Humans by the Numbers: Atoms of the Body

Element A (amu) Symbol # Atoms Mass (kg)   Element A (amu) Symbol # Atoms Mass (kg)   Element A (amu) Symbol # Atoms Mass (kg)
Hydrogen 1.0079 H 4.22E+27 7.06E+00   Rubidium 85.4678 Rb 2.20E+21 3.12E-04   Zirconium 91.224 Zr 2.00E+19 3.03E-06
Oxygen 15.9994 O 1.66E+27 4.40E+01   Strontium 87.62 Sr 2.20E+21 3.20E-04   Cobalt 58.9332 Co 2.00E+19 1.96E-06
Carbon 12.0107 C 8.03E+26 1.60E+01   Bromine 79.904 Br 2.00E+21 2.65E-04   Cesium 132.9055 Cs 7.00E+18 1.54E-06
Nitrogen 14.0067 N 3.90E+25 9.07E-01   Aluminum 26.9815 Al 1.00E+21 4.48E-05   Mercury 200.59 Hg 6.00E+18 2.00E-06
Calcium 40.078 Ca 1.60E+25 1.06E+00   Copper 63.546 Cu 7.00E+20 7.39E-05   Arsenic 74.9216 As 6.00E+18 7.46E-07
Phosphorus 30.9738 P 9.60E+24 4.94E-01   Lead 207.2 Pb 3.00E+20 1.03E-04   Chromium 51.9961 Cr 6.00E+18 5.18E-07
Sulfur 32.065 S 2.60E+24 1.38E-01   Cadmium 112.411 Cd 3.00E+20 5.60E-05   Molybdenum 95.94 Mo 3.00E+18 4.78E-07
Sodium 22.9897 Na 2.50E+24 9.54E-02   Boron 10.811 B 2.00E+20 3.59E-06   Selenium 78.96 Se 3.00E+18 3.93E-07
Potassium 39.0983 K 2.20E+24 1.43E-01   Manganese 54.938 Mn 1.00E+20 9.12E-06   Beryllium 9.0122 Be 3.00E+18 4.49E-08
Chlorine 35.453 Cl 1.60E+24 9.42E-02   Nickel 58.6934 Ni 1.00E+20 9.75E-06   Vanadium 50.9415 V 8.00E+17 6.77E-08
Magnesium 24.305 Mg 4.70E+23 1.90E-02   Lithium 6.941 Li 1.00E+20 1.15E-06   Uranium 238.0289 U 2.00E+17 7.91E-08
Silicon 28.0855 Si 3.90E+23 1.82E-02   Barium 137.327 Ba 8.00E+19 1.82E-05   Radium 226 Ra 8.00E+10 3.00E-14
Fluorine 18.9984 F 8.30E+22 2.62E-03   Iodine 126.9045 I 5.00E+19 1.05E-05            
Iron 55.845 Fe 4.50E+22 4.17E-03   Tin 118.71 Sn 4.00E+19 7.89E-06            
Zinc 65.39 Zn 2.10E+22 2.28E-03   Gold 196.9665 Au 2.00E+19 6.54E-06   TOTAL     6.75E+27 7.00E+01

Atoms of the Body

[Table by Tableizer]

Humans by the Numbers: Cells of the Body

Cells - Types & Numbers

[Table by Tableizer]

Organ/system Cell type Mean total cell number Percentage of Total SD
Gallbladder other stromal cells 8.48E+06 2.29E-05 9.00E+04
Blood Erythrocytes 2.63E+13 7.09E+01 0.51+13
  Leucocytes 5.17E+10 1.39E-01 2.43+10
  Platelets 1.45E+12 3.91E+00 5.70E+11
Bone Cortical osteocytes 1.10E+09 2.96E-03 2.40E+08
  Trabecular osteocytes 7.11E+08 1.92E-03 3.72E+08
Bone marrow Nucleate cells 7.53E+11 2.03E+00 2.18E+11
Heart Connective tissue cells 4.00E+09 1.08E-02 NA
  Heart muscle cells 2.00E+09 5.39E-03 NA
Kidney Glomerulus total cells 1.03E+10 2.78E-02 3.60E+09
Liver Hepatocytes 2.41E+11 6.50E-01 NA
  Kupffer cells 9.63E+10 2.60E-01 NA
  Stellate cells 2.41E+10 6.50E-02 NA
Lungs, bronchi,bronchioles Alveolar cells (type I) 3.86E+10 1.04E-01 9.50E+09
  Alveolar cells (type II) 6.99E+10 1.88E-01 1.45E+10
  Alveolar macrophages 2.90E+10 7.82E-02 7.30E+09
  Basal cells 4.32E+09 1.16E-02 9.50E+08
  Ciliated cells 7.68E+09 2.07E-02 1.62E+09
  Endothelial cells 1.41E+11 3.80E-01 3.00E+10
  Goblet cells 1.74E+09 4.69E-03 5.10E+08
  Indeterminate bronchial/bronchiolar cells 3.30E+09 8.89E-03 1.00E+09
  Interstitial cells 1.37E+11 3.69E-01 1.60E+10
  Other bronchial/bronchiolar secretory cells 4.49E+08 1.21E-03 1.97E+08
  Preciliated cells 1.03E+09 2.78E-03 3.40E+08
Nervous system Glial cells 3.00E+12 8.09E+00 6.60E+11
  Neurons 1.00E+11 2.70E-01 NA
Pancreas Islet cells 2.95E+09 7.95E-03 7.80E+08
Skeletal muscle Muscle fibers 2.50E+08 6.74E-04 NA
  Satellite cells 1.50E+10 4.04E-02 1.70E+09
Skin Dermal fibroblasts 1.85E+12 4.99E+00 2.60E+11
  Dermal mast cells 4.81E+07 1.30E-04 2.82E+07
  Epidermal corneocytes 3.29E+10 8.87E-02 4.70E+09
  Epidermal nucleate cells 1.37E+11 3.69E-01 3.90E+10
  Epidermal Langerhans cells 2.58E+09 6.95E-03 6.50E+08
  Epidermal melanocytes 3.80E+09 1.02E-02 NA
  Epidermal Merkel cells 3.62E+09 9.76E-03 NA
Small intestine Enterocytes 1.67E+10 4.50E-02 7.10E+09
Stomach G-cells 1.04E+07 2.80E-05 3.00E+06
  Parietal cells 1.09E+09 2.94E-03 8.00E+07
Supradrenal gland Medullary cells 1.18E+09 3.18E-03 1.80E+08
  Zona fasciculata cells 6.67E+09 1.80E-02 1.02E+09
  Zona glomerularis cells 1.77E+09 4.77E-03 2.70E+08
  Zona reticularis cells 7.02E+09 1.89E-02 1.10E+08
Thyroid Clear cells 8.70E+05 2.34E-06 NA
  Follicular cells 1.00E+10 2.70E-02 NA
Vessels Endothelial cells 2.54E+12 6.85E+00 1.05E+12
  Total 3.710083E+13 1.000000E+02 2.90E+12

