Click here for my Google Scholar Citations page

Ramirez, R.M. 2019. Implications of revised CO2-CH4 and CO2-H2 absorption for outer edge habitable zone planets. Res. Notes 3, 3, 48

I discuss the effect that recent revisions from laboratory experiments to previously-estimated values of CO2-CH4 and CO2-H2 collision-induced absorption have on the habitability of planets near the outer edge of the habitable zone. I also discuss implications for life. The link above has the most updated version, clarifying the Methods section in the RNAAS version.


Ramirez, R.M. 2018. A more comprehensive habitable zone for finding life on other planets. Geosciences 8(8), 280

I review the literature on habitable zone theory and on planetary habitability in general. I discuss the classical HZ as well as the newer HZ definitions that have improved how this navigational tool is used. Only by utilizing different HZ definitions can we maximize our chances of finding life elsewhere in the cosmos. This is the first review in the burgeoning field of “dynamic habitability.”


Ramirez, R.M. and Craddock, R.A. 2018. The geological and climatological case for a warmer and wetter early Mars. Nature Geoscience 11, 230 – 237

We review the geological and climatological evidence for early Mars and conclude that it does not support an icy climate punctuated by transient warming episodes. Impacts cannot satisfy the observations either. We argue that a warm and semi-arid climate better satisfies the observations, including valley network formation . Perhaps this was accomplished with a volcanically-produced CO2-H2  and/or CO2-CH4  greenhouse.


Ramirez, R.M. and Kaltenegger, L. 2018. A methane extension to the classical habitable zone. The Astrophysical Journal 858, 2

Following in the footsteps of our previous work with hydrogen, we show that the addition of CH4 to the atmospheres of habitable zone outer edge planets produces a strong greenhouse effect for worlds orbiting hotter (> 4500 K) star systems but a cooling anti-greenhouse effect for planets orbiting cooler stars. Thus, the addition of CH4 increases HZ width by over 20% for hotter stars whereas it shrinks by a similar percentage for cooler stars. Such dense CO2-CH4 atmospheres around hotter stars suggest inhabitance, highlighting the importance of devising alternative HZ definitions that include the effects of secondary gases.


Ramirez, R.M. and Levi, A. 2018. The ice cap zone: a unique habitable zone for ocean worlds. The Monthly Notices of the Royal Astronomical Society, 477, 4, 4627- 4640

We use my new coupled 1-D radiative-convective and latitudinally-dependent energy balance climate model to test whether the Levi et al. (2017) mechanism, in which ocean worlds can be habitable in the absence of plate tectonics, is feasible. We find that such ocean worlds may be habitable so long as they rotate at least 3 times faster than the Earth does. We also define a circumstellar region for G – M -stars where such worlds may reside.


Ramirez, R.M. 2018. The moist greenhouse is sensitive to stratospheric temperature. AAS Res. Notes 2,1,6.

I argue that the difference between 1-D and recent 3-D simulations on the moist greenhouse threshold for planets around M-dwarfs revolves around the different stratospheric temperatures either assumed or calculated in their respective analyses. This is my reminder of how sensitive the stratospheric temperature is for determining the moist greenhouse.


Yan J., Ding, F., Ramirez, R.M., et al., 2017. Abrupt climate transition of icy worlds from snowball to moist or runaway greenhouse. Nat. Geosc., doi:10.1038/ngeo2994

As we had shown in Ramirez and Kaltenegger (2016), icy exomoons (and planets) could deglaciate and may become habitable during the red giant phase of stellar evolution. Here we show that runaway greenhouse states are immediately triggered on such icy exomoons orbiting F- and G-stars. However, icy moons orbiting later (K and M) stars, with a carbonate-silicate cycle, or with relatively dirty ice, may remain habitable after deglaciation.


Ramirez, R.M., 2017. A warmer and wetter solution for early Mars and the challenges with transient warming. Icarus 297, 71 – 82

I show that the ice albedo problem makes it much harder to transiently warm a glaciated early Mars, requiring surface pressures that are ~10 – 60% higher than is the case for a warm relatively non-glaciated planet. Mineralogy constraints suggest that temperatures well exceeding ~300 K are needed to produce the distribution of surface clays, requiring surface pressures (usually > 5 – 10 bar) that exceed available constraints under such episodic scenarios. Based on the evidence, a warm (possibly semi-arid) early climate is probably the most likely scenario.


