One of NASA’s greatest and most public goals is to find habitable planets orbiting stars other than the Sun. The stars NASA has identified as the most promising candidates for habitability are M dwarfs, also known as red dwarfs. These stars are less than half the Sun’s mass, less than a fifteenth of the Sun’s luminosity, and capable of sustaining nuclear burning for 40–2,000 billion years. Astronomers estimate that these stellar features would potentially yield conditions suitable for life in associated planetary systems.
M dwarfs are numerous, comprising 70% of all the nuclear-burning stars in the Milky Way Galaxy.1 For example, among the 39 nuclear-burning stars within 13.5 light-years of the Sun, 30 are M dwarfs.2 What also boosts NASA’s interest in planets orbiting M dwarfs (see figure) is this: the James Webb Space Telescope can determine the atmospheric molecular features of such planets.
Figure: Artist’s Conception of a Planet with Two Moons Orbiting an M-Dwarf Star Credit: NASA/Harvard-Smithsonian Center for Astrophysics/David A. Aguilar
Previously Known M Dwarf Planets’ Habitability Problems As noted in previous Today’s New Reason to Believe articles, astronomers have already identified major challenges to the habitability of planets orbiting M dwarfs.3 The following paragraphs provide a brief overview of five such obstacles, for readers’ convenience.
The first and perhaps most severe problem is that for planets orbiting M dwarfs, the liquid water and ultraviolet habitability zones do not overlap. Life requires both, simultaneously. Without incident ultraviolet radiation of the just-right wavelength and intensity, several life-essential biochemical reactions cannot occur. At the same time, too much incident ultraviolet radiation or incident ultraviolet radiation at a too short wavelength is deadly for life. For host stars with effective temperatures lower than 4,600 Kelvin, the outer edge of the ultraviolet habitability zone is closer to the star than the inner edge of the liquid water habitability zone.4 All nuclear-burning M dwarfs have effective temperatures lower than 3,800 Kelvin.5
The second problem is an electrical one. Rocky or terrestrial planets with an atmosphere thicker than 1% of Earth’s and an orbital distance less than 90% of Earth’s distance from the Sun will have an electrically charged atmosphere of more than several volts. Such a strong atmospheric electric field in combination with ultraviolet radiation from the host star will completely dry the planet’s atmosphere and surface over the course of a few tens of millions of years or less.6 For a planet orbiting an M dwarf to possess life-essential liquid water, its orbital distance must be less than 25% of Earth’s distance from the Sun. In such a near orbit, that planet will quickly become bone dry.
A third problem is tidal locking. Since M dwarfs are relatively dim, their planets must orbit closer than 25% of Earth’s orbital distance from the Sun to have any hope of possessing liquid water. However, the tidal force that a star exerts on such closely orbiting planets is extreme. A planet orbiting an M dwarf close enough to possibly possess surface liquid water will be tidally locked,7 with one of its hemispheres always facing its star (just as one hemisphere of the Moon always faces Earth). Thus, one hemisphere will be blistering hot while the other will be freezing cold. The temperature difference between the planet’s hot side and cold side will exceed 400°C (720°F). While liquid water could perhaps exist in the planet’s twilight zone (the transition zone at the edge of stellar illumination), atmospheric transport would move water to the coldest parts of the planet where it would freeze and remain permanently unavailable for life.8
A fourth problem is that all M dwarfs emit powerful ultraviolet and x-ray flares. For even the calmest M dwarfs orbiting within the liquid water habitability zone, the flux of ultraviolet and x-rays (soft and hard) will be more than sufficient to erode away any water and carbon dioxide that might exist in its atmospheres, oceans, and lakes.9
The fifth problem is that planets orbiting M dwarfs are doomed to suffer a catastrophic greenhouse threshold event. For a planet to hold surface water during its star’s nuclear-burning phase, it must also possess water during the star’s pre-nuclear-burning phase, also known as the pre-main-sequence (PMS) phase. An M dwarf is more than 60 times brighter at the beginning of its PMS phase than at the beginning of its nuclear-burning phase. This increased brightness translates to more heat, which vaporizes water in the atmosphere and on the surface of any of the M dwarf’s planets. Water vapor is a greenhouse gas. Consequently, more water vapor leads to a still higher surface temperature that causes yet more liquid water to be transformed into water vapor, producing even higher surface temperatures. Eventually, so much water vapor builds up in the planet’s troposphere that it escapes into the planet’s stratosphere. There the M dwarf’s radiation dissociates the water vapor, splitting it into hydrogen and oxygen atoms. The hydrogen escapes into interplanetary space. In short order, this hydrogen escape results in the complete desiccation of the planet.10
Newly Discovered Habitability Challenges A team of four astronomers headed by Ana Lobo has poured even more cold water—and hot— both figuratively and literally, on the possibility of life on planets orbiting M dwarfs, particularly for life in the twilight zones of any tidally locked planets there.11 Lobo’s team demonstrated that even if a planet orbiting an M dwarf were somehow able to survive the PMS phase with some water remaining, all that water would either be permanently trapped on the planet’s cold nightside or permanently evaporated into interplanetary space.
