When we think of life on other planets, we tend to think of planets orbiting stars, warmed by their energy, with plants (or something analogous to our plants) that are autotrophs, using sunlight to perform photosynthesis to produce food, and animals (or something analogous) that are heterotrophs, unable to produce their own food, obtaining it directly or indirectly from the autotrophs. But what if worlds suitable for such life, or at least what we consider to be advanced forms of it, are rare? What if, contrary to what we have always believed, most planets are located not in solar systems orbiting stars but far out in interstellar space, yet, nevertheless, harboring life forms not entirely unlike our own—based on liquid water and complex carbon compounds, probably organized into cells—with DNA? What if the life forms exist, not on the frozen surfaces of such worlds—but underground, depending on the heat energy produced by the planets? What if such life forms exist also within our own Solar System, including here on Earth beneath our feet?
The overwhelming majority of the stars in our galaxy, as well as in other galaxies, are concentrated near the center and in globular clusters surrounding the center. Here the stars are so closely packed that night skies would not be dark, and in the densest regions temperatures would be too high for life to exist. In addition, such worlds would be hit by radiation from nearby supernovas. But there is an even greater problem: these stars are almost all very old population two stars, which contain almost no elements heavier than hydrogen and helium; planets would be (if they could form at all) balls of hydrogen, with no carbon, no oxygen, no water…and no life.
Most stars throughout our galaxy are binary or multiple stars orbiting a common center of gravity. If the binaries are extremely close together or extremely far apart, planets could form with stable orbits in the “Goldilocks zone” where temperatures are suitable for life…otherwise, forget about it. Stars very much more massive than our own Sun “burn” hydrogen at such a rapid rate that they don’t remain on what astronomers call the main sequence very long, before becoming unstable and exploding; life might form on their planets, but, based on the only model familiar to us, our own Earth, it would seem unlikely that advanced forms would have time to develop. And the overwhelming majority of stars are a good deal smaller than our Sun; these red dwarfs have Goldilocks zones, and they stay on the main sequence for countless billions of years. But planets close enough to be in the zone would be hit by solar flares and heated by tidal stresses. Vulcanism would be extreme and their surfaces might even be molten. Eventually they would become tide locked, with one side always facing their parent star, burning hot and airless, and the other side facing away, with all the air and water frozen there at temperatures close to absolute zero. True, there would be a twilight zone, but it, too, would be airless.
But it is a law of sorts that small things are more common than big things…there are more microbes than whales, more grains of sand than boulders, and so on. Between the stars that we can see are less massive and cooler “brown dwarfs,” emitting in the infrared. And there is every reason to believe that there are planetary-sized objects there as well. In our own Solar System, beyond the orbit of Neptune, ranging from 30 to 50 A.U. (astronomical units, or the mean distance of the Earth from the Sun), is the Kuiper belt, containing planetary sized objects like Pluto, Haumea, and Makemake. These are very small worlds, but there is no reason why larger ones might not also exist. Beyond the Kuiper Belt is the Oort Cloud (its existence is not absolutely proven), which astronomers believe may extend from 2,000 or 3,000 A.U. out to about 50,000 A.U. This is believed to be the source of most of the comets that sometimes come our way. Even some astronomers have recently speculated that there may very well be planetary-sized objects out there as well, perhaps thousands of times as many as orbit stars.
Throughout the universe, large and massive objects generate a great deal of internal heat, and smaller ones generally are cooler. The conventional explanations for this—residual heat from the planet’s formation, slow gravitational compression, and radioactive isotope decay—don’t quite explain all of it. Something else is going on, something that appears to be a challenge to conventional physics.
