Is life everywhere or nearly nowhere?

 kw: book reviews, nonfiction, exobiology, astrobiology, seti, exoplanets, origins of life

The title of a recent book by Nathalie A. Cabrol is astonishing: The Secret Life of the Universe: An Astrobiologist's Search for the Origins and Frontiers of Life. Why astonishing? First, let us consider the limits of what is known.

• In our solar system, until recently, only three planets have been considered habitable at least part of the time since the solar system was formed about 4.6 billion years ago (hereafter Ga): Venus, Earth, and Mars. Both Venus and Mars are considered "almost certainly dead", but hints of continued habitability are discovered from time to time.
• Many "ice moons", such as three of the Galilean moons of Jupiter, Europa, Ganymede and Callisto; Enceladus, a moon of Saturn; and Oberon, a moon of Uranus—all appear to have large subsurface oceans of liquid water, or actually brine, which could sustain life.
• Beyond the solar system, thousands of exoplanets have now been detected. A few hundred of these are at an appropriate distance from their host stars to be habitable, at least at present, if not for the long term.
• Most of the exoplanets so far detected and confirmed are less than 5,000 light years from Earth. A handful (so far) are at distances ranging up to 33,000 light years.

Further interesting information is found in the List of Exoplanet Extremes. 

What do these facts imply? As much as we might like to speculate about life (almost certainly bacterial or some analog thereof) on various solar system bodies, the confirmed occurrence of life in the solar system is found only on Earth. N=1.

Outside the solar system, we have partially probed a volume of space totaling about 80 billion cubic light years (considering the thickness of the galactic disk to be 1,000 light years). That's not bad; it is 1% of the volume of the Milky Way galaxy. However our galaxy is one of at least 200 billion, and probably more than a trillion, galaxies in the visible universe. We don't know how much universe lies beyond our visible horizon. Again, in all that space, known life: N=1.

From a numerical standpoint, the data we have relate to between a quintillionth and a quadrillionth of the known universe. That makes the book's title an astonishingly extreme overstatement.

On the other hand: The author, the director of the Carl Sagan center at the SETI institute, presents the principles by which life is likely to have arisen, and the evidence from all around the universe that the right chemistry to kick-start life exist nearly everywhere. This makes the book's title almost banally obvious! Isn't that great?

Rather than survey all of the author's points, I'll focus on a few of interest, that may be little known. Firstly, note that word "Origins" in the book's subtitle. Life may have started on Earth more than once. It may have arisen, been snuffed out, and arisen again, perhaps several times. Earlier incidences of life may not have been totally snuffed out, and still exist alongside "us".

Firstly, consider that the "standard DNA coding table" doesn't apply everywhere. For example, there are variations in the encoding of certain DNA codons (3-base groups) to amino acids (or to Stop) that are found in mitochondria. Various classes of eukaryotic organisms have different mitochondria, as revealed by their coding tables. Other microscopic critters, not all of them bacteria, have alternate coding tables. So far, 30 alternative coding tables are known, with the "standard table" bringing the total to 31. See List of Genetic Codes for more details.

Let's step back and consider the situation. There are 64 possible DNA codons. All known life on Earth uses 20 amino acids (one bacterial genus may use a 21st amino acid; I can't find out much information about it). There are dozens, perhaps hundreds, more possible amino acids. The 64-to-20 conversion involves numerous duplicate codes, which makes for a robust system. Many single-codon variations (micromutations, which are common), make no change in the protein being produced. How many possible coding tables are there? I am good at many kinds of math, but not the details of "permutations and combinations". The best I can figure, the number is at least 48x10³³ (a 35-digit number), but it could actually be an 84-digit number. Either way, it is a lot!

Is it safe to assume that life elsewhere in the universe also uses DNA and RNA and ribosome decoding to produce proteins from some 20 amino acids? Not really. It is not even safe to assume faraway life requires water. Dr. Cabrol mentions "life as we don't know it" from time to time. She considers places like the Saturnian moon Titan, where water ice is a rock and the primary liquid is methane. What kind of life could arise there? Water (our solvent!) is polar, but methane is nonpolar; perhaps the abundance of ammonia, which is polar, could make methane plus ammonia an appropriate solvent for generating life-as-we-don't-know-it.

I am reminded of the Lensman series of space opera novels by E.E. "Doc" Smith from 1948 to 1954. It concerns intergalactic warfare between water/oxygen-based life and methane/chlorine-based life. I am also reminded of what the character Ian Malcolm said in Jurassic Park, "Life will find a way." I am further reminded of Vital Dust by Christian deDuve, who calls life "inevitable" and "a cosmic imperative." There could be a lot of different kinds of life in the universe, and it's unlikely that we could eat any of it, nor that it could eat us!

