A Hypothesis! question about Life on other planets. I’ve been watching old Star Trek episodes and they feature all sorts of different sentient life forms, some similar to humans and others vastly different. Mr. Spock, one of the Star trek characters, may have said to the captain: “It’s life, Jim, but not as we know it.” My question is: How varied is life? What are the outliers for life. Could life forms exist in the void of space, living on dark matter?
Well, I didn’t know so I asked around at the Science Circle. Dr. William Wall (Syzygy Asymptote in Second Life) developed the section below about what life requires.
“The least hospitable place in the solar system must be the planet Venus, having hellish surface temperatures of 462 C (864 F), a crushingly heavy atmosphere 92 times that of Earth’s, and sulfuric acid in its atmosphere. And yet recent spectroscopic observations of its atmosphere have revealed the presence of the gas phosphine, a possible biosignature. Life in the hot, thick, caustic clouds of Venus?! Maybe, but doesn’t life require much more benign conditions?
What exactly are the conditions necessary for life to exist? Life as we know it requires at least 6 elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Such life also needs liquid water as the solvent for the chemistry of life. Liquid water requires temperatures between 0C (32F) and 100C (212F) and an ambient pressure. So, we expect life to also require those temperatures and an atmosphere to provide that pressure. Or those temperatures and subsurface water. And yet, extremophiles on the Earth are able to live in much higher temperatures, like 120C (248F) for those that live in the submarine hydrothermal vents. So living things can be very hardy indeed. But most life that we know lives at temperatures of 0-100C.
Such conditions exist in many places in our solar system and beyond. Outside the Earth, Mars likely has subsurface liquid water. Such water is also beneath the surface ice on various moons, such as the Jovian moon Europa, and the Saturnian moon Enceladus. Despite their icy surfaces, these moons have liquid water underneath because of the tidal heating effects of the gas giant planets they orbit. They are also rich in the aforementioned 6 elements of life. Even Venus is almost benevolent in its clouds between 50 and 65 km above the surface, with temperatures below 80C (176F) and might accommodate thermoacidophilic extremophile microorganisms. Which might be the sources of the phosphine?
Beyond the solar system there are thousands of known extrasolar planets, or exoplanets, ranging in size from less than Earth-size to many times the size of Jupiter. Roughly one in five earth-size exoplanets are within the habitability zone of their star, where surface water could be in liquid form if there’s the right atmosphere. In our galaxy alone, there could be 11 billion such potentially habitable earth-sized planets. Do any have atmospheres with oxygen? That can be tested by observing the absorption spectrum of the planet when it transits its star, possibly indicating life. But, so far, no actual life beyond the Earth has been unambiguously detected… yet. Nevertheless, the future is hopeful!”
So, life based on the 6 elements common to life on Earth may be found in our universe, but coming back to Mr. Spock, “It’s life, but NOT as we know it.” What might life based on other elements be like? Could silicon be substituted for carbon? Dr. William Schmachtenberg (Dae Miami in Second Life) suggested that before there was oxygen in abundance on earth other elements may have been used. He said, “It is always a fun topic to discuss the requirements for life. I think it is safe to say liquid water seems the basic prerequisite. Oxygen is important but we now know that some organisms flourish on sulfur rather than oxygen. That was good at the beginning of our planet when oxygen values were low and algae terraformed our atmosphere from toxic gases to an oxygen one.”
- Star Trekkin’, Wikipedia.
- Are We Alone? Life in the Universe. Richard Pogge, Astronomy 161: An Introduction to Solar System Astronomy
- Extraterrestrial Life, Wikipedia.
- Venus, Wikipedia.
- Extremophile, Wikipedia.
- Phosphine gas in the cloud decks of Venus, Greaves, J.S., Richards, A.M.S., Bains, W. et al. (2020). arXiv:2009.06593 [astro-ph.EP].
- Image Credit: The Drake Equation from Anybody Out There?, Science Line, 2019.