# Temperature of a substellar point on an airless tidally-locked planet

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If a hypothetical blackbody planet's components were solid at all temperatures, would the substellar point on the airless tidally-locked (1:1) planet eventually heat up to the surface temperature of the star it is orbiting? This material would be a very poor heat conductor.

Also, would the temperature on the opposite point approach the cosmic background temperature 2.7 K?

The planet would reach an equilibrium where the amount of heat absorbed is the same as the amount of heat radiated. If there is no way to transfer heat on the planet (no conduction, no atmosphere), then that condition must apply locally.

The flux radiated from a blackbody surface (in W/m$$^2$$) is given by $$sigma T^4$$, where $$sigma$$ is Stefan's constant and $$T$$ is the temperature.

If the substellar point is a distance $$d$$ from the star, and the luminosity of the star is given by $$L simeq 4pi R^2 sigma T_{*}^4$$ (assuming it too is a blackbody with radius $$R$$ and temperature $$T_*$$) and assuming $$d gg R$$ to avoid some unnecessary geometric unpleasantness, then the flux absorbed at the substellar point is $$L/4pi d^2$$. The flux is all absorbed, since you wish to assume a blackbody.

The temperature of the substellar point at equilibrium is therefore given by $$sigma T^4 = frac{4pi R^2 sigma T_*^{4}}{4pi d^2},$$ $$T = T_* left(frac{R}{d} ight)^{1/2}$$

Since we assume that $$d gg R$$ then clearly $$T < T_{*}$$.

At other points on the planet's surface it will receive a reduced flux from the star, simply becasue the flux from the star is incident at an angle to the exposed surface, so the equilibrium temperature will be lower.

On the unlit side of the planet there is no illumination from the star, but an almost isotropic flux from the cosmic microwave background equal to $$sigma T_{ m CMB}^4$$ over the entre surface. Therefore in the absence of any other source of heat, then that side will assume the temperature of the CMB at equilibrium.

## Habitability Still a Go on Tidally Locked Terrestrial Exoplanets

Title:Water Trapping on Tidally Locked Terrestrial Exoplanets Requires Special Conditions
Authors:
Jun Yang, Yonggang Liu, Yongyun Hu, and Dorian S. Abbot
First Author’s Institution:
University of Chicago
Status: Accepted to The Astrophysical Journal Letters

We’ve talked many times before on Astrobites about habitable exoplanets, defined by whether they can retain liquid water on their surfaces. To first order, this involves the habitable zone, otherwise known as the “Goldilocks zone” or the “just right” distance a planet can orbit its star where water on the planet’s surface will neither freeze solid nor be boiled away. But distance from the star (which translates directly into the amount of stellar radiation received by the planet) is only the first-order approximation to truly understand the state of a planet’s water, we have to understand the details of the planet’s atmosphere and heat circulation. This is mostly beyond today’s observational abilities, but we can apply models to tell us what kinds of planets are good candidates for habitability.

Yang and collaborators explore a specific subset of exoplanets: tidally locked, rocky exoplanets orbiting M-stars. M-stars are the most common type of star by far, and because they are small, cool stars, their habitable zones are located close in. This results in tidal locking for many habitable zone planets. Tidally locked planets orbit such that one side always faces their star, and one side out into space. Consequently, they have huge temperature gradients between their day and night sides. Atmospheric circulation in these planets will tend to transport water from the day side of the planet (where it evaporates) to the night side (where it condenses back out of the atmosphere). Once on the cold night side, it will freeze and potentially be trapped as ice. But how much of this water freezes? Once it freezes, is it lost forever as a liquid? To understand this scenario more fully, atmospheric circulation models must be combined with models describing the flow of oceans as well as both land and sea ice sheets.

Yang and collaborators use models developed to study Earth’s climate, which comprise coupled models to study atmosphere, ocean, sea ice, and land. In all cases they examine a typical super-Earth exoplanet with a period of 37 days, a radius of 1.5 Earth radii, and a gravity 1.38 times that of Earth, but with a variety of continent/ocean configurations. They examine a waterworld with no continents and three different ocean depths, a planet with one supercontinent covering the night side and an ocean on the day side, with uniform elevation and depth, respectively, and a planet that looks like modern-day Earth, with a substellar point in either the Atlantic or Pacific Oceans, or in Africa.

