Astronomy

How close to a host star can a tidally locked planet be and its dark side still maintain a moderate temperature?

How close to a host star can a tidally locked planet be and its dark side still maintain a moderate temperature?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

So, imagine an atmosphere-less planet, tidally locked to a sun-like star. How close to the star can the planet be before its dark side becomes too hot?

I imagine that at some point the rocks on its sunlit side will melt and evaporate so that the dark side would experience rocky precipitation. Would this be true?

Also, at some point the atmosphere of the star itself would engulf the planet.

But at which point would these effects make the conditions on the dark side unbearable?


Your general idea about this process is correct. At close semimajor axis distance, rock can evaporate and will form a Silicate-oxygen atmosphere. For low-mass rocky planets, the condensation flow from day to night-side, as it necessarily is very hot, will have to compete with the possibility of instead escaping vertically from the nightside, instead of precipitating. For higher-mass planets, vertical escape will be too difficult, and the hot Silicate-Oxygen gas will recondense on the nightside and therefore heat via thermal radiation and condensation latent heat.

This is a problem on which current research is in progress, and not many groups have worked on this, with the exception of one fantastic article that came out this year. They glue four different 1-D models together to represent vertical and horizontal for each day and night-side of catastrophically evaporating planets together, in order to create a sort of fake-2D model of the planetary Silicate-atmosphere.
Their nightsides are very hot in general (500-1000K), but they sadly do not give silicate densities or pressures. Hence, without the density, it is difficult to estimate how much the heat transfer between the Silicate gas and a human would be, i.e. how much a human would 'feel'.

If you are very curious though, I am sure you can construct this effect, by assuming Silicate saturation pressures, which are given in this article.


I imagine that a planet will become too hot for Earth type life - or even hypothetical life with different biochemistries might be able to live at much higher temperatures than Earth life - long before it becomes engulfed by its star's atmosphere.

Even the coolest stars have surface temperatures of a few thousand degrees, so a planet which is close enough to the surface of a star to be engulfed n the atmosphere of the star would probably be far too hot for even hypothetical alien biochemistries.

Since your planet is atmosphere less, there would be no e heat exchange by liquids or gases between the day side and the night side. But heat would spread through the rocks from the hot side to the cooler side and heat it up.

But I am unable to calculate what conditions would be necessary for the dark side of the planet to have a specific temperature.

I notice that you ask:

But at which point these effects will make the conditions on the dark side unbearable?

And I have to ask:

"Who or what the dark side conditions would be unbearable for".

Are you asking about human explorers landing in spaceships and building bases and walking around in spacesuits? Do you want to know how hot the surface can get before it becomes too hot for humans in spacesuits and air conditioned vehicles and air conditioned bases?

Or are you asking about what would make the dark side unbearably hot for native life forms of the planet?

On Earth, water exists in three different states, solid, liquid, and gas, and often transitions between different states. And we would expect that any other chemical used a a solvent and medium for alien life would also transition between solid, liquid, and gas at the temperatures suitable for that hypothetical alien life with radically different biochemistry.

As the atmospheric pressure drops, the temperature at which a liquid boils and also drops. At a low enough atmospheric pressure, that liquid will sublimate and transition directly from solid to vapor, with no liquid stage. In a vacuum or near vacuum, all substances will transition directly from solid to gaseous, and any liquid will quickly boil away.

And your planet is defined as:

an atmosphere-less planet, tidally locked to a sun-like star.

So your planet should not have any bodies of any liquid which life forms could use on its surface.

The only way to give your planet any native life is to give it large bodies of liquid on the dark side covered by very thick layers of frozen liquid.

So there could be inland seas of liquid methane, covered by thick sheets of frozen methane. And possibly there could be hypothetical lifeforms which use liquid methane in the methane ocean. As the heat from the day side is conducted through rocks to the night side, the liquid methane ocean will heat up, possibly becoming too hot for methane based life. And eventually the methane ocean will turn into methane vapour and possibly blow open the sheets of methane ice above it.

And maybe there are inland seas of liquid ammonia, covered by thick sheets of frozen ammonia. And possibly there could be hypothetical lifeforms which use liquid ammonia in the ammonia ocean. As the heat from the day side is conducted through rocks to the night side, the liquid ammonia ocean will heat up, possibly becoming too hot for ammonia based life. And eventually the ammonia ocean will turn into ammonia vapor and possibly blow open the sheets of ammonia ice above it.

And maybe there are inland seas of liquid water, covered by thick sheets of frozen water. And possibly there could be hypothetical lifeforms which use liquid water in the water ocean. As the heat from the day side is conducted through rocks to the night side, the liquid water ocean will heat up, possibly becoming too hot for water based life. And eventually the water ocean will turn into water vapor and possibly blow open the sheets of water ice above it.

And maybe there are inland seas of liquid sulfur, covered by thick sheets of frozen sulfur. And possibly there could be hypothetical lifeforms which use liquid sulfur in the sulfur ocean. As the heat from the day side is conducted through rocks to the night side, the liquid sulfur ocean will heat up, possibly becoming too hot for sulfur based life. And eventually the sulfur ocean will turn into sulfur vapor and possibly blow open the sheets of sulfur ice above it.

And of course different rock temperatures on the night side would be necessary for methane based life, or ammonia based life, or water based life, or sulfur based life. Or life using some other solvent.

See:

https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry[1]

I note that your planet is described as:

an atmosphere-less planet, tidally locked to a sun-like star

If the star has the same mass and luminosity as the Sun, the distance at which a planet in our solar system would become tidally locked would be the distance at which your imaginary planet would become tidally locked to its star. And thus the planetary temperature would be similar to that of a planet at that distance from the Sun.

Habitable Planets for Man Stephen H. Dole, 1964, 2007, discusses the various factors making worlds habitable or not for humans.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[2]

On page 71 table 9 lists the calculated strengths of tidal retardation effects on various objects in the Solar System and lists wherever they are tidally locked or not.

Dole concludes on page 70 that when the factor of h squared is somewhere between 1.2 and 2.0, the planet's rotation will be "stopped" (actually tidally locked to its primary).

However, there is a problem with Dole's conclusion. It was written when the planet Mercury, which is much closer to the Sun that Earth and thus much hotter on the day side, was believed to be tidally locked to the Sun, keeping one side eternally facing the Sun.