Humans by the Numbers: The Brain

From the work of Suzana Herculano-Houzel, Brazilian Neuroscientist.

[Table by Tableizer]

Observed and Expected Cellular Composition of the Human Brain      
According to the Cellular Scaling Rules for Primate Brains      
Results are given in billions.      
  Expected Observed Difference
For a primate of 75 kg      
Total brain mass (g) 1,362 1,508 10.70%
Total number of brain cells 170.97 170.68 -0.20%
Total number of brain neurons 78.08 86.06 10.20%
Total number of brain nonneurons 94.28 84.61 -10.20%
For a primate brain of 1,508 g      
Total number of neurons 93.82 86.06 -8.30%
Total number of nonneurons 113.17 84.61 -25.20%
For a primate cortex of 1,233 g      
Total number of neurons 22.36 16.34 -26.90%
Total number of nonneurons 99.02 60.84 -38.60%
For a primate cerebellum of 154 g      
Total number of neurons 77.94 69.03 -11.40%
Total number of nonneurons 11.26 16.04 42.40%
For a primate RoB (Rest of Brain) of 118 g      
Total number of neurons 0.62 0.69 11.30%
Total number of nonneurons 7.17 7.73 7.80%



Fermi Paradox Solved?


A recent preprint which confirms the earlier work of James Annis [available here on the arXiv]:

On the role of GRBs on life extinction in the Universe
Tsvi Piran, Raul Jimenez
(Submitted on 8 Sep 2014)

As a copious source of gamma-rays, a nearby Galactic Gamma-Ray Burst (GRB) can be a threat to life. Using recent determinations of the rate of GRBs, their luminosity function and properties of their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place. Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 Gyr close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last 500 Myr causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, this probability drops below 50%. When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies and life can exist in only ~ 10% of galaxies. Remarkably, a cosmological constant is essential for such systems to exist. Furthermore, because of both the higher GRB rate and galaxies being smaller, life as it exists on Earth could not take place at z>0.5. Early life forms must have been much more resilient to radiation.

That “z>0.5″ is a date, which is model-dependent. Using Ned Wright’s Cosmology Calculator, with the current parameters of the cosmos, the date is 5.093 Gya, not long before the formation of the Solar System c.4.57 Gya. As the abstract discusses, the odds are pretty good that Life on Earth has taken at least one hit from a major Gamma-Ray Burst. Fortunately (?) it was probably aeons ago, before complex Life had evolved. Or did it end Complex Life once before? Is current animal life a Second Genesis? There are trails in mud from the Archean which look like worm-trails from the Cambrian. Could Life on Earth have had its evolution clock ‘reset’?

As an interesting coincidence, if we use the red-shift age relation of Fulvio Melia’s Rh = c.t Cosmology, which is t = Rh/c*(1/(1+z)), then the age at z = 0.5 is 2/3 the Cosmic Age, or just 4.57 Gya…

Terraforming Venus – A Comparison of Methods

Venus has long been considered Earth’s twin, but since the late 1950s we’ve realised she is Earth’s “Evil Twin”, with a 92 bar mostly CO2 atmosphere, very little water and a 116 day ‘sol’. How do we make her a more pleasant place?

