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Challenges 2014, 5(1), 159-174; doi:10.3390/challe5010159

Assessing the Possibility of Biological Complexity on Other Worlds, with an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy

Department of Biological Sciences, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968, USA
Planetary Habitability Laboratory, University of Puerto Rico at Arecibo, Arecibo, Puerto Rico 00163, USA
Department of Astronomy, Cornell University, 426 Space Science Bldg., Ithaca, NY 14853, USA
Centro de Astrobiología, CSIC-INTA, M-108 Km 4, Torrejón de Ardoz 28850, Spain
School of the Environment, Washington State University, Pullman, WA 99164, USA
Center of Astronomy and Astrophysics, Technical University Berlin, Berlin 10623, Germany
Author to whom correspondence should be addressed.
Received: 19 March 2014 / Revised: 19 May 2014 / Accepted: 20 May 2014 / Published: 28 May 2014
(This article belongs to the Special Issue Challenges in Astrobiology)
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Rational speculation about biological evolution on other worlds is one of the outstanding challenges in astrobiology. With the growing confirmation that multiplanetary systems abound in the universe, the prospect that life occurs redundantly throughout the cosmos is gaining widespread support. Given the enormous number of possible abodes for life likely to be discovered on an ongoing basis, the prospect that life could have evolved into complex, macro-organismic communities in at least some cases merits consideration. Toward that end, we here propose a Biological Complexity Index (BCI), designed to provide a quantitative estimate of the relative probability that complex, macro-organismic life forms could have emerged on other worlds. The BCI ranks planets and moons by basic, first-order characteristics detectable with available technology. By our calculation only 11 (~1.7%) of the extrasolar planets known to date have a BCI above that of Europa; but by extrapolation, the total of such planets could exceed 100 million in our galaxy alone. This is the first quantitative assessment of the plausibility of complex life throughout the universe based on empirical data. It supports the view that the evolution of complex life on other worlds is rare in frequency but large in absolute number.
Keywords: exoplanets; biocomplexity; evolution; habitability exoplanets; biocomplexity; evolution; habitability

This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Irwin, L.N.; Méndez, A.; Fairén, A.G.; Schulze-Makuch, D. Assessing the Possibility of Biological Complexity on Other Worlds, with an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy. Challenges 2014, 5, 159-174.

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