But in 1965 it was discovered that Mercury is not tidally locked to the Sun.

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable-at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury's sky.[114]

The rare 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution.[115] In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth-Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to point always at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, enforcing that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.[115]

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days.[22] A sidereal day (the period of rotation) lasts about 58.7 Earth days.[22]

https://en.wikipedia.org/wiki/Mercury_(planet)#Orbit,_rotation,_and_longitude[3]

So it is possible that a planet would have to be even closer to a sun-like star, and thus even hotter, than Mercury to be tidally locked. But it is possible that the reasons why Mercury has a 3:2 spin-orbit resonance would not apply to all planets orbiting at Mercury's distance from sun-like stars, and that some planets could become tidally locked at Mercury's distance and even farther from a sun-like star.

The planets Mercury and Venus are not tidally locked to the Sun, and thus do not have eternal day on one side and eternal night on another side. But they do have very long days and nights.

On Venus, there is little difference in temperature between day and night, because the dense atmosphere spreads heat evenly around the planet.

Mercury has a very long day, but does have alternations of day and night.

The surface temperature of Mercury ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F)[18] at the most extreme places: 0°N, 0°W, or 180°W. It never rises above 180 K at the poles,[12] due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion (0°W or 180°W), but only 550 K at aphelion (90° or 270°W).[73] On the dark side of the planet, temperatures average 110 K.[12][74] The intensity of sunlight on Mercury's surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).[75]

A darkside temperature of 110 Kelvin would equal minus 261.67 degrees Fahrenheit (-163 °C).

So the temperature on a typical point of Mercury will rise and fall by hundreds of degrees every Mercurian day. Mercury rotates with respect to the stars (sidereal day) every 59 Earth days, but makes a full rotation with respect to the Sun (a solar day) every 175.97 Earth days.

So a point on Mercury which has a temperature of about 110 K will have it rise by at least 300 K during a period of 87.985 Earth days, and then fall again by at least 300 K in the next 87.985 Earth days.

I think this means that the rocks and other surface materials on Mercury do not conduct heat very well. And possibly that might mean that the dark side of a tidally locked airless planet might be very, very cold.

When it was believed that Mercury was tidally locked to the Sun, it was logical to assume that the dark side of Mercury was the coldest place in the solar system, and thus Larry Niven's story set on the dark side of mercury was titled "The Coldest Place".

Thus it is possible that a tidally locked airless planet would have to be very close to its star for enough heat from the day side to make it to the night side and raise the temperature to unbearably hot instead of just right or instead of unbearably cold.


How dust could make some exoplanets more habitable

Three computer simulations depicting how airborne dust can be distributed by winds on rocky exoplanets like Earth. Image via Denis Sergeev/ University of Exeter/ ScienceAlert.

What makes a planet habitable? Various factors can affect a planet’s ability to sustain life, such as temperature, amount of water, composition of both the planet and its atmosphere and the amount of radiation from the host star. Last month, researchers in the U.K. said they’ve found that a common component of atmospheres – dust – could increase the habitability of some exoplanets.

The peer-reviewed results were published in Nature Communications on June 9, 2020.

This is a significant finding, since it suggests that planets with a lot of dust in their atmospheres could have habitable conditions farther from their stars than previously thought. This would, in effect, expand the habitable zone, which is basically the region around a star where temperatures on a rocky could allow liquid water to exist.

Researchers from the University of Exeter, the Met Office and the University of East Anglia (UEA) were involved in the new study.

Effects of dust on the climate of planets. For a tidally locked planet (a) and non-tidally locked planet (b), panels a–d show the base state of the planets, e–h show the short- (stellar) and long-wave (infra-red) forcing (change in surface energy balance) introduced by dust and i–j show the resultant effect of the forcing on the surface temperature. Blue arrows show the motion of the planet around the star, and green arrows show the rotation of the planet relative to the star. Image via Boutle et al./ Nature Communications.

Identification of habitable planets beyond our solar system is a key goal of current and future space missions. Yet habitability depends not only on the stellar irradiance, but equally on constituent parts of the planetary atmosphere. Here we show, for the first time, that radiatively active mineral dust will have a significant impact on the habitability of Earth-like exoplanets.

In our own solar system, Mars typically comes to mind when we think of a dusty world, yet it remains a cold, dry planet on the surface due to its very thin atmosphere. But for some exoplanets, especially those that are tidally locked to their stars, it could be a different situation. Ian Boutle, from both the Met Office and University of Exeter and lead author of the study, said in a statement:

On Earth and Mars, dust storms have both cooling and warming effects on the surface, with the cooling effect typically winning out. But these ‘synchronised orbit’ planets are very different. Here, the dark sides of these planets are in perpetual night, and the warming effect wins out, whereas on the dayside, the cooling effect wins out. The effect is to moderate the temperature extremes, thus making the planet more habitable.

The dust factor is especially significant for planets orbiting red dwarf stars, the most common type of star in our galaxy. Many planets around those stars are likely to be tidally locked, orbiting with one side of the planet always facing the star, just as the moon always keeps one side facing Earth. Those planets would have one side always in daylight, and the other always in darkness. If there is a lot of dust, that could help cool down the hotter day side, and warm the colder night side.

Artist’s concept of a cloudy and rocky exoplanet orbiting a red dwarf star. Dust in the atmospheres of planets like this could moderate the temperature extremes if the planets are tidally locked, helping to make them more habitable. Image via L. Hustak/ J. Olmsted (STScI)/ NASA.

In an interesting scenario, dust could help hot planets retain their surface water, if they have any. A planet that is really hot, like Venus, could be cooled down by enough dust in the atmosphere. The amount of dust would then increase as water starts to be lost on the planet’s surface, which, ironically, in a process called negative climate feedback, would then slow down the loss of water. From the paper:

On tidally-locked planets, dust cools the day-side and warms the night-side, significantly widening the habitable zone. Independent of orbital configuration, we suggest that airborne dust can postpone planetary water loss at the inner edge of the habitable zone, through a feedback involving decreasing ocean coverage and increased dust loading.

The amount of energy a planet receives from its star is an important part of assessing habitability, but as Manoj Joshi from UEA noted, the composition of the atmosphere, including dust, is also very important:

Airborne dust is something that might keep planets habitable, but also obscures our ability to find signs of life on these planets. These effects need to be considered in future research.