The first proposal came from the 1930s, when spectroscopes were first turned towards Venus, and no oxygen was drected, and CO2 seemed to be the main component of the atmosphere. The nature of the clouds was unknown, but water seemed likely and the planet was imagined to be a global ocean. Naturally the solution was to generate free oxygen via electrolysis, expelling the excess hydrogen to space. Unfortunately the planet was inhabited and horrendous conflict ensued (Olaf Stapledon’s “Last and First Men”.)

Large amounts of carbon dioxide are incompatible with water, as it dissolves too readily. Thus the next suggestion, from the late 1930s: Venus’s clouds are polymerised formaldehyde dust and the planet is dry due to water being bound chemically in the dust and rocks. The greenhouse effect created surface temperature near 80-100 C could be managed by suitable insulation and cooling systems, but the colonists could employ a catalyst to destroy the formaldehyde and liberate oxygen and water [Poul Anderson’s “The Big Rain”, Cyril Kornbluth & Frederik Pohl’s “The Space Merchants”, early 1950s.]

In the mid 1950s the global ocean returned as a possibility, but so did a hot dusty desert or a hot ocean of crude oil. No particular model seemed more likely than the others, though oxygen was still unobserved (contra many bad SF stories).

Then in the late 1950s the temperature of Venus was observed to be ~300 C in radio frequencies. This suggested that the planet was hot from a massive greenhouse effect and all the planet’s water was in vapour form. Carl Sagan suggested, in 1961, that the planet could be made more Earth-like via seeding the atmosphere with blue green algae, converting the carbon dioxide into oxygen, the algae eventually falling into the hot depths to be reduced to carbon and their water returning to the atmosphere to be cycled again. Eventually a rich oxygen atmosphere would result, with a surface coated in carbon and the clouds condensing as rain… or so it would if the atmosphere was mostly nitrogen as most assumed in the early 1960s. For example, the 300 C Venus with 4 bars surface pressure and atmosphere of 80% nitrogen, would result in a breathable N2/O2 atmosphere after the algae had finished their job.

Unfortunately for Sagan’s scenario, and the 1960s & 70s SF based on it, the Russian and US probes to Venus revealed, by 1970, that the atmosphere was 96.5% carbon dioxide, with a surface pressure of 92 bars and a temperature of 462 C. Another complication was the bright clouds. These had so invitingly looked like water, but proved to be only partly water and mostly sulfuric acid. Yet not much sulfuric acid and not very much water at all, in fact. Venus is fantastically dry.

What can be done to make Venus more Earth-like? Firstly, contra the apparent evidence, Earth has about as much carbon dioxide as Venus – but on Earth it’s bound up in the rocks as carbonate minerals. On Earth the exhalations of the mantle, in the form of volcanic gases, have mostly dissolved in the oceans and have largely been locked up chemically. Venus, in a sense, is Earth absent water and unable to bury her atmosphere.

A common suggestion is to remove the atmosphere of Venus via blowing it or throwing it into space. Venus’s atmosphere masses 478,000 trillion tonnes and to launch it all into space requires a minimum of 54 MJ/kg – the equivalent of 6.1 trillion megatonnes of TNT or the total fusion of 73 billion tonnes of deuterium. However applying all that energy efficiently is a herculean challenge. Using asteroid collisions adds additional heat to the planet, which merely adds to the problem we meant to solve.

However the 127,400 trillion tonnes of carbon locked up in the atmosphere is a resource quite unlike any other. The entire asteroid belt contains a fraction of the total carbon available in the atmosphere of Venus. Eventually the carbon could be exported off world, thus disposed of in time, but how do we get it out of the air and cool the planet down?

One suggestion is to dispose of it the same way Earth did naturally, by turning it into carbonate rock. To do so we could cycle atmosphere through the crust, but there might not be enough lime (calcium and magnesium oxides) to react it with. Alternatively we mine magnesium (from Mercury, for example) and send it to Venus to burn in the atmosphere, then combine more carbon oxide with the resulting magnesium oxide to make magnesium carbonate:

Mg + CO2 => 2 MgO + C

2 MgO + 2 CO2 => 2 MgCO3

Thus for every atom of magnesium, 3 molecules of carbon dioxide are disposed of. Magnesium will also react with the sulfuric acid in the clouds and quickly reduce the greenhouse effect significantly. While the clouds reflect ~75% of the light, they also prevent a significant amount of heat from escaping. Without the clouds the temperature would fall by ~140 degrees to about ~600 K.

Just how much magnesium is required? To combine chemically with ALL the carbon dioxide would require a mass equivalent to 1/3 the mass of carbon dioxide – about 160,000 trillion tonnes. Solar-powered mass-drivers could easily fling it towards Venus, into the orbital path of the planet, to produce brightly burning fireballs in the upper atmosphere as the magnesium burns. The energy to throw from Mercury towards Venus is about 31 MJ/kg, which means the total energy needed to remove Venus’s atmosphere via burning Mercurian magnesium is ~20% of the energy needed to remove the atmosphere kinetically. Plus the carbon resource stays on Venus. The surface will be covered in hundreds of metres of magnesium carbonate and plain carbon powder. And Venus will still be dry.

