Zones where advanced life can exist in the universe just became stricter. Astronomical researchers have discovered that livable neighborhoods must include not only favorable planet-to-star conditions but also galactic and supergalactic features.
Almost all the research and speculation on habitability in the universe has focused on circumstellar habitable zones. Research on these zones attempts to determine at what specific distances from a host star a planet could conceivably maintain conditions which would make the survival of life possible. Though research on circumstellar habitable zones has largely focused on the liquid water habitable zone—the distance from the star where water could conceivably exist in a liquid state—another ten circumstellar habitable zones are known to be critical for the survivability of life to date. I have written about these eleven habitable zones here,1 here,2 and here.3 For a planet to be truly habitable it must reside simultaneously in all eleven of these circumstellar habitable zones.
Circumstellar habitability, however, is not the only requirement for habitability. For a planet to possibly host life it must also reside in the cosmic temporal habitable zone, the galactic habitable zone, and the supergalactic habitable zone. Astronomer Paul Mason has been studying and writing on this subject and has explained his findings.
Cosmic Temporal Habitable Zone
At the January 2017 meeting of the American Astronomical Society, Mason first addressed the subject of cosmic, galactic, and supergalactic habitability in a paper titled “Habitability in the Local Universe.”4 Therein, he pointed out that long-term habitability on the surface of a planet requires a prerequisite minimum abundance of several different elements. Animals, for example, require certain minimum abundances of twenty-two different elements in the periodic table. Mason explains that it takes a minimum amount of time for star formation and ongoing star burning within a galaxy to generate (through nucleosynthesis) the requisite abundances of these life-critical elements.
That minimum time is about nine billion years after the cosmic creation event. Though not mentioned by Mason, there is also a maximum time. Relatively aggressive ongoing star formation is necessary to sustain the spiral structure of a galaxy. When that star formation ceases, the spiral structure collapses and then the average separation between stars becomes too small for life to survive. Furthermore, virtually all planets within such a galaxy become exposed to the deadly radiation from one or more supermassive black holes.
Another problem for life is that the more time that passes, the more merger events with small and large galaxies will occur. Inevitably, one or more of these merger events will be devastating for life within the galaxy.
The window on life in the universe will close when the universe is about 15 billion years old. Since it takes time—over three billion years—for the first life in the universe to prepare a planetary environment for advanced life, the cosmic time window (less than a billion years) for advanced life is much briefer than it is for microbial life (about six billion years).
Galactic Habitable Zones
In a subsequent paper coauthored with Peter Biermann5 and in a paper delivered at the January 2019 American Astronomical Society meeting,6 Mason discussed galactic habitability conditions in addition to the galactic habitable zone. The galactic habitable zone refers to a narrow distance range from the center of a spiral galaxy where a star revolves around the center of the galaxy at virtually the same rate that the galaxy’s spiral structure rotates (see figure 1). Only within this narrow distance range does a star and its system of planets cross spiral arms infrequently enough (less than once per billion years) that it becomes possible for advanced life to exist on one of the star’s planets.
Figure 1: The Galactic Habitable Zone. Only a star and its system of planets located very near the red annulus will experience very infrequent crossings of spiral arms. The yellow dot represents the present position of the solar system. Image credit: NASA/JPL-Caltech/R. Hurt; Diagram credit: Hugh Ross
Mason and Biermann point out that thanks to a relatively high rate of supernova eruption events, our galaxy maintains a relativistic galactic wind. This wind shields our solar system from deadly extragalactic cosmic rays. However, if the supernova eruption rate in our galaxy were any higher, radiation from the supernovae would prove deadly to advanced life on Earth. Fortunately, the supernova eruption rate in our galaxy is just right.
Mason and Biermann also explain how the activity level of our galactic nucleus must be fine-tuned. It takes small dwarf galaxies being regularly absorbed into the nucleus of our galaxy to sustain the ongoing star formation that is critical for maintaining our galaxy’s spiral structure. However, if our galaxy were to absorb or merge with a large dwarf galaxy, that absorption or merger could activate our galaxy’s nucleus. That activation would shower the entire extent of our galaxy with deadly radiation. Fortunately, our galaxy is absorbing dwarf galaxies of the just-right size and at the just-right rate to make possible the survival of advanced life on Earth.
Supergalactic Habitable Zone
Only in a cluster of galaxies will a galaxy like ours have a sufficient supply of dwarf galaxies to sustain its spiral structure for many billions of years. Our galaxy cluster, the Local Group (see figure 2), has the distinction of possessing many dwarf galaxies but no giant galaxies. Our Local Group also has the distinction of residing on the outer fringe of the Virgo Supercluster of galaxies.
Figure 2: The Local Group, Our Galaxy’s Galaxy Cluster. The Milky Way Galaxy is to the lower right. Above it are the Large and Small Magellanic Clouds. To the upper left are the Andromeda Galaxy and its system of dwarf galaxies. Below Andromeda is the Triangulum spiral galaxy. Image credit for the galaxies: NASA/ESA/ESO/JPL-Caltech/R. Hurt; Map credit: Hugh Ross
Mason and Biermann explain how the giant galaxies near the center of the Virgo Supercluster pour out such intense deadly radiation as to eliminate the possibility for advanced life residing in any of the galaxies near the center of the Virgo Supercluster. Fortunately, our Milky Way Galaxy is far enough away from the center of the Virgo Supercluster and it possesses a strong enough relativistic galactic wind that advanced life on Earth is not harmed by the deadly radiation emanating from the giant galaxies in the Virgo Supercluster.
Layers of Design
A star and its system of planets must be exquisitely designed in many different ways for advanced life to be possible on one of the star’s planets. Thanks to the findings of researchers Mason and Biermann, we now appreciate more than we have before that it also takes exquisite fine-tuning of the universe, the planetary system’s host galaxy, the host galaxy’s galaxy cluster, and the host galaxy cluster’s supercluster of galaxies for advanced life to possibly exist and thrive. As the agnostic astronomer Paul Davies wrote in his book The Cosmic Blueprint, “the impression of design is overwhelming.”7
Featured image: Giant Galaxies in the Center of the Virgo Supercluster of Galaxies. The black dots block out foreground stars. Image credit: European Southern Observatory