The researchers performed a series of simulations of rocky Earth-sized planets and found that naturally occurring mineral dust can have a big impact on the habitability of such planets.

Mars is a very dusty place, and massive dust storms are common, but the dust doesn’t warm the planet much since the atmosphere is so thin. Image via SA/ Roscosmos/ CaSSIS/ CC BY-SA 3.0 IGO/ New Scientist.

Duncan Lyster, who ran an undergraduate experiment as part of the overall study (and now builds his own surfboards), also said:

It’s exciting to see the results of the practical research in my final year of study paying off. I was working on a fascinating exoplanet atmosphere simulation project, and was lucky enough to be part of a group who could take it on to the level of world-class research.

The researchers also point out that dust in a planet’s atmosphere must be taken into account when searching for possible biomarkers in that atmosphere. Those biomarkers could include gases such as oxygen, methane and ozone, and if there also was enough dust, the dust could obscure the detection of them, producing a false negative result. If potential biomarkers were missed in that way, the planet might be erroneously characterized as uninhabitable. Such biomarkers, which will be searched for with upcoming space telescopes like the James Webb Space Telescope (JWST) and others, will be a crucial aspect of the search for evidence of life beyond our solar system. Identifying them is already a challenge due to the extreme distances to these worlds, so knowing the amount of dust in a planetary atmosphere will be important as well. From the paper:

The inclusion of dust significantly obscures key biomarker gases (e.g. ozone, methane) in simulated transmission spectra, implying an important influence on the interpretation of observations. We demonstrate that future observational and theoretical studies of terrestrial exoplanets must consider the effect of dust.

Ian Boutle at the Met Office and University of Exeter, lead author of the new study. Image via Google Scholar.

Nathan Mayne from the University of Exeter, who assisted with the study, also noted how astrophysics in general will play a large role. He said:

Research such as this is only possible by crossing disciplines and combing the excellent understanding and techniques developed to study our own planet’s climate, with cutting edge astrophysics. To be able to involve undergraduate physics students in this, and other projects, also provides an excellent opportunity for those studying with us to directly develop the skills needed in such technical and collaborative projects. With game-changing facilities such as the JWST and E-ELT, becoming available in the near future, and set to provide a huge leap forward in the study of exoplanets, now is a great time to study physics!

The new assessment regarding exoplanetary dust will be very beneficial to scientists who will be looking for biomarkers and other evidence for habitable exoworlds, as well as studying how dust can affect a planet’s climate and environment overall.

Bottom line: Atmospheric dust could increase the habitability of some exoplanets.


The Bizarre Planets That Could Be Humanity’s New Homes

What would human civilization look like on a tidally locked world?

Imagine going to live on a planet where the sun never moves in the sky. No sunrise, no sunset.

Several years ago, I became obsessed with tidally locked planets. The notion of a world permanently caught between two extremes—with one half always illuminated, the other always in the dark—took hold of my imagination. I realized that planets like these were the surest bet in the search for Earth-like places that our descendants could settle on. Worlds of eternal darkness and never-ending sunlight could be the future of the human race—if we’re serious about living in other solar systems.

Astronomers believe that most of the planets in our galaxy that have Earth-like temperatures are likely to be tidally locked. Because their orbital period is the same as their period of rotation, these planets will always present the same face to their sun—just as we always see the same side of the moon, as it orbits Earth.

And the reason for this glut of tidally locked worlds is pretty simple. Up to three-quarters of suns in our galaxy are red dwarfs, or “M-dwarfs,” smaller and cooler than our sun. Any planet orbiting one of these M-dwarfs would need to be much closer to its star to support human life—as close as Mercury is to our sun. And at that distance, the star’s gravity would pull it into a tidally locked orbit.

For example, astronomers recently discovered seven Earth-size planets in the habitable zone of the TRAPPIST-1 system, all of which are likely to be tidally locked.

My obsession with these planets led to my new novel, The City in the Middle of the Night. To picture all their strange geological features and weird knock-on effects, I talked to Lindy Elkins-Tanton, the director of the School of Earth and Space Exploration at Arizona State University, as well as other scientists studying them, and I read as much of the latest research as I could. More than anything else, I became captivated by trying to imagine what it would be like for people living on a planet where the sky never changes.

For now, talking about these planets means indulging in speculation—which is the perfect situation for a science-fiction writer. But we are learning enough about the dynamics of tidally locked worlds to start to understand how they would work, and what kind of civilization we could build there.

The first question: Where would humans settle on a tidally locked planet? When I started working on my book, the clearest answer appeared to be the terminator, the strip of twilight between the dayside and the nightside. “That might be the Goldilocks zone,” neither too hot nor too cold, but stuck “between eternal dusk and eternal dawn,” says Daniel Angerhausen, an astrophysicist at the Center for Space and Habitability at Bern University.

In the terminator zone, Angerhausen suggests, humans might be able to generate geothermal energy, using cold water from the nightside and hot water from the dayside in “some kind of thermal reactor.”

To have access to liquid water on a tidally locked world, you need a system to cool down the dayside and heat up the nightside, says Ludmila Carone of the Max Planck Institute for Astronomy. Otherwise, all the liquid might become tied up in ice on the nightside, or worse yet, the atmosphere itself could get frozen in the dark.

“The habitability of these planets hinges very strongly on how well you can transport heat,” Carone says. Her computer models show that a tidally locked planet might have two strong wind jets, one in each hemisphere, that might act a bit like the jet stream here on Earth. But if the planet is too close to the sun, it might have only one wind jet, directly over the part closest to the sun. In that scenario, heat could be trapped on the dayside.

Even a relatively modest temperature differential (say, 50 degrees Fahrenheit) between the two sides could make these planets harder to live on. A comfortably mild climate on the dayside might still leave the nightside cold enough to freeze water, according to Laura Kreidberg, a junior fellow at Harvard University who studies the atmospheres of exoplanets. “Could all the planet’s water freeze out on the nightside? We don’t yet know,” she says. Ocean currents could help transport heat, too, but those effects depend on how much water the planet has to begin with and where the continents are.

One possible scenario for a tidally locked planet is what’s known as the “eyeball Earth” model, in which a planet starts out entirely covered with ice—which then melts on the side facing the sun. To an observer from space, this could look like an eyeball, explains Angerhausen. Or, with an ocean that transports enough heat, you could end up with a lobster-shaped ocean surrounded by ice.