Figure 1. Diagram showing amount of water for three different types of planets. On the left is a waterworld, where water and ice are transported easily between the hot day and cold night sides, resulting in little ice trapping. In the center, a planet with a large continent covering the night side still maintains a sizeable ocean, because the high heat flux of the planet keeps the ice sheet small. In the right panel, also with a night side continent but with a lower heat flux, the continental ice sheet grows, trapping most of the water of the planet. This last is the worst case scenario for habitability, but is only possible when the heat flux is low, the continents are all on the night side, and the overall water reservoirs are small compared to Earth’s water stores. (Credit: Yang et al. 2014)

For the waterworld, they find that the ice on the night side becomes only 5.4 meters thick, leaving plenty of liquid water on the planet. While ice forms on the night side, it is also continuously melted by warm ocean currents circulating from the day side, and by surface winds that push the ice sheets towards the warmer substellar point. Yang adds continental barriers here, running north to south on the eastern and western terminators (the day/night dividing lines, a fixed geographical point on a tidally locked planet), to investigate what happens if the ocean and ice transport is disrupted by a land barrier. In this case, the ice grows to 1000 meters thick, effectively trapping the water as ice.

On a planet with one supercontinent on the planet’s night side, the water that is trapped as continental ice sheets is maximized for a low geothermal heat flux. For a planet with Earth’s water stores and heat flux, roughly half its ocean would be trapped in such a scenario. For a super-Earth, which would likely have a higher heat flux, only a small ocean, a few hundred meters thick, would be trapped. See Figure 1 for a comparison between this scenario and the waterworld.

Figure 2. For a planet with modern-day Earth’s continental configuration, ice sheet thicknesses are shown for the oceans (left) and land (right). The color bar shows the ice thickness, contours show surface air temperatures of 0, 5, and 7 degrees C, and arrows indicate sea-ice velocity. The black dot indicates the substellar point. This is for a planet with a low heat flux. Most tidally locked super Earths would probably have a higher heat flux, resulting in much thinner ice sheets. (Credit: Yang et al. 2014)

If less artificial continents are studied, such as modern-day Earth’s continental configuration, the ice remains

10 meters thick in most areas, though it can grown to

100 meters in a few isolated regions like Baffin Bay or mostly inland seas. Even small passages between continents allow enough transport of sea ice and water currents to avoid trapping a critical amount of water in ice. See Figure 2 for details.

In conclusion, the habitability outlook for these tidally locked planets is pretty good! Ocean planets can efficiently transport ice back to the day side to be melted, and even small breaks in continental coverage are enough to prevent critical amounts of water being trapped in ocean or land ice sheets. It will be difficult to detect the differences between these kinds of planets observationally, but looking at reflectivity measurements could indicate land/water/ice coverage on planets.

Given that the inertia of a planet is pretty darn huge, the sheer amount of energy needed to break out of synchronous rotation would have pretty cataclysmic effects on the geology, geography, ecology and every other aspect of the planet. While you might not end up with a glowing ball of lava, you certainly will be experiencing violent earthquakes, volcanic eruptions, extreme dislocations of the hydraulic and atmospheric cycles and so on. It would be really unpleasant to live there, to say the least.

Since you didn't specify how fast the planet will be rotating afterwards, I will assume that it will be rotating rather slowly (Only a "modest" amount of energy would be needed for this, maybe a passing neutron star or something). Since the gravitational forces operating on the planet from the sun remain constant, and active over the entire lifespan of the planet and star, it seems reasonable to assume that the system will settle down to some other periodic cycle, perhaps like Mercury with it's strangely resonant rotation/orbital period. The tidal forces that locked the planet in synchronous rotation in the first place will still be in operation.

As for any life on the planet, anything which evolved for a sedentary lifestyle based on its location relative to the "hot pole" will most likely become extinct, and most of the larger life forms which grazed on any vegetation that filled that ecological niche, and all the predators, parasites, symbiots etc. that partake of that part of the ecological web will also become extinct as well. Your planet will most likely be reduced to the equivalent of lichen and moss for aeons while the hydraulic and atmospheric cycles are reestablished in a new equilibrium and the ecology can stabilize.

