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Would it be possible for life to develop on a planet with a highly elliptical orbit? (Similar to a comet, but possibly less extreme.) If so, what conditions would be necessary and what would the life look like?

Of course, a highly eccentric orbit would require the planet to go well outside of its habitable zone for extended periods of time, so any life forms would have to be able to survive a "deep freeze" or super hibernative state (bees, for example, can be frozen and will "wake up" again when thawed).

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    $\begingroup$ You might check out antifreeze proteins. en.wikipedia.org/wiki/Antifreeze_protein and water bears en.wikipedia.org/wiki/Tardigrade for inspiration. $\endgroup$
    – David Elm
    Jan 5 '20 at 6:00
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    $\begingroup$ Read "A Deepness in the Sky" by Vernor Vinge. $\endgroup$
    – Dragongeek
    Jan 5 '20 at 15:33
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    $\begingroup$ The Left Hand of Darkness by Ursula K. Le Guin features an icy world with nearly 0º axial tilt and high orbital eccentricity. This means the world as a whole experiences a deeper winter altogether when the planet is at its apex. $\endgroup$ Jan 5 '20 at 21:55
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    $\begingroup$ @Renan: The Left Hand of Darkness is set on the planet Gethen, which is in the grip of a permanent ice age. Perhaps a better match is Le Guin's Planet of Exile, set on the planet Werel, which has an orbital period of 60 years and correspondingly long winters. But in neither case is the cold so extreme as to be uninhabitable. $\endgroup$
    – TonyK
    Jan 6 '20 at 15:12
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    $\begingroup$ @JimCullen I'm willing to discuss pretty much anything, just want to get an idea for what is feasible. The basic premise is that most, if not all, life freezes and goes dormant for a time, then everything thaws and picks up where it left off, more or less, as the planet comes back around for its close approach. Whether it lasts 1000 (Earth) years, or 100 or 10 or even 1, the premise is the same. The exact length of a freeze affects the narrative more than anything. $\endgroup$
    – WillRoss1
    Jan 7 '20 at 0:00

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On such a planet life would probably evolve in different ways than on our planet, and complex life could be different from what we consider as "plants" and "animals". There would be a huge evolutionary pressure for a much more efficient hibernation than we have in real life, or if the conditions are even more extreme, then life could thrive while in the habitable zone, after that everything dies off and leaves spores behind, which then start everything anew in the next cycle.

Certain spores even here on Earth can survive for years even in vacuum. If life on a whole planet was forced by the circumstances, they might even get better at it.

For multicellular life, the star can be huge and hot, so the planet orbits farther away, therefore much slower. A full revolution could last dozens (or hundreds) of Earth-years, out of which several Earth-years are spent in the habitable zone. Enough time for the spores to hatch, thrive, then when the cold season approaches, they will die off leaving their spores for the next generation behind.

Eucalyptus trees integrated forest fires into their natural lifecycle.

Life will find a way.

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    $\begingroup$ This is, IMO, the correct answer. The trouble is you have 1 cycle to come up with a minimal viable product, so to speak, so it can improve itself next cycle instead of starting from scratch. $\endgroup$
    – user39548
    Jan 6 '20 at 17:46
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    $\begingroup$ @Hosch250 Maybe the planet's original life lived very deep in the ocean, where the temperature cycles far above had little effect. There's one theory that says that's how life on Earth got started. $\endgroup$ Jan 6 '20 at 18:09
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    $\begingroup$ @DisplayName : indeed. Simple deep ocean-dwelling life (from geothermal vents, cyanobacteria, etc.) gets washed upon the shore. It accumulates over millions of years. Then in the warm period, some lifeforms crawl out to feed on the plentiful dead organic matter. Then they die when the cold period comes. Those species which can stay longer on the nutrient-rich land when the cooling comes (or can leave behind spores which survive the frozen period) will have an evolutionary advantage. $\endgroup$
    – vsz
    Jan 6 '20 at 22:47
  • $\begingroup$ I doubt if hypothetical life on a planet orbiting a huge and hot star would have enough time to evolve. The heavier the star, the shorter its lifetime is, see e.g. here. $\endgroup$
    – stop-cran
    Jan 7 '20 at 9:37
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    $\begingroup$ An evolutionary stage can be a land-dwelling organism which still lays its eggs in the water. And these eggs will not germinate in the cold periods. The physical mechanism here is that water is most dense at 4 degrees (well above freezing); the eggs sink below the freezing surface water. They need light to germinate, so while the seas are frozen over they remain deeply submerged. $\endgroup$
    – MSalters
    Jan 7 '20 at 11:03
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I think that because of how orbital mechanis work, a planet with an aphelion inside the habitable zone and a perihelion too close to the sun would be better for life. A planet spends a lot more time near its aphelion than near its perihelion. Thus this planet would have moderate weather most of time and scorching weather during a short "season", instead of a short moderate season and a long freezing period.

