Got it! I will conduct a detailed research study on the habitability of tidally locked 'eyeball' Earths, focusing on climate models and potential biospheres. The research will primarily delve into theoretical models, while also briefly discussing detection methods.

I will structure the paper in an academic format, including an introduction, theoretical frameworks, climate models, atmospheric dynamics, possible biospheres, and concluding remarks on the implications for astrobiology.

I'll update you once the research is complete.

Habitability of Tidally Locked "Eyeball" Earths: Climate Models and Potential Biospheres

Introduction

Tidally locked planets are worlds that always show the same face to their star, meaning one side is in perpetual daylight while the opposite side is in eternal night

www.space.com. Around dim red dwarf stars, the habitable zone is so close that any Earth-sized planet would likely become synchronized in this waywww.aanda.org. An "eyeball Earth" is a hypothesized tidally locked planet with a stark day-night dichotomy: a blazing dayside ocean surrounded by ice, resembling an eye, and a frigid nightside capped in permanent icewww.space.com. These exotic exoplanets are of great interest to astrobiology because red dwarf (M-type) stars are the most common stars in our galaxy (making up ~70% of stars) and many host Earth-sized planets in close orbitsphys.org. If even a fraction of these worlds can sustain liquid water, they could vastly expand the number of places life might exist.

Studying eyeball Earths is crucial for understanding habitability under extreme conditions. They present unique challenges and factors influencing habitability, such as:

Extreme Day-Night Environment: The permanent dayside may be intensely hot and the nightside extremely cold, creating a huge temperature gradientnewspaceeconomy.ca newspaceeconomy.ca. This can drive fierce winds and global weather patterns as hot air rushes from the dayside to the nightsidewww.space.com. Without sufficient heat transport, the nightside atmosphere could cool and collapse (freezing out gases)newspaceeconomy.ca.
Stellar Activity: Many tidally locked worlds orbit red dwarfs that are prone to flares and high UV radiation. Violent stellar outbursts could strip atmospheres or irradiate the surface, posing hazards for lifewww.nasa.gov. The high flare rate and energetic particles can also alter atmospheric chemistry (for example, depleting ozone) and must be considered in assessing long-term habitabilitywww.aanda.org.
Climate Stabilization Factors: The delicate balance of greenhouse gases, clouds, and oceans will determine if the dayside can have liquid water or if the planet swings to climate extremes. Too little greenhouse warming and the dayside ocean may refreeze into a global snowball; too much and the planet could overheat into a runaway greenhouse state. We discuss these factors in detail below.

Understanding these challenges through climate modeling and theory helps identify which tidally locked planets might be habitable oases rather than sterile snowballs or hothouses. In the following sections, we review theoretical climate models for eyeball Earths and explore where life might survive on such worlds, before outlining how future observations could detect signs of habitability.

Theoretical Climate Models

Atmospheric Circulation under Synchronous Rotation: In a tidally locked configuration, the climate is dominated by a persistent heating on the dayside and cooling on the nightside. Simulations show strong day-to-night airflow: hot air rises at the substellar point (the point directly under the star) and streams toward the nightside at high altitudes, then sinks on the cold half, producing a global circulation cell

www.space.com. These winds can be intense and even drive surface phenomena like sea-ice transport; for example, on an eyeball planet, winds would push sea ice away from the daylit ocean toward the night sidewww.space.com. The Coriolis forces are weaker on slowly rotating (long-daylength) planets, so the circulation tends to be dominated by a direct flow from day to night rather than Earth-like east-west jet streams. However, some models do predict an eastward equatorial jet (a Kelvin wave) that transports heat around the planetphys.org. This jet can distort the dayside warm region eastwards and was found to create an asymmetrical "lobster-shaped" ocean pattern rather than a perfectly centered eyeball in one simulationphys.org phys.org.

