Let us suppose that instead of the 24 hour cycle we currently enjoy on Earth between day and night, we instead lived on a planet with a day/night cycle that lasted a full 9 years. The noon of the day side reaching temperatures over +50C, while the opposite end of the world reaches beyond -50C. While such a planet would find life hard to evolve, let us suspend out disbelief and accept that life finds a way.

For the sake of simplicity, we'll say that this world mirrors Earth in most ways. It orbits at 1AU from a star equivalent to Sol, has a similar mass and size, but lacks the massive oceans of Earth, allowing animals to migrate along the twilight and dawn sides.

In this a world, what forms of life would find the most success? What biological traits would such an environment favor? In short, what could I expect to see from plants and animals on this world as opposed to our own?

  • 4
    $\begingroup$ Migratory...... $\endgroup$
    – PcMan
    Feb 27, 2021 at 6:30
  • 4
    $\begingroup$ It’s not astronomically plausible. Such slow rotation would quickly get tidally locked to the star, so that one side is always light and one side is always dark. Or if there’s a large moon, the moon would speed up the rotation. There’s no way to have nine-year days that persist for evolutionary timescales. $\endgroup$
    – Mike Scott
    Feb 27, 2021 at 6:37
  • $\begingroup$ @MikeScott I disagree, there's plenty of cases of planets with orbits slower than their year that show no sign of becoming tidally locked (Such as Venus). $\endgroup$
    – TrEs-2b
    Feb 27, 2021 at 6:42
  • $\begingroup$ Maybe there's a way, the giant ocean can retard the planet rotation with the help of moon and then make it loop sided so the tidal locked planet has it's own parent star as it's North Star! Without water I don't know any other ways ;D $\endgroup$
    – user6760
    Feb 27, 2021 at 7:59
  • $\begingroup$ @MikeScott the planet could be tidally locked into its moon, which happens to have a nine-year orbit. I'm not sure about orbit size or required mass to make tidal locking plausible though. $\endgroup$
    – toolforger
    Feb 27, 2021 at 19:19

4 Answers 4


Short Answer: Life on Such a Planet Probably Closely Resembles Life on a Tidally Locked PLanet. See the Long Answer below.

And you might also possibly be interested in descriptions of Moon life in Kepler's Somnium and H. G. Wells's The First Men in the Moon adjusting to changes in temperature.

And if animals migrate to follow the sun, or flee from it, they would have to travel 0.109514 arc degrees in latitude each Earth day. The Earth's circumference around the equator is 40,075.017 kilometers or 24,901.461 miles. So migrating animals at the equator of an Earth sized planet would have to move 12.191046 kilometers or 7.5751649 miles per Earth day, which is 0.5079602 kilometers or 0.3156318 miles per Earth hour.



And you should also look for questions and answers about other planets with long days.




Long Answer in Four Parts.

Part One: A Big Problem.

The idea of a planet whose day is many times as long as its year has many problems. As far as scientists can calculate, a planet would form sprinning rather rapidly, and then tidal interactions with its star, or its moons, or with other planets orbiting its star, would gradually slow down the rotation of the planet.

And the way that tidal interactions work is that they will eventually slow down the rotation of a planet until it has a rotation period equal to the length of its orbital period around its primary, until it becomes tidally locked. And then the tidal interactions can't slow down the rotation of the planet anymore.

So if a planet orbits its star with an orbital period of one Earth year, as would be normal for a planet orbiting it star in the habitable zone and one Astronomical Unit (AU) from the star, the planet could possibly become tidally locked to the star and have a day one Earth year long. But slowing down the planet even more so that it would rotate even slower would thus be impossible.

Could the planet have a moon orbiting it with a period of nine Earth years and be tidally locked to the moon, and thus have a day nine Earth years long? Each astronomical body has a Hill Sphere within which it can hold onto a satellite without losing it to the gravity of some other body. The masses of the two bodies and distance between them determine how far the Hill sphere of the smaller body extends.

In the Earth-Sun example, the Earth (5.97×1024 kg) orbits the Sun (1.99×1030 kg) at a distance of 149.6 million km, or one astronomical unit (AU). The Hill sphere for Earth thus extends out to about 1.5 million km (0.01 AU). The Moon's orbit, at a distance of 0.384 million km from Earth, is comfortably within the gravitational sphere of influence of Earth and it is therefore not at risk of being pulled into an independent orbit around the Sun. All stable satellites of the Earth (those within the Earth's Hill sphere) must have an orbital period shorter than seven months.


