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I know water is required for life on a planet so I got to thinking, is it possible for a world to be a functional planet (plants animals etc.) without polar ice caps and if so, how would it work without the equator being overwhelmingly hot? Also, would this affect the planet's water bodies if they were similar to Earth's oceans?

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    $\begingroup$ Interestingly, the Earth has, for long periods of its history, been ice cap free. Source: ucmp.berkeley.edu/mesozoic/triassic/triassic.php $\endgroup$ – ckersch Oct 6 '15 at 20:49
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    $\begingroup$ If planet rotates around it axis, it has poles (where such axis crosses surface). $\endgroup$ – Peter M. - stands for Monica Oct 6 '15 at 23:06
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    $\begingroup$ You mean like Mercury, Venus, Jupiter, Saturn, Uranus and Neptune? $\endgroup$ – Dale M Oct 7 '15 at 1:38
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    $\begingroup$ @JoshuaTaylor If a planet is rotating, it has poles. Then there are magnetic poles. If it was still and had no magnetic field then there would be no poles... and no life. $\endgroup$ – Keltari Oct 7 '15 at 4:18
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    $\begingroup$ It's not certain that no planetary magnetic field makes a planet incapable of evolving life. It means atmosphere loss is faster, but higher gravity would counteract that, and we don't know that life always takes billions of years to evolve. It means higher radiation on the planet's land surface, but life evolved in the oceans, and some life on Earth is surprisingly tolerant of radiation (cyanobacteria, cockroaches). $\endgroup$ – nigel222 Oct 7 '15 at 18:07
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As @Green says, a slightly warmer planet than earth would prevent ice caps from forming. Although this largely depends on your planet's composition, there are several ways that you can warm your planet to prevent ice caps from forming.

  1. Move it slightly closer to its star. This would warm it up, but it might have unexpected consequences, like heating some parts of the planet too much.
  2. Give it a thicker atmosphere. This will insulate the planet more, warming it up and protecting the poles from getting too cold.
  3. Have some sort of wind / ocean current that cycles between the equator and the poles. This will help to even out the temperatures.

These all work in conjunction, so you can mix and match freely.

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  • $\begingroup$ Water helps keeps constant temperature. Avoid big continents and put a bit more oceans and the difference in temperature between low and high latitudes ill be smaller $\endgroup$ – jean Oct 6 '15 at 20:14
  • $\begingroup$ Would a thicker atmosphere have any effects on the weather or life on the planet other than it being warmer than a thinner one? $\endgroup$ – Sunspear25 Oct 7 '15 at 2:36
  • $\begingroup$ @Sunspear25 The only thing I can think of is that a thicker atmosphere would mean that it is denser than ours - i.e. The pressure it exerts would be greater than 1 standard earth atmosphere of pressure. As for what effect this would have on life, I couldn't say. $\endgroup$ – starbeamrainbowlabs Oct 7 '15 at 6:04
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    $\begingroup$ @Sunspear25 It would depend on what makes the atmosphere thicker. But I would think that the bigger effect on the weather would be the generally warmer temperatures. $\endgroup$ – Jake Oct 7 '15 at 12:38
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A slightly warmer planet than earth won't have ice caps. Part of our worries about global warming come from the fact that the polar ice caps might melt and flood the coastal regions of the world.

Depending on the atmospheric composition, albedo, solar intensity, water content of the planet along with the shapes of the continents and oceans, a planet may not have any permanent polar ice caps.

The lack of polar ice caps won't prohibit the evolution of complex life forms at lower latitudes. The oceans currents may be interesting but you haven't specified any continents and it's a computational complex thing to figure out too.

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It is definitely possible. Like for example the earth at the time of the dinosaurs. Quote (from relatively far down on the page):

As the world entered the Cretaceous Period, Antarctica was very much situated at or near the South Pole. But at least during a major part of this period, there were no polar ice caps anywhere on Earth. And forests penetrated all the way to the South Pole.

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    $\begingroup$ always +1 for "this can happen, and here's what it's like, because it has happened" $\endgroup$ – nitsua60 Oct 6 '15 at 21:07
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It's not only possible, but it's the normal state here on Earth! Throughout the period for which good fossil evidence exists, there has never been ice at sea level on Earth except when there was a continent at or near a pole. Even with such a continent, ice is not guaranteed.

If there's no ice, the planet is indeed warmer than today. The tropics become too warm for large mammals (but great places for giant crocodiles). The poles become Mediterranean. The planet's temperature is stabilised by a negative feedback mechanism. Increasing heat increases water vapour in the atmosphere, which increases cloud cover. Clouds reflect sunlight, reducing warming.

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One might also consider the possibility of a horizontally rotating planet that exists within its star's habitable zone. I'm not sure if life on one with an axis tilted that far would even be possible, but if so, then it is likely that the equator of the planet is not a solid ring of ice.

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    $\begingroup$ While not in the habitable zone and not being an inhabitable "rocky" kind of a planet, Uranus at least shows an axis nearly tilted to the ecliptic. It however also means that one polar regions stays in continuous sunlight for half a year and in darkness for the other half (year wrt to the planets orbit, not earth year). Oh well, scrolling down further, Peter Masiars answer addresses exactly this. $\endgroup$ – Ghanima Oct 6 '15 at 23:26
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Axial tilt is crucial for climate (and polar caps). If Earth's tilt was 90 degrees (like Uranus), equator would be covered by ice, and poles would be (in right season) warm.

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