Heat Distribution and Day-Night Gradients: A key question is how effectively the atmosphere (and oceans, if present) can redistribute the star's heat. With efficient heat transport, the nightside temperature can be raised significantly, preventing the air from freezing out. For instance, one coupled atmosphere-ocean model found that ocean currents would carry warmth to the nightside, averting atmospheric collapse and enlarging the area of open water on the dayside

phys.org. In fact, including dynamic oceans can make a huge difference: without ocean heat transport, a tidally locked planet might look like a sharp-eyed "iris" of water on the dayside, but with ocean circulation, the open water could spread further or even eliminate ice entirely if conditions are warm enoughphys.org phys.org. General circulation models confirm that robust day-to-night heat flow reduces the temperature contrast between the two hemisphereswww.aanda.org. In one set of simulations holding the same stellar flux, a planet around a cooler, redder star had a more uniform climate: more of the star’s infrared energy was absorbed by the atmosphere, leading to fewer clouds on the dayside (hence more surface warming) and more efficient transport of heat to the dark sidewww.aanda.org. As a result, the day-night temperature difference was much smallerwww.aanda.org. By contrast, if heat redistribution is poor or the atmosphere is thin, the nightside could plunge to extremely low temperatures while the dayside overheats, a scenario less favorable for habitability.

Role of Greenhouse Gases and Clouds: Greenhouse gases (like CO₂ and H₂O vapor) and cloud cover are double-edged swords for tidally locked climates. On one hand, enough greenhouse warming is needed to keep the nightside from freezing solid. Studies show that with Earth-like or lower levels of CO₂, an eyeball planet’s dayside ocean might actually shrink or freeze over time – the heat transported by winds can be bled off by melting drifting ice, turning the planet into a global ice-covered snowball

www.space.com www.space.com. Additional greenhouse gases help trap heat, and climate models find that if a planet has sufficiently high CO₂ or methane in its atmosphere, it can avoid falling into a snowball state despite the strong radiative cooling on the dark sidewww.space.com. On the other hand, excessive greenhouse gas input can push the planet past the runaway greenhouse limit. A 3D model by Hu & Yang showed that a vigorous ocean circulation can actually make a planet more prone to runaway warming: by distributing heat so efficiently, it prevents day-side cloud buildup and allows more stellar energy to be absorbed, potentially boiling away all surface waterphys.org. In their simulations, ocean-bearing tidally locked super-Earths closer to the inner edge of the habitable zone were found to enter a runaway greenhouse more readily, implying the habitable zone for red dwarfs may be narrower than previously thought when ocean dynamics are includedphys.org phys.org.

Clouds provide a crucial stabilizing influence. Over the substellar point, where stellar flux is strongest, vigorous convection can form thick high-altitude clouds

news.uchicago.edu. These dayside clouds reflect a lot of incoming sunlight (increasing the planet’s albedo) and thereby cool the surface below. Yang et al. (2013) demonstrated a stabilizing cloud feedback on tidally locked Earth-like planets: in their 3D simulations, high reflective cloud cover on the dayside effectively worked like a planetary thermostat, allowing the planet to tolerate nearly twice the stellar flux previously thought possible before tipping into a runaway greenhousenews.uchicago.edu. In other words, the inner edge of the habitable zone around an M dwarf might be much closer to the star if such cloud cover forms, doubling the number of potentially habitable planets around red dwarfs by some estimatesnews.uchicago.edu news.uchicago.edu. However, cloud dynamics depend on the spectral type of the star: around cooler stars with redder light, water vapor tends to absorb more of the radiation in the lower atmosphere, which can actually reduce cloud formation at the substellar pointwww.aanda.org www.aanda.org. Fewer clouds mean less reflected light and a warmer surface, as noted above. Thus, the net climate effect is a balance—around some stars, clouds might expand the habitable zone, while around others, reduced cloud cover means a planet must orbit slightly farther out to stay temperate.

