Skip to main content Accessibility help
  • Print publication year: 2015
  • Online publication date: July 2015

6 - Number of planets, per solar system, with an environment suitable for life, ne, 1961 to the present



One of the Drake Equation factors that has changed the most since 1961 is ne, the average number of planets per star that can potentially support life. This factor is still evolving.

The definition of conditions for developing life is related to the definition of a circumstellar habitable zone. It is generally defined as the zone in which physical conditions make presence of liquid water possible. As a first approximation, it implies a temperature between 0 and 100 degrees Celsius, the so-called Goldilocks condition. This preliminary estimate is based on the temperature of the star: in other words, its spectral type and the distance between the star and its planet. Changes in the ne term are principally due to unexpected characteristics on one hand of many exoplanets and on the other hand of some objects in the solar system.

We first discuss the basic condition for life: liquid water. Stellar spectral types where life is most likely to emerge and exist are reviewed, and less favorable conditions of hot and cold stars are discussed. We also look at planets orbiting one member of a binary star, or around both stars (“circumbinary planets”). We point out the physical conditions necessary for planets to shelter life, including mass and other physical parameters. Exomoons are interesting objects and are discussed as well. Relations between the star and its planetary system are reviewed. No longer is distance the only parameter to be considered. In the case of terrestrial exoplanets with large eccentricity that cross the habitable zone, life with some phases of hibernation may be possible. Some terrestrial exoplanets orbit so close to their star that they are co-rotating, keeping one face to the star at all times, which implies that a temperate annular zone may exist between the very 115hot face in front of the star and the very cold face on the opposite side. Characteristics of some satellites in the solar system discovered since the space age show that some tidal effects are liable to extend the habitable zone, as can be seen by the detection of oceans flowing below the icy surface of Europa or internal water springing from the geysers of Enceladus. It would be interesting to search for and study as a source of possible life moons of giant exoplanets located in the habitable zone of their star. As a conclusion, the continuously increasing number of small rocky planets provides great encouragement to search for extraterrestrial life. Indeed, they show a high rate per star and satisfy the conditions necessary for producing life.

