Fig 2: Stellar temperature vs. flux at the planet for Earth and several known exoplanets (plotted with various symbols). The shaded green area indicates the boundaries of the habitable zone estimated by this paper. A planet between the dashed lines will be in the habitable zone no matter what the stellar spectral type is. The “early Mars” line is a less conservative outer boundary based on the hypothesis that Mars used to be warm enough to have liquid surface water. The “runaway greenhouse” line is a less conservative inner boundary at which the liquid surface water has completely evaporated. The “recent Venus” line is an even more liberal inner boundary based on the assumption that Venus used to have liquid water and the observation that Venus has been dry for the last 1 Gyr.

Finding the Edges of the Habitable Zone

BY ERIKA NESVOLD
FEBRUARY 7, 2013

Title: Habitable Zones Around Main-Sequence Stars: New Estimates
Authors: Ravi kumar Kopparapu, Ramses Ramirez, James F. Kasting, Vincent Eymet, Tyler D. Robinson, Suvrath Mahadevan, Ryan C. Terrien, Shawn Domagal-Goldman, Victoria Meadows, Rohit Deshpande
First Author’s Institution: Penn State University

Background

The number of confirmed exoplanets on exoplanets.org is up to 684. But how many of those planets could actually harbor life? We can estimate which exoplanets could support life by making the assumption that life requires the presence of liquid water. The habitable zone of a star describes the range of distances to the star in which a planet could orbit and still have liquid surface water. If the planet gets too close to its star, water is lost due to high temperatures. If the planet gets too far away, temperatures are so low that the water freezes. We’ve talked about habitable zones quite a bit here on astrobites, including the habitable zones around binary stars and brown and white dwarfs , and the habitability of rogue planets.

If we’re looking for extraterrestrial life, the next step is determining how many of the currently-known exoplanets orbit in the habitable zones of their stars. But where exactly is the habitable zone for a given star? Most current estimates of habitable zone boundaries use a planetary climate model by Kasting et al. (1993). In this paper, some of the original authors of the Kasting paper join with new researchers to improve the model and find new boundaries for the habitable zone in our Solar System and around other stars. These improved estimates of habitable zone boundaries will allow us to determine which known exoplanets orbit in the habitable zones of their stars, which narrows down the list of which planets which should observe more closely for signs of life.

Read more: Finding the Edges of the Habitable Zone | astrobites.

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