Simulated Climate Regimes: Combining these factors, theoretical studies predict a range of possible climate regimes for tidally locked terrestrial planets:

Snowball State: If stellar irradiance or greenhouse gases are too low, the dayside ocean freezes over entirely, and the planet becomes an ice-covered snowballwww.space.com www.space.com. Drifting ice and lack of sufficient warming can eliminate the open-water "eye" on the dayside, as models by Yang et al. (2019) suggested, leaving a lifeless frozen globe despite being in the habitable zonewww.space.com www.space.com.
Eyeball State: In intermediate conditions, a stable configuration is a "cold eyeball" planet: a circular dayside ocean or region of liquid water surrounded by a thick ice shell extending across the terminator to the nightsidewww.space.com. This scenario, originally proposed for planets like Gliese 581g, is characterized by a habitable enclave on the star-facing side while the rest of the planet is frozen. Climate models without ocean heat transport yielded this classic eyeball pattern (an iris of liquid water)phys.org.
Lobster/Ocean-Redistributed State: Including full ocean dynamics can morph the eyeball pattern. Hu & Yang (2014) found that instead of a neat round "eye", the dayside ocean could become lobster-shaped – extended along the equator with two off-equatorial "claws" – due to currents and equatorial jet streams redistributing warmthphys.org phys.org. The nightside in their simulation was kept much warmer by ocean heat transport, preventing atmospheric collapsephys.org. In some cases, if stellar flux and CO₂ are high enough, the ocean never freezes at all, even on the nightsidephys.org.
Runaway Greenhouse: At the extreme inner edge of the habitable zone, a tidally locked world may absorb enough heat that water evaporates globally, creating a thick steam atmosphere. The additional greenhouse effect from the water vapor leads to a runaway feedback where all surface water boils awayphys.org. This would turn the planet into an uninhabitable hothouse (similar to Venus). Ocean-covered eyeball planets may actually be more vulnerable to this if heat transport prevents local cooling – a cautionary finding that the combined effect of oceans and atmosphere can shrink the safe parameter space for habitabilityphys.org phys.org.

In summary, modern climate simulations paint a complex picture of tidally locked Earth-like planets. Rather than simple eyeballs, these worlds could have diverse climates shaped by their atmosphere-ocean interactions, cloud feedbacks, and stellar environment. Still, there appear to be plausible scenarios where a temperate region with liquid water can exist – a necessary (though not sufficient) condition for life.

Potential Biospheres

Where could life take hold on a world of permanent day and night? Even if a tidally locked planet has some surface liquid water, the environments would be extreme and uneven. Researchers have proposed several potential life-supporting zones on such planets:

Terminator "Ring of Life": The boundary between day and night – the terminator – might offer a compromise of conditions neither too hot nor too cold

phys.org. Along this ring-shaped zone, the sun is perpetually low on the horizon, providing dim illumination and milder temperatures. Recent 3D climate studies by Lobo et al. (2023) showed that a tidally locked planet with limited surface water could maintain a stable climate localized to the terminator regionphys.org phys.org. In their models, the dayside was largely dry and hot, the nightside frozen, but a band around the terminator stayed near the melting point of water – potentially allowing liquid lakes or wetlands in a narrow stripphys.org phys.org. This concept of terminator habitability suggests that even a mostly desiccated planet (one without a global ocean) could host life in pockets. One key finding is that having large landmasses enhances this scenario: if the planet is mostly ocean, the dayside water would evaporate and thicken the atmosphere with water vapor, likely overheating the planet or distributing moisture until it freezes outphys.org. But if there are continental areas at the terminator, water could be retained in smaller basins or ice caps, and the contrast between day and night could sustain a stable climate enclavephys.org. Life in such a region might resemble extremophiles in Earth's high Arctic or high-altitude deserts, enduring low light and large temperature swings. Nonetheless, it is a conceivable refuge where liquid water and moderate temperatures overlap.
Substellar Region (Dayside Ocean or Oases): The eternal day hemisphere is the only place with abundant starlight, which could power photosynthesis – but it also endures intense radiation and warmth. If an eyeball planet has a dayside ocean, as in the classical model, that ocean could be a cradle for life despite the challenges. The center of the dayside, receiving constant noon sunlight, might be too harsh at the surface for unprotected life due to high UV flux and potential high temperatures. However, life could find niches below the surface – for instance, aquatic lifeforms could stay beneath water depths where harmful ultraviolet and radiation are filtered out, much as phytoplankton or algae survive under sea ice or at depth. The edges of the dayside ocean, near the terminator, would be cooler and receive grazing sunlight, possibly analogous to polar seas on Earth that teem with life in summer. If the planet is warmer and in a "hot eyeball" configuration (with a dayside mostly dry), then the most habitable spot might shift toward the edges of the dayside where some water or damp ground exists. Oases could occur around substellar oases or oasis-like regions – for example, near springs or melt zones where subsurface water emerges. Those would be tiny areas relative to the planet, but in principle could support microbial mats or hardy vegetation if the climate allows. Overall, any organisms on the dayside would need to cope with relentless light (no night for respite) and likely high UV. Possible adaptations could include protective pigments (like extreme UV-resistant microbes on Earth have), reflective or shiny surfaces, burrowing behavior, or spore states to survive flare events.
NightSide and Subsurface Life: The permanently dark hemisphere might seem completely inhospitable, but life could persist in protected niches. Without sunlight, any life on the nightside would have to be chemotrophic or geothermal in nature, analogous to Earth's deep subterranean or deep ocean ecosystems. Indeed, scientists point out that on the dark side of a tidally locked planet, organisms might rely on chemosynthesis instead of photosynthesis, using energy from chemical reactions or geothermal heat