As an aside, given the planet's close proximity to the star, the release of enough energy to break the synchronous rotation of the planet will most likely be distributed to the star as well, so in addition to everything else, the star might have much higher activity and sterilize the planet with violent flares, rendering a lot of what was written above moot.

## Temperature of a substellar point on an airless tidally-locked planet - Astronomy

Short-period planets exhibit day-night temperature contrasts of hundreds to thousands of kelvin. They also exhibit eastward hotspot offsets whereby the hottest region on the planet is east of the substellar point 1 this has been widely interpreted as advection of heat due to eastward winds 2 . We present thermal phase observations of the hot Jupiter CoRoT-2b obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These measurements show the most robust detection to date of a westward hotspot offset of 23 ± 4°, in contrast with the nine other planets with equivalent measurements 3-10 . The peculiar infrared flux map of CoRoT-2b may result from westward winds due to non-synchronous rotation 11 or magnetic effects 12,13 , or partial cloud coverage, that obscure the emergent flux from the planet's eastern hemisphere 14-17 . Non-synchronous rotation and magnetic effects may also explain the planet's anomalously large radius 12,18 . On the other hand, partial cloud coverage could explain the featureless dayside emission spectrum of the planet 19,20 . If CoRoT-2b is not tidally locked, then it means that our understanding of star-planet tidal interaction is incomplete. If the westward offset is due to magnetic effects, our result represents an opportunity to study an exoplanet's magnetic field. If it has eastern clouds, then it means that a greater understanding of large-scale circulation on tidally locked planets is required.

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Multiwavelength constraints on the day-night circulation patterns of HD 189733b. / Knutson, Heather A. Charbonneau, David Cowan, Nicolas B. Fortney, Jonathan J. Showman, Adam P. Agol, Eric Henry, Gregory W. Everett, Mark E. Allen, Lori E.

In: Astrophysical Journal , Vol. 690, No. 1, 2009, p. 822-836.

Research output : Contribution to journal › Article › peer-review

T1 - Multiwavelength constraints on the day-night circulation patterns of HD 189733b

N2 - We present new Spitzer observations of the phase variation of the hot Jupiter HD 189733b in the MIPS 24 μm bandpass, spanning the same part of the planet's orbit as our previous observations in the IRAC 8 μm bandpass (Knutson et al. 2007). We find that the minimum hemisphere-averaged flux from the planet in this bandpass is 76% ± 3% of the maximum flux this corresponds to minimum and maximum hemisphere-averaged brightness temperatures of 984 ± 48 K and 1220 ± 47 K, respectively. The planet reaches its maximum flux at an orbital phase of 0.396 ± 0.022, corresponding to a hot region shifted 20°-30° east of the substellar point. Because tidally locked hot Jupiters would have enormous day-night temperature differences in the absence of winds, the small amplitude of the observed phase variation indicates that the planet's atmosphere efficiently transports thermal energy from the day side to the night side at the 24 μm photosphere, leading to modest day-night temperature differences. The similarities between the 8 and 24 μm phase curves for HD 189733b lead us to conclude that the circulation on this planet behaves in a fundamentally similar fashion across the range of pressures sensed by these two wavelengths. One-dimensional radiative transfer models indicate that the 8 μm band should probe pressures 2-3 times greater than at 24 μm, although the uncertain methane abundance complicates the interpretation. If these two bandpasses do probe different pressures, it would indicate that the temperature varies only weakly between the two sensed depths, and hence that the atmosphere is not convective at these altitudes. We also present an analysis of the possible contribution of star spots to the time series at both 8 and 24 μm based on near-simultaneous ground-based observations and additional Spitzer observations. Accounting for the effects of these spots results in a slightly warmer night-side temperature for the planet in both bandpasses, but does not otherwise affect our conclusions.