Venus has plenty of oxygen for making water – what it lacks is hydrogen. If we made water from all the oxygen in the carbon dioxide atmosphere of Venus, then an ocean about 830 metres deep would result. Or about 340,000 trillion tonnes. The nearest source is the Sun, in the form of the solar wind, 90% of which is protons (hydrogen nucleii). However the total mass ejected by the Sun, per year, is about ~22 trillion tonnes. If it could ALL be gathered, which is unlikely, then it’d take ~1,500 years to gather enough. Perhaps ~100 metres equivalent of water would be enough, which would be ~180 years of collecting all the Solar Wind’s output. Creating a magnetic field vast enough to collect a significant fraction seems a larger task than terraforming Venus, but I’ll leave that computation as a task for the Reader…

The only other major sources of free hydrogen is the atmospheres of the Gas Giants, of which Uranus has the most accessible gravity-well. Fortunately Venus is at the bottom of the Sun’s gravity-well, relative to Uranus, which means a net energy gain, if the energy can be recovered from infalling payloads via regenerative braking – albeit on a vast scale. Reacting hydrogen with carbon dioxide, via the Bosch Reaction goes as follows:

CO2 + 2 H2 => 2 H2O + C

…thus adding to the carbon powder created via burning magnesium. Exposed to the present day sun, while the surface is still incredibly hot, and the water vapour will merely add to the greenhouse effect. Some kind of cooling is required. A large soletta reflecting away some of the sunlight seems to be a given, but is it a permanent necessity?

Kim Stanley Robinson’s novels, “Blue Mars” and “2312” makes the Venus Shade Soletta’s presence a major vulnerability of the Venusian terraforming effort. Stan (as he prefers to be called) uses Paul Birch’s approach to Venus’s carbon dioxide – freeze it out by shading the planet 100%. Birch’s scenario then requires burying the dry ice – it’ll build up to ~700 metres thickness over the whole planet – underneath insulated blocks or in carefully chosen “pits” around the planet. Eventually it’s exported off-world or used locally in other forms. Until that burial process is complete, the planet must be kept in the shade. And once it’s buried, the soletta is retained to give Venus an Earth-like 24 hour day and reduce the insolation. Or at least that was the plan until cyber-terrorists almost succeed in destroying it and undoing the whole carbon freeze-out process.

However if we begin by combining the carbon dioxide chemically, then that dramatic scenario is less likely. Once the ocean condenses and percolates into the regolith, with some spectacular geothermal activity as the crust cools, then I wonder if a permanent shade soletta is absolutely required. Recent modelling of cloud formation on slow rotating planets – like Venus – suggest that a vast, stable cloud system will form at the sub-solar point, increasing the average albedo of the whole planet, and suppressing a runaway Greenhouse effect. Once the carbon dioxide is removed, Venus will have an atmosphere of nitrogen giving a surface pressure of ~2.07 bars. Some of the carbon dioxide will be need to make oxygen, via photosynthesis presumably, but the whole planet’s ecology will need to be tweaked to handle the 116 day ‘sol’ (time from sunrise to sunrise, different to the sidereal day, which is 243 days long.) A terrestrial analogy would be arctic vegetation which has a short growing season and a long dormant phase. Perhaps tubers and root plants as well as cold-tolerant species? There would be, effectively, two ‘seasons’ – the day-light Growing Season, and the night-time Dormant Season. Hibernation/torpor/estivation might need to be tweaked into all the animals. No doubt humans will have their own light, but it would be a shame to impose a terrestrial Day/Night artificially, when we could create a planetary-scale experiment on adaptation to such exotic conditions. Migration, following the most congenial temperature and light-level as the planet turns, might also be a reasonable adaptation.


What can we do with carbon dioxide? One possibility, suggested by Stephen Gillett in the late 1990s, is to turn it into the carbon dioxide analogue of silicon dioxide (silica or glass). When Gillett made this suggestion the stuff was purely theoretical. Since then it was first made in a high-pressure laboratory from carbon dioxide in 2006. Called ‘amorphous carbonia’ it really is a glassy solid, just like silica. At present it hasn’t been successfully ‘quenched’ from high pressure, though it has been cooled to room temperature. One day we might discover the trick of making the stuff stable under more reasonable conditions and use it as a construction material. Conceivably, and this was its allure for Gillett, we can imagine a quasi-biological self-replicating ‘organism’ making the stuff out of the air of Venus, and eventually ‘glassing out’ the excess carbon dioxide as the carbonia ‘shells’ of the quasi-organisms. If we can discover how to do this trick via our burgeoning abilities at making artificial lifeforms, then it’ll solve Earth’s excess carbon dioxide problem too.

Blake’s 7 Rewatched… Time Distortion Factors and All That

Blakes 7 Logo by Xeno

“Blake’s 7″ was a British dystopian version of “Star Trek” that aired from 1978 to 1981, for four seasons, and ended in a fashion almost as controversial as the end of the original “Neon Genesis Evangelion” TV series. I won’t give away any spoilers as the series is up on “YouTube” and well worth watching. Or rewatching, as I am doing.