newspaceeconomy.ca newspaceeconomy.ca. For example, heat from the planet’s interior (which could be enhanced by tidal stresses from the star) might drive hydrothermal vents on the ocean floor, if an ocean exists beneath the ice. Just as bacteria thrive around Earth’s deep-sea vents in total darkness, life could congregate around warm, mineral-rich springs under the night-side ice. Even if the surface is frozen, a planet could harbor a liquid subsurface ocean (like Jupiter’s moon Europa) kept warm by geothermal heat or the insulation of ice. There is evidence that some outer exoplanets (e.g. TRAPPIST-1f and g) might be in such snowball states yet could have oceans under iceen.wikipedia.org. In fact, recent JWST observations of the exoplanet LHS 1140b (a super-Earth in the habitable zone of a red dwarf) suggest it could be an eyeball-like world that is either an ice-covered planet with a subsurface ocean or one with a partial open ocean on the daysideen.wikipedia.org. If it is the former, any life would be confined beneath the ice shell, perhaps around hydrothermal vents or wherever nutrients and energy are available. Organisms in such an ocean might resemble Earth’s extremophiles in Antarctic subglacial lakes or deep in hydrothermal vent communities.
Microbial Survival in Extreme Conditions: The prospects for complex life (plants or animals) on eyeball Earths may be limited, but microbial life could be hardy enough to endure the extremes. On the dayside, microbes might endure high UV by living under rocks (as endoliths) or within salty brine inclusions in ice, similar to how certain Antarctic microbes survive. On the nightside, microbes could remain dormant for long periods and metabolize intermittently when conditions allow (e.g. during volcanic eruptions melting patches of ice). Importantly, even during global glaciations ("Snowball Earth" episodes in our planet’s history), life on Earth persisted – likely in the ocean under the ice, or in meltwater pockets, or around volcanic hot spots. By analogy, a tidally locked snowball planet could still support life in pockets. Jun Yang and colleagues noted that photosynthetic organisms could survive in regions with thin ice where some starlight penetrates, or in ice-free areas heated by volcanism

www.space.com. While such regions might be rare, they provide a toehold for life to continue. Meanwhile, non-photosynthetic life could feed on chemicals produced by long-term geochemical cycles. For instance, chemicals produced on the day side by photochemistry (like oxidants created by UV light) might be transported by winds to the dark side, providing an energy source for microbes that can metabolize those compounds. This kind of “shadow biosphere” living off atmospheric chemistry is speculative but conceivable. Ultimately, any biosphere on a tidally locked world might be localized and relatively sparse. Yet, if even a small habitable zone exists (be it a twilight band, a subglacial ocean, or a warm pond at the substellar point), evolution could exploit it. The diversity of extremophiles on Earth – from radiation-loving bacteria to deep crustal microorganisms – gives hope that life could find a way to endure the perpetual day or endless night of an eyeball planet.

