Update (May 2023): I've amended this entry to reflect some stuff I've learned since I posted it. Specifically, the section on dusty plasma, and an expansion of the sulfur-based life section.
Upper Update (December 2023): Isaac Arthur has a video up about silicon-based life. You can watch it here.
So far, most of the characteristics of worlds we've discussed have been the result of blind physical forces: gravity, impacts, stellar heat, the melting and boiling points of various substances. Now it's time to look at the impact of life.
The first question, of course, is what we mean by "life." I personally like the definition that "life is any system which can evolve." In other words, any system involving self-replication of some sort, which allows for evolution by natural selection.
In broad terms we can divide life into "life as we know it" and "life not as we know it." Life as we know it (hereafter just called Earth life) is the familiar organisms based on DNA (or some comparable complex information-storing molecule), with water as its primary solvent, using carbon to build various complex molecules for various purposes. In my entry on Earth life I'll go into why I think that's the most plausible kind of life that may exist in the Universe.
But today let's look at life not as we know it. What forms might it take?
The answers are almost infinite. Anything which one might imagine forming a self-replicating system could be the basis for some bizarre kind of life. Magnetic fields, ionized plasma, silicon compounds, liquid hydrocarbons, liquid helium . . . the list goes on. Whole books have been written on the topic. Some of the better ones are:
Astrobiology: A Very Short Introduction, by David C. Catling
Extraterrestrials: Where Are They, edited by Ben Zuckerman and Michael Hart,
"Not As We Know It," by Isaac Asimov (essay, can be found in his collection View From a Height, also online)
The Search for Life in the Universe, by Donald Goldsmith and Tobias Owen
Xenology, by Robert A. Freitas (available on the Web here: http://www.xenology.info/Xeno.htm)
Now let's look at some potential biochemistries based on different kinds of worlds. We begin with the most exotic environment, life on a star.
Plasma Life: Ionized plasma within s star might host patterns of magnetic force sufficiently complex to form long-lived structures and reproduce themselves. In this context, "long-lived" might be measured in minutes, though presumably over many generations these "plasmobes" might evolve greater stability and longer lifespans. Extremely hot planets, like "hot Jupiters" grazing the photospheres of their parent stars, might also host plasma life.
So what would plasma life be like? To human eyes it would hardly be detectable — masses of ionized hydrogen scarcely distinguishable from the surrounding stellar atmosphere. With devices to measure electric and magnetic fields, however, plasma life would look like incredibly complex structures of fields and currents of ionized matter. I think they'd be big, as their density would be very low. A being as massive as a human would occupy tens of cubic meters. Fortunately stars are big, and these beings would be able to occupy a fairly thick layer, so they'd be like fish in a gigantic sea.
How would these beings get energy? Even though they're literally swimming in an ocean of sunlight, energy may be hard to come by. Living things need an energy gradient — something they can tap and make use of. The super-hot plasma of a star's photosphere is more or less at equilibrium.
I can think of two possibilities. Stellar magnetic fields get concentrated into "knots" which then break apart, causing sunspots and prominences. (I am oversimplifying, of course.) So our plasmobes might feed on the concentrations of magnetic fields. (How? I dunno. I've never met one.) We might see small, hot objects clustering around the edges of sunspots.
Another way they might extract energy is by living right on the boundary of the photosphere, at the layer where photons which have been bouncing around inside the star for millions of years finally reach an optically transparent layer and take off into interstellar space. Organisms might tap that flux — photons passing through a magnetic field can transfer energy to it. The star would look as though its surface was coated with millions, or even billions, of mini-sunspots, no more than a kilometer or so in size.
Naturally, the energy-harvesters could then form the base of a "trophic pyramid" of herbivores, carnivores, scavengers, and so forth. Especially complex plasma life forms would sense and manipulate their environment by magnetic or electric fields.
How would plasma life affect its environment? In the case of the magnetic-feeders, I expect they would damp down on flare activity as the plasmobes drain away that energy for their own use. A suspiciously quiet star might be inhabited.
The photosphere particle-harvesters would affect the star's radiation and particle output, turning high-energy photons into lower-energy ones. If a star's emission lines are oddly red-shifted even though it's not moving very fast, that might indicate plasma life covering its surface.