Dr. Cabrol points out that planets seem to outnumber stars. Perhaps many stars have no planets, but many more stars have at least one planet, and usually more than one. What proportion of these are rocky (not gaseous like Jupiter, which may have no solid surface) and in the habitable zone of their host stars? Is it a percent or so? Exoplanet data so far indicates between two and three percent. A further constraint is that, as a Main Sequence star heats up during its existence, the shift of the habitable zone shouldn't move beyond the planet in less than 5 billion years or so. This is just based on the fact that life on Earth required about 4.5 billion years to produce us. We are still left with several billion possible planets in our galaxy alone that have the potential to produce life that can become "civilized" and sufficiently technological to send signals via radio or laser or something that we could possibly detect if we are close enough. "Close enough" keeps getting farther away as our own technology improves.

Let's consider that 5 billion year figure. Our Sun is a star of type G2, a little larger than average. Something like 75%-80% of all stars are smaller and lighter. The lighter a star is, the longer it burns hydrogen on the Main Sequence. During that period, it gradually gets hotter and brighter as helium accumulates in the core. I am interested in the larger half of the K series of stars. Their mass is between 0.75 and 0.9 solar masses, and they burn hydrogen for between 17 and 35 billion years, compared to the Sun's expected hydrogen burning life of about 10 billion years. Stars lighter than 0.75 solar mass have even longer "lifetimes," but they are more likely to produce large flares, which can damage or extinguish life from the surfaces of any planets in their habitable zones. So I favor focusing efforts such as SETI (Search for ExtraTerrestrial Intelligence) on stars in the range K5 to G2. Even a G3 star probably would have begun to burn us off its surface by now, as our Sun is expected to do starting about a billion years from now.

The author also considers the Drake Equation, which is a thought experiment that helps us consider the likelihood or prevalence of life in our galaxy (or the universe). It consists of a bunch of factors that are multiplied together to produce N, a possible quantity of detectable civilizations "out there". An important factor is, "How long does civilization Z emit a signal that we could detect, if we are close enough and have sufficient technological sensitivity?" Consider Earth. The first radio transmission that reached beyond "local" was in December 1901. Just about 124 years ago. 

At present, there are a few dozen "clear channel" AM radio stations that emit 50,000 watt signals 24/7, a larger number of FM radio stations of similar or even greater power, and many TV stations, mostly below 10,000 watts. However, more and more of our TV watching is moving to cable (including fiber optics), and digital signals are more efficient, so stations that do broadcast are using lower power. I have an in-attic antenna that presently receives more than 60 digital TV stations, so I don't use (expensive!) cable. Radio is beginning to go digital also. I predict that Earth will be largely "radio silent" before the 200th anniversary of Marconi's transatlantic radio transmission.

If an exo-civilization is typically detectable for only 100-200 years, even without extincting themselves, that cuts a big hole in all our speculations using the Drake equation. I'll have to think more about this…

The last chapter deals at length with our own danger of extincting ourselves. The author considers pollution, particularly CO₂ buildup plus methane buildup, an existential threat; she states clearly that our window of opportunity for ensuring long-term survival is small, a matter of decades at most. I agree in part, but my expectation is not so dire. I won't encroach on her thesis, though.

I will close this part with a hearty recommendation of the book. It is full of great ideas and great information, and very well written. A pleasure!

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If you aren't interested in errata, you can stop here. I ran across a few items, equally the fault of the author and the copy editor, that need to be corrected.

Tidal Locking is mentioned just a few times. It is not clearly explained, and I found this on page 97: "Callisto is tidally locked to Jupiter, orbiting around it in the same amount of time it takes Jupiter to rotate." Not so. Callisto takes 17 days to revolve about Jupiter, while Jupiter takes 10 hours to rotate on its axis. Callisto's rotation period is 17 days, so it always presents the same hemisphere to Jupiter. This is the same in principle as our Moon, which both rotates and revolves in 27.5 days (sidereal periods), so we always see the same hemisphere. In the quoted sentence, the second instance of "Jupiter" should be "Callisto". A second instance where the numbers are correct is on page 141: Pluto and its moon Charon are mutually tidal locked, always facing each other the same way, both rotating and revolving in 153 days.

An egregious typo, minor misspelling of a homonym on page 142: "pour" rather than "pore". To study a document is to pore over it, not "pour."

Information Mastery, a la Carl Sagan, is a proposed scale of technological advancement. It is mentioned on page 215, where it is stated that Level A represents 106 "unique bits of information" and Level Z represents 1,031 bits. This is a formatting error, compounded by the insertion of the comma. The two numbers ought to be 10⁶, or one million, and 10³¹, or ten million trillion trillion (a 32-digit number). I suspect a dumb copy-paste removed the exponent formatting. Anyway, the concept is fascinating.

Let us consider where we are as a civilization on Sagan's scale. The venerable Encyclopedia Britannica contains about half a million topics in 40 million words. I suspect that Sagan would consider a "unique bit of information" to represent about a paragraph. These half million topics then are each stated in an average of 80 words, which comes to a smallish paragraph. Worldwide, there are several printed encyclopedias, but they overlap. Thousands, nay, millions of articles and books and journals are published yearly. Then there's Wikipedia, which has (today) 62 million pages, and about 1/8 of that is in 6.9 million formal articles. All told, that puts us in the realm of a Level C or Level D civilization.

There are a couple other typos, but they have less import. I'll leave it at that.