So if a planet is habitable - except for temperature extremes caused by long days - and thus like Earth, it's rotation can not be slowed down by tidal interaction with its moon to produce a day longer than seven earth months long, if that planet orbits its star at the same distance that Earth orbits the Sun.

If the planet orbits its star at a very great distance, and so the gravity of the star is very week, it could have a very large Hill Sphere, so that a moon could orbit it at a distance so great the orbit took nine Earth years. And the same is true of a giant habitable moon orbiting a giant planet. The orbital period of a moon can't be as long as nine Earth years unless the planet and moon orbit very far from their star.

Here is a link to an article discussing the possible habitabilty of hypothetical exomoons orbiting giant exoplanets around other stars.


In section 2. Habitability of Exomoons, page 20, there is a discussion of the possible month (and thus day) length of a tidally locked exomoon moon.

The longest possible length of a satellite’s day compatible with Hill stability has been shown to be about P)p/9, P)p being the planet’s orbital period about the star (Kipping, 2009a)

So the year of the planet should be at least nine times as long as the month of any moon within its Hill Spere and thus with a stable orbit. So if a moon is tidally locked to its planet, and/or a planet to its moon, making the day and month equal, the year of the planet must be at least nine times as long as the day of the tidally locked object(s).

If the month and day are nine Earth years long, the year must be at least eighty one (81) Earth years long. Which is about 29,585.25 Earth days long.

In Habitable Planets for Man, Stephen H. Dole, (1964,2007), the requirements for habitable worlds are calculated. Because a planet has to have relatively stable temperatures for billions of years in order for lifeforms to make an oyxgen rich atmosphere breatheable for humans, the star of that planet has to shine relatively steadily for billions of years. Dole estimated that the minimum possible age for a habitable planet, and thus for its star, would be three billion years.

On page 68 Dole writes:

The only stars that conform with the requirement of stability for 3 billion years are main-sequence stars having a mass less than about 1.4 solar masses - spectral types F2 and smaller - although the relationship between mass and time on the main sequence is probabily not known with great accuracy and is subject to future revisions(see Figure 25).


So the most massive and luminous stars with habitable planets should be spectral type F2V stars.

The answer by user17707 to this question


Includes a table showing the distances at which planets would receive exactly as much radiation from their stars as Earth Gets from the Sun. The table shows that for a spectral type F2V star the Earth equivalent distance would be about 2.236 AU and the orbital period or year would be about 1,018.01 Earth days, which is only 0.003414 of 29,585.25 Earth days or eighty one years. If the planet can receive signifcantly less radiation that earth gets from the Sun, and still be habitable, it can orbit farther out and have a longer year, but not enough longer to make it possible to have a day nine Earth years long.

Part Two: a Possible Solution.

The main problem with your set up is that there are limits to the posssible year length of a planet, especially for a planet in the circumstellar habitable zone of of its star.

However, the thought has occurred to me that the difference betweeen the sidereal day and the solar day of the planet could be important.

The sidereal day of a planet is the time it takes the planet to turn 360 degrees on its axis, so that the distant stars return to the exact same positions in the sky of the planet.


The solar day of a planet is the time it takes for the planet to turn 360 degrees relative to the direction to its star or sun, so that its star or sun returns to the exact same position in the sky of the planet.


Because the planet will move a distance along it orbit around its star during one sidereal day, the difection to the star will change during the sidereal day, and thus the solar day must have a different length than the sidereal day.

Because the Earth rotates in the same direction that it orbits the Sun in, a solar day on Earth is a little longer than a sidereal day - if the Earth rotated in the retrograde or opposite direction, Earth's solar day would be a little less than one sidereal day.

Because the Earth's orbit around the Sun is many times longer than Earth's sidereal day, Earth's solar day is only a few minutes longer than it sidereal day.

And I think that if a planet has a sidereal day which very close in length to its orbital period and year, it should have a solar day which is many, many times as long as either its sidereal day or its year. And possibly someone might be able to calculate an orbit in some star's circumstellar habitable zone with a year and a sidereal day which combine to make the solar day tens, hundreds, or thousands of times longer than the year or the sidereal day, so that the solar day can last for about 9 Earth years.

Nine Earth years are about 3,287.25 Earth days.

If a planet had a solar day about 3,287.25 Earth days long and it was 5 times as long as its orbital period, the orbital period would be about 657.45 Earth days, a little shorter than the year of a planet in the Earth equilvalent orbit around a F5V type star.