Ramirez,  R.M., Kaltenegger, L., 2017. A volcanic hydrogen habitable zone. The Astrophysical Journal Letters, 837, 1

We propose that volcanically-outgassed hydrogen can widen the traditional N2-CO2-H2O habitable zone. We show that such volcanically-outgassed hydrogen can extend the outer edge in our solar system from 1.67 to 2.4 AU. The outer edge extends out similarly for other star types. Planets with sustained volcanic hydrogen outgassing can stay habitable for relatively longer timescales (0.5 – 1 Gyr or even longer) than from the primordial hydrogen accretion mechanism of Pierrehumbert and Gaidos (2011)


Ramirez, R.M., Kasting, J.F., 2017. Could cirrus clouds have warmed early Mars? , Icarus 281, 248 – 261

Urata and Toon  (2013) had argued that cirrus clouds composed of particles ~10 microns and larger could have provided enough of a greenhouse effect to warm early Mars. We show that although possible, this mechanism is very unlikely, because unrealistically high cirrus cloud fractions and carefully chosen parameters are needed to produce warm conditions.


Ramirez, R.M., 2016.  Atmosphere’s solar shock. Nature Geoscience,                                          doi: 10.1038/ngeo2728

I give my summary of Airapetian et al, which argues that regular solar storms from  a much more active younger Sun  (3.8 billion years ago) could have initiated atmospheric reactions that led  to the production of the powerful greenhouse gas, N2O, and HCN, a compound that is vital to life. If their study is correct, I suggest that it may have implications for both early Mars paleoclimate and very active young stars.


Ramirez, R.M., Kaltenegger, L., 2016. Habitable Zones of Post-Main Sequence Stars. The Astrophysical Journal, 823, 6, 14pp

We compute habitable zone boundaries for A5 – M1 stars as they evolve through the post-main-sequence (RGB – AGB).  We assess how  stellar winds and mass loss affect both 1) the retention of planetary atmospheres and how 2) planetary orbits move outward during the entire stellar evolution. The maximum time that a planet can stay within the post-main-sequence habitable zone can be a couple hundred million and last up to several billion years. Such planets are located in the outer reaches of their  stellar systems and can be detected with current direct imaging techniques.


Sloan, G. C., Goes, C., Ramirez, R. M., Kraemer, K. E., & Engelke, C. W., 2015. Infrared Spectral Properties of M Giants. The Astrophysical Journal811, 1, 45.

We observed 20 sample M giants with the infrared spectrometer on Spitzer. There is significant scatter in SiO band strength within a spectral class. All stars show significant OH band absorption. We also find that V-K color may be a better indicator of molecular band strength than spectral class. I computed the strength of water vapor absorption lines at different stellar temperatures and pressures.


Batalha, N., Goldman, Shawn-Domagal, Ramirez, R.M., Kasting, J.F., 2015. Testing the Early Mars H2-CO2 hypothesis with a 1-D photochemical model. Icarus, 258, 337 – 349

The H2-CO2 greenhouse cocktail proposed by Ramirez et al. (2014) to warm early Mars  is put to the test.  We find that at least a couple percent H2 is achievable, with higher concentrations requiring additional H2 sources or a slowing of the hydrogen escape rate below the diffusion limit.


Ramirez, R.M., Kaltenegger, L., 2014. Habitable Zones of Pre-Main-Sequence Stars. The Astrophysical Journal Letters, 797, 2, L25

Habitable zone boundaries for pre-main-sequence F – M stars are derived. We show habitable zone boundaries around these stars are farther out and wider relative to the main-sequence boundaries, allowing next-generation telescopes to resolve potentially habitable environments even around faint, cool M-stars.


Ramirez, R.M., Kopparapu, R., Lindner, V., Kasting, J.F., 2014. Can increased atmospheric CO2 levels trigger a runaway greenhouse? Astrobiology, 14, 8, doi:10.1089/ast.2014.1153

In Kopparapu et al.(2013) we originally showed that the inner edge boundary (0.99 AU) was extremely close to Earth’s orbit. When we apply a more realistic relative humidity profile, we find that Earth is not as close to the inner edge as had been thought, making it less susceptible to either a moist or runaway greenhouse. We argue that further improvements to this problem require it to be revisited in 3-D.


