HEC: Description of Methods used in the Catalog

Last Update: June 18, 2019

Today we cannot be certain that any exoplanet is really habitable, less inhabited. Life depends on many planetary characteristics that are simply not known for exoplanets. Therefore, the list of worlds of the Habitable Exoplanet Catalog (HEC) is only a selection of the best objects of interest based on current observations and knowledge of what makes a planet habitable.

Habitable means suitable for life and not necessarily that life is present. Life could be simple such as bacteria or complex like plants or animals, if any. Millions of years ago Earth was only able to sustain microbial life, today it can also maintain plants and animals and support a more diverse biosphere.

A potentially habitable exoplanet is a planet around another star with the right size and orbit to be rocky and support surface liquid water if an atmosphere is present. The HEC keeps a current list of these planets from the thousands of exoplanets that are being discovered.

The HEC uses the NASA Exoplanet Archive as its main exoplanet data source and incorporates corrections, updates and different analyses to the data, all available here. The HEC is a project of the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo.

Habitability Criteria


Conservative Sample: These are planets with a radius < 1.5 Earth radii or a minimum mass < 5 Earth masses that orbit within the optimistic habitable zone of the star described in Kopparapu et al. (2014) (ArXiV). Planets that meet these criteria are also called warm terrans or warm subterrans in the catalog. These are the best candidates so far of planets that might be rocky and support surface liquid water.

Optimistic Sample: These are planets that have a radius between 1.5 to 2.5 Earth radii or between 5 to 10 Earth masses that orbit within the optimistic habitable zone of the star described in Kopparapu et al. (2014) (ArXiV)Planets that meet these criteria are also called warm superterrans in the catalog. These are objects that are less likely to be rocky or support surface liquid water. Some might turn to be not rocky at all but ocean worlds or small gas planets (i.e. mini-Neptunes).

The HEC considers planets up to 10 Earth masses and 2.5 Earth radii to include water-worlds, Mega-Earths, and the uncertainty of the radius and mass measurements. Therefore, the sample of exoplanets of the catalog is extensive, as not to miss any object of interest, and further filtering can be implemented for a more conservative sample.