“For there is a single general space,
a single vast immensity which we may freely call Void;
In it are an infinity of worlds of the same kind as our own.”
— Giordano Bruno, “On the Infinite Universe and Worlds” (1584)
In his seminal work, titled “On the Infinite Universe and Worlds,” Italian philosopher and astronomer Giordano Bruno pondered among other things the plurality of worlds and their possible habitability throughout an infinite Universe. In part because of these specific cosmological views, he would be accused as a heretic and burned at the stake by the Catholic Church’s Roman Inquisition in 1600. Yet Bruno’s revolutionary ideas would nonetheless help to create a philosophical and scientific paradigm shift that would help to open humanity’s eyes to the possibility that extraterrestrial life might exist among a multitude of planets.
The notion that extraterrestrial life could be widespread throughout the Universe still remains in the realm of just theoretical speculation. Modern scientific thinking on the subject covers the full spectrum of possibilities, with hypotheses ranging from the Mediocrity principle to the Rare Earth Hypothesis. Yet Bruno’s ideas concerning the plurality of worlds would be validated 400 years after his death with the discovery of more than 1,000 extrasolar planets orbiting stars other than the Sun. Challenging the assumptions of the Rare Earth Hypothesis even more, astronomers have calculated based on these revolutionary discoveries that every single star in our Milky Way galaxy must be home to at least one planet, with the habitable ones among them possibly numbering in the billions.
Astronomers and astrobiologists have focused their search thus far for planets that most resemble Earth, where conditions can be just right for the development of life as we know it—namely the presence of organic chemistry, adequate sunlight, and liquid water on their surfaces. To that end, astronomers have advanced the concept of the stellar “habitable zone,” which can be described as the range of orbits that a terrestrial planet around a star can have, where surface temperatures are just right for Earth-like conditions of habitability to be found. According to this concept, a planet with an orbit that lies just outside of the zone’s inner edge, like Venus for instance in our Solar System, would be too hot for life to develop, whereas a planet just outside the habitable zone’s outer edge, like Mars, would be too cold.
Many important discoveries in recent decades have challenged the assumptions behind this concept of habitability. The discovery of microbial life on Earth that thrives on extreme conditions which are hostile to the rest of terrestrial life, and the evidence for the existence of large underground oceans of liquid water, heated by tidal forces on many Solar System moons that lie outside of the conventional “habitable zone,” like Europa, Enceladus, and Triton, have started to reveal a Solar System where life-friendly conditions might be found on a much broader range of environments than previously thought.
A new study titled “Superhabitable Worlds,” published in the January 2014 issue of the scientific journal Astrobiology, similarly challenges the preconceived notions of exoplanet habitability, making a strong case for the need for substantially broadening the search for life to include those places we haven’t thought to look at. Authors René Heller of the McMaster University’s Department of Physics and Astronomy in Ontario, Canada, and John Armstrong of the Weber State University’s Department of Physics in Ogden, Utah, introduce the concept of “superhabitable worlds” to describe planets that can lie outside of the conventional habitable zones of other stars, and that could be more favorable to harboring life, due to tidal heating. As the two authors write in their study: “To be habitable, a world (planet or moon) does not need to be located in the stellar habitable zone (HZ), and worlds in the HZ are not necessarily habitable. [In our study], we illustrate how tidal heating can render terrestrial or icy worlds habitable beyond the stellar HZ. We call these objects ‘super-habitable.’”
To that end, Heller and Armstrong present in their study several characteristics of these so-called “superhabitable” planets. These include factors like the amount of the planet’s land-to-surface fraction, its total surface area, age, temperature, ability to generate a magnetosphere, as well as the parent star’s age and total mass.
A larger number of relatively shallow seas, in contrast to vast, deep oceans, is thought to be crucial by Heller and Armstrong for the development of life. The assumption behind this argument is that shallow seas would create a much bigger biodiversity, offering more chances for life to spread and develop. “Earth’s shallow waters have a higher biodiversity than the deep oceans. Hence, we expect that planets with shallow waters rather than those with deep extended oceans, tend to be superhabitable,” the authors argue.
