The Habitable Exoplanet Catalog (HEC), a project by University of Puerto Rico (UPR) at Arecibo, went online on the 5th of December and after a brief peruse I felt compelled to express my approval and appreciation in blog form as soon as possible! A brief description, taken from the UPR Acecibo site:
The HEC is the first extensive catalog on the number, diversity, and characteristics of habitable exoplanets in the universe. It uses new developments by the PHL [Planetary Habitability Laboratory] like the Earth Similarity Index (ESI), the Habitable Zones Distance (HZD), classification systems, and comparisons with Earth past and present, to help assess the habitability of exoplanets.
The project uses three habitability metrics as well as two classification systems to assess the relative similarity and potential habitability of a number of exoplanets that have been announced and proposed but awaiting confirmation. These indices are built from observational data, but also from model output. The results, which at the moment outline 16 possibly habitable exoplanets, are presented via awesome individual graphics, thus:
This is exoplanet KOI 736.01 (as always, a wonderfully enigmatic name), a Kepler candidate considered to be ‘most similar’ to Earth based on the new habitability rating system. Awesome, right? Along with general planetary characteristics (part observational, part modelled) there are also some interesting classifications (mesoplanet, terran etc.) and some unusual indices (ESI, SPH, HZD) on the right. Let’s explore these in some detail, using KOI 736.01 as our case study. The classification methodology is described by the authors here.
Firstly, the classifications. The Planetary Class (pClass) sorts planets according to three thermal divisions (hot, warm, cold) and seven mass divisions, which are, in ascending order: asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian. KOI 736.01 is a warm terran planet, meaning that it is within the habitable zone of its star and of a comparable mass to Earth. Class M (mesoplanet) alludes to the fact that KOI 736.01 has a surface temperature of between 0 – 50 °C according to the Habitable Class classification (hClass). Other divisions in hClass range from extremely cold hypopsychroplanets through to very hot hyperthermoplanets.
Great so far, now lets turn our attention to the habitability metrics on the right. ESI or the Earth Similarity Index uses several planetary characteristics, namely radius, density, escape velocity, and surface temperature to determine the relative similarity of the planet to Earth on a scale from 0 (completely dissimilar) to 1 (identical). Our chosen planet has an ESI of 0.98, making it very like Earth and therefore likely to harbour a rocky interior and terrestrial atmosphere. This shouldn’t come as too much of a surprise; based on the general characteristics (mass, radius, gravity and period) included in the graphic it is fairly easy to ascertain that this is a planet very similar to our own, with the exception of a very brief orbital period. SPH, or the Standard Primary Habitability is a measure, calculated from surface temperature and humidity, of the ability of the planet to support terrestrial primary producers and in turn estimate the net primary productivity that would be expected. 0 is very unproductive and 1 is more so. With a SPH of 0.63, KOI 736.01 is moderately productive and very similar to the Earth at 0.65. SPH makes the assumption that water would be present, given a surface temperature of between 0 and 100 °C but in my opinion this may not always be the case. The final metric, HZD stands for Habitable Zone Distance, and describes the position of the planet within the habitable zone: +1 is on the very outer edge, 0 in the middle and planets with a HZD of -1 are straddling the inner edge. Accordingly, with a HZD of -0.59, KOI 736.01 is about half-way between the very centre of habitable zone and its inner edge.
All in all, the Habitable Planet Catalog is a wonderful resource with the potential to increase the accessibility of exoplanet science substantially. It also provides an excellent standardisation methodology that I hope will be readily adopted by other scientists for coherence and comparison. Bearing in mind my analysis was only based on one small area of the site dedicated to Earth-similarity, I suggest that anyone interested in the habitability of extra-solar planets takes the time to wonder around this great site and explore it in more detail.
Follow Professor Abel Mendez, the principal investigator at the PHL, on Twitter here.