Planetary Habitability Index Proposes A Less “Earth-Centric” View In Search Of Life


It’s a given. It won’t be long until human technology will expand our repertoire of cataloged exoplanets to astronomical levels. Of these, a huge number will be considered within the “habitable zone”. However, isn’t it a bit egotistical of mankind to assume that life should be “as we know it”? Now astrobiologists/scientists like Dirk Schulze-Makuch with the Washington State University School of Earth and Environmental Sciences and Abel Mendez from the University of Puerto Rico at Aricebo are suggesting we take a less limited point of view.

“In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability.” says Schulze-Makuch (et al). “The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time.”

Right now, an international science team representing NASA, SETI,the German Aerospace Center, and four universities are ready to propose two major questions dealing with our quest for life – both as we assume and and alternate. According to the WSU news release:

“The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbor life,” Schulze-Makuch said. “The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not.”

Within the next couple of weeks, Schulze-Makuch and his nine co-authors will publish a paper in the Astrobiology journal outlining their future plans for exoplanet classification. The double approach will consist of an Earth Similarity Index (ESI), which will place these newly found worlds within our known parameters – and a Planetary Habitability Index (PHI), that will account for more extreme conditions which could support surrogate subsistence.

“The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature.” explains the team. “For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions.”

Assuming that life could only exist on Earth-like planets is simply narrow-minded thinking, and the team’s proposal and modeling efforts will allow them to judiciously filter new discoveries with speed and high level of probability. It will allow science to take a broader look at what’s out there – without being confined to assumptions.

“Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” the paper’s authors write. “For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”

Of course, the team admits an alien diversity is surely a questionable endeavor – but why risk the chance of discovery simply on the basis that it might not happen? Why put a choke-hold on creative thinking?

“Our proposed PHI is informed by chemical and physical parameters that are conducive to life in general,” they write. “It relies on factors that, in principle, could be detected at the distance of exoplanets from Earth, given currently planned future (space) instrumentation.”

Original News Source: WSU News. For Further Reading: A Two-Tiered Approach to Assessing the Habitability of Exoplanets.

16 Replies to “Planetary Habitability Index Proposes A Less “Earth-Centric” View In Search Of Life”

  1. We are not being egotistical in counting the number of earth-like planets. Most of us imagine life may exist in other forms. It is not a bad thing to have a ‘limited point of view’, if we limit ourselves to things we actually know something about.

    Let us take an extreme example. The sun has complex moving magnetic field patterns. These patterns have short and long period behaviours. Could these patterns have intelligence? I would really doubt it, but it is not a trivial job to prove the absense of intelligence in sufficiently complex systems. So life could exist within the sun? Hooray, there are lots of suns, so that must boost the total odds on intelligent life.

    Okay, maybe that’s a bit silly, but you can have a mind that is too open. Particularly when it rains.

  2. …why risk the chance of discovery simply on the basis that it might not happen?

    Another question might be, why spend money looking for something that might turn out to be impossible, at the expense of looking for something that we know is possible?

    Intelligence might exist in the magnetic vortices of stars, who knows. I’m all for keeping our options wide open, but we don’t have infinite money, so let’s prioritise funding in accordance with what can be safely speculated within the bounds of good science.

  3. All of this depends upon the definition of life. If I were to restrict that definition to its building blocks, the I think life involves chemistry or organic chemistry similar to life on Earth. This then probably has parallels to biology on this planet, with some or many of the same building blocks. The building blocks most likely include water, carbon, nitrogen and so forth. This may extend as well to nucleotides, peptides, saccharides, fatty acids as well on a higher level of composition. Life also requires that temperatures be warm enough to permit chemical activity to proceed, but not so hot that information is erased by random thermal fluctuations.

    There likely exist other types of complex systems that are able to replicate themselves by conserving information from one iteration to the next. Whether this is life might just be a matter of definition. It might turn out that the surfaces of neutron stars have a huge amount of such self-replicating complexity, or that rings around planets such as Saturn are self-organizing systems.


    1. “The definition of life” is useful when you want to identify individuals instead of more complex study of populations and their properties (as in the NASA “definition of life”) or when you want to identify the origin and limits of known or putative life.

      It is not so useful when you want to identify biospheres remotely or in situ, or study the actual biological traits and the fossils that they produce. Life is a process of evolution* of (here) biological populations, it is a characteristic of the population (definition of evolution). It should not be confused with the characteristics of individuals (“definition of life”). Say sex, not strictly confined to the individual (as I am happy to observe =D); conversely an isolated sexual individual can’t produce life, biologically it is a “dead man walking”.