In the most extreme scenarios, the heat on the light side becomes so extreme that water can’t exist. But with enough of a temperature difference, it can re-form on the nightside.

That’s what happens on a tidally locked planet called WASP-103b, a “hot Jupiter”–type world. According to Vivien Parmentier at Aix Marseille University, an author (along with Kreidberg) on a recent study of WASP-103b, water molecules are destroyed on the dayside of the planet, only to drift back to the nightside and recombine into water molecules that form clouds . and then the process repeats.

Beyond the problems with finding liquid water, a tidally locked world around a red dwarf could have other issues, says Carone. Red dwarfs are “notoriously temperamental” and tend to go through long phases in which they flare up and eject material into space.

These flare-ups could heat the atmosphere of a planet in the habitable zone, while the star could also eject material that strips away the atmosphere. This happened to Earth early on, when our original atmosphere was torn away from us. Afterward, Earth “sweated out” another atmosphere from trapped carbon dioxide. But on a tidally locked world, a violent-enough solar disruption could get rid of a second atmosphere, too.

Even with an atmosphere, the dayside of the planet could be exposed to deadly radiation, says Parmentier. The light from a red dwarf wouldn’t provide enough of the UV wavelengths needed to make ozone—so this planet, unlike Earth, might not have an ozone layer. (In my novel, direct sunlight isn’t just too hot it actually causes nasty burns, so people have to stay in the shade.)

Any humans living on the planet would also need to eat and breathe, and the physicists Joseph Gale and Amri Wandel of Hebrew University have been studying whether plant life could survive the flares and radiation exposure. At first, plants might evolve in the ocean to take advantage of the protective layer of water. But eventually, if the star became less violent, the planet could develop an atmosphere thick enough to allow plants to grow on land. Gale and Wandel have also calculated that there would probably be enough light in the visible spectrum to allow normal photosynthesis.

With an atmosphere that could sustain life, though, there would also be air currents strong enough to cool the planet’s dayside. The temperature might end up being about the same as in Earth’s tropical regions. An atmosphere could also help create a layer of cloud that would serve as a permanent sun shade. As scientists such as Carone have been making computer models of tidally locked worlds, they increasingly believe humans could live outside the terminator region.

Adiv Paradise, a Ph.D. student in astronomy and astrophysics at the University of Toronto, has a guess at what that could look like: People might live on the dayside, but would need to construct mining and pipeline operations to bring ice over from the nightside. A lot depends on how bad the radiation bombardment on the dayside might be. Paradise also thinks people could learn to live on the frozen nightside: “I’m from Minnesota. People manage to live in all sorts of places astronomers would describe as ‘not habitable.’”

The biggest challenge for humans living in a tidally locked world, says Paradise, could be the very different sky. If they lived on the dayside, they might “lose all knowledge of the universe,” because they would never see the stars. Their perception of the passage of time would also be altered, because “nothing in the sky would ever change.”

Inspired by these concerns, in The City in the Middle of the Night, I created two different human societies with wildly divergent approaches to the problem of circadian rhythms and the passage of time. And my human settlers definitely take advantage of the temperature differentials to create geothermal power, as Angerhausen suggests. Still, my tidally locked world didn’t reflect these more recent computer models and ended up being a little more fanciful in some of the details. There’s always a trade-off between scientific accuracy and storytelling, and in some ways, I may have ended up writing a bit of an exoplanet fable.

But I wanted to help people imagine the strangeness, terror, and splendor of inhabiting a planet that orbits an alien star. I believe that novels about tidally locked worlds will become a fast-growing subgenre as we make more discoveries and gather more observational data. There are so many great stories to be told about visiting these worlds of never-ending sunlight and darkness. And dreaming about life on another planet is a way of thinking about our own place in the universe, as humans, both now and in the millennia to come.


On a tidally locked planet, would time be quantized? [closed]

Want to improve this question? Add details and clarify the problem by editing this post.

Assume a planet that always presents one side to the sun. No moons. The orbit of the planet around the star is essentially spherical, very minor and undetectable aberrations. The orbital period of the planet around the star is in multiples of the lifetimes of the sentient beings. That is, no sentient being lives long enough to go through one complete rotation.

NOTE: this question is not about HOW this would happen, it is to be taken as a given that it DID happen.

A sentient species evolves on this planet.

There are no day/night cycles, no seasons, and no cyclical changes in the sky - it would be constantly different throughout one's lifespan. No repeats. Life evolved without any natural circadian rhythm. Plants grew and died independently of any cycle. No defined growing season. A plant could sprout at any time, and die at any time. No menstrual cycles, no estrus cycles in animals. There would be no biological basis on which to establish any. Life would have evolved without any concept of cyclical time passage. As far as life was concerned, everything was eternally homogenous, time wise. Nothing happened in unison.

To these sentient beings, the passage of time is irrelevant. They have no way to compare the passage of time between people. Heart beats are different, respiration is different, one's pace in walking is different. The interval to travel from one point to another is different. It is human experience that what seems like a long period of time to one person is a short period of time for another person. The beings get to where they are going when they get there, without reference to anyone else's travels. They eat when they need to, irrespective of how long it has taken between meals (think of a snake, that can go for very long periods between meals, depending on how big the last meal was). Gestations are never the same length, so even if two beings got pregnant at the same time, the deliveries would never likely be at the same time. Two objects released from a height at the same moment would be observed to arrive at the ground at the same moment, but there would be no indication that if dropped at some other moment they would arrive at the ground in the same time as these two, without some form of quantized time.

Many of our 'Laws of Physics' require time to be quantized. Humans, of course, had to quantize time before we could develop any theories based on the quantization of time. This is a straightforward 'chicken or egg' thing.

It seems to me that if Galileo had no preconceived idea of the measure of the passage of time (through the passage of seasons, day/night cycles, etc) he would not have been able to discover that the periodicity of a pendulum was constant. Depending on his state of mind, sometimes the swing would appear to take forever, sometimes it would appear to be shorter.

It also seems to me that Newton relied on a distinct concept of the passage of quantized time before hand, in order to develop many of his Laws. (The action/reaction thing, and gravitational attraction thing, would be valid, just not quantized as to the passage of time).

Building structural integrity and engineering calculations for stresses have no time component to them, if built by 'Rule of Thumb' (We built the last one this thick, and it is still standing. The other one was built thinner, and it collapsed.)

