Superhabitability of High-obliquity and High-eccentricity Planets

Planetary obliquity (θ in Figure 1(A)) shapes the spatial and temporal distribution of incident stellar radiation, generating the familiar hemispheric seasons that we experience on Earth. At low obliquity (herein, 0°–23.5°), the equator receives the most stellar energy, while the poles receive relatively little. Seasonality is also limited, particularly at low latitudes. At moderate obliquity (herein, 23.5°–54°), the spatial distribution of stellar energy becomes more uniform on annual average, but with increasing temporal variability. The result is a climate with smaller equator-to-pole contrasts, less ice on annual average, and larger seasonal contrasts at all latitudes—despite receiving the same stellar energy flux on global average. At high obliquity (herein, 54°–90°), the poles receive more stellar energy than the equator, despite increasingly dramatic temporal variability in irradiance (Ward 1974). In this scenario, equatorial ice belts (rather than polar ice caps) may become stable (Rose et al. 2017; Kilic et al. 2018).

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Eccentric orbits (Figure 1(B)) generate seasons as the star–planet separation varies throughout the orbit. In this case, summer occurs when the planet is closer to its host star, while winter occurs while it is further away. Unlike seasons arising from obliquity, both hemispheres will experience the same season as the stellar energy received by the planet changes across the year. Moreover, the summer versus winter seasons will differ in duration. This contrast arises due to evolving gravitational interaction between the planet and star as the orbital distance changes, as described by Kepler's second law. Consequently, the summer will be relatively short as the planet speeds up close to its star and the winter will be relatively long as the planet orbits slower further from its star. Both the magnitude of seasonal flux variations and the asymmetry in season duration increase with increasing eccentricity.

For planets with a combination of both nonzero planetary obliquity and orbital eccentricity, both seasonality effects will manifest. These effects may be additive or opposing, depending on the the timing of obliquity-driven seasons relative to the point in the planet's eccentric orbit closest to the star (periastron) and the point furthest from the star (apoastron). This relationship can be described by the argument of the periastron (ω in Figure 1(C)), i.e., the angle between the orbital position where both hemispheres receive identical instellation during the northern hemisphere (NH) vernal equinox and the periastron.

For ω = 0° or 180°, the NH vernal or autumnal equinox, respectively, will occur at the periastron, with the opposite occurring at apoastron. The effects of eccentricity and obliquity on the seasons will be out of phase from one another, with the global summer and winter generated by eccentricity corresponding with spring and fall generated by obliquity. The resulting seasonality will therefore be subdued.

For ω = 90° or 270°, the NH summer solstice (SS) or winter solstice (WS), respectively, occurs at the periastron, with the opposite occurring at the apoastron (Table 1). The global summers and winters generated by eccentricity and the hemispheric summers and winters generated by obliquity will coincide for one hemisphere and conflict for the other. The resulting seasonality will therefore be amplified for one hemisphere and muted for the other (Figure 2).

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Photosynthesis by marine microorganisms produces biomass via the following chemical reaction:

$$\begin{eqnarray}&&{\mathrm{CO}}_{2}+{{\rm{H}}}_{2}{\rm{O}}\mathop{\to }\limits^{{hv}}{\mathrm{CH}}_{2}{\rm{O}}+{{\rm{O}}}_{2},\end{eqnarray} \tag{ 1 }$$

where CH₂O is geochemical shorthand for biomass that in reality includes a number of other macro- (e.g., nitrogen, phosphorus) and micronutrients (e.g., iron, molybdenum). These nutrients, especially phosphorus, typically limit photosynthetic rates ("primary productivity") on global and long-term average on Earth (Tyrrell 1999), but any of these ingredients can be locally or seasonally limiting. For instance, light may limit productivity during winter darkness while nutrients may limit productivity during photon-replete summers. We expect similar spatial and temporal variability in primary productivity on other worlds experiencing seasons.

The majority of biomass produced by photosynthesis is broken down in the surface wind-mixed layer by respiration:

$$\begin{eqnarray}&&{\mathrm{CH}}_{2}{\rm{O}}+{{\rm{O}}}_{2}\to {\mathrm{CO}}_{2}+{{\rm{H}}}_{2}{\rm{O}}.\end{eqnarray} \tag{ 2 }$$

However, a small portion sinks from the mixed layer into the deeper layers of the ocean. This flux is termed biological "export production," or simply "export," and is measured in teramoles of particulate organic carbon per year (Tmol POC yr⁻¹). Export production is typically greatest when and where primary production is greatest, but export is also sensitive to temperature and thus it is not necessarily a fixed fraction of primary productivity in either space or time.