The scorching season will will probably lead to extreme cloud formation due to the higher temperatures. This means that albedo of the planet would rise, moderating the temperature increase. However, water vapor is also a potent greenhouse gas, in fact the most important one on Earth, so this will probably increase the planets temperature as well. Ultimately this will push the onset of the for every habitable planet inevitable runaway greenhouse effect to an earlier date. Still, don't expect a season of a giant, burning sun, but rather a global monsoon season.

EDIT1: How will this impact life and evolution in general? I'm not even sure how this would impact the weather exactly. At least not beyond the fact that the evaporating water would create a monsoon season. Stronger erosion and thus more nutriants, minerals and salt in the oceans and that local life will somehow use this hot season are the only other general conditions I'd be willing to predict. Weather is a chaotic system, which is extremely hard to predict precisely. Unless I use an example with an number of set parameters, everything is possible. Relevant parameters would be the eccentricity, semi major axis, stellar spectral class, how much surface water there is, how much the temperature will vary, axial tilt (Regular seasons will still mess with the climate and Milankovitch Cycles might be truely nasty on such a planet.), daylength, tidal locking (which spin/orbit resonance, as 1:1 is rather unlikely for such an eccentric planet) and a number of other factors a more educated person than me would come up with.

Life would either use the monsoon as the watering and growing season, if it is moderate, or would have to develope seeds and roots, which can survive what is basically a steam cooker. A desert world like Mars would experice this very differently than a water world and a continental world like Earth would experience something different again. The water would moderate or worsen the temperature swings, depending on the dominant feedback loop. This is the cloud formation increases albedo vs the greenhouse effect story again. If the world in question orbits a red dwarf a lot depends on wether is is a 3:2, 2:1, or 5:2 spin–orbit resonance. It is a common misconception that a tidally locked world always faces the parent body with the same side, aka. is in a 1:1 spin-orbit resonance. If the eccentricity is appreciatably bigger than 0, which it would be in this case, one would have increasingly weirder resonances. Additionally in these cases the monsoon season would be more of a weekly phenomena instead of a yearly one. This would influence the answer to you question a lot.

I believe you see what I'm getting at. One could spend an entire scientific carrier exploring the climate of eccentric exoplanets and its effects on evolution without even scratching the surface of a fraction of the possibilies. If you want, you can give me some parameters and I'd hazard a guess on what kind of critters might populate such a world.

For your own exploration of such a biosphere I'd recommend this video series on crafting fictional alien biosphere.

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  • $\begingroup$ Great point! How do you think the long moderate season and short scorching/monsoon season variant would impact life forms and evolution in general? $\endgroup$
    – WillRoss1
    Jan 6 '20 at 16:04
  • $\begingroup$ @WillRoss1 Answer in EDIT1. $\endgroup$ Jan 6 '20 at 21:58
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Some types of life would be possible.

Considered a highly elliptical orbit where the closest approach is inside the habitable zone, this planet will then spend nearly all of the time outside the habitable zone and warm up only for brief periods.

Now add geothermal springs that maintain life-friendly temperatures year round. Plant life that becomes dormant for the extended periods of little sunlight might be possible. Life similar to our ocean thermal vent bacteria (and related food chain) would be possible.

Temperature at sufficient depth would stay the same warm temperature year-round. Earth has such a ecosystem too.

Complex eco-systems are unlikely.