If this sentient species had no experience that the passage of time was measurable and periodic, would they ever develop a method to quantize it? Would they ever TRY to quantize it, if there was nothing in their environment that was consistently and repeatedly cyclic, that they needed to or even that they could consistently measure? What would that quantization look like?

I submit that this question is NOT the same as this, as many of the answers pertain to some form of naturally occurring cycle observable by humans, and /or are based on human circadian rhythms, which are not experienced by this species. This life form evolved without any ability to determine periodic time spans. Also, that question asks what would they use to quantize time, this asks what would their method of quantizing time look like? Would they even understand that time could be quantized into absolute divisions, that were equivalent between people and between events?

A further corollary question will be something along the lines of 'What would the Laws of Physics look like without time being quantized in the same constant periodic way that humans have quantized it?' But that is NOT in the scope of this question.

The term 'quantized' comes from the digital field, not from quantum mechanics. The first use of quantum, in this regard, was in 1597. For a non-quantum-mechanics elaboration of the term, See Quantization (signal processing)

Quantization, in mathematics and digital signal processing, is the process of mapping input values from a large set (often a continuous set) to output values in a (countable) smaller set, often with a finite number of elements. Rounding and truncation are typical examples of quantization processes. Quantization is involved to some degree in nearly all digital signal processing, as the process of representing a signal in digital form ordinarily involves rounding.

It is impossible for this species to develop any kind of calendar or any kind of measurement for a periodic recurring cycle. No days, months, years, seasons, hunger cycles, menstrual cycles, crop cycles, planting seasons, harvesting seasons, sleep cycles, or any other natural seasonal phenomena'. It takes three or more lifespans for the sky to repeat. The entire life of the planet has evolved without periodic cycles or rhythms. There is nothing to measure it with. There is no way to tell that this current crop is growing faster, slower, or just the same as the last crop. No way to judge or compare the pregnancy duration of the first child, with the length of the pregnancy for the last child. There is no metric, nothing to count, measure, or quantify. Any answer based on comparing the length of a current event to past or future events in any manner that does not end up with a number, or quantity, of some absolutely replicable unit of measurement, of the actual duration of the event, (so for instance two pregnancies, one then and one now, can be compared in length), is immediately discarded. Any comparison to any time duration in the past has to involve some kind of numerical count for the duration of each event.

Way back as far as 1597, the term 'quantum' was used to mean an amount of something. You could have a quantum of wheat, a quantum of distance, a quantum of liquid, a quantum of patience, a quantum of intelligence. a quantum of solace, a quantum of time. Some of these we consider to be measurable, some not. For instance, we talk about someone having more patience than someone else, or that one is 'losing patience', but what is the measure of patience that reduces to zero? For the things that we consider measurable, we have developed a system whereby they can be measured, counted, or quantified. But unless we have the CONCEPT that we can measure it, there has been no real consistent attempt to develop units of measure. The word 'quantum' is sometimes applied to the 'unit' of measurement - a quantum of distance is the foot, or the inch, or the meter, a quantum of weight is the gram, or the ounce, or the stone. Note that each one is different, even for measuring the same thing, so generically the term applied was 'quantum' meaning 'measurable amount'. But not all things that you could have a quantum of, could you have a 'measurable amount' of. Like patience. We can still talk about a quantum pf patience, without really any attempt at measuring, counting, or quantifying it.

Before we can measure, count, or quantify something, we first have to form the concept that it CAN be measured, counted, or quantified. My question is about developing this concept for time. (It could also be asked about temperature as well - we know something can be hotter or colder than something else, butt it took us a long time to develop the concept that the comparison could be MEASURED.) We can understand that time flows, that there are time passages, that the future is not the past, and we are in the present. Having a concept that time passes is not the same as understanding that it is measurable. Can we have the CONCEPT that time is measurable? We can not see, taste, feel, touch, or senese it except through memory, of knowing that the past happened. That is the process I call quantization and Miriam_Webster agrees with me.

Definition of quantize transitive verb

1 : to subdivide (something, such as energy) into small but measurable increments

Okay, so someone appropriated the term 'quantum' and proceeded to minimize it to absurdity, and it is now narrowly defined by some physics purists as ONLY meaning something to do with quantum mechanics. You do not have to agree with the definition or my terminology, you can come up with your own, but it what I am after is NOT the process of measuring, counting, or quantifying something, it is the process of realizing something CAN be measured or quantified in the first place. I do not want to know HOW it is measured, conted, or quantified, just that this species has the understanding and the concept that it CAN be measured, counted, or quantified.

We humans got the concept that time is something that could be measured, from the obvious day/night cycle. Historically, that is the first form of measuring time. We could COUNT days, and COMPARE the count between one event and another. We could RECORD the count, to compare durations in the present to durations in the past. It was not hard to conceive of the periodicity and the regularity of the day/night cycle. We could count the number of days from winter solace to the next winter solace, and see that the count was always the same. We liver f=through enough winter solaces to get the idea. It was natural for us to develop the concept that time could be measured, counted, and quantified.

But what if there was no calendar? Nothing to count? What natural phenomena would develop the concept that time was measurable in any meaningful fashion? ANswering this is fundamental to my corollary question - what would be the limitations on this species on their concepts in physics, based on their concept of the measurability of time? How would they quantify speed, for instance? They could easily compare two people in a race, and determine one was faster than the other, but what would their equations look like, without time? Distance, it is obvious to measure. But time?And a unit for time? Just as improbable, perhaps, for this species as a unit for patience is to ours.

So any answer HAS to include the reason why this species could even conceive that time could be measured in the first place. Again, any answer based on the premise that time HAD to be measured, in order to do something, is immediately disqualified. If there was no concept that time could be measured, there would be no concept that in any particular situation it was needed to be measured. You just don't do any activity that is time dependent if you do not understand the concept of measuring time. 'Meeting someone at such-and-such time' is just something this species would ever think about, because it just couldn't be done. ANything that puts a demand on this culture for time dependency as we know it is just anthropomorphizing Western culture on another alien species. No they DON'T have to behave the same way we do.