The setting is a Future where the ‘Federation’ has become a force against human freedom, some 800 years or so years from now. Roj Blake, the eponymous revolutionary, led the most visible campaign against the Federation some years before the first episode, “The Way Back”, but had been mentally reconditioned and his memories of being a leader had been suppressed. A new resistance/protest movement had recontacted him, hoping to revive his memories… with predictable results. In the first few episodes Blake collects his ‘seven’ – Kerr Avon, computer hacker; Vila Restal, a master thief; Jenna Stannis, a space smuggler/pirate; Oleg Gan, a murderer with an brain-implanted ‘Limiter'; Cally, a telepathic outcast; Zen, a talkative starship computer, and the starship itself, “The Liberator”.


In the first 2 episodes, Blake is sent on an eight month voyage to a prison planet, Cygnus Alpha. The prison ‘barge’ he is transported in is the “London”, which cruises at ‘Time Distort five” through ‘hyperspace’. TV planet names never seem to follow any logical system (real star names are seemingly random to the uninitiated anyway) so suggesting “Cygnus Alpha” is a planet of “Alpha Cygnus[Cygni]” is only a stab in the dark. Alpha Cygni, or Deneb, is a very large, bright star about ~800 parsecs away (2,600 light-years) which implies a cruise speed for the “London” of ~3,900 c. As 5^6 = 3,125, let’s assume that’s how Time Distort factors, as multipliers of lightspeed, are computed and see where that takes us. Thus ‘Cygnus Alpha’ is ~2,100 light-years away, which is within the current distance uncertainty.


The “Liberator” crew talk of ‘speed’ as “Standard speed” with some factor, typically “Standard by 6″ if they’re in a hurry(ish) and “Standard by 12″ (SB 12) when the pedal is to the floor. In Season 2 Episode 1, “Redemption”, the “Liberator” is reclaimed by its Makers, the System, in interceptors able to travel at SB 14.


In one episode, Season 2 Episode 8, “Hostage”, the “Libertor” is pursued by Federation Pursuit Ships, initially at TD 9, but are pushed to do Time Distort 10, at a pinch. They encircle the “Liberator” but the crew fight their way through and pull away at “Standard by 12″, which the Pursuit ship Mutoid crew report as “Time Distort 20″. Interestingly, using TD 20 as (20)^5 = 3,200,000 c = (12.14)^6, thus suggesting the relationship between “TD” and “SB”. Thus TD 10 is SB 6.8, and TD 9 is SB 6.24, which implies Blake’s usual cruising speed of SB 6 is not much slower than the Federation’s nominal top-speed.


In the climactic conclusion to Season 2, “Star One”, the crew have to fly out of the Milky Way to ‘Star One’, the heart of the Federations computer control and its source of power. Blake and crew quickly realise that Star One has been compromised by an extra-galactic enemy, from M31. One might think that the 2.55 million light-years to Andromeda’s Great Spiral Galaxy would be straightforward at Standard by 12, but intergalactic travel is said to take ‘a lifetime’. By the speeds we’ve computed above it’s about 43 years at TD 9, or ~55 years at SB 6. The “Liberator” can’t sustain SB 12 for significant periods of time, without spending days regenerating its powerbanks, suggesting why no one had ever attempted intergalactic travel. Having a ‘power convertor’ blow-out a million light-years from home is a good reason not to push the hyper-drives too hard.

The True Size of M31 in our Sky.
M31 as Big as we’d see it, if bright enough.

Resources of the Solar System: Beyond Our Inner System


Some of the major objects within 100 AU of the Sun are omitted from the above – four major moons of Jupiter, one of Saturn and one of Neptune – all of which are much larger than the “dwarf planets” highlighted above. Several major asteroid populations are subsumed into “the Asteroid Belt” and the “Kuiper Belt Objects”. Jupiter and Neptune have large populations of asteroids sharing their orbits in the leading and trailing Lagrange points, the so-called Trojans. Also the Centaurs range between Jupiter and the Kuiper Belt, grading into the so-called “Jupiter Family Comets”, some of which are sourced from the Kuiper Belt, Neptune Trojans & Centaurs, while the rest are from the Oort Cloud.

The Trans-Neptunian region, beyond the Kuiper Belt, is little known. Several largish objects are known in that region, like Sedna, and their dynamics suggests the existence of one or two Mars to Super-Earth sized planets just beyond them, at perhaps 200 to 250 AU. We’ve discussed such objects here before, so let’s move a bit further out, into the Inner and Outer Oort Cloud. We see comets come falling towards the Sun from somewhere this side of ~22,000 AU. Based on the best orbital data, the quasi-spherical Oort Cloud that the comets must fall from extends out to about ~2/3 of a light-year. Doesn’t sound like much when said like that, but that translates into 44,000 AU – forty-four thousand times the distance between the Earth and the Sun. Our “Inner System” – the spherical volume ringed by the Classical Kuiper Belt at ~44 AU radius – occupies just 1 billionth of the volume. How many comets might it have? How could we even know?

Actually we do have a pretty good idea. Firstly, the Oort Cloud comets give us some statistical meat to chew on, producing estimates between 100 billion and 10 trillion comets. An average comet is somewhere between 1-10 km across and at ~1,000 kg/m3 density, that’s a mass between several times the mass of Earth to several Jupiters. How did it get there? Currently the comets are believed to have been ejected mostly by Jupiter and Saturn as they changed orbits during the early days of the Solar System via interacting gravitationally with billions of “planetesimals”, the leftovers of planet formation which became the comets.