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  • $\begingroup$ Melting point of carbon dioxide is -56.6 degrees Celsius. And, as it turns out, you still need to breathe during hibernation, and plants don't get the benefit of lungs warmed by the body. So, that might be an issue... $\endgroup$ Jan 5 '20 at 13:14
  • $\begingroup$ @JohnDvorak, plant dormancy doesn't use the same mechanism as animal hibernation. Dormant plants can survive just fine when encased in ice or otherwise prevented from accessing the atmosphere. $\endgroup$
    – Mark
    Jan 5 '20 at 21:15
  • $\begingroup$ Please note that on such a planet life would probably evolve in different ways than on our planet, and complex life could be different from what we consider as "plants" and "animals". There would be a huge evolutionary pressure for a much more efficient hibernation than we have in real life, or if the conditions are even more extreme, then life could thrive while in the habitable zone, after that everything dies off and leaves spores behind, which then start everything anew in the next cycle. $\endgroup$
    – vsz
    Jan 5 '20 at 21:26
  • $\begingroup$ @JohnDvorak - CO2 does not simply freeze out of the air at -56 C, gets a lot colder than that in Antarctica without the CO2 dropping on the ground. $\endgroup$ Jan 5 '20 at 21:26
  • $\begingroup$ I question your final comment, "Complex eco-systems are unlikely." Complexity has to be defined. The nature of eco-systems seems to be that any available energy path is filled by an organism that exploits that path. Things eating things. Things using nutrients left over by dead things. Things living in hot water flows that die as the water cools, and feed more complex nutrients to things living in the cooler waters. Life is complex. Eco-systems are intrinsically complex. $\endgroup$
    – cmm
    Jan 6 '20 at 15:17
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It would be possible for life to exist on a planet in a highly elliptical orbit. If the planet was of sufficient mass and had a sufficiently massive heat generating core like the Earth then liquid water could remain a liquid deep under a frozen ocean for thousands of years and life could evolve and live near to oceanic vents.

So a very eccentric orbit would allow deep sea life to evolve without a problem. The lower the degree of eccentricity the greater the chances of life spreading throughout the oceans protected by a variable thickness ice sheet.

If an even less eccentric orbit was allowed then the ice might melt in the equatorial regions during summer and allow life to spread onto land. Plant life that briefly flowered, seeded and then died would be viable as would amphibians that returned to the ocean for winter.

Having land animals survive would be more difficult, but should be possible even with a considerable eccentricity. A planet moving between Mars orbit at aphelion and Venus orbit at perihelion should be able to allow land animals to develop as the oceans and atmosphere would act as immense dampers preventing the worst excess of heat and cold.

It is entirely plausible that land animals could hibernate for longer periods than is seen on Earth with sufficient evolutionary pressure such as an eccentric orbit might provide.

You might be interested in the Heliconia series of books which deal with a similar situation although in this case the planet is found in a binary system and takes many hundreds of years to orbit the primary star.

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An eliptical orbit is a very highly eccentric orbit. A planet in such like orbit sometimes stay very near to its parent star, and sometimes very far. Possibility of existing life or supporting life is very hard in such condition.

Before, scientists assumed that the more elliptical a planet’s orbit is, the higher the planet’s average temperature will be since it’s getting with a lot more energy during its closer flybys. But in a study published in The Astrophysical Journal Letters, a team of researchers from the Planetary Habitability Laboratory and the Arecibo Observatory in Puerto Rico found that this is not always the case — those planets may actually exhibit cooler surface temperatures more amenable to habitability.

A star’s habitable zone is where temperatures and conditions are thought to be most opportune to support liquid surface water, necessary for life to evolve. Planet’s with elliptical orbits are sometimes thought to move in and out of this zone — too inconsistent to support stable conditions for life. The new results suggest it’s possible they may possess the right temperatures to support water and even life.

“Since many planets are in an elliptical orbit, it would be applicable to many objects,” Abel Méndez, a Planetary Astrobiologist at the University of Puerto Rico at Arecibo and lead author of the study, tells Inverse. He and his research team created climate simulations of Earth-like planets that have elliptical orbits, which hinted at the cooling effect.