Sure, there are a lot of situations in which measuring patience is useful, but in all that, we still have no concept that patience can be measured in any absolute way, so we do not consider it a necessity to measure it absolutely. Would time be the same for this species? If not, what would trigger in them the concept that time could be measurable in a quantized (repeatable, replicable, consistent, understandable, quantifiable, countable, measurable, observable, any term you wish to use) way?

So okay, being specific about the question, and explaining it thoroughly and understandably takes a long time. But it has become obvious that TL:DR methods just don't work for many people.

This did not make it into the previous edit.

Take, for instance, the development of the concept that length could be measured. I can measure a length of cloth with my arm, and mark it. Tomorrow, I can measure it, and find I get the same mark. I can measure ten arm's lengths, cut it off, and put it aside. I can take another piece of cloth, measure off ten lengths, and cut it. I can then compare the two lengths of cloth, and see they are both the same length. I can cut another piece of cloth at twenty arm's length, and notice it is as long as the other two pieces of ten arm's length combined. So I know length can be reliably, provably, replicably be measured. I can validate my measurement against something else. I can compare one measured piece with another that I made before and see that they are the same. I have no problem with the concept that length can be measurable. So absent any form of calendar or recurring periodic cycle, how do I get the same concept that time is measurable, can be quantized or sliced up or marked off so it can counted or quantified (whatever you want to call it) and assigned a numerical value, and this value will be the same for any other period of time of the same duration? Length I can see and touch and feel and store and compare between the past and the future. I can not do that with time.


Support Knowable Magazine

Your generosity will help us continue making scientific knowledge accessible to all.

Atmospheric Circulation of Tide-Locked Exoplanets

TAKE A DEEPER DIVE | Explore Related Articles from Annual Reviews

Atmospheric Circulation of Tide-Locked Exoplanets

With better data, astronomers are beginning to enter an era where simulations of conditions on far-off exoplanets are possible, improving understanding of where life might exist in the universe.

Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects

As telescopes have gotten more powerful, astronomers have begun peering into the atmospheres of planets outside of the solar system.

Exoplanet Atmospheres

To help answer the question “Are we alone?” scientists are studying the atmospheres of planets orbiting other stars.


Tidally locked exoplanets may be more common than previously thought

Many exoplanets to be found by coming high-powered telescopes will probably be tidally locked -- with one side permanently facing their host star -- according to new research by astronomer Rory Barnes of the University of Washington.

Barnes, a UW assistant professor of astronomy and astrobiology, arrived at the finding by questioning the long-held assumption that only those stars that are much smaller and dimmer than the sun could host orbiting planets that were in synchronous orbit, or tidally locked, as the moon is with Earth. His paper, "Tidal Locking of Habitable Exoplanets," has been accepted for publication by the journal Celestial Mechanics and Dynamical Astronomy.

Tidal locking results when there is no side-to-side momentum between a body in space and its gravitational partner and they become fixed in their embrace. Tidally locked bodies such as Earth and moon are in synchronous rotation, meaning that each takes exactly as long to rotate around its own axis as it does to revolve around its host star or gravitational partner. The moon takes 27 days to rotate once on its axis, and 27 days to orbit Earth once.

The moon is thought to have been created by a Mars-sized celestial body slamming into the young Earth at an angle that set the world spinning initially with approximately 12-hour days.

"The possibility of tidal locking is an old idea, but nobody had ever gone through it systematically," said Barnes, who is affiliated with the UW-based Virtual Planetary Laboratory.

In the past, he said, researchers tended to use that 12-hour estimation of Earth's rotation period to model exoplanet behavior, asking, for example, how long an Earthlike exoplanet with a similar orbital spin might take to become tidally locked.

"What I did was say, maybe there are other possibilities -- you could have slower or faster initial rotation periods," Barnes said. "You could have planets larger than Earth, or planets with eccentric orbits -- so by exploring that larger parameter space, you find that in fact the old ideas were very limited, there was just one outcome there."

"Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks," Barnes said. "And so when you explore that range, what you find is that there's a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no moon and with an initial 'day' that was four days long, one model predicts Earth would be tidally locked to the sun by now."

Barnes writes: "These results suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future."

Being tidally locked was once thought to lead to such extremes of climate as to eliminate any possibility of life, but astronomers have since reasoned that the presence of an atmosphere with winds blowing across a planet's surface could mitigate these effects and allow for moderate climates and life.

Barnes said he also considered the planets that will likely be discovered by NASA's next planet-hunting satellite, the Transiting Exoplanet Survey Satellite or TESS, and found that every potentially habitable planet it will detect will likely be tidally locked.

Even if astronomers discover the long-sought Earth "twin" orbiting a virtual twin of the sun, that world may be tidally locked.

"I think the biggest implication going forward," Barnes said, "is that as we search for life on any exoplanets we need to know if a planet is tidally locked or not."


Earth-like planets around small stars likely have protective magnetic fields, aiding chance for life

Earth-like planets orbiting close to small stars probably have magnetic fields that protect them from stellar radiation and help maintain surface conditions that could be conducive to life, according to research from astronomers at the University of Washington.

A planet's magnetic field emanates from its core and is thought to deflect the charged particles of the stellar wind, protecting the atmosphere from being lost to space. Magnetic fields, born from the cooling of a planet's interior, could also protect life on the surface from harmful radiation, as Earth's magnetic field protects us.

Low-mass stars are among the most common in the universe. Planets orbiting near such stars are easier for astronomers to target for study because when they transit, or pass in front of, their host star, they block a larger fraction of the light than if they transited a more massive star. But because such a star is small and dim, its habitable zone -- where an orbiting planet gets the heat necessary to maintain life-friendly liquid water on the surface -- also lies relatively close in.

And a planet so close to its star is subject to the star's powerful gravitational pull, which could cause it to become tidally locked, with the same side forever facing its host star, as the moon is with Earth. That same gravitational tug from the star also creates tidally generated heat inside the planet, or tidal heating. Tidal heating is responsible for driving the most volcanically active body in our solar system, Jupiter's moon Io.

In a paper published Sept. 22 in the journal Astrobiology, lead author Peter Driscoll sought to determine the fate of such worlds across time: "The question I wanted to ask is, around these small stars, where people are going to look for planets, are these planets going to be roasted by gravitational tides?" He was curious, too, about the effect of tidal heating on magnetic fields across long periods of time.

The research combined models of orbital interactions and heating by Rory Barnes, assistant professor of astronomy, with those of thermal evolution of planetary interiors done by Driscoll, who began this work as a UW postdoctoral fellow and is now a geophysicist at the Carnegie Institution for Science in Washington, D.C.

