In the world I am designing, the days last 9 years. The main problem I had was how life could have evolved on a world like this. An easy solution is to have the world start having 9 year days after life already evolved. I plan on doing this by having a rogue planet pass by, slowing the day length. Is this idea realistic? If not, what else could cause the planet's day/night cycle to increase?

  • $\begingroup$ Are you planning to have some sort of angular momentum transfer between the rogue planet and the planet? $\endgroup$
    – HDE 226868
    Commented Jun 18, 2016 at 17:27
  • $\begingroup$ @HDE226868 In a way yes $\endgroup$
    – TrEs-2b
    Commented Jun 18, 2016 at 17:34
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    $\begingroup$ Single passing body: Not realistic in the slightest. $\endgroup$
    – JDługosz
    Commented Jun 18, 2016 at 17:59
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    $\begingroup$ But +1 for asking, unlike TV and movie scrip writers... $\endgroup$
    – JDługosz
    Commented Jun 18, 2016 at 18:01
  • $\begingroup$ Our moon is constantly slowing down Earth's rotation since billions of years. Give it some more aeons and Earth will be revolving much slower than today. Maybe the planet's rotation could be gravitationally bound to its sun the same way. $\endgroup$
    – JimmyB
    Commented Jun 19, 2016 at 10:33

2 Answers 2


Any rogue planet passing so close to a regular planet that it has a major impact on its rotation, will have far severer effects on life on that planet. We are talking about 100 meter+ height tsunamis (yes, I wrote 100 meters, not 100 feet), storms with wind speeds exceeding 1000 m/h, severe tectonic activity (if the planet has an active mantle) and long term implications on its shape (faster revolving planets have more flattened poles), seasons and geography. I don't think any multicellular life form (as we know them on Earth) could survive such cataclysmic changes.

I suggest you use a different method. Planets gradually get tidally locked to their parent stars (the lesser the distance, the faster the process) which elongates their day-night cycles. Gravitational interaction with moon(s) can also have major impact on a planet's day-night cycle. For example, when Earth formed, it was spinning extremely fast and a day only lasted 6.5 hours. Later, after the origination of moon (regardless of how it came to become Earth's moon), the gravitational interaction between moon and Earth has been slowing down Earth's rotation speed.

Meteorite impacts also have their impact on a planet's rotation. Instead of one, mega-horrible asteroid, I suggest a series of smaller, more benign asteroids hitting the planet at opposite direction to its direction of rotation. This will help slow down its rotation speed slowly.

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    $\begingroup$ Maybe the reason your hypothetical meteor shower hits only one side of a rotating planet is because the remainder of the shower hit the back side of the planet's moon, which acted like a shield. $\endgroup$ Commented Jun 19, 2016 at 11:11
  • $\begingroup$ Quite likely indeed. $\endgroup$ Commented Jun 19, 2016 at 17:54
  • $\begingroup$ If you want some figures on the amount of meteorite impacts you'd need to do so, this article by Randal Munroe may help. Notice that his goal in his article is to make days shorter, not longer. You just have to hiht the planet with meteorites the other way around to reverse the effect. Notice this is by no means a gentle thing to do in short time. Just slowing the planet enough to add a fraction of a milisecond by this method would end life on Earth, no matter the size of the meteorites. $\endgroup$ Commented Jun 20, 2016 at 0:30
  • $\begingroup$ Haha yes, I have read that article @Renan. Uranus hints of some horrible planetary/mega meteorite impact in the past ... $\endgroup$ Commented Jun 20, 2016 at 4:58

I believe the easy solution is actually to have life apearing on your planet when it already has a nine-years day.

If you are thinking about how an ecossystem would thrive for so long without "plants", remember that photosynthesis is not a requirement for life to exist. Many biologists believe that life on Earth started up in the depths of the ocean, where no light reaches. In such places, the source of energy for the ecosystem is chemosynthesis rather than photosynthesis.

So you could throw in a mix - at the day-side of the planet, life thrives with plants at the base of the food chain. At the dark side, it's chemosynthethic beings - not necessarily microscopic life, you could have chemosynthetic fungi in your world.

When it starts to get dark, plants produce seeds or spores that will stay dormant for nine years before they start growing. And when daylight begins to appears, the chemosynthetic lifeforms produce spores that will stay latent for nine years before they restart their part of the cycle.

This would create an interesting effect... With plants and chemosynthetizers appearing as a "wave" in one side of the planet and dying at the same rate on the other side, at the points where it's dawn or dusk.

Or, you could completely drop photosynthesis and go chemosynthetic all the way.

See also this question about a world where it is perpetually dark. You could get some ideas for survival on the dark side of your planet.

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    $\begingroup$ Locusts (and, I believe, a few other insects) have a multi-year incubation period for their eggs. Bears hibernate for a few months every winter. Lichen grows in caves where daylight never reaches. With some minor variations on these traits, and perhaps some believable additions, it should be possible to produce, if not a rich ecosystem, at least a stable one that could believably exist on a 9-year-day planet. $\endgroup$ Commented Jun 18, 2016 at 22:06
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    $\begingroup$ I'm alive and I hardly ever go outside. $\endgroup$ Commented Jun 19, 2016 at 1:17
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    $\begingroup$ Plants could even develop seeds which are dormant in the darkness, chemosythetic lifeforms spores that are dormant in light (I'm thinking of some chemical triggers). Symbiosis between plants and chemosynthetic lifeforms would be interesting, too. $\endgroup$ Commented Jun 19, 2016 at 11:58

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