Will our cosmic angst ever be relieved?

 kw: book reviews, nonfiction, science, life, extraterrestrial life, SETI

About eleven years ago I posted a review of David Brin's novel Existence. In it I mentioned two books that form the "bookends" to my thinking about exo-life: Rare Earth by Peter Ward and Donald Brownlee (2000), and Vital Dust (1995) by Christian de Duve. I read both those books before I began this blog. It may be that The Possibility of Life: Science, Imagination, and our Quest for Kinship in the Cosmos, by Jaime Green, is a fourth worthy player in this space.

Now that we know at least the existence of more than 5,500 exoplanets, plus more than 8,000 "candidates" that are being vetted, it is clear that planets outnumber stars.  A few hundred planets are in the "earthlike" range of size and mass and surface gravity; as of late July 2023 this Wikipedia page lists 63 "potentially habitable" objects in the "Earth to Super-Earth" size range, small enough to be "probably rocky". That is well over 1% of the known exoplanets, which implies that such planets number between 10 billion and 20 billion in our galaxy (accepting the extrapolations of scientists who publish about this). It is well to note that a rocky planet with 2.5 Earth's size would have 15.6 times Earth's volume and, if of the same density, 15.6 times its mass. The greater radius would mean, by the cube-square law, that the surface gravity would be 2.5 times that of Earth. It is unlikely that the density would be the same, so that's just a very rough starting point.

As the author points out in a late chapter, somewhat channeling Peter Ward, bacterial life (or something very like it) appeared on Earth within half a billion years after it was formed, but since there was a period of "molten crust" for 300-400 million years, it could have come into existence rather rapidly, on cosmic timescales. But then, it took another billion years for photosynthesis to emerge, and another billion or 1.5 billion years for the first eukaryotic (complex, nucleus-bearing) cells to be produced. So even with our whole planet for a chemical-evolutionary playground, it was apparently rather hard for the basics of complex life to arise. Once it did arise, the next period, called the "boring billion", was required for the evolution of multicellular creatures bigger than a poppy seed. So, is de Duve right, that these developments are inevitable, or are Ward and Brownlee right, that the several big steps in this progression are so hard and rare that we cannot logically claim that a sequence like this happened more than once?

To my statistical mind, it seems we need some way to determine the variance in those numbers, applied to billions of planets. For example, let us first suppose that the production of eukaryotic cells takes, on average, five billion years after photosynthesizing bacterial cells originate, with a standard deviation of half a billion years. In such a case, for it to take only 1.5 billion years is a 7-sigma outlier; it can only happen on one planet out of 390 billion. Thus, if there are, to be generous, about 20 billion "habitable" planets per large galaxy, there would only be eukaryotes that arose this "quickly" on one planet per 20 galaxies. Our chances of having a near neighbor are slim indeed! On the other hand, if 1.5 billion is closer to the average, and the standard deviation is half a billion years, then about half the "habitable" planets on which life originates can be expected to develop complex life in 2-4 billion years from the inception of life. The Milky Way is big, with a volume of about 30 billion cubic parsecs. Divide 30 billion by 10 billion: three cubic parsecs per planet, or about 100 cubic light-years. The radius of a sphere of this size is about 3 light years; double this to get 6 light-years from planet to planet. That points to a galaxy littered with inhabited planets. This is no improvement over the Drake equation, which is easy to manipulate to find any result. We still suffer from a dramatic lack of knowledge.

The book has six (longish) chapters, and the tone is wistful and lyrical rather than coldly analytical; more like Vital Dust than Rare Earth. I enjoyed it very much. The big questions, to which Ms Green returns again and again, are how we would recognize life on another planet, from a distance, and then if/when we meet "aliens", how we might communicate. At one point she muses on the problem we are facing to communicate with our descendants, across a gap of 10,000 years—about 350 generations—regarding the danger of radioactive wastes stored at Yucca Mountain or other repositories. Consider how English has changed in just 1,000 years. Old English, also called Anglo-Saxon, cannot be read nor heard intelligibly by anyone without learning it as a foreign language. Although it was a Germanic language, modern German speakers are equally befuddled. That was just 1,000 years ago. Middle English, or Chaucerian, of 500-600 years ago, is barely intelligible; many words we can recognize in print are pronounced differently now than they were then (for example, the "e" that ends so many words such as "made" or "file" was explicitly pronounced). Even the "English" of the King James Bible takes some getting used to, even though the latest edition of the KJB was produced in 1769; the "original" text of 1611 is very difficult for most of us to read. If we have such trouble with human language (and there are hundreds of "extinct" languages that still cannot be read by anyone), how will we ever learn a truly alien language, "spoken" by a creature that is not even as well related to us as an octopus?

Most science fiction stories about contact with ET's sidestep the problem by having the aliens figure out some of our languages before arriving, or by having telepathy, or they have a "universal translator" such as those on Star Trek. I've read one story that took a serious swipe at the problem: human astronauts find an abandoned spaceship, and one of them finds a chemistry handbook and locates the Periodic Table of the Elements. Right away that yields, not just the alien words for about 100 elements, but the symbols for the alien number system. A big leg up. How to go beyond that? It still won't be easy. Maybe an alien version of the CRC Handbook would help…

Nonetheless, I think the occurrence of life is somewhere in the middle, such that at least a few planets within 100 light years (31 parsecs) will be found to have complex life. Whether any of those planets will host technological species is more iffy, but I am hopeful. Life that gets any kind of start is going to diversify. The diversity of plants and animals is astonishing, and in the Bacterial/Archaean realm there may be 100 times as many species. I think that, as the Ian Malcolm character said in Jurassic Park, "Life finds a way." (Picture from Mongabay)