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According to this explanation at the Georgia State University website,

Axis Tilt is Critical for Life

. . .

The Earth's spin axis is tilted 23.5° with respect to the ecliptic, giving moderate seasons and preventing temperature extremes anywhere on the planet. . . . Not only is the Earth's angle of tilt close to the optimum value, it also seems to have been essentially constant. That is crucial for the development of advanced life.

Is this claim correct?

  1. Why does it matter if there are some areas of a planet with extreme temperatures, as long as there are other spots on the planet that are not extreme?
  2. Why are "moderate" seasons required for life to exist?
  3. If humans can live at the equator on Earth where there is the least amount of tilt, why would an exoplanet with less tilt or no tilt be necessarily non-inhabitable?
  4. Even if humans could not live on a planet without axial tilt, are there no other forms of known "advanced" life that can? We know that extremophiles exist, such as tardigrades' ability to survive in the vacuum of space. What is the most "advanced life" that could live on a planet without axial tilt?
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I agree with David Hammen. Hyperphysics is mostly a very good site but they dropped the ball on that page IMHO. Hope you don't mind a partially speculative answer, but here goes:

Why does it matter if there are some areas of a planet with extreme temperatures, as long as there are other spots on the planet that are not extreme?

It shouldn't matter if part of the planet is uninhabitable. There are deserts on Earth which are all but uninhabitable but that doesn't effect life elsewhere. Prior to 5.3 million years ago the Mediterranean sea evaporated and that entire region could have had a salty basin and been hugely hot, but I've not read of life on Earth having any problem with that. Source

Why are "moderate" seasons required for life to exist?

There's a boatload we don't know about the evolution of life on other planets but seems perhaps universally true that life adapts, so I find it difficult to believe that moderate seasons are necessary. Very extreme changes could be difficult, but change can force adaptation.

If humans can live at the equator on Earth where there is the least amount of tilt, why would an exoplanet with less tilt or no tilt be necessarily non-inhabitable?

The tilt is planet wide but the lowest variation happens near the equator, but animals that thrive near the poles adapt by hibernation or migration and smaller stuff can be frozen and then come back to life, so, I don't agree with the article on this point.

Even if humans could not live on a planet without axial tilt, are there no other forms of known "advanced" life that can? We know that extremophiles exist, such as tardigrades' ability to survive in the vacuum of space. What is the most "advanced life" that could live on a planet without axial tilt?

One of the interesting historical facts of life on Earth, at least to me, is how long it took what we might consider advanced life to develop. One celled life in various forms was around for over 3 billion years but the first fossils are about 650 million years old. It took life a very long time on earth to get from too small to see to large enough to leave a footprint . . . but, I digress.

I agree 100%, one celled life or Tardegrades could live on a planet with no tilt or 90 degree tilt. Easy. Ocean life in general should be fine cause oceans are more adaptive. Evaporation keeps ocean surfaces colder than land gets during peak heat and while a completely frozen over ocean isn't great for life, cold oceans hold more oxygen and CO2 which can be good for life. Oceans also circulate as an effective means of temperature moderation and fish don't really care how windy it is or how much or little it rains. The tilt question, I think, is really just about life on land.

Land life could be more vulnerable to high wind, extreme temperature shifts, droughts or floods, which could be driven by greater axial tilt, but I find it hard to believe that Axial Tilt is the be-all and end all. Day length and year length are key factors too.

One point I agree with the article on, is that a close to 90 degree tilt might not be ideal with one part of the planet always facing the sun and the other part never facing it but outside of extreme tilts, I don't see why it would be a big deal.

A thick cloud cover, for example, reduces seasonal changes. There's a number of factors.

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This is a case where the Southern US phrase might could is appropriate. Intelligent life1 might could arise only on terrestrial planets with just the right axial tilt. Then again, intelligent life might could arise on all kinds of other planets as well. Extrapolating from a sample size of one is always a risky endeavor.

The hyperphysics page referenced in the question implicitly poses the rare Earth hypothesis proposed by Peter Ward and Donald Brownlee as if it is fact. It is not fact. It's a hypothesis that attempts to answer the Fermi paradox. There's a lot to be said both for and against the rare Earth hypothesis. Stating it as if it is fact is bad form, particular for http://hyperphysics.phy-astr.gsu.edu, which overall is a very good site.

The hyperphysics page goes even further than do Ward and Brownlee, forgetting that Ward and Brownlee addressed the issue of intelligent life. Could life arise on a Venus-like or Mars-like planet that orbits at just the right distance from it's star? That's a good question, the correct answer to which is "we don't know (yet)." Could intelligent life arise on such a planet? Once again, the correct answer is "we don't know (yet)."

Extrapolating from a sample size of one (the solar system, which contains one planet that harbors intelligent life), life in general and intelligent life in particular might should be quite common. Extrapolating from a sample size of one (the solar system, which contains a very large number of objects that don't harbor life), life in general and intelligent life in particular might should be quite rare. Which is the case is a good question, the answer to which as of now can only be rightfully stated as "we don't know (yet)."


1 In the context of the Fermi paradox, "intelligent life" is defined as life that is capable of communicating with life on planets orbiting other stars.

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    $\begingroup$ From the rare Earth hypothesis Wikipedia link: "Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. . . . A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. . . . a large satellite also stabilizes the planet's tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible." What is the hypothesis' basis for claiming photosynthesis & climate variation are required for any type of "intelligent life"? If this were not true, would tilt be a non-issue? $\endgroup$ – seijitsu Oct 25 '15 at 0:20
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    $\begingroup$ @seijitsu -- That is Ward and Brownlee's conjecture. Others strongly disagree, conjecturing that life is pervasive. Some go so far as to conjecture life on a tidally locked planet orbiting a red dwarf. Which group is right? For now, the right answer is "we don't know (yet)." $\endgroup$ – David Hammen Oct 25 '15 at 4:48

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