Where is everybody? This common query represents the bewilderment of decades of futility in the search for extraterrestrial intelligent life (ETI). Part of the more recent disappointment stems from what scientists call the red sky paradox.1 It refers to at least five enigmatic facts.
The first is that red dwarf stars are by far the most numerous stars in the universe, accounting for 78% of all nuclear-burning stars (stars that are actively fusing lighter elements into heavier elements) in the Sun’s vicinity.2 Second, red dwarf stars sustain nuclear burning much longer than other stars. Compared to the Sun, red dwarf stars maintain nuclear burning 20 times longer on average. Third, red dwarf stars, during their nuclear–burning history, brighten at a much slower pace than all other nuclear-burning stars. This slower brightening pace means more stable temperatures on the surfaces of planets orbiting red dwarf stars.
The fourth fact is that astronomers have observed an abundance of rocky planets orbiting red dwarf stars. About 59% of all rocky planets discovered so far are hosted by red dwarf stars.3 Fifth, about a third of these rocky planets are temperate, meaning they orbit their host stars at distances where it is possible for liquid water to exist on at least small parts of the planets’ surfaces for at least some fraction of their host star’s nuclear–burning history.
Assuming that all temperate rocky planets host microbial life, and further assuming that given sufficient time, namely a few billion years, microbial life will evolve into advanced intelligent life, then we should have discovered ETI by now on a large number of planets orbiting red dwarf stars. The fact that we have not yet discovered ETI constitutes the paradox. In other words, given that red dwarf stars are so predominant, long-lived, luminosity stable (average brightness changes gradually), and prolific in producing rocky planets, why do we not find ourselves orbiting a red dwarf star?
On the other hand, if the evolution of intelligent life from microbial life is not an ensured and universally rapid (occurring within a few billion years) process, then the advantage of red dwarf stars, owing to their much greater longevity, becomes even bigger. The red sky paradox becomes all the more perplexing.
Proposed Resolutions to the Red Sky Paradox
In a paper published in the Proceedings of the National Academy of Sciences USA, computational astrophysicist David Kipping proposed four non-mutually exclusive resolutions to the red sky paradox.4
Proposal #1: The first of his proposed resolutions is that we humans find ourselves orbiting a yellow dwarf star rather than a red dwarf star by an extraordinary stroke of random chance. That is, our star, planet, and species are extreme statistical outliers.
Kipping did not find his first proposal at all satisfying. In his words, this proposal has the effect of “softening the red sky paradox, but exacerbating the classic Fermi paradox.5 The Fermi paradox, named after physicist Enrico Fermi, is the apparent contradiction between the calculated high probabilities for the existence of ETI and the complete lack of substantiated evidence for ETI.
Proposal #2: Kipping’s second proposal is that life on a planet with a red sky (sky illuminated by a red dwarf star) may be inhibited. That is, it could be that life requires significant, enduring exposure to 300–450 nanometer wavelengths (long ultraviolet, visible violet, and blue wavelengths) and not just the 600–800 nanometer wavelengths (visible orange, visible red, and infrared wavelengths) that radiate from red dwarf stars.
Proposal #3: Kipping’s third proposal is that the time window for advanced life on a planet with a red sky (sky illuminated by a red dwarf star) may be brief. Red dwarf stars spend much more time than other stars in the pre-main sequence phase. The pre-main sequence phase is the time between star formation and the ignition of nuclear fusion. Red dwarf stars spend 0.1–1.1 billion years in the pre-main sequence phase. During this phase, red dwarf stars’ luminosities are much higher than they are during the main sequence phase of nuclear fusion burning.6 These greater luminosities imply high probabilities of generating moist greenhouse threshold events on liquid-water-bearing planets hosted by these stars.7
A moist greenhouse threshold event occurs when the brightening of a planet’s host star causes a greater portion of the planet’s liquid water to be transformed into water vapor. Water vapor is a greenhouse gas. More water vapor in the planet’s atmosphere leads to a higher surface temperature, which causes yet more liquid water to be transformed into water vapor, producing even higher surface temperatures, and so on.
Eventually, so much water vapor builds up in the planet’s troposphere that this water vapor escapes into the planet’s stratosphere. The host star’s radiation dissociates this stratospheric water vapor, splitting water molecules into hydrogen and oxygen atoms. The resultant hydrogen escapes to interplanetary space. With sufficient time, this hydrogen escape results in the complete desiccation of the planet.8 Given how long red dwarf stars spend in the pre-main-sequence phase, most liquid-water-bearing planets orbiting red dwarf stars end up bone–dry long before their host stars enter the main sequence phase where the stars’ luminosities become stable enough to make life possible.