While a trillion comets seems an immense resource, they’re spread very thinly – that 44,000 AU radius volume means each comet has, on average, about ~400 cubic AU to itself. Even more widely spread will be the estimated ~1,000 or so Pluto-to-Earth-Moon-sized “planetary embryos” that different planet formation theorists have postulated. These too would’ve been flung from the Inner System by the migrating Gas Giants. At the upper end of the mass scale there might be Mars-to-Earth sized objects. If such objects have significant geophysical heat sources and captured a H2/He envelope from the proto-Sun’s Nebula, then they might retain sufficient heat to have liquid water oceans. Alternatively they might have water-ice crusts and buried oceans, if stripped of their proto-atmospheres.

Could a planet-sized world be cold enough for more exotic oceans, like hydrogen? I have discussed the possibility before, but hadn’t closely examined the requirements of such a state of affairs. Large masses of rock are “warm” compared to cold stuff like liquid hydrogen (~20.3 K), let alone frozen hydrogen (13.8 K) or even liquid helium (~5 K under pressure.) If we assume the usual heat-source suspects, the radioactive decay of Uranium, Thorium and Potassium-40, then planets with significant fractions of rock (which contains U, Th & K naturally) are unlikely to ever get cold enough. Earth, which is 2/3 rock, has an equilibrium temperature of 34.4 K – and no amount of pressure can keep hydrogen liquid at that temperature, as its critical point is 33 K at 13 bar. Outcroppings of colder rock might allow pools of the stuff, but never oceans. Even if Earth’s geothermal output came from just radioactive decay (rather than ~50%) and the heat of formation had dissipated, the equilibrium temperature is still 28.8 K.

If we reduce the rock fraction, then we might be able to get closer to liquid hydrogen conditions. Using the mass-radius relations for different materials, derived by Sara Seager and her team, I’ve computed the equilibrium temperature for different planets. Of course we might posit planets with NO silicates, but no such planet (dwarf or otherwise) is observed in our solar system, and even comets have some fraction of “grit”.

Here’s a summary table for different compositions – a “Mercury-like” planet that’s just 32.5% silicate (i.e. half Earth’s); a pure Silicate planet, and an icy planet that is only 22% silicate. Mass and Radius are in “Earths”, while ‘T(eq)’, is the Equilibrium Temperature in K.

Equilibrium Temperatures

A very Icy planet, as you can see, could easily support liquid hydrogen on the surface, as could the smaller Mercury and Silicate planets. Even an Earth composition would come close. But there’s a complication. Hydrogen is a greenhouse gas and significant amounts of it as gas will quickly raise the surface temperature, holding in the heat from the rocks. Depending on how much you have, it might be warm enough for nitrogen oceans (~63 K) or ammonia (196 K) or even liquid water (~273 K).

A Deeper Future View

UPDATE Centauri Dreams has published this essay here: A View of the Deepest Future

The long-term fate of Life in this Universe is rarely contemplated. A few landmark studies, by Freeman Dyson, then Fred Adams, Peter Bodenheimer & Greg Laughlin, have looked into Deepest Time, long after Matter itself fails and the Void becomes unstable. How far can biological Life extend into the Long Dark? A study by Robin Spivey extends Life’s tenure, in neutrino-annihilation warmed Ocean Planets, to 1025 years – and Beyond. That’s 100 times longer than the 1023 years we’ve reported here previously and some 1,000 trillion times longer than the time the Universe has presently existed. If the current Age of the Universe was a clock tick – a second -, then those 1025 years would be 20 million years.

Spivey discusses his new finding here: Planetary Heating by Neutrinos: Long-Term Habitats for Aquatic Life if Dark Energy Decays Favourably [Open Access article]
Outer-shell electrons of 56Fe (iron) inside the cores of Ocean-Planets become ‘catalyzers’ of Inverse Photo-Neutrino Process (IPP) reactions, annihilating neutrinos and creating a steady heat-flow sufficient to warm the planet at ~0.1 W/m2. This Figure illustrates the flow:

Neutrino-Heated Ocean Planet - Large

Perhaps coincidentally, the inexorable processing of stellar materials in Type Ia Supernovae leads to a chemical mixture which makes Earth-like planets. Each Type Ia Supernova masses about 1.4 Solar Masses, or about half a million Earths, with the ejecta debris being mostly iron, then oxygen and silicon. Earth-stuff. Thus the ‘ashes’ of stars can produce a multitude of Ocean Planets.

To quote Spivey:

Observations have determined that the ejecta of a typical SNIa are, by mass, 18% oxygen, 15% silicon, 13% iron, and 49% nickel (almost all in the unstable form 56Ni which decays radioactively to 56Fe), along with smaller amounts of carbon, calcium, sulphur and magnesium. Elements emerge from SNIa in strata, with the lightest occupying the outermost layers. This provides the oxygen-rich outermost shell with the best opportunities for reacting with hydrogen in the interstellar medium, resulting in the formation of water molecules. On cooling to temperatures found in deep space, ice XI is obtained, whose ferroelectric self-aggregation may be relevant to comet formation [38-40]. The bombardment of protoplanets with comets would be important to the formation of oceanic planets, deferring the delivery of water to their surfaces.

