# Superhabitability of High-Obliquity and High-Eccentricity Planets

Jonathan Jernigan Department of Earth, Atmospheric, and Planetary Science,
Purdue University, West Lafayette, IN 47901, USA  Émilie Laflèche Department
of Earth, Atmospheric, and Planetary Science, Purdue University, West
Lafayette, IN 47901, USA  Angela Burke Department of Earth, Atmospheric, and
Planetary Science, Purdue University, West Lafayette, IN 47901, USA  Stephanie
Olson Department of Earth, Atmospheric, and Planetary Science, Purdue
University, West Lafayette, IN 47901, USA Stephanie Olson
<EMAIL_ADDRESS>
###### Abstract

Planetary obliquity and eccentricity influence climate by shaping the spatial
and temporal patterns of stellar energy incident at a planet’s surface,
affecting both the annual mean climate and magnitude of seasonal variability.
Previous work has demonstrated the importance of both planetary obliquity and
eccentricity for climate and habitability, but most studies have not
explicitly modeled the response of life to these parameters. While exaggerated
seasons may be stressful to some types of life, a recent study found an
increase in marine biological activity for moderately high obliquities <45∘
assuming an Earth-like eccentricity. However, it is unclear how life might
respond to obliquities >45∘, eccentricities much larger than Earth’s, or the
combination of both. To address this gap, we use cGENIE-PlaSim, a 3-D marine
biogeochemical model coupled to an atmospheric general circulation model, to
investigate the response of Earth-like marine life to a large range of
obliquities (0–90∘) and eccentricities (0–0.4). We find that marine biological
activity increases with both increasing obliquity and eccentricity across the
parameter space we considered, including the combination of high obliquity and
high eccentricity. We discuss these results in the context of remote
biosignatures, and we argue that planets with high obliquity and/or
eccentricity may be superhabitable worlds that are particularly favorable for
exoplanet life detection.

Exoplanets (498) — Habitable planets (695) — Planetary climates (2184) —
Astrobiology (74) — Biosignatures (2018)

##  1 Introduction

Figure 1: A: Planetary obliquity ($\theta$) is the angle between a planet’s
orbital and equatorial planes. B: The eccentricity of an elliptical orbit is
the ratio of the distance between one focus and the geometric center of the
ellipse (c) to the semi-major axis (a). C: The argument of the periastron
($\omega$) is the angle between the vernal equinox and the periastron. For
$\omega=282.7^{\circ}$, the present-day Earth value, the Northern Hemisphere
(NH) summer occurs near the apoastron, while the Southern Hemisphere (SH)
summer occurs near the periastron (Table 1).

Planetary obliquity and orbital eccentricity modulate planetary climate, in
part by generating seasons due to time-varying instellation patterns. The
effects of these orbital parameters on climate are modest for Earth, but a
diversity of annual mean climate and seasonality scenarios are possible on
other worlds. The diversity of planetary obliquities is apparent even among
terrestrial planets in our own solar system, with Venus and Mercury exhibiting
obliquities of 180∘ and near 0∘ respectively, while Mars’ obliquity varies
chaotically from 0–60∘ (Laskar & Robutel, 1993). A similarly large range of
obliquities is expected in other stellar system architectures as a result of
complex orbital interactions (Millholland & Laughlin, 2019) and giant impacts
(Li & Lai, 2020). Eccentricity is less varied among observed low-mass planets
to date, including terrestrial planets in the Habitable Zone (HZ) (Van Eylen &
Albrecht, 2015; Eylen et al., 2019; Guendelman & Kaspi, 2020). Nonetheless,
eccentricity varies among solar system planets and known exoplanets (Udry &
Santos, 2007), and these differences in eccentricity are expected to impact
planetary climate, seasonal cycles, and habitability (Williams & Pollard,
2002; Dressing et al., 2010). Considering the diversity of obliquities and
eccentricities anticipated among HZ planets, many potentially habitable
exoplanets may experience amplified seasonality compared to Earth. This
enhanced seasonality may have broad-reaching consequences for planetary
habitability and exoplanet life detection efforts.