Proposal #4: Kipping’s fourth proposal is that the occurrence rate of Earth-sized planets hosted by red dwarf stars where the planets are both temperate and moist may be very low. Currently, the technology available to astronomers, with rare exceptions, is only able to detect planets orbiting the brighter red dwarf stars. It is not known if the number of planets orbiting the brighter red dwarf stars represents the total number orbiting all red dwarf stars. It seems unlikely that planets as large as Earth orbit the smaller and much more numerous red dwarf stars as abundantly as they orbit the largest red dwarf stars.
Astronomers typically assume that planets with surface temperatures that permit the existence of liquid water will indeed possess surface liquid water. For red dwarf stars, this assumption is likely incorrect. The detected population of Earth-sized planets orbiting red dwarf stars could be dominated by photo-evaporated cores of super-Earths—planets that were larger than Earth that have had their atmospheres and oceans stripped away by their host stars’ radiation.
Additional Red Sky Paradox Resolutions
In last week’s article, Eyes, Sun, and Earth Designed to Prevent Myopia, I explained the necessity of regular exposure to visible violet light to maintain the functionality of eyes. In 2016 (Overlap of Habitable Zones Gets Much Smaller), I explained why all life, not just organisms with eyes, requires a certain intensity and wavelength range of ultraviolet light to exist. This requirement means that, for life to possibly exist on a planet, it must orbit its star within the ultraviolet habitable zone. For planets orbiting red dwarf stars, the ultraviolet habitable zone never overlaps the liquid water habitable zone.
Since requirements for every conceivable physical life–form include that it must exist on a planet or moon orbiting its host star in both the liquid water and ultraviolet habitable zones, all red dwarf stars are eliminated as candidates to host life-bearing planets. For this reason alone, the red sky paradox is resolved.
Red dwarf stars are much cooler than the Sun. Therefore, for planets orbiting red dwarf stars to be temperate enough for life, they must orbit at distances much closer than Earth orbits the Sun. This requirement poses a problem. Any planet with an atmosphere thicker than 1% of Earth’s (a requirement for life) that is closer to its host star than about 90% of Earth’s distance from the Sun will very likely possess an atmospheric electric field strong enough to completely dry out the planet.9 Venus, for example, has an atmospheric electric field of 10 volts.10
Additionally, all red dwarf stars emit powerful ultraviolet and X-ray flares. Astronomers used the MUSCLES (Measurements of the Ultraviolet Spectral Characteristics of Low-Mass Exoplanetary Systems) Treasury Survey to establish that, even for the quietest red dwarf stars, the flux of ultraviolet and soft X-rays is more than sufficient to erode away the atmospheres and hydrospheres of planets orbiting these stars that reside inside the liquid water habitable zone.11
Plus, three-dimensional magnetohydrodynamic models of the impact of the stellar winds of red dwarf stars show that, for planets orbiting these stars in the liquid water habitable zone, the stellar wind pressure on the planets is 100–100,000 times greater than the solar wind pressure on Earth.12 This greater stellar wind pressure will compress any possibly existing magnetospheres around the planets to a degree that erosion of the planetary atmospheres cannot be prevented. That is, the greater stellar wind pressure will speed up the erosion of planetary atmospheres and hydrospheres generated by the stellar flux of ultraviolet and soft X-rays.
Lastly, the tidal forces a star exerts on one of its planets increases with the fourth power of the inverse of the distance between the star and the planet. Therefore, a planet need only be slightly closer to its host star than Earth is to the Sun before tidal forces result in the planet’s rotation period becoming equal or nearly equal to its period of revolution. For such a tidally locked planet, one hemisphere will be blistering hot while the other hemisphere will be freezing cold. All planets orbiting red dwarf stars that are within the liquid water habitable zone will be tidally locked.13 While liquid water conceivably could exist in the twilight zone on a planet’s surface (a transition zone at the edge of stellar illumination), atmospheric transport would move water to the coldest parts of the planet where it would freeze.14
The resolution of the red sky paradox means that red dwarf stars cannot possibly host any habitable planets. The combination of no atmosphere, no surface water, deadly persistent X-rays and far–ultraviolet radiation, and frequent major flare events on their host stars renders these planets unfit to support any kind of physical life. There is no escaping the fact that 78% of all stars are noncandidates for hosting planets on which physical life is possible.