AB - We present new Spitzer observations of the phase variation of the hot Jupiter HD 189733b in the MIPS 24 μm bandpass, spanning the same part of the planet's orbit as our previous observations in the IRAC 8 μm bandpass (Knutson et al. 2007). We find that the minimum hemisphere-averaged flux from the planet in this bandpass is 76% ± 3% of the maximum flux this corresponds to minimum and maximum hemisphere-averaged brightness temperatures of 984 ± 48 K and 1220 ± 47 K, respectively. The planet reaches its maximum flux at an orbital phase of 0.396 ± 0.022, corresponding to a hot region shifted 20°-30° east of the substellar point. Because tidally locked hot Jupiters would have enormous day-night temperature differences in the absence of winds, the small amplitude of the observed phase variation indicates that the planet's atmosphere efficiently transports thermal energy from the day side to the night side at the 24 μm photosphere, leading to modest day-night temperature differences. The similarities between the 8 and 24 μm phase curves for HD 189733b lead us to conclude that the circulation on this planet behaves in a fundamentally similar fashion across the range of pressures sensed by these two wavelengths. One-dimensional radiative transfer models indicate that the 8 μm band should probe pressures 2-3 times greater than at 24 μm, although the uncertain methane abundance complicates the interpretation. If these two bandpasses do probe different pressures, it would indicate that the temperature varies only weakly between the two sensed depths, and hence that the atmosphere is not convective at these altitudes. We also present an analysis of the possible contribution of star spots to the time series at both 8 and 24 μm based on near-simultaneous ground-based observations and additional Spitzer observations. Accounting for the effects of these spots results in a slightly warmer night-side temperature for the planet in both bandpasses, but does not otherwise affect our conclusions.

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## Life on a tidally-locked planet?

(EDIT: In this scenario, the planet is tidally locked to the sun.) I'm writing a story and it's set on a planet similar to gliese 581c. I want to make sure that the characteristics of the weather, atmosphere, etc. are accurate. Ultimately, I understand that we can only speculate on the answers until we can begin sending probes to these sort of planets to get an idea of what they are like, but I am trying to be as scientifically accurate as possible. Any help I can receive in making sure I'm describing this sort of hypothetical environment appropriately is appreciated.

I've done research into what would be required for a tidally locked planet to support life. From my understanding of what I've read, a tidally-locked planet would need to be in a tight orbit around a red dwarf star. There would need to be a moon(s) to generate currents and tide in the planet's oceans. These would need to go from night side to day side and back to transfer heat and create a sustainable climate. The core would need to be magnetized in order for the planet to have a thick enough atmosphere to support life, and high global winds would need to travel fast enough to circulate warm and cold air around and prevent temperatures reaching extremes either side of the planet. Most life would exist in a habitable zone along the terminal ring (the "Twilight Zone") with extremophiles habiting the more extreme areas of the planet.

That is more or less the sum of what I've been able to learn through browsing the internet. I'm curious as to how much of that is accurate and how much I have either missed or drawn incorrect conclusions over. Is any of this even accurate?

Now for the more in depth questions that I can't seem to find solid answers to. I apologize if some of my phrasing gets a little repetitive. Feel free to answer as many or as few of these questions as you want.

Would a red dwarf cause colors in plants to tend more towards red, purple, or orange hues as opposed to the greens we associate with healthy foliage? What about people's skin tone? If they're in a limited habitable zone would that create more or less variation?

Would oceans be able to extend across most of the planet or would they be limited in range as well?

Might things be windier on this sort of planet? Would there be wind storms or more frequent rain? Would it be cloudy frequently? Tornadoes? Perhaps some unique weather condition we don't see on Earth?

What sorts of life would be most common? Would life develop in the more extreme regions?

With no day/night cycle, would life develop circadian rhythm? Would we see species that don't sleep?

Once again, I appreciate any help I can receive with this. I want the setting for this story to be as accurate as possible.

The math here and scientific understanding needed is quite impressive and difficult. I have little to no experience, other than understanding tidal locking, but I may be able to help point you in the right direction.

Size of moon: Mutual tidal locking (unsure of technical term, maybe dual), such as Charon and Pluto occur when the two bodies have roughly the same size or gravity. If this is a moon, I hypothesize that it would mean the moon would appear very large. According to this (math is done properly), https://www.quora.com/How-large-does-the-moon-Charon-appear-in-Plutos-sky-and-would-light-from-Charon-be-enough-to-cast-a-shadow-on-Pluto , charon would look to be about 8 times larger give or take than the moon does on Earth. That also means that it would likely be quite a bit brighter than the moon is.