If a planet had a solar day about 3,287.25 Earth days long and it was 10 times as long as its orbital period, the orbital period would be about 328.725 Earth days, a little shorter than the year of a planet in the Earth equivalent orbit around a G5V type star.

If a planet had a solar day about 3,287.25 Earth days long and it was 15 times as long as its orbital period, the orbital period would be about 219.15 Earth days, and it would orbit a star between a G8v and an K2V.

If a planet had a solar day about 3,287.25 Earth days long and it was 100 times as long as its orbital period, the orbital period would be about 32.8725 Earth days, a little shorter than the year of a planet in the Earth equivalent orbit around a M2V type star.

So it hould be possible to give a planet in the habitable zone of its star a solar day nine Earth years long, if that planet has a sidereal day a little shorter than its year.

Part Three: Could a Habitable Planet have a Sidereal Day Similar to it's Year?

If a planet is close enough to its star, the stars tidal force will be strong enough to rapidly slow down it's rotation and tidally lock the planet so that it's sidereal day will be exactly the length of the planet's year, and one side will always face the star and the other side will always face away. And the same thing will happen to a moon orbiting close to its planet.

And that should happen in just millions of years, instead of the billions of years necessary for the planet to become habitable for large land animals needing an oxygen rich atmosphere.

The era when the planet is almost but not yet tidally locked and has solar days nine Earth years long should last just tens of thousands of years, far too short.

But possibly an advanced civilization might want to settle on that planet and so would seed it with lifeforms capable of living on it and give it an oxygen rich atmosphere. Perhaps those aliens come from a very very rare planet which is billions of years old and has developed an oxygen rich atmosphere, but is in just the right position in its unusual solar system to be almost but not yet completely tidally locked and have a solar day nine Earth years long. Maybe those aliens might think that spending centuries terraforming a planet which have a day of the right length for only tens of thousands of eyars will be worth it.

Or maybe the aliens come from a planet which is totally tidally locked, and they think a planet with a solar day nine Earth years long is almost good enough for them, so they seed it with lifeforms from their home planet and decide to colonize in a hundred thousand years when the planet is predicted to become totally tidally locked to its star.

Part Four: Could a Tidally Locked Planet Be Habitable?

If a tidally locked planet can be habitable, aliens might evolve on it and might terraform a planet with a solar day nine Earth years old, and seed it with life, planning to colonize it in a hundred thousand years when it becomes totally tidally locked.

But can a tidally locked planet be habitable? Astronomers and astrobiologists often assumed tidally locked planets would be uninhabitable.

Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.08 to 0.45 solar masses) means that their nuclear reactions proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri[80] (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause tidal locking. One side of the planet would eternally face the star, while the other would always face away from it. The only ways in which potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side, or if there was a gas giant in the habitable zone, with a habitable moon, which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the planet. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis.

This pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO2 and H2O) need only be 100 millibars (0.10 atm), for the star's heat to be effectively carried to the night side.[81] This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of Greenwich Community College, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[82]


So at the present times scientists think it is at least possibly possible for tidally locked planets to have life, possibly even land animals that breath oxygen.

  • $\begingroup$ While I love answers like this, I am a bit confused. You say that a large problem is that tidal interactions slow down the rotation of a planet until it has a rotation period equal to the length of its orbital period around its primary, becoming tidally locked. But then you say it can't be slowed down anymore. The issue is that, as far as I know, this isn't true. Venus for example has a day/night cycle that lasts longer than its year. I might just be misunderstanding though. $\endgroup$
    – TrEs-2b
    Mar 1, 2021 at 4:07
  • $\begingroup$ @TrEs-2b There is a difference, explained in part 2, between the sidereal day of a planet and its solar day. The sidereal should not be longer than the planet's year. But part 2 explains how the solar day could be longer than the year. In the case of Venus, it rotates in a tretrograde direction, and so has a sidereal day longer than its year, but it has a solar day shorter than its year, only 116.75 Earth days long, which you don't want. Mercury has a prograde rotation, a sidereal day 58.646 Earth days, a year of 87.9691 Earth days, and a solar day 176 Earth days long. Continued. $\endgroup$ Mar 1, 2021 at 16:50
  • $\begingroup$ @TrEs-2b So Mercury has a sidereal day shorter than its year, but a solar day which is two Mercury years long. That is much closer to what you want. A planet with retrograde rotation can have a sidereal day longer than its year, but that results in a solar day shorter than its year. As far as I know a planet with prograde rotation can have a sideral day NO longer than its year, but can have a solar day longer than its year, which seems to the only way for a habitable planet to have a day nine Earth years long. Continued. $\endgroup$ Mar 1, 2021 at 16:55
  • $\begingroup$ @TrEs-2b You might want to contact Sean Raymond at his PlanetPlanet blog. planetplanet.net/about He says he is available for help on wortd building projects. $\endgroup$ Mar 1, 2021 at 16:59
  • $\begingroup$ I see, I still don't really understand what the difference is between a solar and Sidereal day? $\endgroup$
    – TrEs-2b
    Mar 2, 2021 at 8:24

Very long days and nights lead, as you state, to extreme temperature differences.

