really interesting questions and to start looking for things that are hard to find.  The presence or the absence of another biosphere in our solar system would be extremely informative.  A second origin of life in our solar system would suggest that biospheres are extremely common, even if you need special circumstances for it to explode in scale and remake a world. Only one origin in this system would be a little data in favor of the origin of life being tougher, but we need a LOT of information before we would be able to say that.  Proving a negative is difficult to say the least, and proving that there was never a biosphere elsewhere in our solar system that died out later is even harder.

Apparently every time I write for this blog, I wind up producing huge essays.  Up next, I want to talk about the history of life on Earth and its main developments over time and what they may (or may not) tell us about the types of biospheres that could exist elsewhere.

Space and Time - Revisited

Well that was a long hiatus. Between pushing out a paper, TA-ing two classes, and getting distracted with fiction writing, this blog went way on the back burner for a couple of months.

I was frankly surprised and flabbergasted by the degree of interest my first post got. I just play an astrobiology type on the internet, I'm really just a molecular biologist with a hobby. I got a number of interesting pointers to recent research, a lot of interesting questions, and spoke to a number of very interesting people actually working in the field of astrobiology! As a result, I'm putting the next topic (our own solar system and its history) on hold for a little bit and going a little more in depth into our solar system's cosmological position in space and time. The more I look into this, the more clear it becomes to me that we are utterly typical in terms of where and when you'd expect a complex biosphere to form – on a coarse scale, at least. Some interesting questions still arise about the sort of star we find ourselves around.

Anyway, in my last post I mentioned a recent study (Sobral 2012, http://arxiv.org/abs/1202.3436) which looked at star formation rates across cosmological time using proxy measurements, finding a precipitous decline in star formation rates for many gigayears. I can do better than this – I think I can put some rough numbers on our star, planet, and biosphere's relative position in time and how typical it is drawing from multiple sources.

Numerous publications for decades at this point have performed similar measurements of star formation rate over time, and all those with sufficient detail reveal one, simple, important fact: the rate of star formation per unit mass in the universe as a whole is decaying exponentially, and has been for most of the history of the universe. I could point to a number of publications for this, but for the purposes of this post I will use Yuskel, 2008: http://arxiv.org/abs/0804.4008. This group collated a number of datasets and fit an equation to the star formation rate going all the way back to when the universe was less than a billion years old. I've reproduced a figure from Horuchi, 2010 (http://arxiv.org/abs/1006.5751) which while not actually being ABOUT star formation rates takes the fit from Yuskel and compares it to multiple data sets, supporting its validity:

There's a couple of things going on here. Firstly, note that the star formation rate is given in terms of something called Z – the redshift. Astronomers love talking about their observations in terms of redshift because it is directly observable and related to both distance and age of what you are looking at. You look and you see something that should be radiating at wavelength X instead radiating at wavelength X*(Z+1). That observation is generally incontestable. But to turn that observation into a distance or time you need to start messing around with cosmological models which, although they have been converging to a pretty tight focus in recent years are still open to refinement and change. It's generally better to keep your data in the raw observational form as a result.

Secondly, note the FORM of the equation of best fit. Since around Z = 1, corresponding to more than eight billion years ago, the star formation rate has been cratering. As the universe has expanded over that timeframe, star formation rate has very closely tracked [size of the universe]^-3.4^th. The plateau of steady high star formation rates lasted only about 3 or 4 gigayears, long before our Sun formed. This represents huge bursts of star formation from young galaxies and giant ellipticals that formed and burned out young.

If I want to get an idea of the total number of stars that have been born and will ever be born, I need to turn this function of star formation rate at a given Z (expressed in solar masses formed per comoving cubic megaparsec per year so as to normalize for the expansion of the universe) into a function of star formation rate at a given absolute time, project it forward with reasonable assumptions, and integrate. Here is a graph of Z with respect to T from the best modern cosmological models:

  Redshift changes with time very differently when the universe is matter-dominated (blue line), has a roughly even amount of matter and dark energy (red line), and when it is dark-energy-dominated (green line). Whatever the case, I can use the relationship between redshift and time to turn the graph of star formation rate across redshift into star formation rate over time for the history of the universe so far:

The red dot represents the formation of the Sun, and the end of the blue line the present day at t = 13.82 gigayears. As you can see, the vast vast majority of star formation occurred before our Sun was born.

