A Database of Potentially Habitable Worlds
The Habitable Worlds Catalog (HWC) lists up to 70 potentially habitable worlds out of over five thousand known exoplanets. Of these, 29 are more likely to be rocky planets capable of surface liquid water (conservative sample). The other 41 might include water worlds or mini-Neptunes, with a lower likelihood of habitable conditions (optimistic sample).
This is a list of exoplanets that are more likely to have a rocky composition and support surface liquid water (i.e., 0.5 < Planet Radius ≤ 1.6 Earth radii or 0.1 < Planet Minimum Mass ≤ 3 Earth masses). They are represented artistically in the image below.
Figure 1. Artistic representation of currently known potentially habitable exoplanets organized in order of their increasing distance from Earth in light-years. They are compared with Earth and Mars for scale. The sizes of planets detected by the radial velocity method were estimated from their mass using a mass-radius relation. CREDIT: PHL @ UPR Arecibo.
Table 1. List of exoplanets in the conservative sample. The Methods section has a description of each parameter.
This is a list of exoplanets that are less likely to have a rocky composition or maintain surface liquid water (i.e., 1.6 < Planet Radius ≤ 2.5 Earth radii or 3 < Planet Minimum Mass ≤ 10 Earth masses). Some of these planets might turn out to be ocean worlds or mini-Neptunes.
Table 2. List of exoplanets in the optimistic sample. The Methods section has a description of each parameter.
Figure 2. All planets near the habitable zone (the darker green shade is the conservative habitable zone, and the lighter green shade is the optimistic habitable zone). Only those planets with less than 10 Earth masses or 2.5 Earth radii are labeled. Credit: PHL @ UPR Arecibo.
Figure 3. Mass-radius plot of exoplanets, including potentially habitable worlds (named in magenta). Other planets in the catalog are not shown because their mass or radius is only known. Here are more plots. Credit: PHL @ UPR Arecibo.
Figure 4. Orbital plots of planetary systems with one or more potentially habitable exoplanets. Most systems have only one planet. TRAPPIST-1 has seven planets with up to four potentially habitable. These figures are available for download at this link. Credit: PHL @ UPR Arecibo.
Figure 5. Location in the night sky of all the known stellar systems with potentially habitable worlds (some systems have more than one planet). Click the image to enlarge. Credit: PHL @ UPR Arecibo, Jim Cornmell.
Figure 6. Interactive sky map. Credit: Aladin Sky Atlas, CDS, Strasbourg Observatory.
Figure 7: A summary of a few properties of all known confirmed exoplanets listed in the NASA Exoplanet Archives. Credit: PHL @ UPR Arecibo.
The HWC includes planets up to 2.5 Earth radii or 10 Earth masses orbiting within the optimistic stellar habitable zone to be as inclusive as possible. They are further divided into a conservative and optimistic sample. The conservative sample includes planets up to 1.6 Earth radii or 3 Earth masses, or those that are more likely rocky. The optimistic sample consists of larger planets and might include super-Earths, ocean worlds, or mini-Neptunes, and therefore less likely to be rocky or support surface liquid water. These samples can be further constrained if only those planets inside the conservative habitable zone are considered. The different limits of the habitable zone are described in Kopparapu et al. (2014). There is no way to tell if a planet is really habitable with this criteria. It only serves as a starting point for exoplanets of interest for further observations.
The HWC uses exoplanet data from the Planetary Systems Composite Parameters Planet Data of the NASA Exoplanet Archive with additions and corrections from other sources, including papers. This data provides a more comprehensive dataset of planetary properties but is more prone to inconsistencies due to multiple independent sources. Therefore, the HWC should be only used as a guideline for further data exploration of the individual planets of interest.
The HWC includes many measured, derived, and modeled stellar and planetary properties available in the full database (see Data section). Here is a description of the properties shown in the summary tables (Tables 1 and 2).
Name - Name of the planet. This links to the data of the planet at the NASA Exoplanet Archive.
Type - PHL's classification of planets that includes host star spectral type (F, G, K, M), habitable zone location (hot, warm, cold), and size (miniterran, subterran, terran, superterran, jovian, neptunian) (e.g., Earth = G-Warm Terran, Venus = G-Hot Terran, Mars = G-Warm Subterran).
Mass - Mass of the planet in Earth masses (Earth = 1.0 ME). Mass was estimated (denoted by ~) from radius when not available using Chen & Kipping (2016). Usually, we only know the minimum mass for radial velocity planets (denoted ≥). Some planets have upper mass measurements (denoted <).
Radius - Radius of the planet in Earth radii (Earth = 1.0 RE). Radius was estimated (denoted ~) from mass when not available using Chen & Kipping (2016).
Flux - Average stellar flux of the planet in Earth fluxes (Earth = 1.0 SE).
Tsurf - The estimated surface temperature in Kelvins (K) assuming an Earth-like atmosphere (i.e., same bond albedo and greenhouse) using Méndez & Rivera-Valentín (2017) (denoted ~). Temperatures could be much larger for thicker atmospheres (Earth = 288 K).
Period - Orbital period in days (Earth = 365 days).
Distance - Distance from Earth in light-years (ly).
Age - Star age in billion years. Some are lower or upper limits (denoted > or <) (Sun = 4.6 Gy).
ESI - Earth Similarity Index, a measure of similarity to Earth's stellar flux, and mass or radius (Earth = 1.0). Results are sorted by this number. Planets that are more similar to Earth on this scale are not necessarily more habitable.