I see two alternatives here for life:

  1. Continuous migration, to stay on the side of the planet which is either between dawn and noon or noon and sunset
  2. Adaptation of their biological cycles so that they thrive from dawn to about 1/3 of the light period, then estivate for another 1/3 of the light period, thrive again for the remaining 1/3 and then hibernate for the dark half of the day. If we stick to the 24 hours day, they would thrive from 6:00 to 10:00, estivate from 10:00 to 14:00, thrive from 14:00 to 18:00 then hibernate from 18:00 to 6:00.

Option 1 leads to a less efficient usage of the resources, because for half of the light period they are left unused. Moreover, should there be an ocean across the planet, more or less like the Atlantic or Pacific oceans on Earth, the migration would be stopped.

Option 2 seems therefore the most logic ending point for life on this planet, with some fringe lifeforms staying active around the local noon.

On Earth we have species with a 17 years life cycle, so it's not implausible.

  • $\begingroup$ Probably creatures go through the polar regions to inhabit the other time, too $\endgroup$
    – Mary
    Feb 27, 2021 at 15:00
  • $\begingroup$ Third alternative: Sea animals could easily migrate. $\endgroup$
    – toolforger
    Feb 27, 2021 at 19:19

Life evolves near the poles

The effect of the sun is proportional to the angle that the sun intersects the surface, so near the poles, the effect of the sun is minimal, day and night are roughly the same temperature.

Carbon based life as we know can live in this temperature. Look at anything living in arctic latitudes here.

They would evolve in the cold and fill up one pole down to a few degrees of latitude, eventually filling both poles.

As they'd advance in intelligence, they'd start building shelter and cooling systems to avoid the fatal heat.

By the time they're a space faring civilisation, there are mirrors covering much of the good locations for cities, allowing the inhabitants to survive the brutal summer months.

Eventually they'll get their own "global warming" from imperfect efficiency in their air conditioning.


Thaw-zone scavengers.

These could be large, perhaps quadrupedal, with a nice thick layer of fat. They'd range the newly thawing frontier, not needing to move quickly but perhaps with front feet adapted for digging snow/shoveling slush. These might just be the longest-lived creatures. They would also require sensitive noses (or equivalent) to detect the smell of thawing carcasses from those fallen beasts that died when winter hit their grounds.

Secondary scavengers.

Smaller, more mobile, perhaps even with flight. Any carcasses uncovered would have the last of their flesh picked-off. They would then need to move on - perhaps attaining altitude - to track their next meal, or the larger creatures that have found it. Their mouth-parts/beaks might be adapted to delicate work, perhaps sucking bone-marrow.

Behind them. There'd be the hardy equivalent of birds of prey, seeking creatures too distracted by their own feast to notice their approach.

There's no reason to suppose that any of these creatures would be parasite-free.

Insects would follow, to finish up the scraps and breed on the remains.

Then there'd be those adapted to seeking out the newly thawing amphibia/insects near the river-beds. Perhaps fox-like.

  • The amphibia/insects themselves might (as L.Dutch sais) be adapted to thriving for a couple of years, then laying eggs which dry out - waiting for the rains to return in the latter years before the next big freeze.

  • There'd be others whose strategy was to bury themselves deep through the driest times, and would lay eggs which would endure the cold, then hatch as the thaw descends ready to eat the fresh shoots - or each other.

To sum-up. Wherever you get life (or death) there'll be creatures that can monopolize the potential of the flesh.

  • $\begingroup$ Scavengers need something to scavenge. That has to come from non-scavenging organisms, which have to outnumber them $\endgroup$
    – Mary
    Feb 27, 2021 at 15:01
  • $\begingroup$ @Mary Yes, quite. I only covered one part of the environment, I imagine there'd be ungulates or equivalent munching away in the spring/autumn zones. $\endgroup$ Feb 27, 2021 at 15:13

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