Notice that the iron component is 62% by mass, thus the very large core in the illustration. Quoting Spivey again:

Based on the composition of type Ia supernova ejecta, a hypothetical oceanic planet of one Earth-mass is projected to consist of a large iron core of radius ~4240 km surrounded by a silicate mantle of thickness ~1300 km through which heat would be transported by advection. External to this inner mantle would be an outer mantle of ice consisting of strongly convective ice VI and VII phases of combined depth ~320 km. A liquid ocean ~50 km in depth covered by a solid crust of ice Ih upwards of 50 m in thickness would overlie the hot ice mantles.

Spivey’s new paper focuses on how the supply of neutrinos can be maintained at the right density to keep planets warm for the maximum amount of time. He posits several, as yet, unobserved processes – the decay of dark energy into neutrinos in less than ~70 billion years and the accelerated decay of black holes, also preferably into neutrinos. Other researchers have posited the existence of ‘sterile’ neutrinos, which Spivey shows improves the characteristics of the neutrino halo surrounding a Galaxy cluster, enabling planets to be warmed in a life-friendly manner in a sphere of 400 thousand light-years radius.

The existence of Dark Matter itself has been called into question by physicists, such as Mordechai Milgrom, who think the evidence for invisible Dark Matter can be equally well explained by modifying Newtonian Gravity to have a minimum gravitational acceleration. This Modification of Newtonian Dynamics (MOND) theory neatly explains the structure of galaxies, but hasn’t been as successful on a cosmological scale. Intriguingly if Galactic haloes are made of sterile neutrinos, then MOND and Dark Matter physics are equivalent in outcomes: Reconciliation of MOND and Dark Matter theory with giant ‘Neutrino Stars’ forming around each large Galaxy. Spivey suggests that a key research priority is determining the properties of neutrinos, to confirm the IPP heating mechanism. Such neutrino studies are important for refining the Standard Model of particle physics – and possibly discovering new physics, such as the masses of the various neutrinos, something not predicted by the Standard Model.

Spivey’s most audacious suggestion is the strategy that Life should adopt in the next few aeons to extend its lifespan. Unfortunately for Life in this Galaxy, our local Group of Galaxies is insufficiently massive to form a large enough neutrino ‘star’ before Dark Energy spreads galaxies too far apart. To survive, Life in our Local Group needs to emigrate to the Virgo Super-Cluster. Although our Milky Way is heading towards Virgo at ~200 km/s, cosmic acceleration, from Dark Energy, is presently pushing us away from Virgo at ~1,000 km/s. Thus we need to launch towards Virgo faster than the Dark Energy pushing us away. Yet the reward is 10 trillion trillion years of Habitable planetary environments, which may well be worth intergalactic migration.

Spivey suggests using antimatter rockets to launch modest payloads. Essentially small Life-seeds, like those proposed by Michael Mautner to seed Life in our own Galaxy, but launched on intergalactic journeys of a hundred or more billennia. Whether the cosmic-ray flux between the Galaxies can be endured for geological epochs is presently unknown and while I wouldn’t rule it out, it seems unlikely at best. A good reference, available online, is still Martyn Fogg’s “The Feasibility of Intergalactic Colonisation and its Relevance to SETI”, which suggests how a mere 5 million year intergalactic voyage might be survived by a bio-nanotech seed-ship.

But we’ve discussed other options in these pages previously. In theory a tight white-dwarf/planet pair can be flung out of the Galactic Core at ~0.05c, which would mean a 2 billion year journey across every 100 million light-years. A white-dwarf habitable zone is good for 8 billion years or so, enough to cross ~400 million light-years. It’d be a ‘starship’ in truth on the Grandest Scale. Perhaps other Intelligences have begun their preparations earlier than us and we should look for very high-velocity stars leaving the Milky Way and Andromeda’s M31. Over the next aeon we might observe many, many stars flinging towards Virgo from the nearby Galactic Core black-holes.

The Verse as an Engineered System


Years ago, when the 1970s “Battlestar Galactica” was the pinnacle of TV Sci-Fi in a 10 year old’s imagination, I wondered at the nature of the 12 Colonies of Man, which all seemed to be in the same star system. The old BSG was a bit confused about “star systems” vs “galaxies” so much so that the interstellar nature of the Colonies and their systems had to wait for the 2004 reboot to be made explicit. Yet the idea stuck with me – could a system have 12 habitable planets? I knew about binary and trinary star systems as well as the possibility of Gas Giant moons being habitable (something I imagined as a child, only to have confirmed by the discovery of habitable zone Gas Giants over the last 19 years.)

Then along came “Firefly” and its Verse, which is in a multiple planet, multiple star, yet single system. Dozens of planets! Could that be possible?

Planet-formation Guru, Sean Raymond, has wondered the same thing and has now posted a series of blog-posts which have made the imagining more concretely physics based.

The discussion begins here: Build a better Solar System, a post in which Raymond ponders how our Solar System could be improved by rearranging the known bodies – he concludes we could have 7 habitable ‘planets’ though several would orbit a more Sunwards Jupiter.

