Astronomers’ quest to find another planet capable of supporting life continues in earnest. As they compile their findings, we all learn more about what life requires. The fact that Earth has avoided a collision with the Moon magnifies our planet’s rarity as an advanced life site.
For advanced life to exist on a planet, that planet must reside simultaneously in 16 habitable zones, which include the known extragalactic habitable zone, the two known galactic habitable zones, and the 13 known planetary habitable zones. I have written extensively about these habitable zones in my books, Improbable Planet and Designed to the Core.1
To date, astronomers have discovered and determined at least some of the orbital and physical features of 5,373 planets.2 Many of these planets reside in the known extragalactic and galactic habitable zones and the liquid water planetary habitable zone, but only one resides in all 16 of the known habitable zones.
Planet-Moon System Habitability Criteria
Since the 1990s, astronomers have understood that advanced life is possible only on a rocky planet that is orbited by a moon that is a substantial fraction of the planet’s mass. In the case of the Earth-Moon system, the Moon’s mass is 1.23% Earth’s mass.3 This small percentage may not seem like much, but relative to its host planet’s mass the Moon is 52 times larger than any other known moon, namely Titan.
It takes the extraordinarily large moon-planet mass ratio of the Earth-Moon system to stabilize Earth’s rotation axis tilt. Earth’s rotation axis tilt regularly oscillates between 22.04 and 24.50 degrees. By comparison, Mars’s rotation axis tilt varies from 0–60 degrees. Thanks to Earth’s stable rotation axis tilt, our climate is sufficiently stable to permit the existence of advanced life.
The combination of the Moon’s high mass relative to Earth and its proximity (see figure) causes powerful lunar tidal forces on Earth. These tidal forces recycle life-critical nutrients all along the continental shelves, thereby enabling the biomass and biodiversity that advanced life needs.
Figure: Earth-Moon System
Earth-Moon separations and sizes are to scale. Diagram credit: Hugh Ross
The Moon’s tidal forces have operated for the past 4.47 billion years4 to gradually slow down Earth’s rotation rate from just 3–4 hours per day to the present 24 hours per day. A 24-hour rotation rate is ideal for advanced life, whereas either a 23-hour or a 25-hour rotation rate would be a huge problem.5 Thanks to the Moon’s highly fine-tuned mass and rate of orbital recession from Earth, Earth attained a 24-hour rotation rate at the same time in solar system history when the Sun’s luminosity became stable enough for advanced life to exist.6
The Moon also rescued Earth from losing all of its atmosphere and surface water. During its first 700 million years, the Sun was blasting out particles and radiation that normally would have sputtered away all Earth’s atmosphere and water. Two of the Moon’s features prevented that outcome. The Moon’s strong magnetic field and proximity to Earth allowed its magnetic field to couple with Earth’s magnetic field. Both the early Earth and early Moon possessed hot liquid iron cores and were sufficiently close to one another that their mutual tidal force exertion circulated the liquid iron in their cores. This circulation generated powerful magnetic fields surrounding both bodies. Nothing less than the coupling together of the two magnetospheres would have preserved Earth’s atmosphere and its surface oceans.7
Thanks to Earth’s amazingly fine-tuned Moon, an abundance of diverse life has thrived for 3.8 billion years on Earth’s surface. The Moon’s exquisitely designed features have protected advanced life and global civilization. However, the Moon will not always be a protector of life on Earth. A lunar doomsday is coming—but the Sun is on schedule to precede the Moon with its own doomsday.
Since its formation 4.47 billion years ago, the Moon has been slowly spiraling away from Earth. Its orbital distance from Earth increases by 3.82 ± 0.07 centimeters/year.8
Earth’s tidal forces exerted on the Moon have resulted in the Moon becoming tidally locked. Those tidal forces have slowed the Moon’s rotation rate to equal its revolution rate, causing one hemisphere of the Moon to always face Earth. At the same time, the Moon’s tidal forces exerted on Earth are slowing down Earth’s rotation rate. Eventually, Earth will become tidally locked to the Moon, but not for another 40–50 billion years. When that occurs, the Moon will stop spiralling away from Earth and begin to spiral inward. This inward spiral will result in the Moon crashing into Earth—the ultimate doomsday scenario for life.