Methods for Detecting Habitability

Characterizing tidally locked exoplanets and assessing their habitability is a formidable challenge, but new observational techniques and missions are making progress. A number of methods can be used to detect signs of a stable, life-supporting environment on these worlds:

Transmission Spectroscopy: When an exoplanet transits in front of its star (as many close-in tidally locked planets do), we can probe its atmosphere by studying starlight filtering through the limb of the planet. Different gases absorb light at specific wavelengths, leaving fingerprints in the star’s spectrum during transit

www.nasa.gov. By using space telescopes like the James Webb Space Telescope (JWST), scientists can look for the spectral signatures of key atmospheric components such as water vapor, CO₂, methane, and even potential biosignatures (like oxygen or ozone) in a transiting planet’s atmosphere. Detecting an atmosphere at all is a big first step; for example, finding evidence of substantial greenhouse gases could indicate the planet has enough atmosphere to distribute heat and avoid freeze-out. On the other hand, the absence of certain gases or presence of others in odd ratios might hint at geochemical or biological processes. Transmission spectra of eyeball Earth candidates (such as TRAPPIST-1e or LHS 1140b) are high on astronomers’ wish lists.
Thermal Emission and Phase Curves: Even without perfect transits, we can learn about an exoplanet’s climate by measuring its infrared thermal emission. A tidally locked planet presents a permanent dayside facing us at certain parts of its orbit, and a nightside at others. By observing the planet’s infrared brightness at different orbital phases (e.g. when the dayside is visible versus when the nightside is), we obtain a phase curve – essentially a mapping of temperature distribution. The JWST’s infrared instruments (like MIRI) can directly measure the day-side emission of rocky exoplanets and see how it deviates from that of a bare rock

astrobiology.com. If the dayside is cooler than a bare rock would be, or if specific infrared absorption features (like CO₂ or H₂O bands) are present, it suggests an atmosphere is absorbing and redistributing some heatastrobiology.com astrobiology.com. Likewise, detecting even a faint glimmer of thermal emission from the nightside is a game-changer: a non-zero nightside temperature means heat transport from the dayside, i.e. an atmosphere (or ocean) at workastrobiology.com. For instance, a phase curve showing a modest day-night contrast (the dayside only slightly warmer than the nightside) would indicate a thick atmosphere smoothing out the temperature differences, a positive sign for habitability. In contrast, an extreme contrast – a roasting dayside and an almost invisible, freezing nightside – might imply a thin or absent atmosphere. Phase curve measurements can also hint at cloud presence. Highly reflective dayside clouds would keep the surface cooler and could be inferred if the observed dayside brightness is unusually low for the expected temperaturenews.uchicago.edu. Upcoming JWST observations are aiming to measure such thermal phase variations for habitable-zone targets like TRAPPIST-1e, which will directly test if these planets have Earth-like heat circulation or are closer to airless rocks.
Future Telescopes and Techniques: The search for habitable tidally locked planets will greatly benefit from the next generation of telescopes. JWST is the first to be capable of characterizing terrestrial planets in the habitable zone of red dwarfs, but its capabilities are stretched to the limit with these small, dim stars. Future ground-based Extremely Large Telescopes (ELTs) equipped with advanced adaptive optics and spectrometers may be able to conduct high-resolution spectroscopy of exoplanet atmospheres, perhaps detecting molecules like O₂ or confirming surface pressure via absorption lines. Concepts like direct imaging of exoplanets usually focus on planets around Sun-like stars, but for nearby red dwarfs, it’s possible that instruments could directly detect the glow of a planet’s day side or the glint of starlight off an ocean. NASA and ESA are also planning missions (e.g., the proposed Habitable Worlds Observatory and ESA’s ARIEL mission) that will specialize in exoplanet atmospheres. These could target dozens of tidally locked habitable-zone planets in the coming decades, looking for consistent signs of habitability: for instance, the simultaneous presence of water vapor, carbon dioxide, and perhaps a sign of disequilibrium like abundant oxygen. Additionally, observing variability over time can hint at climate: for example, periodic changes could indicate weather patterns or seasonal redistribution of clouds (though seasons in the traditional sense may not occur without axial tilt or orbital eccentricity on these planets). In summary, a combination of transit spectroscopy, thermal emission mapping, and eventually direct observations will allow us to infer whether a tidally locked planet has a thick atmosphere, clement temperatures, and maybe even oceans or clouds – the preconditions for a living world.