The process by which export POC is transferred from the surface to depth is referred to as the "biological pump." The net consequence of the biological pump is to concentrate bioessential nutrients at depth while depleting them at the surface. As a result, the biosphere depends on vertical mixing to replenish nutrients to the sunlit regions of the water column where photosynthesis is viable, also referred to as the photic zone. Greater nutrient recycling ultimately allows greater biological productivity, increasing the production (but not necessarily the survival) of biogenic gases like O₂ and CH₄ that may enter the atmosphere and serve as "biosignatures" for life. See Olson et al. (2020) for a further review of marine biogeochemical cycles as they relate to exoplanet habitability and biosignatures.

Surface air temperature (SAT) decreases on annual and global average by ∼5°C with increasing obliquity from 0° to 90° for fixed eccentricity (Figure 3). Cooling on global average reflects a ∼18°C decrease in equatorial SAT, but SAT at the poles actually increases by ∼20°C with increasing obliquity, leading to reductions in ice cover and planetary albedo as in previous studies.

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Globally and annually averaged SAT increases dramatically with eccentricity for fixed obliquity. SAT increases by ∼5°C between 0 and 0.2 eccentricity and further increases by ∼13°C between 0.2 and 0.4 eccentricity on global average. Warming is more pronounced at the poles, which warm by ∼20°C over this same eccentricity range. This warming on annual average arises despite longer winters and lower annual mean instellation on high-eccentricity planets (Table 3).

SAT varies only with latitude for planets with both zero obliquity and eccentricity. For all other orbital scenarios, SAT varies both spatially and temporally. Peak SATs surpass 50°C during the summers for orbits with an eccentricity of 0.4 or an obliquity of ≥60°, rendering land habitats in the polar regions of planets with high obliquity and the equatorial regions of planets with the combination of high eccentricity and low obliquity habitable only for thermophilic life by Earth standards. Seasonal temperature contrasts of ≥60°C under these same orbital scenarios may exacerbate this stressor for life on land. However, simulations with both low to moderate obliquity and eccentricity of ≤0.3 generate SATs <50°C year-round at all latitudes, with seasonal temperature variations <20°C for equatorial and middle latitudes.

Sea surface temperature (SST) more directly impacts marine life than SAT. Globally and annually averaged SST decreases by ∼5°C from 0° to 90° obliquity for fixed eccentricity. Globally and annually averaged SST increases by ∼4°C between 0 and 0.2 eccentricity, and it further increases by ∼10°C between 0.2 and 0.4 eccentricity, for fixed obliquity.

Assessing the habitability of marine environments also requires consideration of how seasonal maxima and minima may affect life locally. Even for the most extreme orbits we consider here, 2 week average SST does not exceed 46°C at any latitude, suggesting that high obliquity and/or high eccentricity does not preclude marine environments suitable for life. The 2 week average SST above 40°C occurs only during the summers of simulations with orbits of 0.4 eccentricity and 0°–30° obliquity, where it ranges from 43 to 45°C. This muted seasonal maxima relative to SAT arises due to the high heat capacity of water, which makes the ocean more resistant to temperature swings than the atmosphere. The seasonal minima is also muted because SST cannot fall below the freezing point of seawater regardless of atmospheric temperature, but we note that permanent sea ice is eliminated for obliquities greater than 45° and eccentricities greater than 0.2 in our simulations. In combination, seasonal SST contrasts are significantly reduced relative to seasonal SAT contrasts. Simulations with low to moderate obliquity and ≤0.3 eccentricity display seasonal SST variations of <10°C. For simulations with both high obliquity and 0.4 eccentricity, seasonal temperature variation in the southern hemisphere middle and polar latitudes approaches 20°C. These seasonal effects are large compared to the Earth, but elimination of polar sea ice nonetheless increases open ocean habitats for life.

