No Place Like Home?

Are There Planets More Habitable Than Earth?

With all the problems humans face managing Earth's resources for the benefit of all, many people wonder about the feasibility of abandoning Earth and moving to a superior home. Are there, as some researchers suggest, planets better suited for habitability than Earth? Three astronomers at the University of Washington think so. They have published a paper listing 24 candidates as possibilities for "superhabitable" planets.¹ Naturally, their paper has caused quite a stir, especially on the internet.

Superhabitability Criteria

The three astronomers argue that if certain characteristics of the Sun-Earth-Moon system were altered, our planet would be even more suitable for us humans and our living companions than it already is. The search for such a residence would focus on the following combination of features:

Smaller Sun

The rate at which a star burns through its nuclear fuel is proportional to its mass. Therefore, the research team speculated, life would be better off on a planet orbiting a star less massive than the Sun. Such a star would spend more of its history in a temperature range that's appropriate for the needs of life on a nearby planet. The star must not be too small, however, given that stars with less than half the Earth's mass frequently emit deadly superflares.

So the team concluded that superhabitable planets would orbit stars between 0.5 and 1.0 solar mass, stars more than one billion years old, and stars that are well into their more stable burning cycle.

Larger Planet

Biomass and biodiversity are constrained, obviously, by the amount of habitable surface area. Thus, the team concluded that life would be better off on a planet that is larger than Earth. They also argued that a more massive planet would possess a thicker atmosphere, which would enable more flying species to exist. Research shows, however, that planets 50 percent more massive than Earth possess ultra-thick atmospheres, such as Neptune's, with the wrong composition to meet living creatures' respiratory needs.

So the team concluded that superhabitable planets would be about 10 percent larger than Earth, with masses measuring 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 suggested that life on a planet could benefit from stronger tidal forces than those the Moon currently exerts on Earth

Other Characteristics

In addition to these three primary superhabitability features, the astronomers suggested that life would benefit from the following alterations: (1) more, or all, of the planet's surface should be as moist and warm as the Amazon jungle; (2)  the planet's atmospheric oxygen level should be a few percent higher, and (3) the planet's landmasses should be broken up into smaller continents and islands, with large areas of shallow seas between them. The astronomers also noted the crucial requirements of a strong, sustained (for several billion years) magnetic field to shield the planet from cosmic radiation, as well as strong (but not too strong) and enduring plate tectonic activity.

Prospective Planetary Candidates

As of October 7, 2020, the Exoplanet Catalog listed 4,357 extrasolar planets observationally confirmed to exist.2 From an analysis of this catalog, as well as catalogs of unconfirmed extrasolar (possible) planets, the team produced a list of 24 planetary candidates. Exercising considerable caution (given the large probable errors in the measurements of these exoplanets' characteristics), the researchers referred to these planets as "possible candidates" only. More importantly, only two of the 24 planets—Kepler 1126b and Kepler 69c—have been statistically confirmed to exist.

Here's what we know about these two planets' characteristics:

Kepler 1126b

This planet's host star has a mass equal to 0.92 solar masses, within the desired range. But the planet itself has an orbital period of just 108.6 days. This orbital period implies a proximity to the host star that would make tidal locking unavoidable. Moreover, the planet's diameter is 69 percent larger than Earth's, and its mass is estimated to equal 3.64 Earth masses.3 According to these details, while the host star of Kepler 1126b meets the hoped-for criteria, the planet itself departs from them. As an additional challenge, its lack of gas giant companions means that Kepler 1126b is likely subject to intense bombardment by asteroids and comets.

Kepler 69c

This planet's host star has an appropriate mass (0.81 solar masses), but it lacks other essentials. For one thing, it is only about 400 million years old. It is also significantly less rich than the Sun in elements heavier than helium. The planet itself has a diameter equal to 1.69 Earth diameters, and an estimated mass equal to 6 Earth masses. Its orbital period is 242.5 days, and its orbital eccentricity level is 0.14. At this eccentricity level, its proximity to its host star is highly variable and, thus, so is its degree of incident radiation. So, in the case of Kepler 69c, both the planet and its host star fail to meet the superhabitability criteria.

Questionable Criteria for Candidacy

For Stars:

Research does indicate that a star more massive than the Sun consumes its nuclear fuel too quickly to allow for any microbial life on a nearby planet being able to prepare the planet's surface for more advanced life. However, stars that are either slightly more massive or slightly less massive than the Sun will never possess a time window of sufficient duration to meet the needs of advanced life, including a period of extreme luminosity (brightness), stability, and very limited flaring activity. Where a star lacks sufficiently stable brightness (a contributor to appropriate surface temperature) and produces overly abundant flaring activity (a contributor to deadly or disruptive radiation bursts), the development on an orbiting planet of conditions approximating those needed for a balanced ecosystem, or conducive to civilization, would likely be impossible.4

Note that a star's luminosity falls exponentially (by about the fourth power) in relation to its mass. So a planet orbiting a star less massive than the Sun must be closer to its host star to enjoy sufficient luminosity for life. But this lesser distance also means that the planet will be subject to more intense radiation from the star's flares, radiation harmful to life.

