New definition may decrease the number of potential habitable planets

Earth_HZThe number of potentially habitable exoplanets will be impacted
A team of astronomers from Penn State led by Ravi Kumar Kopparapu and Ramses Ramírez, also PHL collaborators, announced a redefinition of the classical stellar habitable zone. Their study has strong implications on how we search and study potentially habitable exoplanets.
The main conclusion of their study is that the habitable zones are actually farther away from the stars than previously thought. Interestingly, Earth appears to be now situated at the very inner edge of the habitable zone.
The new study builds on the previous work of James Kasting, Evan Pugh Professor of Geosciences at Penn State, also one of the co-authors. Currently, all definitions of the habitable zone were based on Dr. Kasting’s pioneering work.
Estimates with the new habitable zone suggest that some exoplanets previously believed to be in the habitable zone may not be. However, the study has not as yet taken into account the complicated feedback of clouds, which also help to stabilize climates.
The PHL’s Habitable Exoplanet Catalog (HEC) will certainly be impacted by the new definition of the habitable zone. This will affect the classification of a number of potentially habitable exoplanets out of those nearly 900 confirmed and the over 2,700 NASA Kepler exoplanets candidates.
Recently, the Earth Similarity Index (ESI), a measure of Earth-likeness, and other habitability metrics were updated at the PHL. The ESI does not rely on the definition of the habitable zone but other considerations were used to improve it.
A major release of HEC, reflecting all these results, will be presented via a public lecture at the Arecibo Observatory on Saturday, February 16, 2013.
Credit: The Planetary Habitability Laboratory @ UPR Arecibo (
habitable planets

Extension of the new habitable zone as a function of distance for cool to hot stars. Credit: Chester Harman, Penn State

Habitable zone

Extension of the new habitable zone as a function of stellar flux (relative to Earth 100%) for cool to hot stars. Credit: Chester Harman, Penn State

About the author

Recipient of many prestigious NASA Awards including the Exceptional Public Service Medal and the Robert H. Goddard award. Experience includes working for NASA, as a contractor, in satellite design, construction and operations. Expert in the satellite operations concepts and ground systems including command, control, and science data processing.