The math needed for accuracy and everything you want is a bit ridiculous: The mathematical problem of the size, density, and relative distances of the sun, planet, and moon is hard enough on its own. You say you want multiple moons so as to help circulate the air better, but I don't think you can do that. It's either tidally locked or not. I think you are asking for it to basically switch its tidal locking daily. That sort of thing takes a very, very long time. On the order of hundreds of thousands to hundreds of millions of years for equalish sized bodies. I am NOT an expert, though.

Find better places, more specific places: I would suggest you bring this up to https://astronomy.stackexchange.com/ or other astrophysics or astronomy forums. You may even reach out to local professors.

Planetary conditions: I have a hard time believing the planet would not experience extreme winds. It depends on the heat of the sun and size of the planet. Imagine the planet-moon are tidally locked and face eachother. Imagine there is a sun and a concentric ring around it. Along the concentric ring, the planet-moon are spinning/dancing while locked to eachother (days) and spinning at the same speed as they travel along the ring at another speed (years). This would allow for the sun to heat the planet. I don't know if this is making sense. It's hard to explain it. There would be a time where the moon would be in the way, partially of the sun which would mean it would be cooler during a certain season. I DO NOT now if this three-body problem is possible mathematically/physically, but I can at least picture it ocurring.

I hope some of this helps at all.

Well, it certainly gives me material to work with from a writing standpoint. I also appreciate the direction it gives me. So thanks.

As a correction. "Mutual tidal locking" (tidal equilibrium) can and will always happen between two bodies given a long enough time. The smaller body will lock to the larger one. Then there will be a continued exchange of orbital and spin momentum of the bodies until the larger one is locked to the smaller.

Isn't OP talking about tidal locking to the star, and not between the moons and the planet?

A moon probably isn't necessary. However if it does have a large and close enough moon, I think it might avoid tidal locking to the sun if it locks to the moon instead.

The sunlight would not look much different from ours. It would be a warmer shade of white, similar to an incandescent light bulb (the filament temperature is comparable to that of the surface of a red dwarf star). The red dwarf sun itself would be larger than our sun appears in the sky. Solar flares will likely be much stronger than we experience, so I would expect frequent auroras visible over much of the night side of the planet.

Plants could be green, or some other color. I don't know that there is any reason to think plants that evolved under a red dwarf sun would prefer different colors. I'm not sure about UV levels at the surface. That depends on the atmosphere too. But yes I would expect less variation among those on the day side of the habitable zone, but perhaps some live just on the night side where the temperature is still warm enough and there's some light, but they never see the sun.

The ocean can be as extensive as you want. It can cover the whole planet or there may be small seas. Just depends on how much water the planet got. The oceans on the night side would likely be frozen, and the day side would be very hot, maybe similar to the Sahara. It would very likely be overcast over much of the day side, which would help to keep temperatures moderate. There would be winds transferring heat from the day to night side, but the simulations I've seen don't have extreme winds.

Life on the night side couldn't rely on photosynthesis. It might get its energy from undersea volcanoes, as some life on Earth does, or eat things that ocean currents bring over from the day side, as life at the bottom of our ocean eats things that fall from near the surface. I think it's possible a tidally locked red dwarf planet might have life all over its surface, and it's also possible it may have large uninhabitable regions.

If you haven't read it yet, this article may be useful for an overview, and for the papers in the references section:

This is quite helpful! Thank you.

The star doesn't necessarily have to be a red dwarf. Weɽ expect habitable zone tidal-locked planets to be more common around such stars, but it could conceivably happen around a larger star if the planet forms with a slow initial rotation rate.

The moon isn't necessary to circulate heat. The atmosphere will form its own convection currents that keep the nightside above around 240 K. This would entail some constant winds but not the intense gales we used to think were necessary. Habitability could extend well past the twilight zone in principle the entire dayside could be habitable to some extent.

The presence of a large moon might actually prevent tidal-locking to the star, or if not the process of tidal-locking may cause the moon to be ejected from orbit, so I wouldn't expect them to be common for such planets.