Exaggerated seasonality on high-obliquity and/or high-eccentricity worlds
could be challenging for life. However, higher obliquity planets have warmer
poles and more equable climates than lower obliquity planets due to decreased
planetary albedo from a reduction in both ice and cloud cover, which may allow
higher obliquity planets to maintain surface liquid water at farther distances
from their host star (Williams & Kasting, 1997; Williams & Pollard, 2003;
Spiegel et al., 2009; Dressing et al., 2010; Armstrong et al., 2014; Ferreira
et al., 2014; Linsenmeier et al., 2015; Wang et al., 2016; Kilic et al., 2018;
Nowajewski et al., 2018; Guendelman & Kaspi, 2019; Kang, 2019a; Colose et al.,
2019; Palubski et al., 2020; Komacek et al., 2021). Some authors have even
suggested that planets with higher obliquities may be ‘superhabitable’ worlds
more suitable for life than Earth (Heller & Armstrong, 2014; Olson et al.,
2020), but most models for planetary habitability to date lack explicit
representation of life.

Recent biogeochemical modeling suggests that moderately high planetary
obliquity enhances the productivity of marine life (Barnett & Olson, 2022).
However, Barnett & Olson (2022) only considered obliquities up to 45∘. It is
thus unknown whether marine life would respond positively to even higher
obliquities, or if there exists an optimal obliquity beyond which strong
seasonal contrasts begin to negatively affect marine life. Their study also
neglects the effects of seasonality arising from eccentricity.

The mechanism by which obliquity influenced biospheric productivity in Barnett
& Olson (2022)’s study was through enhanced nutrient recycling due to seasonal
breakdown of the ocean’s thermal stratification, which otherwise tends to
limit vertical mixing. This same phenomenon may occur on planets with
eccentric orbits, but asymmetry in season duration may yield different
dynamical and biological effects. Planets on highly eccentric orbits can also
move into and out of their star’s HZ with each orbit bearing unclear
consequences for habitability and life. Ultimately, it is unknown how marine
life might respond to the large range of orbital scenarios we expect to find
with current and next-generation observatories.

We address this gap here by using an atmospheric GCM (PlaSim) coupled to a 3D
biogeochemical model (cGENIE) to simulate the climates of high-obliquity and
high-eccentricity worlds and characterize the response of an Earth-like
biosphere to each orbital scenario. We review how obliquity and eccentricity
drive seasonality and provide an introduction to key biogeochemical processes
regulating the biospheric response to seasons in Section 2. We then describe
our climate and biogeochemical modeling approach in Section 3. We present our
results regarding the response of surface environments and life to seasons in
Section 4, and we discuss implications for exoplanet habitability and life
detection on high-obliquity and high-eccentricity worlds in Section 5.
Finally, we summarize our findings and offer recommendations for future work
in Section 6.

##  2 Background

###  2.1 Effects of Orbital Parameters on Planetary Climate and Seasonality

Planetary obliquity ($\theta$ in Figure 1A) 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).

Eccentric orbits (Figure 1B) 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 vs. 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 non-zero 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 ($\omega$ in
Figure 1C), i.e, the angle between the orbital position where both hemispheres
receive identical instellation during the NH vernal equinox and the
periastron.

For $\omega=$ 0∘ or 180∘, the Northern Hemisphere (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 autumn generated by obliquity. The
resulting seasonality will therefore be subdued.

For $\omega=$ 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).

Table 1: Timing of Orbital Events (Month, Day) Eccentricity | Periastron | Apoastron | SS (NH) | WS (NH)
---|---|---|---|---
0.0 | - | - | (6, 20) | (12, 20)
0.1 | (1, 13) | (7, 14) | (6, 28) | (1, 3)
0.2 | (1, 24) | (7, 24) | (7, 6) | (1, 16)
0.3 | (2, 4) | (8, 4) | (7, 11) | (1, 27)
0.4 | (2, 14) | (8, 13) | (7, 15) | (2, 8)

###  2.2 Characterizing biological activity

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

$CO_{2}+H_{2}O\xrightarrow{hv}CH_{2}O+O_{2}$ (1)

where CH2O is geochemical shorthand for biomass that in reality includes a
number of other macro- (e.g., N, P) and micro-nutrients (e.g., Fe, Mo). These
nutrients, especially P, 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:

$CH_{2}O+O_{2}\rightarrow CO_{2}+H_{2}O$ (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 further review of marine biogeochemical
cycles as they relate to exoplanet habitability and biosignatures.

