August 15, 2006 | David F. Coppedge

Stellar Habitable Zones: Don’t Forget the Sunscreen

Astronomers concerned with the origin of life on earth have long thought about the “habitable zone” (sometimes called continuously habitable zone, or CHZ) of our solar system.  They’ve discussed this aerobee-shaped zone around our sun – or any star – mainly in terms of locations where the temperature would permit water to exist as a liquid.  But stars put out more than heat.  Our sun emits prodigious amounts of ultraviolet radiation that could be hostile to life.  Some stars put out a great deal more UV than does our sun.  What does UV do to the CHZ?
    Three Argentinian astronomers studied this factor and published their calculations in Icarus.1  It was not their intent to question the abiotic origin of life when suitable conditions would be found on a planet.  They came up with criteria for modeling suitable sweet spots where UV might provide a source of energy strong enough to fortify a primordial soup without killing any incipient organisms in the process:

Ultraviolet radiation is known to inhibit photosynthesis, induce DNA destruction and cause damage to a wide variety of proteins and lipids.  In particular, UV radiation between 200 and 300 nm becomes energetically very damaging to most of the terrestrial biological systems.  On the other hand, UV radiation is usually considered one of the most important energy source on the primitive Earth for the synthesis of many biochemical compounds and, therefore, essential for several biogenesis processes.  In this work, we use these properties of the UV radiation to define the boundaries of an ultraviolet habitable zone.  We also analyze the evolution of the UV habitable zone during the main sequence stage of the star.  We apply these criteria to study the UV habitable zone for those extrasolar planetary systems that were observed by the International Ultraviolet Explorer (IUE).  We analyze the possibility that extrasolar planets and moons could be suitable for life, according to the UV constrains [sic] presented in this work and other accepted criteria of habitability (liquid water, orbital stability, etc.).

This kind of modeling required making some assumptions, about which they were forthright in the introduction:

The so-called “Principle of Mediocrity” proposes that our planetary system, life on Earth and our technological civilization are about average and that life and intelligence will develop by the same rules of natural selection wherever the proper conditions and the needed time are given (von Hoerner, 1961 and von Hoerner, 1973).  In other words, the conditions that give place to the origin and evolution of life on Earth are average, in comparison to other worlds in the universe.
    This hypothesis is in the “hard core” (Lakatos, 1974) of all the research programs that search for life in the universe, which during the last fifty years were known within the scientific community as exobiology, bioastronomy, astrobiology, CETI, SETI, etc.
    Using the “Principle of Mediocrity,” we speculated about the existence of Earth-like planets, which must have liquid water on its surface, comparable surface inventories of CO2, H2O, N2 and other biogenic elements, an early history allowing chemical evolution that leads to life, and subsequent climatic stability for at least 4.5 Gyr [i.e., 4.5 billion years] (Lineweaver, 2001 and Owen, 2000).  We also speculated about the possible universal mechanisms for the origin of life, about universal mechanisms of Darwinian natural selection and for the appearance of intelligence and technological civilizations, and about how to detect primitive life and advanced technological civilizations beyond our home planet (Shklovskii and Sagan, 1996 and Lemarchand, 1992).