Some people have speculated that plants on a planet around a red dwarf would be black, to absorb as much visible light as possible, but we're not too sure. It's not clear to what extent there's a reason green plants are dominant on Earth and to what extent it's just sort of a historical accident. Red and purple photosynthesizers have been more prevalent at certain points in the distant past.

Human skin tone appears to have little function other than to manage our UV exposure making sure its not high enough to damage our skin but still enough to help in vitamin D production. A red dwarf produces little UV light, so humans coming from Earth might need supplements or artificial UV light, but any human-like creatures that evolved there would never come to rely on sunlight for vitamin production in the first place. Other than that, animal color schemes might be overall shifted into red, which will show up better, but not as much as you might think an old incandescent light bulb produces the same spectrum of light as a typical red dwarf star (human color perception tends to adjust to the environment).

Oceans can extend as far or as little as you want. The planet could be completely covered in ocean or water could be limited to a few small lakes with islands of habitability around them (bizarrely the latter case may be more likely to support life, but for esoteric reasons). Any oceans on the nightside will naturally be covered in a layer of ice, but there could be some life there around hydrothermal vents on the ocean floor.

In terms of weather, you would expect a more-or-less permanent cloud formation around the substellar point, at the center of the day side. The high albedo of this cloud formation actually does a lot of the work of keeping the global climate stable and reducing temperature variation. If the orbital period is short, and therefore rotation is relatively fast, this formation can be "smeared" across the equator to some extent. That first source I linked has some helpful diagrams. Other than that, there are various controls on the climate that could cause rain and storms to be more, less, or as common as they are on Earth.

We can't really predict exactly what life would develop. On the dayside, it could more-or-less resemble what we see on Earth, but on the nightside it would never be more than extremophiles.

No day/night cycle means no circadian rythm (Though if the planet has some axial tilt or orbital eccentricity it could have a day/night cycle in thin strips in the twilight zone, with the star oscillating just above and just below the horizon over the course of the year). We don't know if sleep is necessary for intelligent life, but if it is we might expect such life on this world to have sleep patterns like a dolphin, where some section of the brain is always aware at any one time.

## Researchers extend capabilities of computer simulation of tidally locked exoplanets

Spatial distributions of sea-ice fraction and surface air temperature. (Left) Sea-ice fraction (unit, %) (Right) surface air temperature (unit, °C) (Upper) 355 ppmv CO2 and (Lower) 200,000 ppmv CO2. In A and B, arrows indicate wind velocity at the lowest level of the atmospheric model (990 hPa), with a length scale of 15 m s−1 . In C and D, arrows indicate ocean surface current velocity, with a length scale of 3 m s−1 . Note that the color scale for surface air temperature is not linear. The substellar point is at the equator and 180° in longtitude. Credit: PNAS, Yongyun Hu, doi: 10.1073/pnas.1315215111

(Phys.org) —A pair of researchers at Peking University in Beijing China, has extended the capabilities of an existing computer simulation that is used to study tidally locked exoplanets. In their paper published in Proceedings of the National Academy of Sciences, Yongyun Hu and Jun Yang describe the improvements they've made and also how those improvements give a new perspective on the range of possible tidally locked exoplanets that may be habitable.

Prior to this new effort, most computer models that sought to recreate the conditions that exist on exoplanets that are tidally locked (they don't spin, thus only one side ever faces their star) relied mostly on the impact of atmospheric conditions. The new enhancements include possible impacts of ocean currents.

The main goal of the upgraded model is, like many others, to allow for predicting the likelihood of life existing somewhere other than here on Earth. Tidally locked exoplanets present a challenging prospect—on one hand, the side that points towards the star is likely warm enough to support life—on the other, the cold side may be so cold that gases freeze and are lost to space preventing the evolution of an atmosphere.

To try to get a better handle on what may go on with such exoplanets, the researchers extracted parts of models that try to predict ocean behavior here on Earth. Those parts were then modified to more accurately reflect what has actually been observed, namely, smaller, colder and less feature rich worlds.