Much more problematic, however, would be the planet's subjection to its star's much stronger tidal forces. These tidal forces rise exponentially (by the fourth power) as the distance between the star and the planet decreases. Thus, a star need be only slightly less massive than the Sun before its tidal force on a candidate planet would slow that planet's rotation period to a level too low for life's survival. (Slower rotation means greater temperature extremes between day and night.) 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.

For Moons:

A moon more massive than our Moon would seem likely to generate stronger tides, thus providing more habitable space for certain marine animals. However, a more massive Moon would also slow the rotation period of its host planet. And a slower rotation rate would eliminate other habitable space. Indeed, if it were much slower, it would eliminate all habitable space. If Earth's Moon were even two percent more massive, it would destabilize the tilt of Earth's rotation axis such that, instead of tilting back and forth by only two degrees, our planet's rotation axis would tilt back and forth by tens of degrees.5 Such violent movement would generate climatic alterations catastrophic to life.

For Planets:

As with the Moon, increasing a planet's own mass would not lead to improved habitability. A more massive planet will accrete a much thicker atmosphere. Even Earth, as massive as it is now, had an original atmosphere and hydrosphere that were about 200 times thicker than they are today. It took an extremely rare and highly fine-tuned event—proto-Earth's collision with a planet 11–15 percent as massive as Earth is today—to reduce Earth's atmosphere and hydrosphere down to their current thinness. (This same collision formed our Moon.)

A slightly thicker atmosphere might make it easier for certain animals to fly, but the thicker the atmosphere, the more energy is required for animals to breathe. Lungs fail to function altogether when the atmospheric pressure increases by three times its current level.

Earth's Extraordinary Habitability

In addition to offering the appropriate size, tides, atmosphere, and orbital characteristics, Earth may be considered extraordinarily suitable for life in other ways. For example, several of its exceptional interior features keep Earth's internal heat flow at an extremely high level,6 allowing for the long-term plate tectonic activity that built continents and continues to recycle nutrients.

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 crust of rigid solid plates. The existence of such an asthenosphere requires a specific quantity of water to be moving in a continuous flow from the planet's surface into the mantle, where it serves as a lubricant. Moreover, for this recycling system to work, there must be a certain (high) abundance of available aluminum.7 And, as it turns out, Earth's crust has 31.9 times as much aluminum, and its mantle 8.3 times as much aluminum, as the average rocky body in the universe.

Even if another planet somehow matched Earth's internal heat flow, aluminum abundance, and fine-tuned ­water subduction, its asthenosphere would have to reside at a just-right depth, and its depth would depend on the mass of the planet. For planets larger than Earth, the asthenosphere would be at too great a depth to facilitate the necessary movement; for planets smaller than Earth, the asthenosphere would be at too shallow a level.

The three astronomers' research paper on alternative planetary habitats—superhabitable ones, at that—understandably generated internet buzz, but more importantly, it brought attention to the very real constraints on a planet's habitability. Thus, it can help us appreciate the exceptional characteristics of our home—a planet like no other, orbiting a star like no other, and orbited by a moon like no other.  

Notes
1. Dirk Schulze-Makuch, René Heller, and Edward Guinan, "In Search for a Planet Better Than Earth: Top Contenders for a Superhabitable World," Astrobiology 20, published ahead of print (Sept. 18, 2020): liebertpub.com/doi/10.1089/ast.2019.2161.
2. Exoplanet TEAM, The Extrasolar Planets Encyclopaedia, "Catalog," updated October 7, 2020: http://exoplanet.eu/catalog.
3. NASA, "Exoplanet Catalog: Kepler-1126 b" (web page), accessed October 1, 2020, https://exoplanets.nasa.gov/exoplanet-catalog/4949/kepler-1126-b; Exoplanet TEAM, "Catalog."
4. Hugh Ross, "It Takes a Dull Star to Have a Great Party," Today's New Reason to Believe (blog), (July 6, 2020): https://reasons.org/explore/blogs/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2020/07/06/it-takes-a-dull-star-to-have-a-great-party.
5. Dave Waltham, "Anthropic Selection for the Moon's Mass," Astrobiology 4 (Winter 2004): https://pubmed.ncbi.nlm.nih.gov/15684727; Hugh Ross, Improbable Planet (Baker, 2016), 56–58.
6. Hugh Ross, "Earth's Furnace Is Ideal for Life," Today's New Reason to Believe (blog), (Jan. 20, 2020): https://reasons.org/explore/blogs/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2020/01/20/earth-s-furnace-is-ideal-for-life.
7. Katrin Mierdel et al., "Water Solubility in Aluminous Orthopyroxene and the Origin of Earth's Asthenosphere," Science 315, no. 5810 (Jan. 19, 2007): https://science.sciencemag.org/content/315/5810/364.full.