With all the problems humans face managing Earth’s resources for the benefit of all life, many people wonder if we can abandon Earth and move to a superior planet. Are there planets better suited for habitability than Earth? Three astronomers at the University of Washington have published a paper in which they list 24 candidate superhabitable planets.1 Naturally, their paper has caused quite a stir on the internet.
The three astronomers argued that if certain characteristics of the Sun, Earth, and Moon were altered, Earth would be even more habitable than it already is. Hence, searching for star-planet-moon systems with these superior features would yield superhabitable worlds.
The rate at which a star burns through its nuclear fuel is proportional to its mass. Therefore, the research team presumes life would be better off on a planet orbiting a star less massive than the Sun. That planet conceivably would have more time where the surface temperature is suitable for life. However, stars less than half Earth’s mass exhibit frequent deadly superflares. The three conclude superhabitable planets will orbit stars between 0.5 and 1.0 solar masses that are older than 1 billion years.
Biomass and biodiversity are constrained by the habitat surface area. Thus, the team concludes life would be better off on a planet larger than Earth. They also argue that a more massive planet would possess a thicker atmosphere and, therefore, enable more flying species to exist. However, it is known that planets 50% more massive than Earth all possess very thick atmospheres, like Neptune, with the wrong composition for life. The three conclude that superhabitable planets will be about 10% larger than Earth and between 1.0 and 1.5 Earth masses.
Greater Tidal Forces
The Moon exerts tidal forces on Earth that create diverse habitats along the seashores of continents and islands. On this basis, the trio suggests that life on a planet could benefit from stronger tidal forces than what the Moon currently exerts on Earth.
In addition to these three primary superhabitable features, the astronomer team suggests that life would benefit if: (1) more, or all, of the planet’s surface were as moist and warm as the Amazon jungle, (2) the atmosphere had a few percent more oxygen, and (3) the landmasses were broken up into smaller continents and islands with large area shallow seas between them. They also note that it is crucial for the planet to sustain a strong magnetic field for several billion years and strong, but not too strong, plate tectonic activity.
Superhabitable Planet Candidates
The Exoplanet Catalog (updated October 7, 2020) listed 4,357 extrasolar planets that are observationally confirmed to exist.2 The team analyzed this catalog and several catalogs of extrasolar candidate (unconfirmed) planets to produce a list of 24 “superhabitable” planets. They caution readers, however, that given the large probable errors in the measurements of the planets’ characteristics, their 24 planets are “possible candidates” only. Also, only 2 of the 24 planets, Kepler 1126b and Kepler 69c, have been statistically confirmed to exist.
Here’s what we know about the two planets’ characteristics. Kepler 1126b’s host star is 0.92 solar masses. Kepler 1126b is the only known planet in the system, meaning for certain that there are no gas giant planets in the system. Kepler 1126b has an orbital period of 108.6 days, a diameter 69% larger than Earth’s, and an estimated 3.64 Earth masses.3 Thus, Kepler 1126b’s host star meets the astronomers’ criteria, but the planet itself fails all their planetary criteria. Also, its lack of gas giant companions means that it will be subject to intense bombardments by asteroids and comets.
The host star of Kepler 69c has a mass = 0.81 solar masses, but the star’s age is only about 400 million years and it is significantly less rich in elements heavier than helium than is the Sun. Kepler 69c’s diameter = 1.69 Earth diameters, its orbital period = 242.5 days, and its orbital eccentricity = 0.14. Its roughly estimated mass = 6 Earth masses. Thus, both Kepler 69c and its host star fail to meet the three astronomers’ criteria.
How Valid Are the Superhabitability Criteria?