However, long before this catastrophe occurs, the Sun will become a red giant star and incinerate both the Moon and Earth. This incineration event will occur in about another 4.5 billion years.
Unstable Moons in Exoplanet Systems
Another rare feature of Earth’s Moon is that it has been dynamically stable over the entire period of Earth’s conceivable habitability. University of California at Los Angeles theoretical physicist Bradley Hansen, in a recently published paper, has demonstrated three ways that it may be common for large moons orbiting rocky planets in extrasolar planetary systems to be dynamically unstable.9
The rocky extrasolar planets discovered so far orbit their host stars at distances considerably smaller than Earth’s orbital distance from the Sun. Hansen first cites research that establishes that the rate of tidal evolution of planet-moon systems is inversely proportional to the planet’s orbital distance from its host star, to the host star’s mass, and to the planet’s mass.10
Second, Hansen cites other research showing that it takes a collision between two or more rocky planets to form a large moon that subsequently orbits a rocky planet.11 Such a collision causes the planet to rotate more rapidly than it did before the collision. However, if the subsequent planetary rotation rate is too low, then the planet will extract angular momentum from its moon’s orbit, causing the moon to spiral inward and relatively quickly collide with its planet. On the other hand, if the planet’s subsequent rotation rate is sufficiently high, then angular momentum will be transferred to the moon, resulting in the moon spiraling outward and escaping the gravitation grip of its planet. For situations similar to the Earth-Moon system, where the host star exerts a substantial tide on the planet, the outward spiraling of the moon eventually reverses, resulting in a planet-moon collision.
Third, Hansen calculated that even in the case where the large moon escapes the gravitational grip of its planet, a collision of the moon with its original host planet is highly likely. When the moon spirals outward to the point where it no longer orbits its planet, it nonetheless continues to share the planet’s orbit about their host star. As such, the escaped moon will make repeated close encounters with its planet. Hansen demonstrated that “nearly all of the freed exomoons will eventually collide with the original host planet.”12
What effect would planet-moon collisions have on another star like the Sun? In 2015, the F-type main-sequence star, KIC 8462852, investigated by a team of astronomers led by Tabetha Boyajian, became a media sensation.13 Unusual luminosity fluctuations in the star that included dimming of up to 22% caused astronomers and laypeople alike to speculate that an extraterrestrial intelligent civilization had constructed a Dyson sphere around the star. This speculation garnered KIC 8462852 the nickname, Tabby’s star.
A Dyson sphere is a megastructure, hypothesized by physicist Freeman Dyson, encompassing a star that captures a large percentage of the star’s power output and transforms that power into a useful energy source for an intelligent civilization. Follow-up observations of KIC 8462852 extending through the end of 2017, made by Boyajian and her team showed wavelength-dependent dimming consistent with huge clouds of dust periodically blocking out some of KIC 8462852’s light, but not with an opaque alien megastructure, which would equally block out all wavelengths of light.14
Hansen’s study provides a reasonable explanation for KIC 8462852’s luminosity variations. One or more moons colliding with planets orbiting stars like KIC 8462852 would generate huge clouds of dust orbiting such stars that would well explain the observed dimming variations.
Yet More Lunar Design
As Hansen explained, collisions between rocky planets and their large moons may sterilize otherwise habitable planets. Hansen has added yet another habitability requirement for planets. Advanced-life-habitable planets must be rocky planets that simultaneously reside in all 13 known planetary habitable zones and possess a moon that is about 1% the planet’s mass where that moon remains in a stable orbit about the planet for a long enough time period for advanced life to exist.
The probability for all these requirements to be met without invoking supernatural, super-intelligent Agency is absurdly remote.15 Thanks to Hansen’s research, the case for a supernatural, super-intelligent Agent has become scientifically stronger, demonstrating that the more scientists learn about nature the more evidence they uncover for the handiwork of a personal God.