Table 2: Parameters Varied Parameter | Value
---|---
Obliquity |  low: 0∘, 15∘
|  moderate: 30∘, 45∘
|  high: 60∘, 75∘, 90°
Eccentricity | 0, 0.1, 0.2, 0.3, 0.4

##  3 Methods & Model Description

Our experiments leverage cGENIE, a 3D marine biogeochemical model, coupled to
PlaSim (Holden et al., 2016). PlaSim is a reduced-complexity 3D atmospheric
circulation model, built around the PUMA atmospheric model described by
Fraedrich et al. (2005). PlaSim replaces the 2D energy-moisture balance model
(EMBM) typically used in cGENIE simulations, and it provides cGENIE with
interactive wind forcing, temperature, humidity, surface pressure, divergence
and vorticity. The module also includes fractional cloud cover from relative
humidity and precipitation. Time step length is 12 hours for cGENIE and 45
minutes for PlaSim; coupling inputs are averaged over the 16 previous PlaSim
time steps to account for this difference (Holden et al., 2016).

Table 3: Instellation Relative to Earth $(S_{\oplus})$ Eccentricity | Instellation ($S_{\oplus}$)
---|---
0.0 | 1.000
0.1 | 1.054
0.2 | 1.082
0.3 | 1.195
0.4 | 1.136

Ocean circulation in cGENIE is handled by C-GOLDSTEIN and GOLDSTEINSEAICE.
C-GOLDSTEIN is a reduced physics, frictional geostrophic 3D ocean circulation
model that uses advection and diffusion to transport heat, salinity, and
biogeochemical tracers (Edwards & Marsh, 2005). Net precipitation minus
evaporation (P–E) is represented by a proportionate salinity flux. C-GOLDSTEIN
calculates the mixed-layer depth considering the strength of density
stratification arising from temperature and salinity gradients and wind stress
from PlaSim. GOLDSTEINSEAICE is a dynamic-thermodynamic sea-ice model.
Dynamical equations are solved for the percentage of ice cover and the height
of ice in each cell. Sea ice growth and decay is governed by heat flux from
the atmosphere and ocean. C-GOLDSTEIN, GOLDSTEINSEAICE, and PlaSim are coupled
through heat transfer, wind forcing, and the drifting of sea ice along ocean
currents (Holden et al., 2016).

Chemistry and biology in cGENIE are handled by the ATCHEM and BIOGEM modules
for atmospheric chemistry and marine biogeochemistry, respectively (Ridgwell
et al., 2007). ATCHEM includes parameterized chemistry involving O2, O3, CH4,
and CO2 assuming a Sun-like stellar spectrum (Reinhard et al., 2020). ATCHEM
passes pCO2 to PlaSim for calculation of surface temperature, but PlaSim does
not currently receive pCH4. ATCHEM and BIOGEM are coupled through spatially
resolved (2D) sea-air gas exchange that varies with local sea surface
saturation state for each species, but ATCHEM assumes a well-mixed atmosphere
and homogenizing gaseous species with each time-step. BIOGEM includes
photosynthesis, aerobic and anaerobic respiration, methanogenesis (Olson et
al., 2013), aerobic and anaerobic methanotrophy (Olson et al., 2016),
nitrification and sulfide oxidation. Photosynthesis is limited by the
availability of both N and P, but N2 fixation occurs when N is scarce relative
to P (Ridgwell et al., 2007). Photosynthesis is further constrained by the
availability of photosynthetically active radiation (PAR), which is calculated
as a fixed fraction of incident radiation. Light is then attenuated in the
water column according to an imposed e-folding depth. This calculation
implicitly assumes a Sun-like spectrum and that exo-photosynthesis prefers
photons of the same wavelength as photosynthesis on Earth.