A second research effort learned the effects of stellar flaring activity on habitability. While no telescope is dedicated to studying ultraviolet and x-ray flares from M dwarfs, the Transiting Exoplanet Survey Satellite (TESS) completed an all-sky survey mission between April 2018 and July 2020. (TESS is currently conducting a second all-sky survey.) From that survey, 1,276 two-minute-cadence light curves are available for each of 200,000 preselected targets.
A team of astronomers led by Beate Stelzer identified 112 M dwarfs among these TESS targets bright enough to reveal the number and intensity of their flaring events.16 Stelzer’s team found that 2,500+ optical flare events had occurred within the light curves of these 112 M dwarfs. Given the existing evidence that nearly every optical flare is associated with an x-ray flare, the team concluded that M dwarf x-ray flares occur more frequently than anticipated. These more frequent x-ray flares pose more serious risks than previously acknowledged to the erosion of the atmospheres and hydrospheres of planets orbiting within the liquid water habitability zones of M dwarfs.
A third team of twenty astronomers, led by Kevin France, investigated this scenario: a planet orbiting a 10-billion-year-old M dwarf experiences unlikely late tectonic events that release gases to form a second-generation atmosphere.13 The motivation for their investigation was strong observational evidence that M dwarfs exhibit high stellar activity throughout their PMS phase and during the first 5 billion years of their nuclear-burning phase. France’s team hoped that stellar activity levels for old M dwarfs might be low enough for life to possibly exist on planets orbiting these stars. This team’s observations of Bernard’s Star (GJ 699), a 10-billion-year-old M dwarf, showed that the fraction of time the star was in a state of high-flare activity was roughly 25%. Evidently, even very old M dwarfs have more flaring activity than the existence of life would require. Life needs atmospheric stability that flaring disrupts.
A fourth team of astronomers, led by Rodolfo Garcia, developed computer simulations of M dwarf star formation and of the thermal evolution of planets in their orbit.14 These computer models showed that if any such planet contains water during its star’s nuclear-burning phase, it will experience a runaway greenhouse phase during the star’s PMS phase. The team established that “if the planet has outgassed enough carbon dioxide during its runaway greenhouse phase, surface temperatures will be too high for outgassed water to condense and the planet will not be habitable.”15 They concluded that only if the planet begins with very little carbon dioxide and thereafter experiences almost no volcanism can there be any hope that during the nuclear-burning phase of its host star it will possess surface liquid water and possibly be habitable. A far more likely outcome, they demonstrated, is that rocky planets orbiting M dwarfs will “become Venus-like worlds with thick carbon dioxide atmospheres and no liquid water.16
Magnetosphere Problem Three-dimensional magnetohydrodynamic models of the stellar winds emanating from nuclear-burning M dwarfs show that, for planets orbiting these stars in the liquid water habitability zone, the wind pressure on the planets would be 100–100,000 times greater than the solar wind pressure on Earth.17 This greater stellar wind pressure would compress any possibly existing magnetospheres around the planets to such a degree that erosion of water and carbon dioxide from the planets’ atmospheres, oceans, and lakes would be inevitable.
