Habitable zones, and our idea of what constitutes them, change over time. We know, for example, that the habitable zone around a given star should migrate outward as main sequence stars become brighter with age. Thus the notion of the ‘continuously habitable zone’ (CHZ) has emerged, the region where a planet remains in habitable conditions for a specified period of time. If you want to look for technological civilizations, that time frame might be 4 billion years, paralleling the experience of life on our own planet. If you’re content to look for microbes, as little as a billion years might suffice, perhaps less.
Among the numerous factors involved in creating the CHZ, ultraviolet radiation is significant. A new paper points out the need to assess UV and the limits it places upon emerging biospheres. Get too much of it and you inhibit photosynthesis, as well as damaging DNA and various proteins. Get too little and you dampen a primary energy source for the synthesis of biochemical compounds. So just as older markers like the presence of liquid water can define a habitable zone, so too can the properties of ultraviolet radiation constrain the formation of life.
In defining the UV habitable zone, Andrea Buccino (Instituto de Astronomía y Física del Espacio, Buenos Aires) and colleagues try to establish both an inner limit (not so close to the star as to damage DNA) and an outer (beyond which there is not sufficient UV for biogenesis to occur). They then apply their criteria to all nearby stars with exoplanets that have been studied in UV by the International Ultraviolet Explorer (IUE). This provides a dataset of 23 stars harboring 32 different planets.
Intriguingly, in most cases the ultraviolet habitable zone is closer to the star than the traditional habitable zone. “In those cases, UV radiation inside the traditional HZ would not be an efficient source for photolysis, and therefore the formation of the macromolecules needed for life would be much more difficult, if not completely impossible,” the authors write. In fact, stars like 51 Peg and HD160691 show no overlap between the UV region and the habitable zone; fully 59 percent of the sample fits in this category.
The authors find seven cases where the traditional habitable zone and the UV zone overlap at least partially, allowing the presence of a habitable planet. But in three of these cases, the presence of a giant planet would make terrestrial-style orbits unstable. Five extrasolar systems have giant planets inside the traditional habitable zone but four of these are at the extremes of the ultraviolet zone. The conclusion:
Applying all these criteria to those stellar systems whose central star has been observed by IUE, we obtained that an Earth-like planet orbiting the stars HD216437, HD114752, HD89744, τ Boo and Rho CrB could be habitable for at least 3 Gyr. A moon orbiting υ And c would be also suitable for life. While, in the 59% of the sample (51 Peg, 16CygB, HD160691, HD19994, 70 Vir, 14 Her, 55 Cnc, 47 UMa, ǫ Eri and HD3651), the traditional HZ would not be habitable following the UV criteria exposed in this work.
And this about F stars: the two studied in the sample, HD114762 and τ Boo, would demand layers of atmospheric protection far higher than that of the early Earth to protect against life-threatening UV.
Centauri Dreams‘ take: How planets cope with incoming ultraviolet is also an area that needs more research. The authors point out possible attenuating effects from planetary atmospheres, oceans, orbital factors and more, all of which have bearing on the result. It is clear that our traditional notions of what makes for habitability are in a state of revision as we learn more about our own ecosphere. The paper, slated for publication in Icarus, is Buccino, Lemarchand and Mauas, “Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones,” available here.