The depth of the wind-mixed layer varies spatially and temporally with SST (Figures 4 and 5). The mixed layer is shallowest when and where SST is highest, and the mixed layer is deepest when and where SST is lowest. This relationship reflects density stratification of the ocean that opposes vertical mixing, the strength of which varies with latitude and season as the surface of the ocean warms/cools in response to spatially and temporally variable instellation.

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At low obliquities, the mixed layer is shallowest at the equator and deepest at the winter pole. As obliquity increases, the equatorial mixed layer deepens as thermal stratification weakens as the poles begin to receive more stellar energy at the expense of the equator. The depth of the mixed layer also becomes increasingly variable at all latitudes as seasonal temperature contrasts increase. At high obliquities, the depth of the equatorial mixed layer exceeds that of the summer pole (Figure 5).

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Both obliquity and eccentricity influence biological export production. However, obliquity effects are much larger within the parameter space we investigated. At lower obliquity and eccentricity values, increases in both obliquity and eccentricity have large positive effects on export production, but increases in either parameter are less significant at higher values of one or both (Figure 6). We use our 0° obliquity and 0 eccentricity simulation, which yields 1600 Tmol POL yr⁻¹ export production, as a baseline for comparing the biospheric response of worlds experiencing seasons.

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For simulations with 0 eccentricity, export production increases nearly linearly with obliquity from 1600 Tmol POC yr⁻¹ (baseline) at 0° obliquity to 5200 Tmol POC yr⁻¹ (3.3× baseline) at 90° obliquity. Simulations with fixed eccentricities of 0.1 and 0.2 yield small increases in export production over low obliquity ranges from 2200 and 2600 Tmol POC yr⁻¹ (1.4 and 1.6× baseline, respectively), converging on 2700 Tmol POC yr⁻¹ (1.7× baseline) at 30°. At moderate and high obliquities, export production again increases nearly linearly, reaching 5500 Tmol POC yr⁻¹ (3.4× baseline) at 90° for these same eccentricities. Simulations with higher eccentricities of 0.3 and 0.4 yield 3300 and 4100 Tmol POC yr⁻¹ (2.1× and 2.6× baseline) at 0° obliquity, decrease approximately linearly by 200–300 Tmol POC yr⁻¹ over low obliquity ranges, then increase to 5300–5400 Tmol POC yr⁻¹ (3.3–3.4× baseline) at 90° (Figure 6).

For simulations with low obliquity, export production increases nearly linearly with eccentricity, increasing from 1600 and 1900 Tmol POC yr⁻¹ (1.0 and 1.2× baseline, respectively) at 0 eccentricity and converge to 4100 Tmol POC yr⁻¹ (2.6× baseline) at 0.4 eccentricity.

For moderate and high obliquities, export production is initially higher at 0 eccentricity and is less sensitive to eccentricities ≤0.2, changing by ≤4%. In simulations with 30°–60° obliquity, export production then increases nearly linearly from 0.2 up to 0.4 eccentricity, yielding 3800–4300 Tmol POL yr⁻¹ (2.4–2.7× baseline) at 0.4 eccentricity. Meanwhile, export production in higher-obliquity simulations with 75° and 90° obliquity remains relatively insensitive to eccentricity. Export increases by only 100 Tmol POL yr⁻¹ in our 75° simulation, while export decreases by 400 Tmol POL yr⁻¹ in our 90° simulation, with both scenarios converging to ∼5200 Tmol POL yr⁻¹ at 0.4 eccentricity (Figure 6).

The distribution of export production is closely related to mixed-layer depth, both spatially and temporally. In every simulation, nearly all export production occurs at latitudes with high mixed-layer depth seasonality, during and immediately following the transition from deep to shallow mixed layer as thermal stratification develops with surface warming (Figures 5 and 7), a phenomenon referred to as "spring blooms" on Earth. This relationship between productivity and mixed-layer depth arises due to seasonal differences in nutrient availability as ocean stratification varies (Barnett & Olson 2022).

