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Ultraviolet Limits on Habitable Worlds

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.

Comments on this entry are closed.

  • Asa May 9, 2006, 1:20

    Thanks for another great post on habitable zones.

    It’s pretty wild that in just the last 10 years or so, we’ve amassed enough of a catalog of extra solar planets that we can start to discuss habitability characteristics and still have a decent sample of dozens of planets that fit various habitability criteria.

    – Asa

  • ljk April 2, 2008, 10:09

    Search for extrasolar planets with high-precision relative astrometry

    Authors: T. Roell, A. Seifahrt, R. Neuhäuser

    (Submitted on 28 Mar 2008)

    Abstract: We present our search program for substellar companions using high-precision relative astronomy. Due to its orbital motion around the star, an unseen substellar companion would produce a periodic “wobble” of the host star, which is the astrometric signal of the unseen companion. By measuring the separation between the components of stellar double and triple systems, we want to measure this astrometric signal of a possible unseen companion indirectly as a relative and periodic change of these separations. Using a new observation mode (the “cube-mode”) where the frames were directly saved in cubes with nearly no loss of time during the readout, an adaptive optics system to correct for atmospheric noise and an infrared narrow band filter in the near infrared to suppress differential chromatic refraction (DCR) effects we achive for our first target (the double star HD 19994) a relative precision for the separation measurements of about 100…150 micro-arsecond per epoch. To reach a precision in the micro-arcsec-regime, we use a statistical approach. We take several thousand frames per target and epoche and after a verification of a Gaussian distribution the measurement precision can be calculated as the standard deviation of our measurements divided by the square root of the number of Gaussian distributed measurements. Our first observed target is the stellar binary HD 19994 A & B, where the A component has a known radial velocity planet candidate.

    Comments: 4 pages, IAUS 249 conference

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.4159v1 [astro-ph]

    Submission history

    From: Tristan R\”oll [view email]

    [v1] Fri, 28 Mar 2008 16:39:20 GMT (1389kb,D)