Leaving our Solar System behind, he ponders the “Ultimate Solar System”, which squeezes the maximum number of planets into orbit around the best kind of star (an M0-M1 star at about 0.5 Solar Masses probably.)

Ultimate Solar System, Prelude: Building the ultimate Solar System

Ultimate Solar System, Part 1: Building the ultimate Solar System part 1: choosing the right star

As mentioned the right star is an M star, but not one that is too small, as smaller stars mean planets are subject to much higher tidal forces. For example, the strength of tides varies with the inverse cube power of the orbital radius, while the luminosity of stars varies with the 4th-3rd powers. For a 0.5 Solar mass star, the luminosity is about (0.5)^4 = 0.0625 times the Sun, so the habitable radius is at about 0.25 AU, but the tidal forces are ~(0.5)*(1/0.25)^4 times what Earth experiences from the Sun i.e. 32 times stronger. Meaning instead of half metre tides, the planet might get 16 metre tides, though it’d also be rotating slower due to more tidal-braking.

Ultimate Solar System, Part 2: Building the ultimate Solar System part 2: choosing the right planets

Naturally enough, since we’ve picked a long-living star, we want long-living planets – not too small, else they’ll lose their atmospheres. However, since we’re engineering things, selecting a low-EUV activity star will aid in keeping the atmosphere. Another concern is the inexorable decline in geological activity. Bigger planets last longer.

Ultimate Solar System, Part 3: Building the ultimate Solar System part 3: choosing the planets’ orbits

Raymond discusses habitable zone orbits and orbital stability – spaced properly and the system is stable against mutual perturbations for aeons, at least.

Ultimate Solar System, Part 4: Building the ultimate Solar System part 4: two ninja moves — moons and co-orbital planets

This is where the real innovation appears – firstly, twin planets – equal mass planets orbitting each other; then two pairs sharing an orbit 60 degrees apart. This allows 24 planets to be squeezed in, on six mutually stable orbits. Alternatively there could be four Gas Giants in 3/2 resonance orbits, each with five habitable moons, and each Gas Giant with two Trojan pairs in the L4/5 positions, thus 9 habitable planets per orbit for a total of 36.

Ultimate Solar System, Part 5: Building the ultimate Solar System part 5: putting the pieces together

Finally the Ultimate System is a Binary – one with the double Trojan Pairs in six orbits (24 total) and the other with Gas Giants and their retinues of Trojans and Moons – for a Grand Total of 60 habitable planets. The binary stars are at least 100 AU apart, to avoid exciting each other’s systems into too much Chaos. Of course we know of widely separated stellar systems with higher multiplicity. We could have two 100-AU apart binaries, themselves orbitting another binary at a distance of ~2,000 AU. Now that’d be a setting for some cool SF!

Lockstep Empire


Karl Schroeder’s imaginary “Lockstep” Civilization or Empire is a group of thousands of human-inhabited planets which periodically go into synchronous suspended animation for 30 years at a time, then revive for a month – a ratio of 360:1. This conserves resources and allows them to travel ‘quickly’ (from a subjective point-of-view) between worlds. Intriguing the planets are those possibly thousands which are sprinkled through the dark void between every star. The suggested number is ~hundreds of thousands of Pluto-to-Moon sized objects and thousands of Earth-sized objects for every star in the sky. A vast territory, all within a few light-years at most.

The Lockstep Empire is a reasonable solution to the problem of interstellar travel & communications – if we can’t develop warp-drive. It’s also a good exposition of the inter-stellar (“between the stars”) territory that civilizations could access and populate. Schroeder has rightly noted that many thousands of large objects probably exist between the stars, though maybe not so many Neptune to Jupiter sized ones, however, as WISE has demonstrated there seem to be fewer brown-dwarfs and super-Jovians Out There.

Here’s a short-story set in the Lockstep universe, from Tor: Jubilee, by Karl Schroeder

The kind of materials captured by the small denizens of inter-stellar space depends on how they formed. Two main options: via direct collapse out of protostellar nebulas or via scattering from forming planetary systems. In either scenario they might capture large amounts of the primordial nebula’s hydrogen/helium gas, along with ices, silicates and metals. One could have a large population of “micro-Neptunes” – Mars-to-Earth mass objects with gas-planet atmospheres. Internal geothermal heat will make it very hard for such objects to condense their helium atmospheres, but hydrogen might cool sufficiently to form oceans and ice-caps. Imagine such dark, eerie oceans of ultra-cold liquid, lit by stars twinkling through helium atmospheres…


Marshall Eubanks notes in the the comments:

In a short submission to Nature in 1999 (, David Stevenson pointed out that an Earth-like nomadic planet in deep space, with an Earth-like amount of radioactive heating, could have a thick hydrogen-helium atmosphere and a surface temperature high enough to support liquid water on the surface, even though there was no Sun and the planet would appear to have a temperature (as seen from the outside) of only 35 K. So, there might be “dark, eerie oceans” of regular old water on some dark Earths, oceans we might conceivably be able to sail without much protection, although we would need some sort of breathing apparatus.

Stevenson’s paper is available here, slightly longer than the “Nature” edit, on the author’s web-site: Interstellar Planets