Dusty Plasma: This is a variant on plasma-based life, in which the ionized hydrogen incorporates microparticles of carbon and silicon. The particles can hold electric charges (the familiar "static electricity" phenomenon), and interactions between them and the ionized gas might allow for energy and information storage. One might expect dusty plasma organisms to exist on the hottest rocky planets, where stellar heat has baked away all the lighter elements — but hydrogen from the star's outer atmosphere can linger on the surface and interact with fine dust. They would likely be big: living storm systems, miles across. These star-kissed worlds would probably be tidally locked, and the dusty plasma organisms would never dare to venture onto the night side. If the planet has a magnetic field, the magnetic poles would either be energy-rich environments full of life — or possibly dangerous disruptive effects, avoided as much as the dark side.
Silicon Life: On very hot planets, with surface temperatures over 1000 degrees C or 1270 Kelvins, liquid (or at least "squishy") silicon compounds could act as a liquid solvent for complex silicon-carbon ("silicone") or silicon-sulfur molecules serving as the basis for life. Simple silicon molecules are annoyingly stable, but compounds of silicon with other elements, especially carbon and metals, might have more flexible properties.
IMPORTANT NOTE: I am not a chemist so I am probably getting a lot of this wrong, or oversimplifying. If you know more about chemistry than I do, feel free to comment and I will add edits to the text.
Silicon-based organisms could base their ecosystem on photosynthesis, just a very different kind from what plants do on Earth. They'd use sunlight to build long-chain silicon-carbon-sulfur compounds for energy storage. Or they could conceivably evolve biological photovoltaic collectors, turning sunlight directly into electric currents which could then power their cells.
Life derived from silicon would almost certainly not look like animated quartz crystals, any more than carbon-based life looks like blocks of graphite or diamond. They would be pretty massive and dense, though, since they're using molten glass as circulatory fluid. Sheer viscosity suggests that silicon life would be large.
Silicon life using photovoltaic power would be hard to detect, as the organisms wouldn't be transforming their environment very much. Storing up electrons in biological capacitors doesn't alter a planet's atmosphere the way plant photosynthesis did on Earth.
Sulfur Life: Sulfur is liquid from 386 to 718 Kelvins. In oceans of liquid sulfur one can imagine life forms made of silicon or silicone (silicon-carbon) compounds. Sulfur can also form long chains on its own. Again, the basis of their ecology could be photosynthesis or photovoltaics. Liquid sulfur is somewhat gooier than water (about twice as dense) so I expect sulfur life would tend to the large side.
Changes to the environment from sulfur life might be subtle: a liquid-sulfur ocean consisting entirely of monatomic or diatomic sulfur instead of larger isomers might indicate something pushing things out of equilibrium. Most rocks are combinations of silicon and oxygen, so building silicon-sulfur molecules would release free oxygen. If oxygen is rocket fuel on Earth, imagine how reactive it would be at 500 Kelvins in a sulfur ocean! It would form sulfur dioxide gas, which is a pretty plausible atmosphere component for any world with oceans of sulfur anyway.
Sulfuric Acid Life: Sulfuric acid is liquid from 280 to 600 Kelvins, a much bigger range than water. It could act as a solvent for life based on silicon or silicones, although existence would be an endless fight against corrosion. But on Earth all life fights an endless war against oxidization, so that's not very different. Worlds like Venus might have seas of sulfuric acid in which such life might evolve.
I expect that silicon plants doing photosynthesis in a sulfuric acid environment might wind up generating free hydrogen. An atmosphere rich in diatomic hydrogen on a hot rocky world would be a huge "biosignature." It would also be devastating over the long term, as the planet would lose hydrogen until its acid oceans dried up. Maybe that's what happened to Venus.
Further research has made me aware that there already are bacteria on Earth which generate energy using a sulfur plus hydrogen sulfide cycle, so this would be an obvious mechanism for life at the cooler end of the sulfuric acid spectrum. The big problem is that you can have oceans of sulfuric acid, or you can have liquid water, but you can't have both on the same world. Maybe a sulfur-hydrogen sulfide cycle is possible without water.
Next time we'll look at alternative kinds of life for the liquid-water zone, and talk about why carbon compounds in water are the best bet for life.
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