Inhabitability of Planets Orbiting Red Dwarfs
Scripture declares that God knows the number of stars in the universe and that he has assigned a name to each star (Psalm 147:4). Astronomers do not know the exact number but they have a rough estimate: 50–60 billion trillion stars. Of those 50–60 billion trillion stars, astronomers have assigned proper names to only a few hundred.
Astronomers know the relative numbers of stars for each mass range. They know that high-mass stars are rare and low-mass stars are common. Stars range in mass from 0.075 times the Sun’s mass to over 60 times the Sun’s mass. Stars less than one-half the Sun’s mass comprise a little more than 80% of all stars. Stars more than twice the Sun’s mass comprise just 0.13% of all stars. Apparently, God really likes small stars, but do they allow habitable planets to exist?
Astronomers also know that the smaller a star’s mass, the more likely it will possess planets—and that there is an inverse correlation between the average number of planets per star and the star’s mass. Therefore, it is highly probable that at least 90% of all planets orbit stars that are less than half the Sun’s mass.
The great preponderance of planets orbiting small mass stars explains why astrobiologists have been so focused on these planets in their habitability search. In previous blogs1 and in my book Improbable Planet,2 I have described research findings that show just how unlikely such planets are to be habitable by any scientist’s definition of habitability. In this blog I will describe two recent research findings that add to the weight of evidence that planets orbiting small mass stars are not habitable.
The first of these recent findings arose from the MUSCLES Treasury Survey (MTS). MUSCLES is an acronym for Measurements of the Ultraviolet Spectral Characteristics of Low-Mass Exoplanetary Systems. The goal of the MTS is to measure the ultraviolet characteristics of known planet-hosting small mass stars in order to better determine the atmospheric features of the planets orbiting these stars. The MTS uses the Hubble Space Telescope, the Chandra X-Ray Telescope, and the XMM-Newton telescope to make its measurements.
The MTS team used their measures of the chromospheric calcium II K spectral line in small mass stars to determine the stellar surface ultraviolet flux for these stars. The team also observed the soft x-ray and proton flux during flare events for small mass stars.
The MTS team showed that for even the low mass stars that are the quietest at optical wavelengths, the flux of ultraviolet and soft x-rays is sufficient to erode the atmospheres of planets orbiting these stars that reside in the liquid water habitable zone.3 They also demonstrated that these stars emit powerful ultraviolet flares.4 For one small mass star, GJ 876, the MTS team showed that the planets of GJ 876 that reside in the liquid water habitable zone are impacted by Carrington events. These are flares with x-ray fluxes exceeding a milliwatt per meter squared and proton fluxes exceeding 100 proton flux units at a rate that is ten thousand times more frequent than that experienced by Earth.5
The one known time Earth experienced such an intense Carrington event was on September 1–2, 1859. That flare generated auroras so bright that people living in the northern contiguous 48 states of America were awakened in the middle of the night by its light, thinking it was daylight. Telegraph systems all over the world failed. Some telegraph poles caught fire. The United States Atmospheric and Environmental Research Institute calculated that if such an event were to happen today the recovery cost, (independent of the very significant loss of agricultural productivity) for the US alone would be 1–3 trillion dollars.6
The second recent finding was the result of a detailed three-dimensional magnetohydrodynamic modeling of the impact of the stellar wind from the TRAPPIST-1 star on its system of seven planets. The team of five astronomers who calculated the model showed that the TRAPPIST-1 planets experience a stellar wind pressure between 1,000 and 100,000 times greater than the solar wind pressure on Earth.7 They also established that for all plausible planetary magnetic field strengths, the magnetospheres of the TRAPPIST-1 planets would be so compressed that they would not prevent the direct flow of stellar wind particles upon the planetary atmospheres. Consequently, all the TRAPPIST-1 planets have had their atmospheres and surface water stripped away.
No atmosphere, no surface water, deadly persistent x-rays, far ultraviolet radiation, and very frequent Carrington events add to the already long list of deadly features of planets orbiting small mass stars. Today, there is no escaping the fact that the vast majority of planets are uninhabitable even if one only considers the liquid water habitable zone as a criterion for habitability. Of course, as any reader of my previous blogs is aware, there are other criteria. For a planet to be truly habitable it must simultaneously reside in at least nine different habitable zones.8 Of the nearly 3,637 planets now known to exist,9 only one simultaneously resides in all nine known habitable zones. That one testifies of the exquisite design by the Creator God of the Bible and the hundreds of different features of the universe, the Milky Way galaxy, the solar system, and Earth that make advanced life possible.
- Hugh Ross, “Earth’s Seven Sisters: Are They Really Similar?” Today’s New Reason to Believe (blog), Reasons to Believe, February 23, 2017, https://www.reasons.org/blogs/todays-new-reason-to-believe/earths-seven-sisters–are-they-really-similar; Hugh Ross, “Overlap of Habitable Zones Gets Much Smaller,” Today’s New Reason to Believe (blog), Reasons to Believe, December 27, 2016, https://www.reasons.org/blogs/todays-new-reason-to-believe/overlap-of-habitable-zones-gets-much-smaller; Hugh Ross, “Long-Term Habitability Requires a Sun-Earth Twin,” Today’s New Reason to Believe (blog), Reasons to Believe, August 8, 2016, https://www.reasons.org/blogs/todays-new-reason-to-believe/long-term-habitability-requires-a-sun-earth-twin.
- Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016): 78–93.
- 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): id. 31, doi:10.3847/1538-4357/aa76dd.
- Trevor Maynard, Neil Smith, and Sandra Gonzalez (Atmospheric and Environmental Research), “Solar Storm Risk to the North American Electric Grid,” (2013), Solar Storm Risk to the North American Electric Grid (PDF).
- Cecilia Garraffo et al., “The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets,” Astrophysical Journal Letters 843 (July 12, 2017): id. L33, doi:10.3847/2041-8213/aa79ed.
- Hugh Ross, “‘Electric Wind’ Becomes 9th Habitable Zone,” Today’s New Reason to Believe (blog), Reasons to Believe, July 4, 2016, https://www.reasons.org/blogs/todays-new-reason-to-believe/electric-wind-becomes-9th-habitable-zone; Hugh Ross, “Astrosphere Habitable Zones Display Fine-Tuned Characteristics,” Today’s New Reason to Believe (blog), Reasons to Believe, July 7, 2014, https://www.reasons.org/articles/astrosphere-habitable-zones-display-fine-tuned-characteristics; Ross, Improbable Planet, 78–93.
- Exoplanet TEAM, The Extrasolar Planets Encyclopedia, Catalog (July 26, 2017), https://exoplanet.eu/catalog/.