Habitable zones around main-sequence stars: New estimates

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

Research output: Contribution to journalArticle

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Abstract

Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et al. However, this model used band models that were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in the Kasting et al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67 AU. Here an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars. New H2O and CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water-loss (inner HZ) and maximum greenhouse (outer HZ) limits for our solar system are at 0.99 and 1.70 AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 and 7200 K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water-loss limits for stars with T eff ≲ 5000 K, which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending on the host star's spectral type. We suggest that conservative estimates of the HZ (water-loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on η, so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

Original languageEnglish (US)
Article number131
JournalAstrophysical Journal
Volume765
Issue number2
DOIs
StatePublished - Mar 10 2013

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main sequence stars
greenhouses
water loss
estimates
climate models
stars
K stars
M stars
terrestrial planets
planet
extrasolar planets
solar system
radial velocity
climate modeling
planets
Kepler mission
G stars
habitability
F stars
space missions

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Kopparapu, R. K., Ramirez, R., Kasting, J. F., Eymet, V., Robinson, T. D., Mahadevan, S., ... Deshpande, R. (2013). Habitable zones around main-sequence stars: New estimates. Astrophysical Journal, 765(2), [131]. https://doi.org/10.1088/0004-637X/765/2/131
Kopparapu, Ravi Kumar ; Ramirez, Ramses ; Kasting, James F. ; Eymet, Vincent ; Robinson, Tyler D. ; Mahadevan, Suvrath ; Terrien, Ryan C. ; Domagal-Goldman, Shawn ; Meadows, Victoria ; Deshpande, Rohit. / Habitable zones around main-sequence stars : New estimates. In: Astrophysical Journal. 2013 ; Vol. 765, No. 2.
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Kopparapu, RK, Ramirez, R, Kasting, JF, Eymet, V, Robinson, TD, Mahadevan, S, Terrien, RC, Domagal-Goldman, S, Meadows, V & Deshpande, R 2013, 'Habitable zones around main-sequence stars: New estimates', Astrophysical Journal, vol. 765, no. 2, 131. https://doi.org/10.1088/0004-637X/765/2/131

Habitable zones around main-sequence stars : New estimates. / Kopparapu, Ravi Kumar; Ramirez, Ramses; Kasting, James F.; Eymet, Vincent; Robinson, Tyler D.; Mahadevan, Suvrath; Terrien, Ryan C.; Domagal-Goldman, Shawn; Meadows, Victoria; Deshpande, Rohit.

In: Astrophysical Journal, Vol. 765, No. 2, 131, 10.03.2013.

Research output: Contribution to journalArticle

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AU - Kopparapu, Ravi Kumar

AU - Ramirez, Ramses

AU - Kasting, James F.

AU - Eymet, Vincent

AU - Robinson, Tyler D.

AU - Mahadevan, Suvrath

AU - Terrien, Ryan C.

AU - Domagal-Goldman, Shawn

AU - Meadows, Victoria

AU - Deshpande, Rohit

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Y1 - 2013/3/10

N2 - Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et al. However, this model used band models that were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in the Kasting et al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67 AU. Here an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars. New H2O and CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water-loss (inner HZ) and maximum greenhouse (outer HZ) limits for our solar system are at 0.99 and 1.70 AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 and 7200 K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water-loss limits for stars with T eff ≲ 5000 K, which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending on the host star's spectral type. We suggest that conservative estimates of the HZ (water-loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on η⊕, so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

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