The Habitable Universe
Last Update: July 20, 2015
The PHL's Habitable Universe web page provides estimates on the number of habitable worlds in the universe. Here we use the latest data and models to estimate the number of potentially habitable exoplanets, those Earth-size planets orbiting within the habitable zone of stars, starting from the nearby stars, our home galaxy, to the whole observable universe. All estimates show both conservative and optimistic values, which are based on corresponding definitions of the habitable zone. So far, only up to 30 potentially habitable exoplanets have been identified, none yet Earth-like. This page is only updated as new information is available.
Stellar Neighborhood
The Stellar Neighborhood, around 10 parsecs from our Sun (33.6 light years), consists of 259 known stellar systems with a total of 322 nearby stars, mostly M-dwarf stars. Most confirmed exoplanets detected so far are less than 318 parsecs away (1,037 light years).
Estimated Number of Habitable Worlds
132 - 160
Around M-Dwarf Stars
126 - 151
Around Solar-like Stars
6 - 9
Stellar Systems within 10 parsecs from the Sun (center). Systems are colored by main star spectral type and identify with the number of components, if more than one ('P' for those systems with planets). More details about this image are available here. CREDIT: RECONS Project.
Milky Way Galaxy
Our home galaxy the Milky Way has a radius of 34 kiloparsecs (110,000 light years) and contains 100 to 400 billion (B) stars. The Stellar Neighborhood is a small region at a distance of 8.33 kiloparsecs (27,200 light years) from its center.
Estimated Number of Habitable Worlds*
40B - 49B
Around M-Dwarf Stars
38B - 46B
Around Solar-like Stars
2B - 3B
* B = billions, 109
Artistic representation the Milky Way, created by combining real shape data of our galaxy with an actual image of a similar galaxy (Messier 51). CREDIT: Nick Risinger.
Observable Universe
The Observable Universe has a radius of 14 billion parsecs (46 billion light years) and contains about 100 billion galaxies and other forms of matter and energy. From our point of view the universe appears as a spherical volume regardless of its actual shape.
Estimated Number of Habitable Worlds*
4.2T - 5.3T
Around M-Dwarf Stars
4T - 5T
Around Solar-like Stars
0.2T - 0.3T
* T = trillions, 1012
Computer simulation of the large scale structure of the dark energy of the universe, which constitutes 68% of its composition. CREDIT: DEUS Consortium.
Important Notes
The number of potentially habitable worlds shown in this page are calculated from estimates of ηE (Eta Earth) for M-dwarf stars from Kopparapu (2013) and for Solar-like (GK) stars from Petigura et al. (2013). The population of the Stellar Neighborhood is based on the RECONS 10 Parsecs Sample. More details about this particular analysis is available here.
The numbers of stars in the Milky Way was conservatively assumed to be 100 billion stars with and average star distribution similar to our Stellar Neighborhood (75% M-Dwarf stars, 20% GK-Stars). These estimates does not consider that some regions of our galaxy are better than others to maintain habitable worlds, therefore they could be considered upper limits.
The number of galaxies in the universe was assumed 100 billion with an average number of 100 billion stars. The distribution of stars was again assumed similar to our Stellar Neighborhood but there should be large differences between young and older galaxies.
Earth-size Planet = a planet with a radius between 0.5 to 2.0 Earth radii or a mass between 0.1 to 10 Earth masses.
Habitable Zone (HZ) = the region around a star where an Earth-size planet could support liquid surface water. The size of this region depends on the luminosity and temperature of the star. There are two size definitions of the HZ, the Conservative Habitable Zone and the Empirical or Optimistic Habitable Zone.
References
Leconte, Jérémy, Forget, Francois, Charnay, Benjamin, Wordsworth, Robin & Pottier, Alizée 2013, Nature Increased insolation threshold for runaway greenhouse processes on Earth-like planets
Kasting, James F., Kopparapu, Ravi, Ramirez, Ramses R., & Harman, Chester 2013, ArXiv e-prints Remote Life Detection Criteria, Habitable Zone Boundaries, and the Frequency of Earthlike Planets around M and Late-K Stars
Kopparapu, Ravi Kumar 2013, The Astrophysical Journal A Revised Estimate of the Occurrence Rate of Terrestrial Planets in the Habitable Zones around Kepler M-dwarfs
Petigura, Erik A., Howard, Andrew W., & Marcy, Geoffrey W. 2013, ArXiv e-prints Prevalence of Earth-size planets orbiting Sun-like stars