Conclusion

Tidally locked "eyeball" Earths represent a fascinating alternative path for planetary habitability. Their unique configuration – perpetual day on one side, perpetual night on the other – leads to climate regimes with no direct analog on present-day Earth. However, by leveraging advanced 3D climate models, scientists have begun to sketch out how such planets might behave. Key findings indicate that atmospheric and oceanic circulation can profoundly moderate the extremes: with a thick atmosphere (and especially with oceans), heat can be redistributed enough to keep the nightside atmosphere from freezing and to sustain liquid water on the dayside

phys.org www.aanda.org. Cloud cover can act as a planetary thermostat, reflecting starlight and preventing runaway heatingnews.uchicago.edu, while sufficient greenhouse gases are needed to avoid a deep freezewww.space.com. The potential climate outcomes range from globally frozen snowballs to temperate eyeballs to steaming greenhouses, depending on the balance of incoming flux and climate feedbacks.

In terms of astrobiology, eyeball planets challenge our notions of a traditional biosphere. Instead of a globally clement Earth, life might be confined to a narrow twilight fringe or an ocean beneath ice. Yet, as this paper discusses, there are plausible habitats even on these extreme worlds – from the terminator zones that could host liquid water lakes

phys.org phys.org, to substellar oceans teeming with marine life, to dark-side chemosynthetic communities feeding off geothermal energynewspaceeconomy.ca newspaceeconomy.ca. Life, if it exists on such planets, may be highly specialized and localized, but not impossible. The history of our own planet shows that microorganisms can endure prolonged darkness, extreme cold, high radiation, and other challenges analogous to those on tidally locked exoplanetswww.space.com.

Observationally, the coming years are exciting. With JWST and other telescopes, we are now obtaining the first data on atmospheres and temperature maps of rocky exoplanets in habitable zones. These observations will test the theoretical models discussed here. A detection of an atmosphere (or lack thereof) on a TRAPPIST-1 planet, for example, will immediately tell us if these worlds tend toward Earth-like circulation or more resemble airless Moon-like bodies. In the more distant future, we may be able to pick up direct indicators of habitability – such as a spectral signature of water clouds, or even tentative biosignatures – on a tidally locked planet. Each such discovery will refine our understanding of how climate and life interact on these alien yet common worlds.

In conclusion, tidally locked eyeball Earths underscore the adaptability of the concept of habitability. They force us to broaden the conventional Goldilocks zone notion to include planets with permanent days and nights, where habitability might be a localized phenomenon. The research so far suggests that while challenges are myriad (from flaring stars to ice feedbacks), there are pathways for these planets to maintain stable, life-friendly niches. Many open questions remain: How does magnetic activity and stellar wind erosion impact long-term atmospheres? Can a magnetosphere or ionosphere shield the dayside from flares? What compositions of atmosphere and ocean are most favorable for a large day-night habitable area? Future climate simulations, coupled with actual data from exoplanet observations, will help answer these questions. Tidally locked worlds will likely be the first Earth-sized exoplanets we characterize in detail, and they may surprise us by how different – or how familiar – their patterns of clouds, climate, and possibly life are compared to our own planet. The study of eyeball Earths thus sits at the intersection of planetary science, climate science, and astrobiology, and promises to enrich our understanding of the diverse conditions under which life could exist in the universe.