Is it really true that stars a little less massive than the Sun offer their planets a superior habitability prospect? It is true that stars more massive than the Sun run through their nuclear fuel before there is adequate time for microbial life to prepare a planet for advanced life and the equivalent of human beings. However, stars that are either the tiniest bit more or less massive than the Sun will never possess a time window of sufficient length to allow for two crucial developments: the star’s luminosity (brightness) needs to be stable enough and the star’s flaring activity low enough for civilization on one of its planets to be possible. I presented details on the reasons why and confirming observations in a previous article.4
A second reason why a star cannot be less massive than the Sun is that a star’s luminosity falls with about the fourth power of its mass. Therefore, for a planet orbiting a star less massive than the Sun to be habitable, it must be closer to its host star. But that lesser distance means the planet will be subject to more intense radiation from the star’s flares.
Much more problematic, though, is that the planet will be subject to much stronger tidal forces from the star. These tidal forces rise with the inverse fourth power of the distance between the star and the planet. Therefore, a star need only be slightly less massive than the Sun before the tidal forces exerted by the star on the potentially life-supporting planet slow the planet’s rotation period down to a duration that rules out life on that planet. As it is, Earth is at the minimum distance from the Sun to avoid such a catastrophe. Venus, with a rotation period of 243 days, is not.
A moon more massive than our Moon would generate stronger tides that could provide more habitat space for certain marine animals. However, a more massive Moon also would slow down the rotation period of its host planet. A slower rotation rate would eliminate much more habitat space and, if much slower, all habitat space, than any additional marine habitat space generated by stronger tides.
Another problem with a more massive Moon is that if Earth’s Moon were even just 2% more massive, Earth’s rotation axis tilt would become unstable. Instead of tilting back and forth by only ±1 degree, the planet’s rotation axis would tilt back and forth by tens of degrees.5 Such violent movement would generate climatic alterations that would be catastrophic for life.
As with the moon, making the planet larger does not make it more habitable. A more massive planet will accrete a much thicker atmosphere. As it is, Earth is so massive that it began with an atmosphere and a hydrosphere about 200 times thicker than it possesses now. It took an extremely rare and highly fine-tuned event—the collision of a planet 11–15% of Earth’s present mass with the proto-Earth—to strip Earth’s atmosphere and hydrosphere down to its very thin layers. (This collision also formed our Moon.) A slightly thicker atmosphere might make it easier for certain animals to fly, but a much thicker atmosphere will not. The thicker the atmosphere, the more energy it takes for animals to breathe. Lungs fail to function at an atmospheric pressure three times greater than Earth’s present atmosphere.
Earth’s Own Superhabitability
When we consider features other than size, tides, atmospheres, and distances, Earth is extraordinary. As a result of several exceptional interior design features, Earth’s internal heat flow is still extremely high.6 This feature is key because long-term habitability requires strong, enduring plate tectonic activity. For such activity to be possible, a thin, low-viscosity, semi-molten asthenosphere layer must reside at a fine-tuned depth below the planet’s surface where the crust consists of rigid solid plates. For such an asthenosphere and a set of plates to exist there needs to be a fine-tuned amount of water being continuously subducted from the planet’s surface into the mantle. There the water serves as a lubricant as the mantle experiences a very high heat flow from the core-mantle boundary to the asthenosphere. Furthermore, for the water being subducted into the mantle to bring the viscosity of the asthenosphere down to a low enough level, both the crust and the mantle must possess an extremely high abundance of aluminum.7
Earth is extremely aluminum-rich. Earth’s crust has 31.9 times and its mantle 8.3 times as much aluminum as the average for rocky bodies in the universe. Even if another planet somehow matched Earth’s internal heat flow, its aluminum abundance, and the fine-tuned amount of water being subducted a just-right depth of the asthenosphere depends on the mass of the planet. For planets larger than Earth, the asthenosphere will be at too deep a level for the necessary crustal plates to exist and move relative to one another. For planets smaller than Earth, the asthenosphere will be at too shallow a level.
This research may have generated internet buzz, but ultimately it helps constrain scientific characteristics of habitability. There does exist a superhabitable planet, one like no other orbiting a star like no other orbited by a moon like no other. It doesn’t require a wild guess to know which planet fits that description.