Related content

Powered by UNSILO
Benest, Daniel. 1991. “Habitable Planetary Orbits around α Centauri and Other Binaries.” In Bioastronomy: The Search for Extraterrestrial Life: The Exploration Broadens, ed. Jean Heidmann and Michael J. Klein, 44–47. Proceedings of the Third International Symposium on Bioastronomy 1990, Lecture Notes in Physics 390.
Briot, Danielle, and Schneider, Jean. 2010. “Occurrence, Physical Conditions, and Observations of Super-Ios and Hyper-Ios.” In Pathways Towards Habitable Planets, ed. Vincent Coudé du Foresto, Dawn M. Gelino, and Ignasi Ribas, 409–10. Astronomical Society of the Pacific Conference Series 430.
Briot, Danielle, Lellouch, Emmanuel, and Schneider, Jean. 2011. “What Could be Observed in the Case of Super-Ios and Hyper-Ios ?” In Molecules in the Atmospheres of Extrasolar Planets, ed. J. P. Beaulieu, S. Dieteres, and G. Tinetti, 155–58. Astronomical Society of the Pacific Conference Series 450.
Cassen, Patrick M., and Reynolds, Ray T.. 1979. “Is There Liquid Water on Europa?Geophysical Research Letters 6: 731–34.
Dole, Stephen H. 1964. Habitable Planets for Man. New York: Blaisdell.
Dumusque, Xavier, Pepe, Francesco, Lovis, Christophe, Ségransan, Damien, Sahlmann, Johannes, Benz, Willy, Bouchy, François, Mayor, Michel, Queloz, Didier, Santos, Nuno, and Udry, Stéphane. 2012. “An Earth-Mass Planet Orbiting α Centauri B.” Nature 491: 207–11.
Galilei, Galileo. 1610. Sidereus Nuuncius. Venice: Thomas Baglioni.
Grasset, Olivier, Dougherty, M. K., Coustenis, A., Bunce, E. J., Erd, C., Titov, D., Blanc, M., Coates, A., Drossart, P., Fletcher, L. N., Hussmann, H., Jaumann, R., Krupp, N., Lebreton, J.-P., Prieto-Ballesteros, O., Tortora, P., Tosi, F., and Van Hoolst, T.. 2013. “Jupiter Icy Moons Explorer (JUICE): An ESA Mission to Orbit Ganymede and Characterise the Jupiter System.” Planetary and Space Science 78: 121.
Hart, Michael H. 1979. “Habitable Zones around Main Sequence Stars.” Icarus 37: 351–57.
Hohlfeld, Robert Grant, and Terzian, Yervant. 1977. “Multiple Stars and the Number of Habitable Planets in the Galaxy.” Icarus 30: 598600.
Howe, Herbert Alonzo. 1885. “The Habitability of Other Worlds.” Sidereal Messenger 4: 294–98.
Huang, Su-Shu. 1959. “The Problem of Life in the Universe and the Mode of Star Formation.” Publications of the Astronomical Society of the Pacific 71: 421–24.
Huang, Su-Shu. 1960. “Life Supporting Regions in the Vicinity of Binary Systems.” Publications of the Astronomical Society of the Pacific 72: 106–14.
Kasting, James F., Whitmire, Daniel P., and Reynolds, Ray T.. 1993. “Habitable Zones around Main Sequence Stars.” Icarus 101: 108–28.
Laskar, Jacques, Joutel, Frédéric, and Robutel, Philippe. 1993. “Stabilization of the Earth's Obliquity by the Moon.” Nature 361: 615–17.
Leitner, Johannes J., Schwarz, R., Funk, B., Pilat-Lohinger, E., Firneis, M. G., Dvorak, R., Eybl, V., Eggl, S., and Aittola, M.. 2008. “Alternative Solvents as a Basis for Life Supporting Zones in Planetary Systems.” European Planetary Science Congress. Proceedings of the Conference Held 21–25 September 2008 in Münster, Germany.
Mayor, Michel, and Queloz, Didier. 1995. “A Jupiter-Mass Companion to a Solar-type Star.” Nature 378: 355–59.
McFarland, Robert White. 1883. “Habitability of the Planets.” Sidereal Messenger 2: 217–18.
Peale, Stanton J., Cassen, Patrick M., and Reynolds, Ray T.. 1979. “Melting of Io by Tidal Dissipation.” Science 203: 892–94.
Quintana, Elisa V., Barclay, Thomas, Raymond, Sean N., Rowe, Jason F., and Bolmont, Emeline. 2014. “An Earth-Sized Planet in the Habitable Zone of a Cool Star.” Science 344: 277–80.
Reynolds, Ray T., Squyres, Steven W., Colburn, David S., and McKay, Christopher P.. 1983. “On the Habitability of Europa.” Icarus 56: 246–54.
Roemer, Ole. 1676. “Démonstration touchant le mouvement de la lumière trouvé par M. Romer de l'Academie Royale des Sciences.” Journal des Sçavans, 233–36. Académie royale des sciences, 16 décembre 1676.
Schneider, Jean. 1977. “A Model for a Non-Chemical Form of Life: Crystalline Physiology.” Origins of Life 8: 3338.
Schneider, Jean, Dedieu, Cyrill, Le Sidaner, Pierre, Savalle, Renaud, and Zolotukhin, Ivan. 2011. “Defining and Cataloging Exoplanets: The DatabaseAstronomy & Astrophysics 532: A79A89.
Schneider, Jean, Lainey, Valéry, and Cabrera, Juan. 2015. “A Next Step in Planetology: Exomoons.” International Journal of Astrobiology 14: 191.
Shklovskii, Iosif Samuilovic, and Sagan, Carl. 1966. Intelligent Life in the Universe. San Francisco: Holden-Day.
Williams, Darren M., Kasting, James F., and Wade, Richard A.. 1997. “Habitable Moons around Extrasolar Giant Planets.” Nature 385: 234–36.