Okay, so I have an earthlike planet and another, bigger planet which orbits the sun really slowly and once every few millennium it crosses in front of the earthlike planet, blocking out the sun. would life on the earthlike planet be wiped out when the other planet eclipses the sun?

Heres a simple diagram of the proposed orbit of both planets: Obit

Thanks for all the answers. You've me helped immensely.

  • $\begingroup$ Welcome to Worldbuilding. Please take a minute or two to tour the help center section. As it stands, your question will soon be deleted, because it is too broad and quite unclear, and possibly off-topic, too. Also, it might be a good idea to elaborate a bit. $\endgroup$
    – Burki
    Oct 21, 2015 at 10:01
  • $\begingroup$ Burki is right about dropping the hammer on your question. You should either be specific about what aspect of effects you are concerned with (social, biological etc) or you should provide us with a setting of defined planets and their orbits so that details can be figured out. $\endgroup$ Oct 21, 2015 at 10:10
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    $\begingroup$ You also may want to consider a reality-check tag. An orbital period consistent with "every few millennium" may not be close enough to support life. Your smaller planet has to be outside your larger planet. But not that far outside. Would it be brought into orbit around the larger planet? Are they in different ecliptics? Making the base scenario realistic would be challenging. $\endgroup$
    – Brythan
    Oct 21, 2015 at 11:40
  • $\begingroup$ As a consequence of the laws of orbital motion, if planet A is closer to the sun than planet B, which is a requirement for planet A to be able to eclipse the sun as viewed from planet B at all (partially or completely), then planet A must move faster than planet B. Orbital lineups can still certainly happen, but as described by Matt, the eclipsing planet would have to be truly gargantuan to eclipse the sun for any significant length of time when viewed from the outer planet. $\endgroup$
    – user
    Oct 21, 2015 at 11:43
  • $\begingroup$ @Brythan Indeed. Even Pluto has an orbital period of "only" about 248 Earth years. I know OP is talking more about orbital lineups than about the orbital period of the inner planet, but for some idea of how often these might happen, we can look to for example the Grand Tour utilized by the Voyager 1 and Voyager 2 probes. $\endgroup$
    – user
    Oct 21, 2015 at 11:45

3 Answers 3


If planet B approached your Earth like planet A to the point that B's apparent diameter was the same as, or larger than the apparent diameter of the primary star, it would be close enough to cause gravitational interaction with A.

Occluding the star for three days would cause sharp global temperature drops, which would in turn cause global weather, like storms and hurricanes. Lots of snow would fall, changing the albedo of some areas and possibly causing temperature effects that would last for years. It might even trigger a mini ice age each time.

Large bodies of water won't change temperature much. They hold an amazing amount of heat. But surface waters will freeze. This means that ponds and small lakes will freeze over, large lakes and even sea shores will freeze to some extent.

All in all, I don't think it would destroy life on A. Earth has certainly endured worse. If there's a technological civilization, they're likely to be prepared for it.

If B is fairly small, the gravity effect would probably be that B would either become a moon of A or just smack into it. If it was moving fast enough, the net result might be that both A and B's orbit would change. But if it's moving that fast, it's not likely to hang around for days or even hours.

If B is fairly large then either it will suck up A or A will become a moon of B. Another possibility would be that A's orbit will change, making life there either more difficult or impossible. I suppose it's possible that it might shift A to an even more temperate orbit.

Another possibility could be that A is already a moon of B with an orbit so highly inclined that it rarely passes through B's shadow. Depending on its orbit, it might dip into the penumbra occasionally, and once in a millennium or so, pass directly behind B. Having this period last hours or days sounds pretty reasonable. It would mean that A's orbit was wide enough to keep it from being heavily affected by B's tidal forces and magnetosphere.

Have you ever seen the movie "Pitch Black"? The story takes place on a moon of a gas giant. It has constant daylight moon wide for 22 years then an extended period of darkness. This scenario required more than one local star, though.

  • $\begingroup$ In "Pitch Black", original colonists, who have disappeared, constructed an orrery to predict when the dark period would occur. An orrery is an analog computer. It was interesting to see in the movie, but awfully anachronistic. $\endgroup$ Oct 21, 2015 at 14:00
  • $\begingroup$ I don't buy that 3 days of darkness is going to cause an ice age. All you're missing is 3 cycles of daylight, the normal nighttime is exactly the same. Consider that for a location at 45 degrees of latitude, the winter sun is only about 1/3 the power of the summer sun. Dropping 3 days of winter sun in the middle of summer will make it cold, but not climate-system-destroyingly cold. Putting those days in the middle of winter won't change much at all. On another note, B must be larger than A, or you won't get a total eclipse on A. $\endgroup$ Oct 21, 2015 at 15:16
  • $\begingroup$ @Matt I mentioned a possible mini ice age because if a huge portion of the Earth were to suddenly turn white, it would begin to reflect a lot of sunlight back into space. I think you're right about B being larger, because the umbral shadow has to be as wide as the planet. If it was the same size as the planet, it would have to pass within the Roche limit. The larger it is, the farther away it can be. $\endgroup$ Oct 21, 2015 at 15:52

It would get dark.

But really, it wouldn't be all that different from your typical solar eclipse. Keep in mind that to cast the whole planet into darkness, your shadow-casting planet has to be either REALLY big or REALLY close. It must be larger than the earthlike planet, and have an angular diameter greater than the star. The orbital mechanics of keeping everything in syzygy for several days would be rather tricky as well.

  • $\begingroup$ +1, just for using a word I can use with the letters I always get in Scrabble. $\endgroup$
    – Frostfyre
    Oct 21, 2015 at 12:37
  • $\begingroup$ Frederick Pohl wrote a novel called 'Syzygy'. $\endgroup$ Oct 21, 2015 at 14:57
  • $\begingroup$ There'd be gravitational effects to consider too. Tidal forces could cause earthquakes and volcanic eruptions. $\endgroup$ Oct 21, 2015 at 20:58

Not necessarily. It really depends on the interval of the eclipse, the area of umbra and penumbra and the extent of water bodies on the home planet (the target of eclipse).

It happens that water bodies (oceans, seas) hold a large amount of heat energy which they would slowly release in case the primary heat source (sun) is blocked out. That's what happens during night when sea breezes blow.

The next thing is the extent of eclipse. If you have an eclipse over half of the planet's bright side that lasts for days, there will probably be some strong gales and storms due to the temperature difference between the bright and dark spots on the daylight side.

Once the seas have finished their stored energy reserve and fallen below freezing point, you are in for an ice age that would at least last for a few months. I can't say if it will wipe out life entirely from the planet, but it won't be a pleasant place to live on!


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