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Barnett & Olson (2022) showed that annual biospheric productivity increases with obliquity in cGENIE simulations, and they argued that moderately high-obliquity planets may thus be particularly attractive candidates for exoplanet life detection. However, limitations to their modeling methodology precluded them from investigating obliquities higher than 45°, and it was unclear whether the trend of increasing productivity with obliquity would continue to more extreme orbits. We have improved upon their initial study by coupling the PlaSim atmospheric GCM to cGENIE to allow for seasonal reversal of surface winds, and we demonstrate that biospheric productivity increases with obliquity up to 90° (Figure 6), suggesting that high-obliquity planets may be superhabitable (Heller & Armstrong 2014). We also show that seasonality arising from high-eccentricity orbits also favorably influences biospheric productivity (Figure 6). Perhaps most surprising, the combination of high obliquity and high eccentricity supports marine biospheres that are more productive than present-day Earth in our model (Figure 6)!

As concluded in previous work, ocean life responds favorably to "extreme" orbits in our simulations due to changes in the distribution and recycling of bioessential nutrients in the oceans (Olson et al. 2020; Barnett & Olson 2022). Enhanced nutrient availability in our simulations is the consequence of seasonal breakdown of density stratification within the ocean (Figure 5), allowing nutrient-rich deep waters to mix with nutrient-depleted surface waters and stimulating photosynthetic activity.

Higher productivity on high-obliquity and high-eccentricity planets may favorably influence the "detectability" of remote biosignatures. Higher productivity results in greater production of photosynthetic O₂, a frequently discussed biosignature that signals the presence of life on Earth (Meadows 2017; Meadows et al. 2018; Schwieterman et al. 2018). Moreover, higher rates of photosynthesis ultimately increase the potential rates of other metabolisms in the ocean interior and/or marine sediments—and thus the production of other metabolic waste products that may also serve as gaseous biosignatures. This relationship arises because photosynthesis translates stellar energy to chemical energy that is then propagated through the biosphere in the form of chemical disequilibrium (Krissansen-Totton et al. 2018). This energy flux on Earth is orders-of-magnitude greater than geological energy inputs, with the consequence that Earth's biosphere is ultimately solar-powered despite a diversity of nonphotosynthetic metabolisms. Photosynthetic rates thus throttle the production of most biogenic gases that may serve as remote biosignatures, including those not directly involved in photosynthesis (e.g., CH₄, N₂O, and H₂S).

Biosignature gases may be produced in large quantities without influencing planetary spectra, potentially resulting in a "false negative" for life (Reinhard et al. 2017). As an example, microbial sulfate reduction produces large fluxes of H₂S in marine sediments on present-day Earth (Canfield 1991), but the resulting H₂S forms pyrite (FeS₂) within sediments or is reoxidized before it can reach the atmosphere and influence the spectral appearance of our planet. Likewise, the Black Sea—an analog environment for Earth's ancient oceans—supports high rates of methanogenesis, but it is ultimately a negligible source of CH₄ to the atmosphere due to oxidation of CH₄ within the water column (Schmale et al. 2011). Alien observers expecting to find H₂S or CH₄ on inhabited planets may thus be mislead by their low abundances in our atmosphere.

Exoplanets with large seasonal cycles may be less vulnerable to these types of false negatives. In addition to replenishing nutrients, seasonal breakdown of the thermal stratification that inhibits vertical mixing in the ocean on high-obliquity and high-eccentricity worlds may increase the fraction of biosignature gases produced in the ocean interior and marine sediments that ultimately make it to the surface and accumulate in the atmosphere where they may affect planetary spectra. The combination of greater biosignature production and enhanced communication between the ocean interior and the atmosphere may thus increase the detectability of life on worlds experiencing large seasonal cycles.

In addition to increasing the atmospheric accumulation of biosignature gases, seasonality may also serve as an independent beacon for life (Olson et al. 2018; Mettler et al. 2022). The composition of Earth's atmosphere oscillates seasonally, reflecting seasonally variable gas fluxes as life responds to its changing environment. Most famously, CO₂ levels rise and fall at Mauna Loa as the balance between CO₂ fixation into biomass by photosynthesis and CO₂ release by respiration shifts with the seasons (Keeling et al. 1976). Similar fluctuations on other worlds may provide a temporal biosignature that reveals the presence of exoplanet life.

