Habitability vs. Colonizability

“Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.”

- St. Albertus Magnus (13th century)

In the article “A Tale of Two Worlds” by novelist Karl Schroeder, the author states that in the detection and characterization of planets around other stars, habitability and colonizability are not the same thing. NASA’s Kepler space telescope has shown that Earth-size planets that are neither too hot nor too cold to support life are surprisingly common. These potentially habitable planets may at first seem to be where humans and their machines could one day settle. However, Schroeder mentions that the current definition of whether a planet is habitable has nearly nothing to do with its colonizability.

Take the exoplanets Kepler-62e and Kepler-78b as examples. Kepler-62e is a super-Earth in orbit around a star that is somewhat cooler than the Sun. It has 1.61 times the Earth’s diameter and is located at a comfortable distance from its parent star such that temperatures are just right to support life. Kepler-62e possesses the right properties for it to be a potentially Earth-like habitable planet. In contrast, Kepler-78b, formerly known as KIC 8435766 b, is an Earth-size planet in an extremely close-in 8.5-hour orbit around a Sun-like star. This planet is expected to be tidally-locked with one side permanently facing it parent star and experiencing hellish temperatures of 2300 K to 3100 K. Being so close to its parent star, any breathable atmosphere or liquid water is unlikely to be present on Kepler-78b. Nevertheless, the permanent night side of Kepler-78b is believed to be much cooler and temperatures there may even dip below freezing in the absence of any appreciable atmosphere to transport heat over from the scorching dayside.

Figure 1: Four potentially habitable exoplanets shown to scale alongside the Earth. Left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f, and Earth (except for Earth, these are all artists’ renditions). Credit: NASA Ames/JPL-Caltech.

 Figure 2: Artist’s depiction of Kepler-62e, a super-Earth in the habitable zone of a star that is smaller and cooler than the Sun. Credit: NASA Ames/JPL-Caltech.

Figure 3: Artist’s depiction of Kepler-20e - a planet with a smaller radius than Earth in a close-in orbit around a Sun-like star. Kepler-20e is believed to be tidally-locked with the same hemisphere always facing its parent star. The planet’s dayside temperature is estimated to be over 1000 K while its night side is much cooler. Kepler-78b is quite similar to Kepler-20e, just that it has a much hotter dayside. Credit: NASA/Ames/JPL-Caltech.

Kepler-62e is a potentially habitable planet while Kepler-78b is most certainly not. However, this may not imply that Kepler-62e is more colonisable than Kepler-78b. In fact, Kepler-78b may be more promising when it comes to colonizability. Assuming that Kepler-62e has the same density as Earth, its surface gravity will be 1.6 times of Earth’s. The stronger gravity will place an unavoidable permanent strain on humans and their machines. Even if the stronger gravity may be bearable, getting stuff off the surface of Kepler-62e into space will require exponentially more energy compared to Earth.

A study by L. Kaltenegger et al. (2013) suggests that planets in the size range of Kepler-62e are likely to be completely covered by ocean with no land in sight. The absence of land may yet again lower its potential for colonization even though the planet’s ocean may be a perfect environment for its local life. Actually, if life exists on a planet, it may immediately deem the planet unsuitable for colonization, regardless of the planet’s physical properties. This is because life on another world is likely to operate on a different biochemistry that is incompatible and possibly hostile to Earthly life. Furthermore, colonization also raises the problem of contaminating a pristine alien biosphere. Based on these considerations, an Earth-like habitable planet that is teeming with life (i.e. an Earth analogue) is almost certainly unsuitable for colonization.

Figure 4: Artist’s impression of an Earth-like planet.

Figure 5: Artist’s impression of a potentially habitable planet.

Compared to Kepler-62e, the planet Kepler-78b may appear inhospitable due to its superheated dayside. However, Kepler-78b is tidally-locked and the other half of the planet never faces its parent star. One can imagine conditions there being somewhat like within the cold permanently shaded craters at Mercury’s poles, but encompassing half a planet. An airtight habitat containing a breathable atmosphere could easily find its place on the cool night side of Kepler-78b. On a side note, the sight of its parent star from the stupendously hot dayside of Kepler-78b would certainly be terrifyingly spectacular. The huge temperature difference between the dayside and night side of Kepler-78b provides an enormous potential to move heat around, thereby generating power. Additionally, Kepler-78b is approximately the same size as Earth and this makes getting stuff off the planet’s surface into space no more difficult than it is for Earth, unless Kepler-78b is unusually dense.

Although Kepler-62e is undoubtedly well suited to support life as a habitable planet, the seemingly inhospitable Kepler-78b looks more promising with regard to its colonizability. In short, besides habitability, colonizability should also be used to judge the value of planets around other stars. Nevertheless, Kepler-62e and Kepler-78b are mere examples to distinguish between habitability and colonizability. Both planets are in no way prime interstellar destinations since they are located several hundred light years away. From here to there, there are innumerable stars with planets just like Kepler-62e and Kepler-78b.

With regard to habitability, the ‘habitable zone’ is generally defined as a region around a star where temperatures are neither too hot nor to cold for a planet to have liquid water on its surface and thus capable of supporting life. On the contrary, a ‘colonizable zone’ does not have the same limitations as a ‘habitable zone’ since it depends on a planet by planet basis and may not be required to be around a star at all. A study by Strigari et al. (2012) show that for ever star in the galaxy, there may be as many as ~10,000 unbound objects with masses ranging from Pluto to somewhat larger than Jupiter. These objects are sometimes termed free-floating planets or rogue planets. Such worlds may serve as colonizable “pit stops” in the vast distances between stars.

Figure 6: Artist’s impression of a Pluto-like object and its large moon, orbiting far from its parent star.

In the solar system, objects including Mercury, Earth’s Moon and Pluto may turn out to be excellent places for colonization in a novel method proposed by K. L. Roy et al. (2009). The authors propose creating habitable environments for humans by enclosing airless and otherwise sterile planets, moons, large asteroids, and even free-floating planets within engineered shells. Within such a shell, an environment could be created that is similar in almost all respects to that of Earth except for gravity. These “shell worlds” could be constructed just about anywhere with a suitable planet, moon or large asteroid. It allows humans and their machines to colonize any star system without interfering with or contaminating a planet that has already developed life (i.e. a habitable planet).

References:

-  W. J. Borucki et al. (2013), “Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone”, arXiv:1304.7387 [astro-ph.EP]

- Sanchis-Ojeda et al. (2013), “Transits and Occultations of an Earth-Sized Planet in an 8.5-Hour Orbit”, arXiv:1305.4180 [astro-ph.EP]

- L. Kaltenegger et al. (2013), “Water-Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f”, arXiv:1304.5058 [astro-ph.EP]

- Strigari et al. (2012), “Nomads of the Galaxy”, arXiv:1201.2687 [astro-ph.GA]

- K. L. Roy et al. (2009), “Shell Worlds: An Approach to Terraforming Moons, Small Planets, and Plutoids”, JBIS Vol. 62, pp. 32-38