How to project this plot forward into the future in order to determine how many stars will ever live? I have made two executive decisions. Firstly, I have decided to simply project the exponential decay that has been occurring since Z=1 forward into the future, for reasons that I will get to later. Secondly, I have decided to break with a slavish devotion to the redshift numbers of the far future, once the universe has turned completely dark-energy dominated. Instead, I will keep the same function of Z with T and therefore the same function of star formation over time that has held since approximately Z=1, because I fail to see how simply increasing the distance between galaxy clusters more and more rapidly as will happen once the universe is dark energy dominated will change star formation rates within the clusters. It doesn't make a huge difference, going one way or the other changes the final numbers by less than five percent.

Anyways, after much obsessive faffing around with these equations in matlab to deal with the above set of assumptions, I projected star formation rates forward into the future to get the following graph. The red dot continues to represent the formation of the Sun, and the green dot the present day:

The decline is striking.

By integrating this curve forward, we can get an idea of where the Sun lays in the final star-order of the universe. I have normalized the graph to the (finite) number of stars that will ever exist when you integrate out to infinity:

In short, we find that the Sun was born when ~79% of stars that will ever exist already existed, and at the present moment ~90% of all stars that will ever exist already exist. Thus, the sun is a relative but by no means extreme latecomer to the universe, and despite existing near the beginning of an apparently open-ended universe its time of formation is not terribly special.

The universe is full to bursting with hydrogen and helium, only the tiniest fraction having been converted into heavy elements by being consumed in stars. Naively one might assume that all this gas would eventually condense down into stars one day. Recent results in astrophysics are suggesting reasons, however, that this probably won't happen – reasons for my continued use of the exponential decay of the universal star formation rate in my above analysis, as processes happening today continue. In my last post, I mentioned the life histories of various types of galaxies and how they suggested that star formation might be closer to finishing than starting. I feel compelled to go into a little bit more detail here, speaking more in terms of what happens to all the gas that could form stars but doesn't using recent astrophysical results.

When you look out into the universe, the vast vast majority of elliptical type galaxies are very red due to the age of their stars and are not forming stars, whether they have an internal reservoir of gas within their dark matter halo or not – see http://www.dailygalaxy.com/my_weblog/2014/02/giant-elliptical-galaxies-why-are-they-red-and-dead.html for a discussion on this. Big spirals are mostly forming stars at a steady clip, with only a few tapering down and turning 'green' or eventually 'red' from their initial 'blue' status. Recently, a project called GalaxyZoo which has automated and crowdsourced the analysis of huge numbers of new galaxies observed in the Sloan Digital Sky Survey has taken a very quantitative look at star formation across galaxy types in the universe, and come up with some striking conclusions:

http://blog.galaxyzoo.org/2014/02/21/the-green-valley-is-a-red-herring/

http://arxiv.org/abs/1402.4814

http://arxiv.org/abs/1501.05955

These studies were able to get more information than the instantaneous rate of star formation, and look back along the history of the galaxies by looking at light of different frequencies – huge stars that don’t live long make lots of ultraviolet, stars like our Sun peak in the green light, while long lived stars peak in the red. They were able to see that among elliptical galaxies, the tiny fraction that are star-forming mostly show evidence of recently being involved in mergers, and that all those that are red and green colored show spectral patterns indicative of very rapid shutdown of star formation, faster than can be accounted for by star formation eating up available gas. They call this fast star-formation shutdown 'quenching'. Something about their formation, either primordially or via mergers of spirals, puts their gas into forms that cannot form stars. The prime suspect is the initiation of regular energetic outbursts from their large central black holes, heating the gas and rendering it too turbulent.