In addition, many moons are heated by tidal stresses if they orbit near their parent body, like Jupiter’s moon Io. Europa, another moon of Jupiter, is covered by ice but astronomers are now reasonably sure that, deep beneath the ice, is an ocean of liquid water. Given that all the elements needed for life, and even organic compounds, have been found in meteorites and in comets and even in interstellar gas clouds, it seems likely that such compounds, and perhaps even living organisms, might exist on Europa. Saturn’s moon Titan is bitterly cold on its surface, with an atmosphere of nitrogen and lakes of liquid methane; but, again, astronomers believe that warmer temperatures exist deep below the surface. Unmanned spacecraft found ice volcanoes on Saturn’s moon Enceladus, Neptune’s moon Triton, and some evidence for ice volcanoes on Titan, on Jupiter’s moon Ganymede, and Uranus’s moon Miranda. This indicates liquid water beneath the surface. Depending on how hot a world is, livable conditions might be found anywhere from near the core in smaller and cooler ones to near the surface in warmer ones.
On Earth, extremophile bacteria live in temperatures ranging from near freezing to 252 degrees Fahrenheit, well above the boiling point of water at sea-level pressure. Water under pressure deep underground or around deep-sea hydrothermal vents can remain liquid at higher temperatures. Many of these bacteria are not photosynthetic, like plants, but chemosynthetic, deriving energy from hydrogen gas in hot springs or from complex chemical reactions involving iron, potassium, and sulfur. There seems no reason why such organisms could not live within Europa, for example, or within planets far out in interstellar space. Since we don’t really understand the source of heat within planets, no one knows how small a world might be and still have livable conditions deep inside. But bacteria (some biologists classify some of these extremophiles in another kingdom, archaea) are one thing, even if they are organized into multi-cellular organisms. There seems little chance that these autotrophs, analogous to our plants, could develop any degree of consciousness. Since animals need oxygen to survive, there would be no animals…or so we thought.
Recently, animals have been discovered that need no oxygen. Tiny creatures, one millimeter long, of the phylum Loricifera, have been found in marine sediment and gravel at a depth of 3,000 meters in the L’Atalante basin in the Mediterranean. Instead of the organelles known as mitochondria that produce energy in most organisms’ cells, these creatures have organelles called hydrogenosomes. Through an anaerobic process, they produce energy by breaking glucose down into two pyruvate molecules, which then are converted into lactic acid. In addition, a nematode worm Halicephalobus mephisto, only some one-fiftieth of an inch long, has been found living in rock pores at a depth of 9,000 feet in a South African gold mine; it is suspected that they may live at even greater depths. Completely anaerobic, needing no oxygen, it feeds on chemosynthetic bacteria which also live at that depth. Theoretically at least, such animals might exist within the moons of our outer planets (or even deep within our own Moon) or inside the planets in the dark regions between the stars. While the outer planets and moons are relatively deficient in heavier elements compared to Earth, we know that they contain water, methane, nitrogen, and all the elements necessary for our type of life.
Moving deeper into the realm of speculation, an Earth-sized planet far out in interstellar space might have a very small and very hot, perhaps molten, nickel-iron core surrounded by a relatively thin mantle of silicates. Above that would be a very deep ocean of liquid water, hot at the bottom and cold at the top. Above that would be a thick shell of water ice and above that, perhaps layers of frozen methane and nitrogen.
Moving even further from proven fact and into pure speculation, perhaps anaerobic animals exist within some of these worlds, and perhaps some of them are fairly large multi-cellular organisms. Could they develop any degree of intelligence? As one who has been convinced by the evidence that intelligent design, not the Darwinian version of evolution, explains the diversity of life forms here on Earth, how can I, or anyone, deny the possibility? But that’s all it is…a possibility. It seems unlikely that such animals could develop, living in darkness underwater, any kind of technology or have knowledge of the universe above the ice, unless they have psychic abilities. (If so, this raises some rather unsettling, even perhaps sinister, possibilities if such creatures exist deep under our own Earth’s crust and can interact with us.)
Assuming our own civilization doesn’t self-destruct in the very near future, which seems increasingly likely, new discoveries will eventually be made, and perhaps some of these questions can be answered.