Their simulations ranged from one stellar mass -- stars the size of our sun -- down to about one-tenth of that size. By merging their models, they were able, Barnes said, "to produce a more realistic picture of what is happening inside these planets."

Barnes said there has been a general feeling in the astronomical community that tidally locked planets are unlikely to have protective magnetic fields "and therefore are completely at the mercy of their star." This research suggests that assumption false.

Far from being harmful to a planet's magnetic field, tidal heating can actually help it along -- and in doing so also help the chance for habitability.

This is because of the somewhat counterintuitive fact that the more tidal heating a planetary mantle experiences, the better it is at dissipating its heat, thereby cooling the core, which in turn helps create the magnetic field.

Barnes said that in computer simulations they were able to generate magnetic fields for the lifetimes of these planets, in most cases. "I was excited to see that tidal heating can actually save a planet in the sense that it allows cooling of the core. That's the dominant way to form magnetic fields."

And since small or low mass stars are particularly active early in their lives -- for the first few billion years or so -- "magnetic fields can exist precisely when life needs them the most."

Driscoll and Barnes also found through orbital calculations that the tidal heating process is more extreme for planets in the habitable zone around very small stars, or those less than half the mass of the sun.

For planets in eccentric, or noncircular orbits around such low mass stars, they found that these orbits tend to become more circular during the time of extreme tidal heating. Once that circularization takes place, the planet stops experiencing any tidal heating at all. The research was done through the Virtual Planetary Laboratory, a UW-based interdisciplinary research group funded through the NASA Astrobiology Institute. "These preliminary results are promising, but we still don't know how they would change for a planet like Venus, where slow planetary cooling is already hindering magnetic field generation," Driscoll said. "In the future, exoplanetary magnetic fields could be observable, so we expect there to be a growing interest in this field going forward."


An alien planet

So what's the deal with this Earth-sized planet? Proxima b is "one of the most interesting planets known in the solar neighborhood," Alejandro Suarez Mascareño, the lead author on this study, said in the same statement.

This strange alien planet orbits Proxima Centauri, the closest star to our sun. Because the planet orbits right in the middle of its star's habitable zone, it's possible that liquid water &mdash and potentially even life &mdash could exist there. Because of its Earth-like mass, scientists believe that, not only could liquid water exist on Proxima b, it could also be a rocky, terrestrial planet similar to Earth.

But Proxima b orbits around a star that, while close to our solar system, is also much dimmer, and much less massive than our sun. Researchers think that the exoplanet is tidally locked and in synchronous rotation with its star, meaning that one side is always facing the star and one is always facing away: a light side and a dark side.

In addition, it's unclear if, Proxima b has an atmosphere. The planet lies very close to its star, completing one orbit every 11 Earth days. So, some researchers think that radiation coming from Proxima Centauri might have stripped away Proxima b's air, making it impossible for the alien planet's surface to hold onto liquid water. As scientists continue to study this system with new and better technology, we will be able to better understand what it's really like on Proxima b.

This new study was published May 26 to the preprint server arXiv and accepted to the journal Astronomy & Astrophysics.

Editor's Note: A previous version of this article stated that researchers had pinpointed Proxima b's mass. Instead, they changed the minimum possible mass for the alien planet.

For a limited time, you can take out a digital subscription to any of our best-selling science magazines for just $2.38 per month, or 45% off the standard price for the first three months.View Deal

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected]

FYI. Proxima b has been in the news a number of times. Here is a statement indicating the host star likely produces 400x more flares on the planet than our Sun does on Earth. ESPRESSO confirms the presence of an Earth around the nearest star, "Although Proxima b is about 20 times closer to its star than the Earth is to the Sun, it receives comparable energy, so that its surface temperature could mean that water (if there is any) is in liquid form in places and might, therefore, harbour life. Having said that, although Proxima b is an ideal candidate for biomarker research, there is still a long way to go before we can suggest that life has been able to develop on its surface. In fact, the Proxima star is an active red dwarf that bombards its planet with X rays, receiving about 400 times more than the Earth. "Is there an atmosphere that protects the planet from these deadly rays?" says Christophe Lovis, a researcher in UNIGE's Astronomy Department and responsible for ESPRESSO's scientific performance and data processing. "And if this atmosphere exists, does it contain the chemical elements that promote the development of life (oxygen, for example)? How long have these favourable conditions existed? We're going to tackle all these questions, especially with the help of future instruments like the RISTRETTO spectrometer, which we're going to build specially to detect the light emitted by Proxima b, and HIRES, which will be installed on the future ELT 39 m giant telescope that the European Southern Observatory (ESO) is building in Chile."

http://exoplanet.eu/catalog/proxima_centauri_b/ shows the surface temperature is 216K, -57C. Very cold :)

Oxygen in the atmosphere is NOT needed to promote life. The Earth did not have oxygen in its atmosphere until about 1.5 billion years AFTER life got started. Life did just fine with a CO2 atmosphere for over a billion years.

In fact, biologists refer to the development of plants as "The Oxygen Catastrophe" These new organisms called "plants gave off a poison gas call oxygen that killed most (yes most) existing life. We descended from those few who could tolerate this new poison. Later animals evolved to make use of this very reactive gas but O2 came later and life had to adapt.

A planet does NOT need a protective atmosphere for life. Life can live 100 feet underground in aquafers ad use chemical energy. It wil not be as energetic are we are but still life.

My guess is that almost all life in the universe is anaerobic bacteria that lives underground.

Oxygen in the atmosphere is NOT needed to promote life. The Earth did not have oxygen in its atmosphere until about 1.5 billion years AFTER life got started. Life did just fine with a CO2 atmosphere for over a billion years.

In fact, biologists refer to the development of plants as "The Oxygen Catastrophe" These new organisms called "plants gave off a poison gas call oxygen that killed most (yes most) existing life. We descended from those few who could tolerate this new poison. Later animals evolved to make use of this very reactive gas but O2 came later and life had to adapt.

A planet does NOT need a protective atmosphere for life. Life can live 100 feet underground in aquafers ad use chemical energy. It wil not be as energetic are we are but still life.

My guess is that almost all life in the universe is anaerobic bacteria that lives underground.


