HEC: Science Plots

Here is a selection of scientific plots generated from the analysis of the Habitable Exoplanets Catalog (HEC). They are updated with new exoplanets data as available. Many more plots will be added later. Additional explanations are at the end of this pageOther source of pre-generated plots is the NASA Exoplanet Archive, or use an interactive plotter like the ones at the Exoplanets Data Explorer or the Extrasolar Planets Encyclopaedia (correlations, histograms).
Last Update: June 12, 2012

The Habitable Zone

Figure 1. Confirmed exoplanets as a function of mass and location with respect to their stellar habitable zone (HZ). Red dots are exoplanets and blue dots Solar System planets (Jupiter, Saturn, Uranus, and Neptune are off the scale to the right of the plot). There are 4 superterrans within the HZ (green shade). Only planets up to 10,000 Earth masses are shown.

Figure 2. NASA Kepler candidates as function of radius and location with respect to their stellar habitable zone (HZ). Red dots are candidates, green dots those already confirmed (so far 85 including community efforts), and blue dots Solar System planets (Jupiter, Saturn, Uranus, and Neptune are off the scale to the right of the plot). There are 28 superterrans and one terran within the HZ (green shade). Kepler-22 b and Kepler-16 (AB) b are the only two confirmed Kepler exoplanets within the habitable zone. Only planets up to 10 Earth radii are shown.

Basic Definitions

HEC proposes a few new names for classifying exoplanets that are as simple as possible and preserve most of the common usage in the astronomical community (Table 1). We try to avoid the use of the words 'Earths' or 'Super-Earths' to describe exoplanets because they generally cause confusions to the general public. We prefer the word 'terran' because it conveys the message without the more 'Earth-like' connotation.

Table 1. The Planetary Class (pClass) is a simple mass/size classification for planetary bodies including the Solar System and exoplanets. Radius is used instead for the classification when the mass is not available.



 Mass Range (ME)

Radius Range (RE)*




 0 — 10-5

 0 — 0.03

 Usually irregular bodies. (no exoplanets examples yet).
 Mercurian M 10-5 — 0.1 0.03 — 0.4 Mercury-size bodies. Usually spherical and atmosphere-less.
 Subterran T- 0.1 — 0.5 0.4 — 0.8 Mars-size bodies.
 Terran T 0.5 — 2 0.8 — 1.25 Earth-size bodies.
 Superterran T+ 2 — 10 1.25 — 2.6 aka Super-Earths
 Neptunian N 10 — 50 2.6 — 6 Neptune-size bodies.



 > 6

 Jupiter-size bodies.

* used if mass is not available.

We also use the Habitable Zone Distance (HZD) as a standard to measure the orbit of a planet in relation to the star's habitable zone (HZ), with -1 being the inner edge, zero the center, and +1 the outer edge of the HZ (Méndez et al., 2011). This is a very practical quantity because it allow to easily plot together all their planets with respect to their HZ. This can't be done if equilibrium temperature or stellar flux are used as this quantities miss the effect of the effective temperature of the star on the HZ. Here are the orbits of many exoplanets plotted in HZD units together with the HZ. The HZD is part of a proposed Habitability Coordinate System.

Important Note

All the previous plots use estimated mass instead of minimum mass based on a mean 60° orbital inclination when the system inclination was not available like in most cases. Mass or radius was also estimated when necessary with an empirical mass-radius relationship. The data for these plots combines data from the Extrasolar Planets Encyclopaedia, NASA Kepler, SIMBAD, and in a few cases directly from the papers.

If you use any of these plots please acknowledge as Habitable Exoplanets Catalog (HEC), Planetary Habitability Laboratory, University of Puerto Rico at Arecibo or simply credit as PHL @ UPR Arecibo. A research paper is being prepared (Méndez et al., 2012).


  • Méndez et al. (2012) The Habitable Exoplanets Catalog (in preparation).