This magnetosphere compression is most intense when the M dwarf is in its first several million years of nuclear burning. It is exponentially more intense during the M dwarf’s PMS phase. M dwarfs spend a long time (0.1–1.1 billion years) in the PMS phase. During this phase, the emission of particles and radiation that sputter away the atmospheres and hydrospheres of any nearby planets is hundreds to thousands of times greater than at any time during the nuclear-burning phase.
Another problem for life is that at the beginning of an M dwarf’s PMS phase, its brightness is at least 60 times greater than during its nuclear-burning phase. The relatively rapid dimming that occurs during the PMS phase rules out any realistic origin-of-life or survival-of-life scenario. Long before life can possibly originate or survive on a planet orbiting an M dwarf, resources essential for life are expelled from the planet.
Planets that form more distantly from an M dwarf and then later migrate into the liquid water habitable zone still face the problem of the collapsing magnetosphere problem. During an M dwarf’s PMS and early nuclear-burning phases, the danger zone for planetary magnetosphere compression extends far beyond the liquid water habitability zone. Furthermore, planetary migration models consistently demonstrate that planetary migration occurs early in the planetary system formation.18 For M dwarfs, it occurs long before the end of the PMS phase.
M Dwarf Planetary Systems as Candidates for Life? For the past two decades, the quest to find habitable planets has focused almost entirely on planets orbiting M dwarf stars. Given the accumulation of evidence challenging the potential of M-dwarf-hosted planets as candidates for support of any kind of life for any length of time, the time to take M dwarf planets off the list seems to have come.
To be clear, astronomers would do well to continue the study of M dwarfs and their planets. However, research on these stars and planets will likely prove significantly more productive if habitability is not the driving factor behind the research.
To date, astrobiologists have yet to find any evidence for the existence of life beyond Earth. Thus, astrobiology remains, in one sense, a data-free discipline. As the research focus on M dwarf planets repeatedly argues against their capacity for life support, a growing body of evidence shows that more than a dozen habitability zones must exist and overlap for life to possibly exist on a planet.19 Astronomical research has powerfully revealed that for a planet to be truly habitable it must be Earth or Earth’s twin.
My book Designed to the Core provides a list and descriptions of the six best Earth analogues discovered to date.20 The one that far outstrips the other five in matching Earth’s life-critical features is Venus. Yet, Venus is so far from possessing the features essential for the support of life that no research astronomer would classify Venus as habitable.
Despite these pessimistic prospects for finding life beyond Earth, astrobiologists are likely to find indisputable evidence for the remains of life beyond Earth. Thanks to millions of meteoritic impacts, Earth has been dispersing its microbes throughout the solar system. For example, over 200 kilograms of Earth’s surface material resides on every square kilometer of the lunar surface.21 Every kilogram of Earth’s surface material contains, on average, about 100 trillion microbes. While conditions on the Moon do not permit the survival of viable microbes, they do permit the preservation of microbial fossil remains. Therefore, instruments on board a spacecraft will soon be sufficiently sensitive to detect the remains of Earth-transported microbial life on the Moon, Mars, and other solar system bodies.
The Value of a Null Result In science research, a “no” is considered just as important and viable as a “yes.” Astrobiologists and laypeople need not be disappointed if future astronomical research affirms what astrophysicist Neil deGrasse Tyson said when he declared that, beyond Earth, “The universe is a deadly place.”22 This powerfully affirmed fact speaks about the exquisite design of our galaxy cluster, galaxy, star and planetary system, and even our moon to make possible our existence here on Earth. We can see and know that the existence and flourishing of human beings are made possible only by the One who created everything and invites us into a personal, never-ending love relationship.
Endnotes
John J. Bochanski et al., “The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk. II. The Field,” Astronomical Journal 139, no. 6 (June 2010): 2679–2699, doi:10.1088/0004-6256/139/6/2679.
Jianpo Guo et al., “Habitable Zones and UV Habitable Zones around Host Stars,” Astrophysics and Space Science 325 (January 2010): 25–30, doi:10.1007/s10509-009-0173-9.