With these assumptions established, they reviewed the literature on habitable zones.  They agreed that our sun’s lies between Venus and Mars, with Earth in the sweet spot.  Previous work, however, failed to consider that “The ultraviolet radiation emitted by a star can also be important to determine the suitability of extrasolar planets for biological evolution and for the subsequent adaptation of life in exposed habitats.”  It also would have affected Earth’s atmospheric stability and composition.  The trick is to find a formula for the radius from any star where UV is strong enough to generate biologically-interesting compounds, without so strong as to tear them apart or shred a planet’s atmosphere.  This defines the UHZ, the ultraviolet habitable zone, or UV-HZ, as they call it.
    Their calculations for inner and outer boundaries of the UV-HZ considered the work of many other researchers on the effects of UV on molecules and atmospheric dynamics, and the changes in UV output over time from various star types.  They also considered how much UV would reach a planetary surface in the absence of an ozone layer.  As expected, UV considerations narrow down the traditional CHZ, and rule out some known extrasolar systems for the life lottery.  The UV-HZ usually lies closer in for most stars, they found.  What effect does this have?  “In those cases, UV radiation inside the traditional HZ would not be an efficient source for photolysis,” they reason, “and therefore the formation of the macromolecules needed for life would be much more difficult, if not completely impossible.”
    And now, the numbers: “In near the 41% stars of the sample … there is no coincidence at all between the UV region and the HZ.”  Scratch those as lively places, in other words.  In some others where there is overlap between the UV-HZ and the traditional HZ, it doesn’t last long enough to give life a foothold.  This ups the losers to 59% of stars sampled.  Alas, three others have a giant planet in the way, making any earthlike planet’s orbit unstable.  Some others have giant planets sitting in the sweet spot.  That means only a moon orbiting the gas giant could host a Darwinian game show.  Around a couple of F-type stars, “there is a region in the HZ where complex life would be burnt by UV radiation,” they found.  “In both cases, an atmospheric protection much larger than that of early Earth would be needed to make the traditional HZ suitable for life.”
    This sounds pretty depressing for earlier speculations about the CHZ. 

In this work we present a more restrictive criteria to habitability of an extrasolar Earth-like planet than the traditional liquid water one presented by Kasting et al. (1993), as we analyze the biological conditions to the origin and the development of life once the liquid-water scenery is already satisfied.
    Until an atmospheric protection would be built, a planetary surface would be exposed to larger amounts of UV radiation, which could act as one of the main source in the synthesis of bioproducts and, in a certain wavelength, could be damaging for DNA.

Bottom line: 59% of the 21 stars considered for the study were ruled out by UV considerations: “the traditional HZ would not be habitable following the UV criteria exposed in this work.”  Of the remaining candidates, five had zones of overlap where (if an Earth-size planet orbited therein) a cozy radiation bath would endure for at least 3 Gyr.  A couple of gas giants might also have moons in the safe zone.  All in all, despite the “principle of mediocrity,” Earth looks like a winner.


1Buccino, Lemarchand and Mauas, “Ultraviolet radiation constraints around the circumstellar habitable zones,” Icarus, Volume 183, Issue 2, August 2006, Pages 491-503, doi:10.1016/j.icarus.2006.03.007.

Die-hard naturalists will respond that 5 out of 21 isn’t so bad, considering how many quintillions of stars are out there.  There could still be billions of advanced civilizations.  But notice how all the prior speculation ignored this crucial factor, that narrows the playing field considerably.  How many other factors are being ignored in the hopes of winning the cosmic lottery?  What about the geological composition of the planet, the presence of an adequate moon, a suitable tilt and rotation rate, a magnetic field, ability to avoid other types of radiation and flares, location in the galactic habitable zone, and many other factors?
    If China was a riddle wrapped in an enigma, this is a twiddle wrapped in a dogma.  Remove the dogmatic assumption that chemicals will self-organize and breathe into themselves the breath of life, and the whole story evaporates anyway.  Suppose I started a story with, “assuming a pile of turtles…. ” and then built an elaborate model of earthquakes, tides, and atmospheric dynamics on it, complete with tensor calculus and detailed graphs.  No amount of hand-waving will cover up for crazy assumptions.
    These guys have their history wrong, too.  Their “hard core” hypothesis, the “so-called Principle of Mediocrity,” is mostly empty space in the head.  Copernicus didn’t believe it, and neither did Kepler or Galileo.  These Argentinian scientists need to watch The Privileged Planet or get the book.
    There’s an upside to papers like this.  Realistic investigations into the requirements for life underscore the hopelessness of explaining our planet and its life by chance, without factoring in a Designing intelligence who intended for Earth to be inhabited (e.g., Isaiah 45).  We’ve just learned He has given us enough light to enjoy plants and sunsets without being burnt to a crisp by UV.  That’s all the more reason to sing, “Count your blessings, name them one by one, and it will surprise you what the Lord hath done.”

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