We run a total of 35 experiments, varying obliquity from 0∘ to 90∘ and
eccentricity from 0 to 0.4 (Table 2). Although obliquity simply shapes the
spatial distribution of instellation, annual average instellation varies with
eccentricity ($e$) by a factor of $(1-e^{2})^{-1/2}$ for planets with
equivalent semi-major axes (Table 3) (Laskar et al., 1993). All other
parameters—such as day length, surface pressure, and atmospheric pO2 among
others—are set to present-day Earth values. We also assume a present-day Earth
continental configuration, which allows us to compare the habitability of land
vs. marine environments. Finally, we use the Sun’s spectrum, which may limit
the relevance of our work to more common M-dwarf planets that receive
relatively more IR photons and less of the visible photons used by
photosynthesis on Earth. This choice is due to current limitations of the
photochemistry and photosynthesis codes described above, but it is also
physically motivated because HZ planets around M-dwarfs are likely to be
tidally locked with near zero obliquity and eccentricity (Heller et al.,
2011). Model runs are spun up for 10,000 years until reaching steady-state
with respect to surface temperature and chemical tracers in the ocean. Each
experiment requires only 1 CPU and completes in 10 days. Data is output every
two weeks, and all data presented herein is averaged over the last decade of
each simulation.

##  4 Results

Figure 2: Zonally averaged solar short wave forcing at top of atmosphere
across the seasonal cycle for varied obliquity (columns) and eccentricity
(rows). Time advances along the x-axis and latitude is represented on the
y-axis for each subpanel. Solar forcing peaks at periastron and reaches a
minimum at apoastron, the timing of which varies with eccentricity.

###  4.1 Extreme surface air temperatures occur locally and seasonally at high
obliquity and high eccentricity

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

Globally and annually averaged SAT increases dramatically with eccentricity
for fixed obliquity. SAT increases by $\sim$5∘C between 0 and 0.2 eccentricity
and further increases by $\sim$13∘C between 0.2 and 0.4 eccentricity on global
average. Warming is more pronounced at the poles, which warm by $\sim$20∘C
over this same eccentricity range. This warming on annual average arises along
with higher annual-mean instellation on high-eccentricity planets despite
asymmetry in season duration (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 $\geq 60^{\circ}$, 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 $\geq 60^{\circ}$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 $\leq$0.3 generate
SATs <50∘C year-round at all latitudes, with seasonal temperature variations
<20∘C for equatorial and middle latitudes.

Figure 3: Zonally averaged surface air temperature (SAT) across the seasonal
cycle for varied obliquity (columns) and eccentricity (rows). Time advances
along the x-axis and latitude is represented on the y-axis for each subpanel.
SATs exceed 50∘C during summers for high-obliquity and/or high-eccentricity
orbits.

###  4.2 Sea surface temperatures remain hospitable globally at high obliquity
and high eccentricity

Sea surface temperature (SST) more directly impacts marine life than SAT.
Globally and annually averaged SST decreases by $\sim$5∘C from 0∘ to 90∘
obliquity for fixed eccentricity. Globally and annually averaged SST increases
by $\sim$4∘C between 0 and 0.2 eccentricity, and it further increases by
$\sim$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, two-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. Two-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 43–45∘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 can not 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 $\leq$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 SH 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.

Figure 4: Zonally averaged sea surface temperature (SST) across the seasonal
cycle for varied obliquity (columns) and eccentricity (rows). Global average
SST increases with increasing eccentricity and decreases slightly with
increasing obliquity. Time advances along the x-axis and latitude is
represented on the y-axis for each subpanel. Two-week average SST does not
exceed 46∘C for any latitude.

###  4.3 Seasonality in the depth of the ocean mixed-layer increases with
obliquity and eccentricity

The depth of the wind-mixed layer varies spatially and temporally with SST
(Figure 4, 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.

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).

Figure 5: Zonally averaged mixed layer depth across the seasonal cycle for
varied obliquity (columns) and eccentricity (rows). Time advances along the
x-axis and latitude is represented on the y-axis for each subpanel. Seasonal
temperature variations in high-obliquity and, to a lesser extent, high-
eccentricity simulations generate extreme fluctuations in mixed layer depth at
middle and polar latitudes.

###  4.4 Marine biological productivity increases with obliquity and
eccentricity

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.

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 (3.3x 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 1.6x baseline,
respectively), converging on 2700 Tmol POC yr-1 (1.7x baseline) at 30∘. At
moderate and high obliquities, export production again increases nearly
linearly, reaching 5500 Tmol POC yr-1 (3.4x 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 (2.1x and 2.6x 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 (3.3–3.4x baseline) at 90∘ (Figure
6).