Ramirez, R.M., Kopparapu, R., Zugger, M., Robinson, T.D.,Freedman, R., Kasting, J.F., 2014. Warming early Mars with CO2 and H2. Nat. Geosc., 7, 59 – 63

In this paper, we show how a CO2-H2 greenhouse for early Mars could have raised mean surface temperatures above the freezing point of water, generating a long-lived greenhouse state. The associated warm temperatures would produce enough rainfall to form the ancient valleys.


Kopparapu, R. K.,Ramirez, R.M., Kasting, James, Eymet, V; et al, 2014. Habitable zones around main-sequence stars: dependence on planetary mass ApJ Letters , 787, L29, doi:10.1088/2041-8205/787/2/L29

We demonstrate that changing the planetary mass has a modest affect on the location of the inner edge of the HZ. The impact on the outer edge is trivial. We also rederive new inner edge limits based on recent 3-D results.

Kasting, J.F., Kopparapu, R., Ramirez, R.M., Harman, C., 2013. Remote life detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late-K stars.PNAS , doi: 10.1073/pnas.1309107110

We argue that the traditional liquid water habitable zone (HZ) (Kasting et al., 1993; Kopparapu and Ramirez et al., 2013) should be the standard definition used for habitable exoplanet detection (Although I do not hold this limited stance anymore, see Ramirez, 2018).  We also explain why astronomers should work in stellar fluxes rather than effective temperatures. I had made the relative humiditiy comparison between our model and that of Zsom et al. (2013) as well as helped derive the relative humidity parameterization we used in that paper.


Kopparapu, R.*, Ramirez, R.M.*, Kasting, J., et al., 2013. Habitable zones around main-sequence stars: New Estimates. ApJ, 765, 2, 131

*Both authors contributed equally to this work.

We completely update Kasting et al. (1993) with revised habitable zone (HZ) boundaries. Major differences to the previous work are the HITEMP database and new water continuum absorption for the inner edge. For the outer edge, longer line shapes are used for CO2. Parameterizations are derived so that astronomers can more easily calculate the HZ for main-sequence stars.
Kopparapu and I contributed equally to this work, with me in charge of the climate model research and development.


Greeley et al., 2008. Mars Aeolian Features seen from the ground and orbit. . J. Geophys. Res., 113, E06S06, doi:10.1029/2007JE002971

This was my research assistantship at ASU. I assisted in data collection and analysis. We measured lengths of wind features, created Rose diagrams, and inferred prevailing wind directions on Mars. This work confirmed that the prevailing wind direction in Columbia Hills was from the north-northwest, although the topography can influence the wind regime into other directions.

Selected White papers

Ramirez, R.M. et al. 2019. Habitable zone predictions and how to test them. arXiv:1903.03706 [astro-ph.EP]

This white paper was submitted to the Astro-2020 decadal survey. Here, we argue that next-generation (and beyond) space missions have the unique opportunity to test whether common Earth-based assumptions made in classical habitable zone theory (e.g. CO2/H2Oare the key greenhouse gases, habitable planets orbit main-sequence stars, the carbonate-silicate cycle is universal) hold for other potentially habitable planets. These tests would allow us to improve the tools we use to find life on other planets. Thus, the future of habitability studies will require moving away from making extrapolations based on our knowledge of the Earth to a more robust first principles approach.


Ramirez, R.M. et al. 2018. The continued importance of habitability studiesarXiv:1803.00215 [astro-ph.EP]

This is a white paper submitted to the National Academies “Exoplanet Science Strategy” call. Here, we summarize recent advances in planetary habitability studies. Interactions between climate modelers are necessary to resolve outstanding research questions. Although some observations can be made with present capabilities, technological advances are necessary to maximize the utility of climate models used for planetary habitability studies.


Airapetian V., Ramirez, R.M. et al. 2018. Exploring extreme space weather factors of exoplanetary habitability. arxiv: 1803.03751 [astro-pH:EP]

This white paper submitted to the National Academies “Exoplanet Science Strategy” call summarizes why space weather studies are important to understanding planetary habitability. It concludes with key research goals for the next 20 years.