Hugh Ross, Improbable Planet (Grand Rapids: Baker, 2016), 81–93; Hugh Ross, Designed to the Core (Covina, CA: RTB Press, 2022), 132–181; Hugh Ross, “Complex Life’s Narrow Requirements for Atmospheric Gases,” Today’s New Reason to Believe (blog), Reasons to Believe, July 1, 2019; Hugh Ross, “Tiny Habitable Zones for Complex Life,” Today’s New Reason to Believe (blog), Reasons to Believe, March 4, 2019; James Green et al., “When the Moon Had a Magnetosphere,” Science Advances 6, no. 42 (October 14, 2020): id. eabc0865, doi:10.1126/sciadv.abc0865.
Eoxplanet TEAM, “Catalog,” The Extrasolar Planets Encyclopaedia, accessed April 24, 2023, http://exoplanet.eu/catalog/.
Mark A. Wieczorek et al., “The Constitution and Structure of the Lunar Interior,” Reviews in Mineralogy and Geochemistry 60, no. 1 (January 1, 2006): 221–364, doi:10.2138/rmg.2006.60.3.
Robin M. Canup et al., “Origin of the Moon,” (March 2, 2021): 13–14, arXiv:2103.02045. This paper is a book chapter in New Views of Moon II, which will be published as special volume of Reviews in Mineralogy and Geochemistry.
Ross, Designed to the Core, 173; Dave Waltham, “Anthropic Selection for the Moon’s Mass,” Astrobiology 4, no. 4 (December 20, 2004): 460–461, doi:10.1089/ast.2004.4.460.
Ross, Designed to the Core, 117–129.
Green et al., “When the Moon Had a Magnetosphere; Hugh Ross, “Earth-Moon Coupled Magnetosphere Paved the Way for Life,” Today’s New Reason to Believe (September 20, 2021); Hugh Ross, “Moon’s Early Magnetic Field Made Human Existence Possible,” Today’s New Reason to Believe (November 16, 2020).
J. O. Dickey et al., “Lunar Laser Ranging: A Continuing Legacy of the Apollo Program,” Science 265, no. 5171 (July 22, 1994): 482–490, doi:10.1126/science.265.482.
Bradley M. S. Hansen, “Consequences of Dynamically Unstable Moons in Extrasolar Systems,” Monthly Notices of the Royal Astronomical Society 520, no. 1 (March 2023): 761–772, doi:10.1093/mnras/stac2847.
Takashi Sasaki, Jason W. Barnes, and David P. O’Brien, “Outcomes and Duration of Tidal Evolution in a Star-Planet-Moon System,” Astrophysical Journal 754, no. 1 (July 20, 2012): id. 51, doi:10.1088/0004-637X/54/1/51; Takashi Sasaki and Jason W. Barnes, “Longevity of Moons around Habitable Planets,” International Journal of Astrobiology13, no. 4 (October 2014): 324–336, doi:10.1017/S1473550414000184; Armen Tokadjian and Anthony L. Piro, “Impact of Tides on the Potential for Exoplanets to Host Exomoons,” Astronomical Journal 160, no. 4 (October 6, 2020): id. 194, doi:10.3847/1538-3881/abb29e.
Robin M. Canup, “Lunar-Forming Impacts: Processes and Alternatives,” Philosophical Transactions of the Royal Society A 372, no. 2024 (September 13, 2014): id. 20130175, doi:10.1098/rsta.2013.0175; Canup et al., “Origin of the Moon.”
Hansen, “Consequences of Dynamically Unstable Moons,” 762.
Tabetha S. Boyajian et al., “Planet Hunters IX. KIC 8462852—Where’s the Flux?” Monthly Notices of the Royal Astronomical Society 457, no. 4 (April 21, 2016): 3988–4004, doi:10.2093/mnras./stw218.
Tabetha S. Boyajian et al., “The First Post-Kepler Brightness Dips of KIC 8462852,” Astrophysical Journal Letters 853, no. 1 (January 20, 2018): id. L8, doi:10.3847/2041-8213/aaa405.
Independent of Hansen’s study, I demonstrate this remote possibility in my books, Improbable Planet and Designed to the Core.
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