Figure 6: Annual and global average biological export as a function of
obliquity and eccentricity. Colored lines represent simulations with identical
eccentricities for varied obliquity (left) and identical obliquities for
varied eccentricity (right). Export production increases with both increasing
obliquity and eccentricity. Eccentricity effects are significant at both low
obliquity and eccentricity but drop off as either parameter increases.

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

For moderate and high obliquities, export production is initially higher at 0
eccentricity and is less sensitive to eccentricities $\leq$0.2, changing by
$\leq$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 (2.4–2.7x 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 $\sim$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, 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).

Figure 7: Zonally averaged export density across the seasonal cycle for varied
obliquity (columns) and eccentricity (rows). Time advances along the x-axis
and latitude is represented on the y-axis for each subpanel. Nearly all
biological export occurs in latitudes where the depth of the mixed layer
varies seasonally, just after the transition from a deep to a shallow mixed
layer.

##  5 Discussion

###  5.1 Support and extension of previous work

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.

###  5.2 Implications for exoplanet life detection

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 non-photosynthetic 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, 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 re-oxidized
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
catalogue, 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.

###  5.3 Caveats and opportunities for future work

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 5’ 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; Guendelman & Kaspi, 2019; Kang,
2019a; Colose et al., 2019; 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.

##  6 Conclusions

We used cGENIE-PlaSim, a 3D marine biogeochemical model coupled to an
atmospheric GCM, to explore the habitability of high-obliquity and high-
eccentricity planets. We considered a large range of orbital scenarios
including simulations with 0–90∘ obliquity and/or 0–0.4 eccentricity. Sea
surface temperatures consistently remained within the bounds considered
hospitable for mesophilic life on Earth at all latitudes for the entire year
in nearly all of our model scenarios, despite extreme surface air temperatures
in some simulations. We therefore conclude that marine life could thrive year-
round on high-obliquity and/or high-eccentricity worlds.

In fact, our simulations suggest that seasonal variations driven by obliquity
and/or eccentricity can be beneficial for life. Biospheric productivity on a
high-obliquity planet typically exceeds that on an otherwise identical low-
obliquity planet in our model. The same holds true for high-eccentricity
versus low-eccentricity worlds, although the observed increase in productivity
with increasing eccentricity is comparatively subdued. In both cases, the
mechanism by which productivity increases is enhanced nutrient recycling via
seasonal breakdown of thermal stratification within the ocean, consistent with
previous studies (Olson et al., 2020; Barnett & Olson, 2022). Higher rates of
photosynthesis introduce more chemical energy to the global biosphere,
benefiting many metabolisms in addition to photosynthesis. High-obliquity and
high-eccentricity planets may therefore be ‘superhabitable’ worlds capable of
supporting larger and possibly more diverse biospheres than Earth.

Our results also suggest that high obliquity and high eccentricity can
favorably influence biosignatures in addition to habitability. Higher
biospheric productivity may ultimately increase the annual production of
biosignature gases such as O2 and CH4 on high-obliquity and high-eccentricity
planets, minimizing the risk of false negative non-detections. At the same
time, seasonal oscillations in biological activity on these planets may
manifest as remotely detectable oscillations in atmospheric composition. This
time-variability provides an independent temporal biosignature unique to
planets experiencing seasons that may mitigate against false-positives and
false-negatives (Olson et al., 2018; Schwieterman et al., 2018).

Our study suggests that high-obliquity and high-eccentricity planets may be
appealing targets for exoplanet life detection. However, there are still some
open questions that require urgent attention. For instance, the extent to
which seasonality affects the development of complex life remains to be
explored. Long-term water loss and its implications for both habitability and
biosignature false positives on high-obliquity planets may also be a concern.
Moving forward, observationally constraining planetary obliquity should also
be a priority. With these caveats, we argue that high-obliquity and high-
eccentricity planets may be among our most promising targets for the search
for exoplanet life.

SLO acknowledges support from the NASA Exobiology, Habitable Worlds, and ICAR
programs under grants 80NSSC20K1437, 80NSSC20K1409, and 80NSSC21K0594,
respectively. We also thank Christopher Colose and René Heller for providing
helpful comments to improve this manuscript. This project benefited from
participation in the NASA NExSS and NASA NOW Research Coordination Networks.

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