The Red Sky Paradox

 The Sun & Sky Should Be Red—But They Aren't

Perhaps you learned in elementary school that the sky is blue because the Sun is so bright. The Sun looks yellow or orange, depending on what particles its light passes through on its way to our eyes. Without all this "interference" near Earth's surface, we would see that the Sun's light is actually white. Oddly enough, white light is not what astronomers would expect of a star that is host to a life-bearing planet. Red light is, and for several reasons.

First, stars called "red dwarfs" (stars just hot enough to sustain nuclear burning) are the most numerous in the universe. They account for 78 percent of all nuclear-burning stars.¹ As slower nuclear burners, they continue to burn longer—some 20 times longer, on average, than the Sun. What's more, during their nuclear-burning phase, red dwarfs brighten at a much slower pace than do stars like our Sun. This slower brightening means longer-term temperature stability for planets orbiting them.

In their search for planets outside our solar system, astronomers observe that red dwarf stars host 59 percent of all the rocky planets discovered to date.² About a third of these planets orbit their host stars at a distance (and thus, at a temperature) that would allow for the existence of liquid water on at least some portion of their surface at some brief moment in their history.

According to the naturalistic assumption that all temperate, rocky planets host microbial life capable of evolving—given sufficient time—into advanced intelligent life, by now we should have discovered extraterrestrial intelligent life (ETI) on one or more planets orbiting red dwarf stars. The fact that we have not seems puzzling. And why do we not find ourselves orbiting a red dwarf star, looking up at a red sky? This enigma is referred to as the red sky paradox.

If the evolution of intelligent life from microbes is not an assured, relatively rapid process, then red dwarf stars have an even greater advantage, given their greater longevity, and the red sky paradox seems all the more perplexing.

Proposed Resolutions to the Red Sky Paradox

Computational astrophysicist David Kipping has proposed four resolutions to the red sky paradox.³

• Proposal #1—Just Lucky I Guess:

We humans find ourselves orbiting a star much brighter than a red dwarf star by an extraordinary stroke of random chance. Our star, planet, and species are extreme statistical outliers. As Kipping noted, however, this proposal exacerbates the classic Fermi paradox.⁴ Named after physicist Enrico Fermi, this paradox contrasts the high probability of ETI's existence (according to some calculations) with the total absence of evidence of actual ETI.

• Proposal #2—Short Waves:

Perhaps a red sky inhibits the development of life on a red dwarf's planets. Life likely requires significant, enduring exposure to 300-450 nanometer light waves, not just the 600-800 nanometer wavelengths that predominate in the radiation from red dwarf stars.

• Proposal #3—Not Enough Time:

The time window for advanced life to exist on a planet with a red sky may be brief. Red dwarf stars spend more time (0.1-1.1 billion years) in the pre-main-sequence phase—that is, the time between formation and ignition of nuclear fusion—than do stars like the Sun. During this phase, their luminosity is much higher than it is during the main sequence phase.⁵ Higher luminosity brings a higher probability of generating a moist greenhouse threshold event on a red dwarf's liquid-water-bearing planet(s).⁶

A moist greenhouse threshold event occurs when the brightening of a planet's host star transforms much of the planet's liquid water into water vapor. Because water vapor is a greenhouse gas, its increase leads to a higher surface temperature on the planet, which, in turn, transforms yet more liquid water into water vapor, producing yet higher surface temperatures that result in yet more water vapor. Eventually, water vapor in the planet's troposphere builds up to such a level that it escapes into the planet's stratosphere. There, the host star's radiation dissociates it into hydrogen and oxygen atoms, and the hydrogen escapes into interplanetary space. Over time, this hydrogen escape causes the complete desiccation of the planet.⁷

Given the extended time red dwarf stars spend in the pre-main-sequence phase, the vast majority of liquid-water-bearing planets orbiting such stars end up bone dry before their host stars enter the main sequence phase, when their luminosity finally becomes stable enough to allow for life's possible existence.

• Proposal #4—Not So Abundant After All:

The number of Earth-sized, temperate, moist planets hosted by red dwarf stars may be lower than anticipated. With rare exceptions, astronomers have detected planets orbiting only the brightest red dwarf stars. Whether planets orbiting brighter red dwarf stars represent the total planet population orbiting red dwarfs remains unknown. But it seems unlikely that Earth-sized planets orbiting the more numerous smaller and dimmer red dwarf stars would be as abundant.

Astronomers typically assume that planets with surface temperatures permitting liquid water's existence do, in fact, possess surface liquid water. For red dwarf stars, this assumption may be incorrect. The detected population of Earth-sized planets orbiting red dwarf stars could be dominated by photo-evaporated cores of super-Earths (planets once larger than Earth but stripped of atmospheres and oceans by their host stars' radiation).

More Plausible Resolutions

Regular exposure to visible violet light is necessary to maintain eye functionality,⁸ and all life, not just organisms with eyes, requires a certain intensity and wavelength range of ultraviolet light.⁹ This requirement means that for life to exist on a planet, the planet must orbit its star within the appropriate ultraviolet range, or ultraviolet habitable zone. For planets or moons orbiting red dwarf stars, this zone never overlaps the liquid water habitable zone.¹⁰ Thus, red dwarfs' candidacy for hosting life-supporting planets appears ruled out.