Tidally locked exoplanets generally exist close to a red dwarf star—they get locked because they move so close to their star. This means that the amount of heat hitting the star is much less, relatively speaking, than it would be for a planet that wasn't locked, because its star is colder. Space scientists tend to refer to such planets that might hold the potential for life as an "Eyeball Earth," because the dark side resembles a pupil.

The enhanced model, the team reports, allows for changing parameters (such as CO2 levels) and then for allowing many simulated years to pass to see what evolves as a result. Doing so, the team says, shows that given the right set of circumstances, heat from oceans can be transported around the globe allowing for a warmer planet than has been predicted before, though most outcomes suggest a narrower habitable zone.

The researchers note that much more work needs to be done on their model and others, noting that many are still too simplistic to render true approximations. One area of concern is that most models don't take into account land formations or uneven ocean bottoms, both of which can impact ocean currents and hence heat transfer.

Abstract
The distinctive feature of tidally locked exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the nightside for terrestrial exoplanets in the habitable zone around M dwarfs. In the present paper, we carry out simulations with a fully coupled atmosphere–ocean general circulation model to investigate the role of ocean heat transport in climate states of tidally locked habitable exoplanets around M dwarfs. Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an "eyeball." For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs' habitable zone. This study provides a demonstration of the importance of exooceanography in determining climate states and habitability of exoplanets.

## Scientists Mull the Astrobiological Implications of an Airless Alien Planet

Astronomer Laura Kreidberg admits she was initially a bit worried about her latest results. Examinations of a planet orbiting the red dwarf star LHS 3844 seemed to indicate that the rocky super-Earth, 30 percent larger than our world, possessed little or no atmosphere.

Kreidberg&rsquos concern stemmed from the fact that researchers are in the midst of a heated debate about the habitability of planets around red dwarfs, which make up 70 percent of the stars in our galaxy. A universe teeming with life is more likely if the worlds orbiting these diminutive entities, which are smaller and cooler than our sun, could be a good abode for biology.

But red dwarfs are harsh hosts, emitting frequent flares containing x-rays and ultraviolet radiation that could sterilize a planet, as well as energetic stellar winds that can strip it of its protective atmosphere. Kreidberg and her colleagues&rsquo findings, which appeared in August in Nature, could be seen as a mark against the idea that planets around small red stars could provide a nurturing environment.

In recent years, astronomers have announced numerous exciting discoveries regarding red dwarfs, such as Proxima Centauri b, a potentially habitable planet orbiting our sun&rsquos nearest star, and the TRAPPIST-1 system, which contains a whopping seven Earth-sized worlds. Red dwarfs are not only abundant but are also the longest-living stars, with a lifetime that can span 10 trillion years&mdash1,000-fold longer than that of our sun. Should a biosphere arise on a red dwarf world, it might stick around for an exceptionally long time.

Astronomers are therefore interested to know whether or not red dwarfs&rsquo planets are good places to go looking for living creatures. &ldquoTo have life as we know it, you need to have liquid water,&rdquo says Abraham Loeb, a co-author of the Nature study and an astrophysicist at the Center for Astrophysics at Harvard University and the Smithsonian Institution (CfA). &ldquoIn order to have liquid water, you need an atmosphere.&rdquo

Kreidberg, who is also at the CfA, has been in the daily habit of checking for new results from NASA&rsquos Transiting Exoplanet Survey Satellite (TESS), a space-based observatory hunting for nearby planets that &ldquotransit&rdquo their host stars&mdashflitting across the faces of those stellar hosts and casting shadows toward our solar system. Among TESS&rsquos first discoveries was the rocky world LHS 3844 b, located just under 49 light-years away, and Kreidberg quickly recognized that it was in an ideal position to test the atmospheric-retention capabilities of red dwarf exoplanets.

LHS 3844 b orbits incredibly close to its parent star, zipping around in a mere 11 hours. This orbit more or less guarantees that the star&rsquos gravitational pull has tidally locked the planet, meaning one side of the world always faces the star. The exoplanet&rsquos dayside is scorching, while its space-facing hemisphere sits out in the cold.