Thought Experiment: Habitable Moon Around a Gas Giant

This year I hope to write a few thought experiments, and from these come up with plausible science fiction stories. I’ll start with one I’ve been thinking about for awhile: habitable moons.

First off, the orbit of the moon is absolutely critical. It cannot be too close to the gas giant, because of the gravity stresses from the gas giant will the moon to become more of an ellipsoid, the internal stresses from the planet’s gravity causing heavily volcanic regions. The moon, Io, that orbits close to Jupiter is an excellent example of this. Because of its close proximity, the effects of gravity has squashed it into an ellipsoid, and the internal stresses from gravity has caused over four hundred volcanic regions to form across its surface. This renders it inhabitable. Another important point to consider is that the orbital period for the moon to remain in a stable orbit needs to be 40 to 65 days or less, according to simulations by scientists, if the gas giant or massive planet is within 1 astronomical unit from the host star. (1 AU equals the distance from our sun to our Earth.) Also important is that the moon needs to be well outside the Roche limit the Roche limit is how close an object can be before tidal forces from the planet or star tears the object apart.

Another problem one may encounter is the albedo of the gas giant may reflect even more sunlight upon the moon. In addition to the planet’s thermal heating, which all planet’s release some heat to space over time, these two effects plus the effects from gravity provide additional thermal heating to the moon. This may cause a runaway greenhouse effect, which will also make the moon relatively inhabitable.

What else can go wrong? Space radiation. Three sources of space radiation bombard the moon, and any planetary body without an adequate magnetosphere: stellar wind, cosmic radiation, and in the case of moons, particles trapped within the host planet’s magnetosphere. If a moon (or a planet for that matter) is to be habitable, it needs protection from the massive amounts of space radiation that may barrage it. Over time, space radiation can strip away the atmosphere. This may be part of the reason why Mars has such a tiny atmosphere. It’s magnetic field is relatively weak, and thus it cannot provide enough protection against the solar radiation that barrages it every minute of every day of Mars’ existence.

Although moons may have their own intrinsic magnetosphere, it is unlikely to be strong enough to protect itself adequately from the intense barrage of the three sources of possible space radiation, particularly if the moon’s orbit puts it on the outer edge of the host planet’s magnetosphere. This means the third source — particles trapped within the host planet’s magnetosphere — may wreak havoc with the moon’s atmosphere and its ability to retain it over time. The moon needs to be close enough for the host planet’s magnetic field to actually protect it adequately, but it also needs an orbit far enough away from the planet to avoid the other negative effects I’ve already discussed. It’s very much a delicate balancing act to find that perfect habitable zone for moons around suitable gas giants. The good news is that for large gas giants, like Jupiter, the magnetosphere of the planet can extend up to fifty times the size of the planet itself. This provides some room for moons to orbit at a safe distance for habitability.

However, now we have a different issue magnetospheres are not a constant size over the course of the planet’s existence. As the pressure of the stellar wind decreases over time, which it can as the star grows older, the magnetosphere will increase in size. In the following study, Jorge Zuluaga and Rene Heller determined that for a Jupiter sized gas giant, it would take 4.3 million years for the moon to be safely embedded within the planet’s magnetosphere. It would take even longer for a Saturn sized gas giant with the time span increasing the smaller in size the host planet is.

What the above study shows us is that it is indeed theoretically possible for a moon to be adequately shielded by the host planet’s magnetosphere. It may take some time for the magnetosphere to increase enough to enshroud the moon, depending on the mass of the host planet, but it is theoretically possible.

So now that we know a habitable moon can exist. What would it be like? First off, the moon would be tidal locked, as in one side would face the host planet. The host planet would eclipse the host star on a regular basis. Because of this, days could last for half of an orbital period, where the orbital period can range from 10 to 60 days depending on the above discussed factors. In order for the moon to have seasons, it would need some tilt to its axis of rotation, but that’s wouldn’t be the only effect to its seasons: how elliptical the host planet’s orbit is around the host star can also influence the seasons. Estimates from other studies, show that a tidal locked moon may end up with a fairly moderate climate, and have a fairly stable axial tilt due to its being tidally locked. Although the dark side may be somewhat colder than the side facing the planet, if there is suitable carbon dioxide amounts in the atmosphere, then this may cause enough of a warming effect for liquid water to exist on the dark side as well.

Another important aspect to the moon would be plate tectonics, which may be caused by heating within the moon itself — left over form its formation days — and tidal effects from the host planet. Again, there’s a delicate balance between too much tidal effects, which would cause the surface to be heavily chaotic and volcanic like Jupiter’s Io, and if the moon is too far away, and the effects aren’t enough to sustain geologically activity.

The moon’s atmosphere also has to be dense enough to support life, and it needs protection from space radiation in order to retain its atmosphere. geological activity within the moon can aid in replenishing the atmosphere as well. Also, the moon itself needs to be dense enough to hold onto important atmospheric gases like nitrogen, oxygen, and water vapor, so the moon’s own gravity needs to keep the escape velocity for gases high enough to avoid losing important gases necessary for life. In the above articles, scientists have determined that if a moon has the density of Mars, it needs to have at least 7% of Earth’s mass in order to retain an atmosphere for several billion years, necessary for life to evolve on the moon.

All these factors need to be examined and assessed when a writer is world-building a habitable moon around a gas giant. It takes a little bit of research and a small amount of math, and presto! You can create an possible habitable moon around a fairly large planet.

What is even more interesting is because of the above features, some interesting societies may develop. For instance, would the biorhythms of the alien life on this moon be linked to the orbital period of the moon? Similar to how there is some link with a lunar cycle on earth to biorhythms of some animals? What significance will the planet play in the developing societies myths and religions? What significance will the dark side and the light side of the moon play in such a society? Especially since the tidally locked side will experience more eclipses with the planet, which the dark side will rarely if ever see the planet at all. This may cause some interesting myths, religions, and practices to develop amongst a sentient society. There’s a lot to examine here, and I think, in time, I may come back to this idea through short stories or novellas. In the meantime, I hope the above helps other writers develop tools to help them build more diverse and interesting worlds in their fiction. The science may seem daunting, but if you break it into smaller chunks, you can tackle each problem one at a time, and build up your world from there. It’ll not only make your world fairly accurate science-wise, but may lead you to discover interesting effects that can influence the development of your societies, providing more inspiration to further stories.