| The concept of life hopping from planet to planet, from one star system to another, may be as old as humanity’s interest in the stars — 25 centuries ago, the Greek philosopher Anaxagoras named this notion panspermia. The ancients imagined creatures much like us making great voyages across the skies. But after telescopes revealed the nature of outer space and microscopes revealed microorganisms, modern panspermia proponents promote spores drifting through space, ultimately landing on a habitable planet, and seeding life. The discoveries of 4000+ planets beyond our Solar System, and of objects from other star systems entering ours, such as ‘Oumuamua, sparked renewed interest in panspermia. Four astronomers, R. Gobat from Chile, S. E. Hong and S. Hong from Korea, and O. Snaith from France, hereafter GHHS, analyzed panspermia in model galaxies similar to our Milky Way. Their presentation is certainly more “scientific” than those of ancient philosophers, but many critical numbers remain highly speculative. GHHS list these essential steps for panspermia: 1) Life develops on a habitable planet 2) Meteor impacts eject surface matter with viable spores 3) Spore-bearing matter is ejected from host star system 4) Spores survive in outer space for a sufficient time 5) Spore-bearing matter captured by new star system 6) Spores land on habitable planet and seed new life The simplest process may be #4. GHHS cite brief lab studies of spores shielded within inorganic matter. Extrapolating these studies many orders of magnitude, GHHS conclude spores might survive 1 million years in the hard vacuum, extremely low temperature, and harsh radiation of interstellar space. GHHS present extensive calculations for several of the 6 essential processes. In the absence of reliable data, they are forced into many debatable, simplifying assumptions. Ultimately, they concede that definitive answers do not exist for any of the 6 processes. They hope their analyses can highlight trends. The GHHS assumption I most object to regards the habitability of planets orbiting M dwarfs (aka red dwarfs). Despite the fact that a great many other factors are essential for life, in the parlance of most astronomers, “habitable” merely means a planet’s surface temperature might possibly sustain liquid H2O. Recent enthusiasm for “habitable” planets focuses on M dwarf systems, since these are the most common — 50 of the 60 stars nearest our Sun are M dwarfs, as are an estimated 75% of all stars in the Milky Way. Due to their low mass, M dwarfs radiate very little light, so their “habitable” zones are extremely close to the star. Trappist-1, for example, has three planets in its “habitable” zone that orbit 21, 26, and 34 times closer to their star than Earth does to the Sun. Proxima Centauri b, another favorite, orbits 20 times closer than Earth. Thus, the stellar radiation these exoplanets receive is 400 to 1100 times greater than they would receive in an Earth-sized orbit. Unfortunately for potential life, M dwarfs have frequent, intense flares and CMEs (coronal mass ejections). As this log-log chart shows, M dwarfs radiate 1000 times more CME mass and 100,000 times more flare energy than our Sun, on an average basis.
The closer orbits and greater stellar activity mean planets in the “habitable” zones of the most common stars can be exposed to tens of millions of times more hazardous radiation than is Earth. Those intense radiation spikes, even intermittently, can sterilize a planet’s surface and erode its atmosphere. GHHS address this by modestly increasing the average luminosity of M dwarfs, which masks this critical issue. Spectacular flares, lasting 1 week and occurring every 1000 years, could well preclude life, whereas increasing the average starlight by 1 part in 52,000, which GHHS say is equivalent, has no meaningful effect. GHHS do not quote individual numerical estimates for each essential process, but do claim the probability of life from one habitable planet reaching another habitable planet within a spore’s lifetime is 1 in a million. GHHS arrive at the following conclusions: 1) Panspermia is most likely near the galactic core, where star density is greatest, and is much less likely far from the core 2) Habitability is less likely near the galactic core, which has the most hazardous supernovae, and becomes more likely far from the core. 3) The probability of life developing independently on a planet seems much higher than the probability of it being seeded by panspermia. If their last conclusion is valid, the answer to how inert chemicals become alive is not in our stars. Best Wishes, Robert January 2022 Note: Previous newsletters can be found on my website. | 