“I had a hard time believing it myself,” Méndez says. “We found ways to evaluate that in the solar system, especially for Mars, and also we started to see a similar trend in the climate simulation of planets.”

According to Edgard Rivera-Valentín, a planetary scientist at the Arecibo Observatory and co-author of the study, if scientists followed previous models, Mars, which has an eccentric orbit, should have an equilibrium temperature of about -43 degrees Celsius. In reality, it’s -63 degrees, closer to the model his team came up with. This is colder than scientists previously would’ve expected.

Earth also has an elliptical orbit, but its orbit is nearly circular, and this cooling effect doesn’t really affect planets with nearly circular orbits. In the future, the research team plans to work on a paper focusing specifically on planets in habitable zones. And who knows — planets like the ones orbiting red-dwarf star Wolf 1061 or even exomoons around Jupiter-like planets might actually be ideal places for aliens to evolve.

“The outcome would be to understand the climate of many planets, not just those that are habitable, and also evaluate whether some are habitable and some are not,” Méndez says.

Abstract: There exists a positive correlation between orbital eccentricity and the average stellar flux that planets receive from their parent star. Often, though, it is assumed that the average equilibrium temperature would correspondingly increase with eccentricity. Here we test this assumption by calculating and comparing analytic solutions for both the spatial and temporal averages of orbital distance, stellar flux, and equilibrium temperature. Our solutions show that the average equilibrium temperature of a planet, with a constant albedo, slowly decreases with eccentricity until converging to a value 90% that of a circular orbit. This might be the case for many types of planets (e.g., hot-jupiters); however, the actual equilibrium and surface temperature of planets also depend on orbital variations of albedo and greenhouse. Our results also have implications in understanding the climate, habitability and the occurrence of potential Earth-like planets. For instance, it helps explain why the limits of the habitable zone for planets in highly elliptical orbits are wider than expected from the mean flux approximation, as shown by climate models.

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Life as we know it is possible in a quite narrow range of parameters, which can be basically limited to those allowing for liquid water to exist.

As such, as long the orbit swipes within the boundaries of the so called habitable zone, defined as the range of distances from the main star where liquid water can exist, the planet could host life.

Too close to the star and the planet would be a sauna, too far and it would be a freezer.

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    $\begingroup$ I don't think this is answering the question. It is apparent to the OP that life can survive during the period when the planet orbits into the habitable zone. OP is interested in how that life can handle entering and exiting the habitable zone for extended periods, and specifically whether it can evolve in the first place in such an environment. $\endgroup$
    – JBentley
    Jan 5 '20 at 19:50
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The answer to this question lies in chemistry and energy.

An orbit can be highly elliptical and still remain (mostly) within the habitable zone - In our own solar system the habitable zone extends from near the edge of Venus' orbit out to the inner edge of Mars' orbit. If the nature (mass, atmospheric composition, geological processes, magnetosphere etc) of Venus or Mars were more compatible with the energy they receive from the Sun for their orbit, they may actually have been habitable still.

Parameters that could impact habitability of a planet in a highly elliptical orbit:

  • Tidal forces
    • Does the planet have moon/s that generate energy as heat within the planets core? Is the planet a moon of a larger planet generating heat from interactions with that planet, and (potentially) benefiting from the protection of that planets magnetosphere?
  • 'Protective' influence of parent/ satellite bodies as orbit nears the sun
    • (frequency in which the planet as a satellite is in shadow during orbit around parent planet) or (frequency of eclipses/ number of satellites if the planet is the parent body).
  • Chemical composition of the planet. Does the planet have an abundance of gasses that provide greenhouse effects - (heating or cooling). Are some of these compounds involved in a self regulating cycle - (chemical compounds that evaporate as the planet heats up near the sun, providing a temporary cooling effect, then precipitate once the planet moves far enough away from the sun).
  • The impact of the life cycle itself on habitability. When conditions on the planet are most favorable for life, what effect does the bloom of life have on the planet? (In terms of Earth-like life, more plant life = more carbon. More trees = more oxygen, more animal life = more methane)
  • The type of star the planet orbits. Stellar class impacts energy output in a multitude of ways - Heat - Radiation - Magnetic activity.
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  • $\begingroup$ These are some really great ideas! Thanks! I love the idea of using moons and tidal forces to generate heat in the planets core. $\endgroup$
    – WillRoss1
    Jan 6 '20 at 15:52
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Here on Earth we have the wood frog, an amphibian that has evolved to be able to survive an Arctic winter, frozen into immobility and complete or almost-complete metabolic stasis.