But while the exoplanet experiences 70 times more radiation than Earth, Kreidberg says it would not necessarily lose its atmosphere at this distance. For instance, an envelope of thick carbon dioxide could be heavy enough to endure the bombardment from the nearby star. Or the world might have once contained a vast ocean that was boiled off by the intense starlight, which also would have split the water into its constituent molecules. The lighter hydrogen could have drifted away, leaving an atmosphere of pure oxygen.

Although the researchers could not directly see the planet, using NASA&rsquos infrared Spitzer Space Telescope, they were able effectively take its temperature, detecting a periodic variation in the thermal emissions from its host star that was caused by the planet&rsquos orbital movements. Much like the moon in our sky, LHS 3844 b shows different faces to observers on Earth as it sweeps through its orbit: at turns, it displays its hotter dayside or its colder nightside, which subtly alters the amount of infrared radiation astronomers see emanating from the star. The planet also passes completely behind its star for a portion of its orbit, as seen from Earth, entirely removing its heat from view and allowing scientists to determine its total contribution to the star&rsquos thermal emissions. Based on these measurements, Kreidberg&rsquos team estimated the temperature of the planet&rsquos nightside as a freezing &ndash273 degrees Celsius and that of its days as a fiery 767 degrees C.

The presence of a regulating atmosphere should allow heat to transfer between hemispheres, reducing such extremes. But computer models suggested that LHS 3844 b&rsquos temperature differences could only arise and persist if the planet had an extremely thin atmosphere, with, at most, a 10th of the pressure of Earth&rsquos and likely none at all.

A great deal of theoretical work has already implied that worlds orbiting red dwarfs would have a hard time forming or retaining significant atmospheres because of the extreme environment, says Colin Johnstone, an astrophysicist at the University of Vienna, who was not involved in the new study. But what the characteristics of a close-in planet such as LHS 3844 b means for places such as TRAPPIST-1&rsquos worlds or Proxima Centauri b, which orbit farther from their parent star, is not entirely clear.

&ldquoIt&rsquos one more piece of evidence suggesting that these stars aren&rsquot going to have habitable planets,&rdquo Johnstone says, though he cautions against making sweeping judgments based on a single example.

Because LHS 3844 b is far inside the traditional habitable zone&mdasha region around a star where a planet is sufficiently warmed by starlight to have liquid water on its surface&mdashthe null result does not much phase Tiffany Jansen, an astronomy Ph.D. candidate at Columbia University, who also was not involved in the recent work.

&ldquoThe discovery of a lack of an atmosphere on this planet doesn&rsquot make it any less likely that planets in the habitable zone would have an atmosphere,&rdquo she says.

But Loeb counters that what happens in the immediate vicinity of a red dwarf star is relevant to more remote planets. He has previously done theoretical calculations suggesting that red dwarfs are prone to blow away the atmospheres of exoplanets in their habitable zone. Even though LHS 3844 b is a single example and is much closer to its star than a habitable planet could be, it provides important evidence that atmospheric stripping takes place. And extrapolations imply similar outcomes can be expected farther out, Loeb says.

The discussion will probably rage on until astronomers can examine more cases. The upcoming James Webb Space Telescope (JWST), an infrared observatory whose mirror will have 6.25 times the light-collecting power of the Hubble Space Telescope, will be revolutionary in its ability to measure heat from distant exoplanets, Kreidberg says.

Other teams have already committed to using time during JWST&rsquos first year to examine the temperature of the planets TRAPPIST-1 b&mdashfound in the TRAPPIST-1 system&mdashand Gliese 1132 b&mdashwhich also orbits a red dwarf. The telescope is currently scheduled to launch in 2021, and it will be joined by powerful 30-meter-class ground-based observatories, expected to come online early next decade, that can conduct similar research.

Kreidberg&rsquos preliminary disappointment about LHS 3844 b eventually dissipated. &ldquoIf you were an alien looking at our solar system and saw Mercury, you&rsquod be a little discouraged,&rdquo she says, but our cosmic backyard contains a wide diversity of atmospheres.

Researchers are still coming to understand just how planetary atmospheres arise, and a great deal remains unknown. &ldquoFor every idea for how to get rid of an atmosphere on a planet, there&rsquos another for how to keep it or make a new one,&rdquo Kreidberg says. &ldquoI don&rsquot think this counts as a victory point for the naysayers just yet.&rdquo