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Of Exoplanets and Habitability: Exploring the Possibility of ‘Superhabitable Worlds’

Artist's impression of an exoplanet with a mass approximately 3.6 times that of Earth, discovered orbiting just inside of the habitable zone of the Sun-like star HD 85512. Are 'Super-Earths' such as this, the most promising candidates for harboring extraterrestrial life? Image Credit: ESO/M. Kornmesser
Artist’s impression of an exoplanet with a mass approximately 3.6 times that of Earth, discovered orbiting just inside of the habitable zone of the Sun-like star HD 85512. Are “Super-Earths” such as this the most promising candidates for harboring extraterrestrial life? Image Credit: ESO/M. Kornmesser

“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.

A comparison of habitable zones (green color) around different types of stars. Red color signifies the set of orbits where an exoplanet is too hot to sustain water on its surface, while blue color represents the orbits where it is too cold. Image Credit: NASA/Kepler Mission/Dana Berry
A comparison of habitable zones (green color) around different types of stars. Red color signifies the set of orbits where an exoplanet is too hot to sustain water on its surface, while blue color represents the orbits where it is too cold. Image Credit: NASA/Kepler Mission/Dana Berry

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.

NASA's Kepler space telescope has discovered 3.538 candidate exoplanets up to date, the vast majority of which fall into the 'Super-Earth' category. Image Credit: NASA
NASA’s Kepler space telescope has discovered 3,538 candidate exoplanets up to date, the vast majority of which fall into the “Super-Earth” category. Image Credit: NASA

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.”

An artist's impression of the planetary candidate orbiting Alpha Centauri B, the closest exoplanet to Earth ever discovered. Image Credit: ESO/L. Calçada
An artist’s impression of the planetary candidate orbiting Alpha Centauri B, the closest exoplanet to Earth ever discovered. Image Credit: ESO/L. Calçada

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.


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  1. Then, perhaps, is there fossils or life expressions elsewhere? But, isn’t the emergence and maintenance of life a process of radical contingency? That is, is a unique and unrepeatable past totally necessary? Or does life emerge through space like mushrooms when some conditions are present? So, how many conditions are necessary: three, four, trillions, infinite? Only one, water or any sort of God? Is God the word that means infinite conditions, absolute necessity? Anyway, how did the life that emerge in a given conditions resist when switching to a different moment? How does life resist time itself, the effects of entropy? But, is it possible for human beings to recognise a simpler life than their own brain only? On the other hand, beyond likeness, is it possible to recognise a complex life than their brain, is this the extra-terrestrial life that some people are searching unsuccessfully? However, is there an origin of life or would it be as finding a cut in the material history of the universe, an infinite void that human language patches now? Along these lines, there is a peculiar book, a short preview in Just another suggestion in order to free-think for a while

    • You make some very exciting and brilliant questions, editor-b. The fact of the matter is that we don’t know the answers yet, and that’s the reason we’re searching. Is life on Earth a typical example of life elsewhere in the Universe, or is it just a part of a richer tapestry of life and intelligence? Could we understand and perceive with our senses extraterrestrial intelligence if we encountered it?

      The only example for life in the Universe that we currently know of, is that on Earth, so we can’t really categorise it and make any comparisons. When the time comes that we discover extraterrestrial life, then we can begin to answer some of these questions.

  2. Exceptionally well-written and informative articles such as those you gift us with Leonidas only heighten our anticipation for the marvels that our James Webb Space Telescope will most certainly reveal. In either case, if intelligent life is commonplace throughout our solar system, or if we are indeed all that there is, the ramifications are awesome. Incidentally, please put me on the list for an autographed copy of your first book Leonidas (I’ve got the feeling that Tom is going to want one too!) 🙂

    • Thank very much Karol!

      Indeed, I too can’t wait for the launch of the James Webb telescope. The discoveries it will help to make, will be really something!

      You know, the idea for a book is going round my head, ever since my partner first suggested that I should really write one. Rest-assured, that when the time comes, you’ll be one of the first to receive it!

  3. When we do discover life on other planets (and we will), humanity will breathe a collective (but momentarily) “sigh of relief” knowing that the Great Question has been answered. But then the real work begins – finding the countless variations of life and all that goes with it. It will be a “reboot” of scientific thought that will boggle the mind.
    Put me on the list for an autographed copy of Leonidas’s book as well!

    • Exactly right Tom. I think it might also be a “reboot” of religious thought that will boggle the mind as well. Imagine the discovery of intelligent life on a distant planet light-years away with the physical appearance of say, a large squid, whose most revered holy book or bible emphatically stated that God created them “in his own image”. (Not to mention how they might feel about our fondness for calamari).

      • I can’t agree more, Tom and Karol.

        I will not hide the fact that while doing my research for writing the article, I kept myself wake at night from excitement, thinking about the possibilities and the significance of discovering extraterrestrial life. I believe that the significance of this, will be up there with the Copernican revolution of the 16th century.

        Indeed, Karol. Personally as an agnostic, I feel that this day can’t come soon enough.

  4. I’d like to see some research done on Gilese 710, that is due to plow into the Oort Cloud in a million years or so (DM 61 366/HIP 9481?)

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