As for how life gets started on such a world: unless it has a completely alien biochemistry, it needs a place where liquid water exists for a very long time. That somewhere might be deep underneath an ice sheet, where geothermal heating and concentration of minerals preserves water in the liquid phase under a deep ice sheet. We speculate that life might have evolved on Europa in exactly this situation. It then radiates out, gradually evolving an ever more freeze-tolerant chemistry, until eventually it is able to completely suppress the formation of lethal ice crystals. At this point the stage is set for complex life outside of its initial warm niche, that reaps the reward of abundant solar energy when the planet is close to the star, and then "hibernates" for the long period when the planet is distant from its star. (Actually, not hibernate. Freezes into a glassy state courtesy of cryoprotectants in its tissues)

The wood frog has gone part way down this postulated evolutionary route. It has not been driven any further because Arctic winters do not get cold enough for long enough to drive such evolution to its end point. So, of course, have all cold-climate trees and evergreens.

Elsewhere I've read speculation that cosmic and ambient radiation would be the limitation on human beings in suspended animation. Lacking any ability to self-repair while suspended, radiation damage would accumulate, and be experienced as if caused by an intense flash of radiation at the time of re-animation. 100 years worth is survivable (the radiation dose would make one feel quite ill). 1000 years worth is a "certain death" dose. So with any orbital period longer than 100 years, life will have to have specially dealt with this added problem.

Do read Vinge's a Deepness in the Sky but be aware that the thaw-freeze cycle is not caused by an eliptical orbit, and the evolution or even the biochemistry of the inhabitants of this planet may not be entirely natural. (a Fire upon the Deep contains some hints).

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You can make the orbit extended how much you want, given the star's heat would be mostly constant on the surface. I came up with two ideas:

  1. Give the star its hot-cold intervals that would perfectly synchronise with the planet's movement. The star would go colder when the planet is near and hotter while the planet would move away. It's silly how improbable that would be, but you can always lean on the anthropic principle.

  2. Give the planet rotation of day=year so that the habitable half would be hidden from heat while near the sun and it would face the sun while away.

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If the planet is small enough or rotates slowly enough, migration could be a viable strategy. Life would prefer to stay on the sunny side while distant from the sun, but would hide on the shadowed side during the short time of nearest approach. Plants might adapt by spreading faster, through radical asexual seeding or rapid spreading through root sprouting, especially if the dormant roots can survive temperature extremes by living (at least in part) deep underground. (Plus you might get some mileage off the ominous and vaguely familiar warning "Summer is coming.")

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A planet with a thick, dense atmosphere orbiting a dwarf star with a very near habitable zone might go past its orbital extremes so quickly that its temperature would never have time enough to stray too far from the average. For example, the orbital periods of TRAPPIST-1'S planets range from only 1.5 days to 18.8 days. If this is "too easy," you could suppose planets that have somewhat longer orbits and range just far enough to get interesting temperature excursions.

TRAPPIST's planets are close enough together to affect each other's orbits, so there might not be very many stable configurations of multiple planets with highly eccentric orbits possible.

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  • $\begingroup$ A planet with an atmosphere like Venus' could probably go many years at the far end of its orbit without the surface temperature changing much. $\endgroup$
    – Space Guy
    Jan 7 '20 at 0:41
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How "highly elliptical" do you want it? Venus is a hundred million km, Mars 220. That should be no problem.

But you want a bit more, I guess. If you let "winter" (short days/long nights) in the north (or south) coincide with the aphel of the orbit, you could keep that one hemisphere somewhat temperate. The other one would then swing between the extremes, of course.

Another option would be to have a dense could cover during aphel, and a much lower albedo around the perihel. How that would keep stable during a whole evolution taking at least two billion years, I don´t know;).

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