This actually dovetails interestingly with another problem in astrophysics: the 'cooling paradox'. As I mentioned, about 90% of the baryonic mass of the universe is in the form of X-ray hot gas clouds blanketing entire galaxy clusters (largely outside the dark matter halos of individual galaxies). This gas is ridiculously thin and immensely hot, and radiating energy rapidly in the X-rays. It turns out that when you figure out how much mass is in these gas clouds and how much energy they are radiating in the X-rays, they should cool and sink down to the centers of the clusters on a timescale of gigayears, probably turning into cool gas flows onto the large galaxies at the centers of these clusters. But they don't. Looking back in time across the universe they are at more or less the same temperature now as they always have been and never seem to cool despite the fact that they are radiating energy. In recent years, for various reasons (images of turbulence in the gas, calculations of the available energy) the prime suspect for the energy source keeping these gas clouds energized has become supermassive black hole jets.

Anyways, as for spiral galaxies, they were able to model the distribution they saw (most of which are forming stars at a steady rate, some of which are tapering off, and some of which are red and dead) as a mixture of populations. One population is forming stars at a steady slowly decreasing rate, much like ours. Another is quenching on a much slower timescale than ellipticals, indicative of a cut-off of gas inflow into their star-forming discs and star formation then slowly depleting their reservoir of gas over a 1-2 gigayear timescale, likely caused by events in their immediate galactic neighborhood disrupting the inflows of cool gas within their dark matter halos onto their star forming discs.

All of this suggests to me that the decline in star formation rate seen across the cosmological past represents larger and larger fractions of galaxies quenching along with slow decreases in star formation rates within individual galaxies, and that the numbers I produced above have at least some semblance of validity.  I can't, of course, rule out the possibility that galaxy quenching is a temporary or cyclical phenomenon  on very long timescales, or that there's some special subset of galaxies that will never quench and will use up all their gas.  But these numbers are a good start.  

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These numbers are, however, numbers about STARS and the Sun's position in star-order.  And as vital as stars are for life, we don't live on a star.  We live on a terrestrial planet.  And this makes a difference.

The huge bolus of star formation early in the universe consisted of many low-metallicity stars at the starts of spiral galaxies and the fast star formation bursts of elliptical galaxies.  Many of these stars are probably not suitable for the creation of terrestrial planets and thus biospheres-as-we-know-them.  In order to get a handle on Earth's position in planet-order we need to normalize this.  I am utterly unprepared to do this rigorously since the astronomical community as a whole hasn't got a good handle on planet formation - if there's one incontrovertible takeaway from the Kepler mission, this is it.  I can, however, pull up some numbers  that are better than nothing and layer them on top of my star formation numbers and see what comes out.

I will be taking data from a paper (Behroozi & Peeples, 2015: http://arxiv.org/pdf/1508.01202v1.pdf) that made the rounds last year suggesting that Earth came in the 8th percentile of terrestrial planets (that is, 92% come after us).  Their conclusion is suspect because it comes from the assumption that ALL gas within galactic dark matter halos will eventually form stars.  However, the authors include a nice set of metallicity normalizations that they apply to the early universe that I will take in its entirety, as it is far too complicated for someone like me who doesn't study cosmology professionally to critique.

In this work, all stars are assumed to form terrestrial planets with a power-law dependence on metallicity and a sharp metallicity cutoff for 'gas planet' formation is assumed.  I will use both these functions, on the chance that either of them is relevant - again, nobody really understands planet formation.  I find myself suspecting that the latter is more relevant due to some talks I've seen on solar system structure and planet formation modeling, but I really don't know.

This work finds that using a power law metallicity dependence, the Eartth is younger than ~83% of currently existing planets, and that with a sharp metallicity cutoff our planetary system is younger than ~64% of currently existing systems (both of these numbers shamelessly scraped from graphs).  Now I apply an almost certainly oversimplified and wrong assumption:  that the fraction of star systems that have been planet-forming has been constant since the formation of the Sun, on the theory that the pollution of heavy elements is more or less complete.

Applying these numbers, I estimate the following positions of the Earth and by extension our biosphere in planet-order:

Under the power-law metallicity assumption, Earth shows up as younger than 72% of planetary systems that will exist.

Under a sharp metallicity-cutoff assumption, Earth shows up as younger than 51% of planetary systems that will exist.  