Plate tectonics are considered another important factor for habitability. On Earth, the fragmentation of the lithosphere into many larger and smaller plates, and the plates’ subsequent motion on top of a viscous mantle, drive our planet’s bio-geochemical carbon cycle which is key to sustaining all terrestrial life. Our planet is big and massive enough to undergo plate tectonics activity, compared to Mars for instance which displays no such signs. On the other hand, there is a mass limit above which a planet cannot maintain any plate tectonics activity, due to very high pressures inside the mantle that accumulate as the mass of a planet increases. Heller and Armstrong in their study consider planets with masses approximately up to two to three times that of Earth to be optimal superhabitable candidates. The internal heat and energy of such massive planets, besides plate tectonics, would help to maintain a strong magnetic field as well, shielding all potential lifeforms on the surface from all the lethal cosmic and stellar radiation coming from space.
The fact that most of the 3,538 exoplanet candidates discovered to date by NASA’s Kepler space telescope fall into the “Super-Earth” category, with masses ranging between two and 10 times that of Earth, gives more credence to the idea that such Super-Earths could turn out to be superhabitable, as described by Heller and Armstrong: “A more uneven surface, or simply a larger planet with more space for living forms, could make a planet superhabitable. Note that Earth, the only inhabited planet known so far, is the largest terrestrial planet in the Solar System.”
Something that has proven to be an important driver for potential habitability is tidal heating. A dramatic display of this phenomenon can be observed on the moons of the outer Solar System. The interior of Jupiter’s moon Io is melted by tidal friction forces caused by the presence of the nearby massive Jupiter. This results in Io being the most actively volcanic body in the Solar System, with the moon’s molten interior finding its way to the surface through the dozens of volcanoes that have been discovered there. A similar process is believed to heat up the interiors of Europa (another one of Jupiter’s moons) and of Saturn’s moon Enceladus. The latter two, although well outside the Solar System’s habitable zone, are considered to be prime candidates for harboring extraterrestrial life in the liquid water oceans that are thought to exist below their surfaces.
The same mechanism could drive the development and evolution of life on extrasolar planets and moons, outside of their stars’ habitable zones as well, according to Heller and Armstrong: “In exomoons beyond the stellar habitable zone, tidal heat could even become the major source of energy to allow for liquid water—be it on the surface or below.”
The authors also draw the conclusion that the best stars in the galaxy to search for superhabitable planets might not be Sun-like, G-type stars at all, but K-class ones. These are orange dwarf stars with intermediate sizes between that of the Sun and of the smaller red dwarfs. Although slightly cooler than the Sun, they live far longer. “These so-called K dwarf stars have lifetimes that are longer than the [current] age of the Universe. Consequently, if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and—once occurred—it would have had more time to ‘‘tune’’ its ecosystem to make it even more habitable,” write Heller and Armstrong. One of the best such known stars is Alpha Centauri B, part of the triple Alpha Centauri star system—the closest star system to the Sun, located just 4.37 light-years away. Astronomers have also announced the discovery of a planetary candidate around Alpha Centauri B in 2012, making it the closest exoplanet to the Solar System ever to be discovered. Although its existence hasn’t yet been confirmed by follow-up observations, it nevertheless opens up the possibility for even more planets in more distant orbits around our closest stellar neighbor. According to Heller and Armstrong, these other planets around Alpha Centauri B, if they exist, may turn out to be the superhabitable worlds described in their study.
The history of astronomical discovery has been a continuing series of dethroning of our various anthropocentric notions regarding our place in the Universe. “Eventually, just as the Solar System turned out to be everything but typical for planetary systems, Earth could turn out to be everything but typical for a habitable or, ultimately, an inhabited world,” say Heller and Armstrong.
Giordano Bruno’s hypothesis about the plurality of worlds turned out to be true centuries after his death, due to the incredible advancements made by modern science and technology. Although the existence of extraterrestrial life still remains within the realm of theoretical speculation only, a series of newer ground- and space-based observatories scheduled to come online in the near future hold the promise of turning it from speculation to reality. We may not have to wait very long until the first telescopic pictures of a far-off inhabited world will turn another of Bruno’s ideas, that of the plurality of beings on other worlds, from science fiction to science fact.