      This is not merely semantics, but it isn’t all that useful either – except in cases of astrobiological or biological methods. Looking for water based cells can be constrained by cellular productivity indexes. On habitability zones; on cellular products (oxygen and nitrous oxides) for remote sensing; on cellular resource cycling (carbon/carbonates et cetera) for in situ sensing.

      * Because evolutionary populations compete static individuals to extinction, they tend to exclude other forms of potential life over longer periods of time.

      Not that eternal or say asexual individuals are unobserved in biology despite the observed flexibility of sexual populations. I am reminded of presumed rejuvenating jellyfish for the former and bdelloid rotifers for the latter. They are just rarer.

      1. There is the potential for a huge range of possibilities. Slime molds have a three-way system for sex. Yes, that means what amounts to 3 sexes. When it comes to other planets I am reminded of Stanislaw Lem’s Solaris, which was a fictional planetary body that was a single integrated living system. In some sense life on Earth is similar, for ecosystems are integrated systems of organisms that have self-regulating capacity.


      2. Slime molds have a three-way system for sex. Yes, that means what amounts to 3 sexes.

        Apparently, there are three sexes among humans, according to some registration forms where it asks to state your gender:
        “Male”, “Female”, “Rather Not Say”.

      3. Some humans prefer to ignore the Victorian era obsession with labels that still permeates today.

      4. Hi, LC, yes I liked the idea behind SOLARIS too. We have organisms on our Planet which are an assortment of interaction and mutual benefit.
        We have to be open minded to the incredible possibilities the universe offers.

  4. As I mentioned recently, I am a fan of Méndez work on habitability indexes.

    A huge fan actually, originating from Nancy’s article on Universe Today on Mendez’s Standard Primary Habitability (SPH) index.

    The SPH is based on the primary two quantities of temperature and water availability, and is well validated by predicting Earth’s Net Primary Productivity (NPP) of biologists both over Earth surface and its biosphere history since the Cambrian. (I refer to the link I gave in my earlier comment.)

    The SPH would be universal with cellular life out of water, which have a maximal productivity ~ 40 degC and 100 % water availability (oceans).

    A concern would be that these are the observations of conditions in the Archean oceans when modern redox metabolism was established. (Independent thermometers of salt inclusion’s and cherts’ isotope ratios from ~ 3.2 – 3.5 Ma bp.) But there are other results that IIRC point to 40 degC as an optimal point for the crowded chemistry of cellular compartments.

    No doubt you can make similar indexes for cellular life based on other liquids. But using the known parameter set as a basis for “a perturbation analysis” is good heuristics often used in both science and engineering. I.e. start with the known and test the limits.

    That it was a recent mentioning is not a coincidence. Méndez et al will launch an online Habitable Exoplanet Catalog (HEC) on December 5, 2011. This work is likely correlated to that effort.

  5. The central question here is at what point will technology advance to a level where any of this even can be considered? It seems we are a long way from indentifying the indicators for any type of life on exoplanets. Nonetheless, and interesting discussion.

  6. Life results when simple molecules start to reproduce themselves. Evolution then takes over to produce organisms able to survive in the local conditions, whatever they may be. So I agree, we are being very Earth-centric to say life can only exist on Earth-like planets.

    However, if we want to find beings we can possibly communicate with, or even recognise, they may only exist in conditions somewhat similar to ours.

    Incidentally, if there are planets with conditions similar to here, given that we have seen the same basic organic molecules out there, the same physical laws apply everywhere, chemistry will work in the same way, and natural selection has discovered the same solutions for survival many different times independently on Earth, such as wings, eyes and so on, creatures on similar planets out there may well even look much like us.

    1. right on, Gusssss! I believe we should focus our efforts on finding brothers and sisters, and still do the other thing…;-)

  7. Consider what life may have evolved to in the current era had earth not experienced one or two mass extinction events.
    I’m picturing dinosaur like creatures sitting at their computers commenting on this UT article.

  8. While I presume that looking for any form of life may be rewarding, I still believe that we should focus on finding intelligent life, brothers and sisters, while looking for places in our neighborhood as similar to Earth as possible… as one day we may need to use them to evacuate to. Do we have the time to do this? I hope, but I think not.

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