These seasonal changes are small on present-day Earth but may be exaggerated on worlds with larger seasonal cycles. However, it remains to be seen whether obliquity- or eccentricity-driven seasonality in atmospheric composition will be more detectable. Our simulations suggest that obliquity is more likely to result in large-magnitude biogenic seasonality compared to eccentricity, but characterizing seasonality on these worlds is complicated by the details of viewing geometry (Olson et al. 2018; Mettler et al. 2022). In some scenarios, views that blend opposing seasons between hemispheres may mask the biogenic signal in disk average. Global seasons on planets with eccentric orbits may thus be easier to characterize than hemispheric seasons resulting from obliquity for many viewing geometries, but our results imply that detectable seasonality may require very high eccentricities compared to Earth's.

A recent study by Schulze-Makuch et al. (2020) generated a list of two dozen potentially superhabitable exoplanets and exoplanet candidates from the Kepler catalog, taking into account various planet and host-star properties previously known to favorably influence habitability (Heller & Armstrong 2014). Our results suggest that high-obliquity and/or high-eccentricity worlds should also be considered among the most favorable targets for life detection. By extension, observationally constraining obliquity and eccentricity will be critical for characterizing habitability and for the interpretation of biosignatures. While eccentricity can be derived from radial velocity measurements and low-resolution light curves prior to atmospheric characterization measurements, constraints on planetary obliquity require more detailed and frequent measurements (Kipping 2008; Van Eylen & Albrecht 2015; Kane & Torres 2017). Previous studies have derived planetary obliquity from full thermal phase curves of large hot Jupiters (Adams et al. 2019). Thermal emission may also reveal the obliquities of directly imaged young Jovian planets (Cowan et al. 2013), but resolving the obliquities of mature planets with less internal heat is complicated by several additional factors (Gaidos & Williams 2004). The obliquities of Earth-sized rocky exoplanets are inaccessible with current instrumentation (Kane & Torres 2017), but full-orbit reflected light curves obtained with future instrumentation may one day constrain the obliquities of potentially habitable worlds (Kawahara & Fujii 2011). Knowledge of planetary obliquity could provide important context for interpreting atmospheric observations relating to both planetary habitability and biosignatures. This context may provide an opportunity to evaluate exoplanets for future observations and prioritize those that have the highest potential for the detection of biosignatures. Future work and/or instrument design should consider constraining the obliquities of rocky planets in the HZ of Sun-like stars a priority.

A potential caveat is that the same compositional seasonality that may signal the presence of exoplanet life may also represent a challenge for certain types of life. For example, seasonally variable ocean chemistry may threaten the development of animal-grade complexity if such life requires stable oxygenation of its environment (Catling et al. 2005; Reinhard et al. 2016). Indeed, ocean deoxygenation is associated with several of the "big five" mass extinctions on Earth, highlighting the potential impact of varying oxygen levels on marine animals (Meyer & Kump 2008). However, if evolution via natural selection favors individuals tolerant of seasonal changes in ocean composition on high-obliquity and/or high-eccentricity worlds, then marine animals may be more resilient against the types of perturbations associated with mass extinctions on Earth. Future work should investigate how seasonality in the composition of the ocean-atmosphere system may specifically affect animals, which are not represented in our simulations.

The long-term effect of obliquity on planetary habitability is also an area of ongoing research. Previous studies have shown that increasing planetary obliquity can extend the outer edge of the HZ through enhanced heating at the poles due to increased seasonality, thereby increasing the orbital distance an exoplanet with high obliquity can occupy around its host star and remain habitable (Williams & Kasting 1997; Spiegel et al. 2009; Dressing et al. 2010; Armstrong et al. 2014; Linsenmeier et al. 2015; Wang et al. 2016; Kilic et al. 2018; Nowajewski et al. 2018; Colose et al. 2019; Guendelman & Kaspi 2019; Kang 2019a; Palubski et al. 2020; Komacek et al. 2021). However, a GCM study by Kang (2019b) showed that planets inside the HZ can experience higher stratospheric water vapor concentrations due to the effects of high obliquity. Higher concentrations of stratospheric water vapor could impact the long-term habitability of high-obliquity planets through enhanced water loss (Kasting 1988; Meadows 2017). While high-obliquity planets are attractive candidates for future observation because of their potential to increase biological productivity, enhanced water loss could create false-positive biosignature detections from the subsequent accumulation of abiotic oxygen or potentially threaten long-term habitability (Meadows et al. 2018). Future work is needed to constrain the effect of obliquity on planetary water loss to discern the relative advantage of higher obliquity on overall planetary habitability.