S. Cassisi and M. Salaris, “Effective Temperature-Radius Relationship of M Dwarfs,” Astronomy & Astrophysics 626 (June 2019): id. A32, doi:10.1051/0004-6361/201925468.
Glyn A. 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,” Geophysical Research Letters 43, no. 12 (June 28, 2016): 5926–5934, doi:10.1002/2016GL068327.
Rory Barnes, “Tidal Locking of Habitable Exoplanets,” Celestial Mechanics and Dynamical Astronomy 129 (December 2017): 509–536, doi:10.1007/s10569-017-9783-7.
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, no. 1 (June 28, 2017): id. 31, doi:10.3847/1538-4357/aa76dd.
Hugh Ross, “Moist Greenhouse Threshold Doomsday,” Today’s New Reasonto Believe (blog), Reasons to Believe, March 11, 2019; 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): id. 129, doi:10.3847/1538-4357/aaea5f.
Ana H. Lobo et al., “Terminator Habitability: The Case for Limited Water Availability on M-Dwarf Planets,” Astrophysical Journal 945, no. 2 (March 16, 2023): id. 161, doi:10.3847/1538-4357/aca970.
B. Stelzer et al., “Flares and Rotation of M Dwarfs with Habitable Zones Accessible to TESS Planet Detections,” Astronomy & Astrophysics 665 (September 7, 2022): id. A30, doi:10.1051/0004-6361/2021142088.
Kevin France et al., “The High-Energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?” Astronomical Journal 160, no. 5 (October 30, 2020): id. 237, doi:10.3847/1538-3881/abb465.
R. Garcia et al., “Carbon Dioxide Outgassing Constrains the Habitability of Rocky Planets after Their Host M Dwarf’s Pre-Main Sequence Phase,” Eighth American Astronomical Society Topical Conference Series Meeting, Habitable Worlds 2021, id. 1211, Bulletin of the American Astronomical Society 53, no. 3 (March 19, 2021): e-id 2021n3i1211.
Garcia et al., “Carbon Dioxide Outgassing.”
Garcia et al.
Cecilia Garraffo et al., “The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets,” Astrophysical Journal Letters 843, no. 2 (July 12, 2017): L33, doi:10.3847/2041-8213/aa79ed.
Alexandre Emsenhuber, Christoph Mordasini, and Remo Burn, “Planetary Population Systhesis and the Emergence of Four Classes of Planetary System Architectures,” European Physical Journal Plus 138, no. 2 (February 27, 2023): id. 181, doi:10.1140/epip//s13360-023-03784-x; Toshinori Shimizu et al., “High-Contrast Imaging around a 2 Myr-Old Cl Tau with a Close-In Gas Giant,” Astronomical Journal 165, no. 1 (December 19, 2022): id. 20, doi:10.3847/1538-3881/ac9fd1; Sahl Rowther and Farzana Meru, “Planet Migration in Self-Gravitating Discs: Survival of Planets,” Monthly Notices of the Royal Astronomical Society 496, no. 2 (June 8, 2020): 1598–1609, doi:10.1093/mnras/staa1590; Craig B. Agnor and D. N. C. Lin, “On the Migration of Jupiter and Saturn: Constraints from Linear Models of Secular Resonant Coupling with the Terrestrial Planets,” Astrophysical Journal 745, no. 2 (January 12, 2012): id. 143, doi:10.1088/0004-637X/745/2/143.
John C. Armstrong, Llyd E. Wells, and Guillermo Gonzalez, “Rummaging through Earth’s Attic for the Remains of Ancient Life,” Icarus 160, no. 1 (November 2002): 183–196, doi:10.1006/icar.2002.6957; John C. Armstrong, “Distribution of Impact Locations and Velocities of Earth Meteorites on the Moon,” Earth, Moon, and Planets 107 (December 2010): 43–54, doi:10.1007/s11038-010-9355-2.
In the climax of Disney’s Enchanted, Queen Narissa in dragon form balances precariously on the top of New York City’s Woolworth Building, gloating about...