In the words of the first person I showed these calculations to, these numbers are very interesting in that they are very boring.  We find ourselves in an unremarkable position in terms of planet-formation order. We are not early and other explanations for the so-called Fermi pardox must be invoked.

Wow this post has gotten long.  I still have some thoughts on other large-scale considerations about our position in space and time - namely why we find ourselves around a star as large and relatively short-lived as the Sun (larger than most stars) and not, say, 30 billion years into the lifespan of a small star born at the same time as the Sun but with a 40 billion year lifetime.  I will save that for another revisit to this topic.  Stay tuned for my original intent, talking about our own solar system.  

Space and Time

Those of an anthropic bent have often made much of the fact that we are only 13.7 billion years into what is apparently an open-ended universe that will expand at an accelerating rate forever.  The era of the stars will last a trillion years; why do we find ourselves at this early date if we assume we are a ‘typical’ example of an intelligent observer?  In particular, this has lent support to lines of argument that perhaps the answer to the ‘great silence’ and lack of astronomical evidence for intelligence or its products in the universe is that we are simply the first.  This notion requires, however, that we are actually early in the universe when it comes to the origin of biospheres and by extension intelligent systems.  It has become clear recently that this is not the case. 

The clearest research I can find illustrating this is the work of Sobral et al, illustrated here http://arxiv.org/abs/1202.3436 via a paper on arxiv  and here http://www.sciencedaily.com/releases/2012/11/121106114141.htm via a summary article.  To simplify what was done, these scientists performed a survey of a large fraction of the sky looking for the emission lines put out by emission nebulae, clouds of gas which glow like neon lights excited by the ultraviolet light of huge, short-lived stars.  The amount of line emission from a galaxy is thus a rough proxy for the rate of star formation – the greater the rate of star formation, the larger the number of large stars exciting interstellar gas into emission nebulae.  The authors use redshift of the known hydrogen emission lines to determine the distance to each instance of emission, and performed corrections to deal with the known expansion rate of the universe.  The results were striking.  Per unit mass of the universe, the current rate of star formation is less than 1/30 of the peak rate they measured 11 gigayears ago.  It has been constantly declining over the history of the universe at a precipitous rate.  Indeed, their preferred model to which they fit the trend converges towards a finite quantity of stars formed as you integrate total star formation into the future to infinity, with the total number of stars that will ever be born only being 5% larger than the number of stars that have been born at this time. 

In summary, 95% of all stars that will ever exist, already exist.  The smallest longest-lived stars will shine for a trillion years, but for most of their history almost no new stars will have formed.

At first this seems to reverse the initial conclusion that we came early, suggesting we are instead latecomers.  This is not true, however, when you consider where and when stars of different types can form and the fact that different galaxies have very different histories.  Most galaxies formed via gravitational collapse from cool gas clouds and smaller precursor galaxies quite a long time ago, with a wide variety of properties.  Dwarf galaxies have low masses, and their early bursts of star formation lead to energetic stars with strong stellar winds and lots of ultraviolet light which eventually go supernova.  Their energetic lives and even more energetic deaths appear to usually blast star-forming gases out of their galaxies’ weak gravity or render it too hot to re-collapse into new star-forming regions, quashing their star formation early.  Giant elliptical galaxies, containing many trillions of stars apiece and dominating the cores of galactic clusters, have ample gravity but form with nearly no angular momentum.  As such, most of their cool gas falls straight into their centers, producing an enormous burst of low-heavy-element star formation that uses most of the gas.  The remaining gas is again either blasted into intergalactic space or rendered too hot to recollapse and accrete by a combination of the action of energetic young stars and the infall of gas onto the central black hole producing incredibly energetic outbursts.   (It should be noted that a full 90% of the non-dark-matter mass of the universe appears to be in the form of very thin X-ray-hot plasma clouds surrounding large galaxy clusters, unlikely to condense to the point of star formation via understood processes.)  Thus, most dwarf galaxies and giant elliptical galaxies contributed to the early star formation of the universe but are producing few or no stars today, have very low levels of heavy element rich stars, and are unlikely to make many more going into the future.