Because red dwarf stars are much cooler than the Sun, any temperate planets orbiting them must be in closer orbit to their stars than Earth's orbit is to the Sun. This requirement, too, poses a problem. Any planet with an atmosphere thicker than 1 percent of Earth's atmosphere (the minimum essential for life) whose orbital path is closer to its host star than 90 percent of Earth's distance from the Sun will likely possess an atmospheric electromagnetic field strong enough to completely dry out the planet.¹¹

Astronomers have used the MUSCLES (Measure-ments of the Ultraviolet Spectral Characteristics of Low-Mass Exoplanetary Systems) Treasury Survey to discover another way red dwarfs dessicate their -planets. They have found that the ultraviolet and soft x-ray flux for even the quietest red dwarfs is more than sufficient to wipe out their planets' atmospheres and hydrospheres.¹²

The wind pressure exerted by red dwarfs on planets orbiting within the liquid water habitable zone is 100-100,000 times greater than our Sun's wind pressure on Earth.¹³ This greater pressure would compress any planetary magnetosphere (if it were to exist) to such a degree that its erosion would be inevitable. Greater stellar wind pressure also speeds up the erosion of any planetary atmospheres and hydrospheres caused by the star's ultraviolet and x-ray flux.

The tidal forces a star exerts on its planets increase exponentially as the distance between the star and its planets decreases. A planet need be only slightly closer to its host star than Earth is to the Sun before tidal forces would cause the planet's rotation period to become equal (or nearly equal) to its revolution period. On such a planet, one hemisphere would be blisteringly hot while the other would be freezing cold. Any planet orbiting a red dwarf within the liquid water habitable zone would be so close as to experience this tidal locking.¹⁴ While liquid water might exist in the twilight zone (the edge of stellar illumination), atmospheric transport would move water to the planet's coldest parts, where it would freeze.¹⁵

Philosophical Implications

With no atmosphere, no surface water, deadly ultraviolet and x-ray showers, and blasts from frequent major flares, planets in orbit around red dwarf stars are unfit hosts for physical life. Thus, 78 percent of all stars are noncandidates for hosting physical life.

Our star, planet, and species are, indeed, extreme statistical outliers—in which case, to propose the involvement of a purposeful Creator in positioning our planet at the just-right distance from a just-right star where all of life's essential requirements are met seems far more plausible than random chance. Despite the discovery of nearly 5,000 planets to date,¹⁶ only one meets the known and still growing criteria for life, especially life as complex and advanced and fragile as human life.¹⁷ Our existence—with a beautiful blue sky overhead—testifies to the exquisite power and care of the Creator God of the Bible.

Notes
1. Glenn LeDrew, "The Real Starry Sky," Journal of the Royal Astronomical Society of Canada 5 (February 2001): 32-33.
2. "Catalog," The Extrasolar Planets Encyclopaedia, Exoplanet TEAM (updated July 15, 2021): http://exoplanet.eu/catalog.
3. David Kipping, "Formulation and Resolutions of the Red Sky Paradox," Proceedings of the National Academy of Sciences USA 118, no. 26 (June 29, 2021): pnas.org/content/118/26/e2026808118.
4. Kipping, "Formulation and Resolutions."
5. Chushiro Hayashi, "Stellar Evolution in Early Phases of Gravitational Contraction," Publication of the Astronomical Society of Japan 13, no. 4 (August 1961): https://ui.adsabs.harvard.edu/abs/1961PASJ...13..450H/abstract.
6. Peter Gao et al., "Stability of CO2 Atmospheres on Desiccated M Dwarf Exoplanets," Astrophysical Journal 806, no. 2 (June 20, 2015): doi:10.1088/0004-637X/806/2/249.
7. Hugh Ross, "Moist Greenhouse Threshold Doomsday," Today's New Reason to Believe (March 11, 2019): https://reasons.org/explore/blogs/todays-new-reason-to-believe/moist-greenhouse-threshold-doomsday.
8. Hugh Ross, "Eyes, Sun, and Earth Designed to Prevent Myopia," Today's New Reason to Believe (Aug. 16, 2021): https://reasons.org/explore/blogs/todays-new-reason-to-believe/eyes-sun-and-earth-designed-to-prevent-myopia.
9. Hugh Ross, "Overlap of Habitable Zones Gets Much Smaller," Today's New Reason to Believe (Dec. 27, 2016): https://reasons.org/explore/blogs/todays-new-reason-to-believe/overlap-of-habitable-zones-gets-much-smaller.
10. Ross, "Overlap of Habitable Zones."
11. 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): doi:10.1002/2016GL068327.
12. 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 28, 2017): doi:10.3847/1538-4357/aa76dd.
13. Cecilia Garraffo et al., "The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets," Astrophysical Journal Letters 843 (July 12, 2017): doi:10.3847/2041-8213/aa79ed.
14. Rory Barnes, "Tidal Locking of Habitable Exoplanets," Celestial Mechanics and Dynamical Astronomy 129 (December 2017): doi:10.1007/s10569-017-9783-7.
15. Kristen Menou, "Water-Trapped Worlds," Astrophysical Journal 774, no. 1 (September 2013): doi:10.1088/0004-637X/774/1/51.
16. "Catalog," The Extrasolar Planets Encyclopaedia.
17. Hugh Ross, "Complex Life's Narrow Requirements for Atmospheric Gases," Today's New Reason to Believe (July 1, 2019): https://reasons.org/explore/blogs/todays-new-reason-to-believe/complex-life-s-narrow-requirements-for-atmospheric-gases; "Tiny Habitable Zones for Complex Life," Today's New Reason to Believe (March 4, 2019): https://reasons.org/explore/blogs/todays-new-reason-to-believe/tiny-habitable-zones-for-complex-life; "Moon's Early Magnetic Field Made Human Existence Possible," Today's New Reason to Believe (Nov. 16, 2020): https://reasons.org/explore/blogs/todays-new-reason-to-believe/moons-early-magnetic-field-made-human-existence-possible-2.