For a planet to be truly habitable, it must reside not only in the temperate and liquid water habitable zones but also in all the other known planetary habitable zones. Thirteen such habitable zones are now known to exist.15 Of the 4,801 planets discovered to date,16 only one simultaneously resides in all 13 known planetary habitable zones. This planet also has all the design features needed to make advanced life and a global, high-technology civilization possible. It is Earth—the one planet that testifies of the exquisite handiwork, intelligence, and power of the Creator God of the Bible.
- Jacob Haqq-Misra, Ravi Kumar Kopparapu, and Eric T. Wolf, “Why Do We Find Ourselves Around a Yellow Star Instead of a Red Star?” International Journal of Astrobiology 17, no. 1 (January 2018): 77–86, doi:10.1017/S1473550417000118; David Kipping, “Formulation and Resolutions of the Red Sky Paradox,” Proceedings of the National Academy of Sciences USA 118, no. 26 (June 2021): e2026808118, doi:10.1073/pnas.2026808118.
- Glenn LeDrew, “The Real Starry Sky,” Journal of the Royal Astronomical Society of Canada 95 (February 2001): 32–33, https://rasc.ca/sites/default/files/publications/JRASC-2001-02.pdf.
- “Catalog,” The Extrasolar Planets Encyclopaedia, Exoplanet TEAM, updated July 15, 2021, http://exoplanet.eu/catalog/.
- Kipping, “Formulation and Resolutions.”
- Kipping, “Formulation and Resolutions,” 1.
- Chushiro Hayashi, “Stellar Evolution in Early Phases of Gravitational Contraction,” Publication of the Astronomical Society of Japan 13, no. 4 (August 1961): 450–452, https://ui.adsabs.harvard.edu/abs/1961PASJ…13..450H/abstract.
- Peter Gao et al., “Stability of CO2 Atmospheres on Desiccated M Dwarf Exoplanets,” Astrophysical Journal 806, no. 2 (June 2015): 249, doi:10.1088/0004-637X/806/2/249.
- Hugh Ross, “Moist Greenhouse Threshold Doomsday,” Today’s New Reason to Believe (blog), Reasons to Believe, March 11, 2019, https://reasons.org/explore/blogs/todays-new-reason-to-believe/moist-greenhouse-threshold-doomsday; Illeana Gómez-Leal et al., “Climate Sensitivity to Carbon Dioxide and the Moist Greenhouse Threshold of Earth-Like Planets under an Increasing Solar Forcing,” Astrophysical Journal 869, no. 2 (December 2018): 129, doi:10.3847/1538-4357/aaea5f.
- Glyn Collinson et al., “The Electric Wind of Venus: A Global and Persistent ‘Polar Wind’-Like Ambipolar Electric Field Sufficient for the Direct Escape of Heavy Ionospheric Ions: Venus Has Potential,” Geophysical Research Letters 43 (June 2016): 5926–5934, doi:10.1002/2016GL068327.
- Collinson et al., “The Electric Wind.”
- Allison Youngblood et al., “The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca II K, and Energetic Particle Fluxes from M Dwarfs.” Astrophysical Journal 843 (June 2017): 31, doi:10.3847/1538-4357/aa76dd.
- Cecilia Garraffo et al., “The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets,” Astrophysical Journal Letters 843 (July 2017): L33, doi:10.3847/2041-8213/aa79ed.
- Rory Barnes, “Tidal Locking of Habitable Exoplanets,” Celestial Mechanics and Dynamical Astronomy 129 (December 2017): 509–536, doi:10.1007/s10569-017-9783-7.
- Kristen Menou, “Water-Trapped Worlds,” Astrophysical Journal 774, no. 1 (September 2013): 51, doi:10.1088/0004-637X/774/1/51.
- Hugh Ross, “Complex Life’s Narrow Requirements for Atmospheric Gases,” Today’s New Reason to Believe (blog), Reasons to Believe, July 1, 2019, https://reasons.org/explore/blogs/todays-new-reason-to-believe/complex-life-s-narrow-requirements-for-atmospheric-gases; Hugh Ross, “Tiny Habitable Zones for Complex Life,” Today’s New Reason to Believe (blog), Reasons to Believe, March 4, 2019, https://reasons.org/explore/blogs/todays-new-reason-to-believe/tiny-habitable-zones-for-complex-life; Hugh Ross, “Moon’s Early Magnetic Field Made Human Existence Possible,” Today’s New Reason to Believe (blog), Reasons to Believe, November 16, 2020, https://reasons.org/explore/blogs/todays-new-reason-to-believe/moons-early-magnetic-field-made-human-existence-possible-2.
- “Catalog,” The Extrasolar Planets Encyclopaedia.