Habitable Zone Boundaries: Implications for our Solar System and Beyond Jacob Haqq-Misra, Ravi Kopparapu, Natasha Batalha, Sonny Harman, and James Kasting Department of Geosciences Penn State University

Liquid water is essential for life (as we know it) Clever biochemists have suggested that non-carbon-based, non- water-dependent life could possibly exist Nonetheless, the best place to begin the search for life is on planets like the Earth This means that we should look within the conventional habitable zone around nearby stars

Boundaries of the habitable zone (HZ) Inner edge determined by loss of water via runaway or moist greenhouse effect –‘Moist greenhouse’ is when the stratosphere becomes wet even though an ocean is still present Outer edge is determined by the point at which CO 2 condensation/Rayleigh scattering begin to overwhelm the CO 2 -H 2 O greenhouse effect –We call this the ‘maximum greenhouse’ limit

The carbonate-silicate cycle The HZ is widened by the negative feedback between atmospheric CO 2 levels and climate (Walker et al., 1981; numerous Berner papers) Atmospheric CO 2 should build up as the planet cools Higher CO 2 in the distant past is at least part of the solution to the faint young Sun problem

The ZAMS habitable zone Diagram adapted from J. F. Kasting et al., Icarus (1993) When CO 2 -climate feedback is taken into account, one gets a habitable zone that is fairly wide compared to the mean planetary spacing Figure applies to zero-age-main-sequence stars; the HZ moves outward with time because all main sequence stars brighten as they age

ZAMS habitable zone Kasting et al., Icarus (1993) Older diagram: Not quite as pretty as the other one, but it illustrates the tidal locking problem for planets around late K and M stars

Recent improvements to models of the habitable zone My students, Ravi Kopparapu, Ramses Ramirez, and others have refined and extended these original HZ calculations Colin Goldblatt (U. Victoria) showed that new H 2 O absorption coefficients (from HITEMP, rather than HITRAN) make a big difference in warm, moist atmospheres, moving the inner edge outwards Recent studies show that 3-D climate modeling leads to new insights –Leconte et al. (Nature, 2013) showed that the inner edge moves inwards because of ‘radiator fin’ behavior of the tropical Hadley cells (an idea borrowed from Ray Pierrehumbert) –Yang et al. (Ap.J., 2013) showed that the inner edge moves way inwards for tidally locked planets orbiting M stars because they become nearly 100% cloud-covered on their daysides

Updated habitable zone (Kopparapu et al., 2013, 2014) Note the change in the x-axis from distance units to stellar flux units. This makes it easier to compare where different planets lie The exoplanets represent objects identified either by ground-based RV measurements or by NASA’s Kepler Space Telescope Figure credit: Sonny Harman

But, this analysis overlooks a phenomenon that could have been important on early Earth and that should be important on at least some Earth-like planets around other stars…

A new paper by Kristen Menou shows that planets near the outer edge of the habitable zone should not have stable, warm climates, despite the influence of the carbonate-silicate cycle See also Kadoya and Tajika (ApJ, 2014), along with earlier papers by Tajika, referenced therein Menou used a parameterized version of an energy-balance climate model (EBM)

Menou’s new model One needs to simultaneously solve for surface temperature, T surf, as a function of pCO 2 and for pCO 2 as a function of T surf The radiation balance is done using a fit to Darren Williams’ 1997 EBM The EBM parameterization itself was created by fitting results from our own 1-D radiative-convective climate model

Menou’s new model (cont.) The CO 2 model balances removal by weathering, W, with production from volcanism, V The weathering rate parameterization is from Berner and Kothavala (2001)

pCO 2 dependence of the weathering rate The key parameter here, is , the power law dependence of the weathering rate on pCO 2 W  pCO 2   = 0.5 if weathering is proportional to dissolved [H + ]   0 today because soil pCO 2 is thought to be independent of atmospheric pCO 2 The terrestrial biological pump: Vascular plants enhance soil pCO 2 through root respiration and release of humic acids

Limit cycles on poorly lit planets When Menou put all of this together, he found that stable steady states are not achieved under some circumstances, e.g., low stellar heating or low rates of volcanism Instead, the planet’s climate is predicted to undergo limit cycles of global glaciation followed by deglaciation So, we tried this ourselves… IR cooling Solar heating Different weathering rates Snowball Earth Present Earth Limit cycles K. Menou, EPSL (2015)

Limit cycling near the HZ outer edge Our own EBM predicts limit cycles near the outer edge of the HZ – Radiative transfer in this model is parameterized based on Kopparapu et al. (2013) A planet in this region remains frozen much of the time, but warms occasionally near the equator The results depend, however, on the spectral distribution of the parent star’s radiation Haqq-Misra et al., PNAS, submitted T surf pCO 2

Spectral distribution of stellar radiation When we do this calculation for planets around other stars, we need to account for the spectral distribution of the star’s radiation – Hotter, blue stars emit more radiation in the visible and UV – Cooler, red dwarf stars emit more of their radiation at longer, near-IR wavelengths – Snow and ice are less reflective in the near-IR, so limit cycling is less likely to occur Segura et al., Astrobiology (2005)

Limit cycling near the HZ outer edge When limit cycling is included, the outer HZ edge moves well inwards for F- and early G-type stars This phenomenon may have implications for the existence of complex (animal) life, including intelligent life Haqq-Misra et al., PNAS, submitted Figure credit: Sonny Harman

Conclusions The habitable zone for simple life is relatively wide because of the negative feedback between CO 2 and climate Habitable zones for complex (animal) life are narrower because of limit cycling behavior in the outer regions of the HZ Complex life, and intelligent life, is not necessarily rare, but it may be restricted to the inner parts of the HZ around G and early K-type stars

Backup slides…

Our new limit cycle figure We have tried to illustrate this behavior in a different way The figure at the right is from a paper on early Mars – Stable climate states are achieved when the surface temperature curves (green and red) intersect the weathering rate curve above the freezing point of water – Limit cycles are predicted when the intersection occurs below the freezing point Batalha et al., submitted to Science (Natasha Batalha is my graduate student from Astronomy)