Spiral galaxies are different.  Their distinguishing feature is the way they accreted – namely with a large amount of angular momentum.  This allows large amounts of their cool gas to remain spread out away from their centers.  This moderates the rate of star formation, preventing the huge pulses of star formation and black hole activation that exhausts star-forming gas and prevents gas inflow in giant ellipticals.  At the same time, their greater mass than dwarf galaxies ensures that the modest rate of star formation they do undergo does not blast nearly as much matter out of their gravitational pull.  Some does leave over time, and their rate of inflow of fresh cool gas does apparently decrease over time – there are spiral galaxies that do seem to have shut down star formation.  But on the whole a spiral is a place that maintains a modest rate of star formation for gigayears, while heavy elements get more and more enriched over time.  These galaxies thus dominate the star production in the later eras of the universe, and dominate the population of stars produced with large amounts of heavy elements needed to produce planets like ours.  They do settle down slowly over time, and eventually all spirals will either run out of gas or merge with each other to form giant ellipticals, but for a long time they remain a class apart.

Considering this, we’re just about where we would expect a planet like ours (and thus a biosphere-as-we-know-it) to exist in space and on a coarse scale in time.  Let’s look closer at our galaxy now.  Our galaxy is generally agreed to be about 12 billion years old based on the ages of globular clusters, with a few interloper stars here and there that are older and would’ve come from an era before the galaxy was one coherent object.  It will continue forming stars for about another 5 gigayears, at which point it will undergo a merger with the Andromeda galaxy, the nearest large spiral galaxy.  This merger will most likely put an end to star formation in the combined resultant galaxy, which will probably wind up as a large elliptical after one final exuberant starburst.  Our solar system formed about 4.5 gigayears ago, putting its formation pretty much halfway along the productive lifetime of the galaxy (and probably something like 2/3 of the way along its complement of stars produced, since spirals DO settle down with age, though more of its later stars will be metal-rich).

On a stellar and planetary scale, we once again find ourselves where and when we would expect your average complex biosphere to be.  Large stars die fast – star brightness goes up with the 3.5^th power of star mass, and thus star lifetime goes down with the 2.5^th power of mass.  A 2 solar mass star would be 11 times as bright as the sun and only live about 2 billion years – a time along the evolution of life on Earth before photosynthesis had managed to oxygenate the air and in which the majority of life on earth (but not all – see an upcoming post) could be described as “algae”.  Furthermore, although smaller stars are much more common than larger stars (the Sun is actually larger than over 80% of stars in the universe) stars smaller than about 0.5 solar masses (and thus 0.08 solar luminosities) are usually ‘flare stars’ – possessing very strong convoluted magnetic fields and periodically putting out flares and X-ray bursts that would frequently strip away the ozone and possibly even the atmosphere of an earthlike planet. 

All stars also slowly brighten as they age – the sun is currently about 30% brighter than it was when it formed, and it will wind up about twice as bright as its initial value just before it becomes a red giant.  Depending on whose models of climate sensitivity you use, the Earth’s biosphere probably has somewhere between 250 million years and 2 billion years before the oceans boil and we become a second Venus.  Thus, we find ourselves in the latter third-to-twentieth of the history of Earth’s biosphere (consistent with complex life taking time to evolve).

Together, all this puts our solar system – and by extension our biosphere – pretty much right where we would expect to find it in space, and right in the middle of where one would expect to find it in time.  Once again, as observers we are not special.  We do not find ourselves in the unexpectedly early universe, ruling out one explanation for the Fermi paradox sometimes put forward – that we do not see evidence for intelligence in the universe because we simply find ourselves as the first intelligent system to evolve.  This would be tenable if there was reason to think that we were right at the beginning of the time in which star systems in stable galaxies with lots of heavy elements could have birthed complex biospheres.  Instead we are utterly average, implying that the lack of obvious intelligence in the universe must be resolved either via the genesis of intelligent systems being exceedingly rare or intelligent systems simply not spreading through the universe or becoming astronomically visible for one reason or another. 

In my next post, I will look at the history of life on Earth, the distinction between simple and complex biospheres, and the evidence for or against other biospheres elsewhere in our own solar system.