Can an eccentric planet be habitable?

[willstaruss22]

Let's assume an planet with Earths mass and radius orbits a G2V type star at 1 au. This planet would have an the same tilt, rotation rate and atmosphere of 1 bar as Earth. However the planets eccentricity would be 0.3. How would this greater eccentricity effect this planet?

 

[mathman]

The main effect would be on the weather. I suspect a detailed study would be needed to get a full picture.

 

[vociferous]

I do not believe there is an a priori reason that an eccentric planet would not be habitable so long as the orbit spent most of its time in the habitable zone. Now, if you are asking would it be less likely to support basic or advanced life, I suspect you already have a hypothesis.

Try to construct a simple computer model to compare total solar irradiance and the irradiance at their extremes to Earth and see what you find out.

 

[DrStupid]

Try to construct a simple computer model to compare total solar irradiance and the irradiance at their extremes to Earth and see what you find out.

I'm afraid that wouldn't be sufficient. This problem cries for a climate model including oceanic circulations. And the result will strongly depend on several environmental conditions such as distribution of continents and surface water or composition and pressure of the atmosphere.

 

[willstaruss22]

I'm curious how extreme the temperatures would be.

 

[marcus]

I'm curious how extreme the temperatures would be.

You are saying eccentricity 0.3,
so distance varies between 1.3 AU and 0.7 AU

One can ignore effect of atmosphere, greenhouse, circulation, and heat storage in ocean if there is ocean. One can for instance just calculate the variation of the theoretical EQUILIBRIUM TEMPERATURE in direct light from the star at a certain distance.

Picture a dark unreflective flat rock with star directly over head, in vacuum. How hot will it get before it reaches equilibrium and is radiating out into space the heat equivalent to what it is receiving from starlight?

This is one kind of temperature you can calculate.

Or picture a dark unreflective SPHERE, whose surface area for radiating off energy is 4 times its cross section area for absorbing. How hot will it get before the surface area is radiating energy as fast as the cross section is absorbing?

In absolute temperature terms, the equilibrium varies as the FOURTH ROOT OF THE ILLUMINATION, which varies as the inverse square of the distance. So you have to calculate (1.3)^-.5
and (0.7)^-.5
which are 0.877 and 1.195

So it gets about 20% hotter sometimes and 12% colder sometimes, than it would in circular orbit R = 1 AU.

For more detail about equilibrium temperature of planet:
http://lasp.colorado.edu/~bagenal/3720/CLASS6/6EquilibriumTemp.html

This article says equilibrium temp of Earth is 280 Kelvin and 20% of that is 56 degrees K.
That is quite a lot.

However in reality lots of other factors come in, like clouds could increase the "albedo" or reflectivity so not as much light would be absorbed, and so on. they include the "albedo" in their equations at that Colorado website.

 

[vociferous]

I'm afraid that wouldn't be sufficient. This problem cries for a climate model including oceanic circulations. And the result will strongly depend on several environmental conditions such as distribution of continents and surface water or composition and pressure of the atmosphere.

I think that is a little beyond the scope of what we can reasonably predict, but in any case, figuring out the solar irradiance is always going to be the first step. You can extrapolate to whatever additional models you want to use after that.

 

[mfb]

A google search for "eccentricity habitability" gives tons of papers discussing that. There is certainly a non-trivial limit on eccentricity (it can be larger than 0, but not .99), but it is unclear where this is - that shouldn't be surprising, even the habitable zone for circular orbits is still under discussion.

 

[willstaruss22]

Wait so it would get 20% hotter which is an extra 56k at perhilion and 12% colder which is an extra 33.6 k at aphelion? That scary.

 

[mfb]

In equilibrium, and without any feedback loops like clouds etc.. The planet won't fully reach equilibrium within a year especially in (and close to) the oceans.

 

[lpetrich]

There's been some research on this issue. Look in arxiv.org, and you'll find lots of papers on this subject.

Like:
[1002.4875] Habitable Climates: The Influence of Eccentricity
[1401.5323] Climate of Earth-like planets with high obliquity and eccentric orbits: implications for habitability conditions
[1401.5323] Climate of Earth-like planets with high obliquity and eccentric orbits: implications for habitability conditions
[1002.4877] Generalized Milankovitch Cycles and Longterm Climatic Habitability
Habitability of Earth-like planets with high obliquity and eccentric orbits: results from a general circulation model
Cambridge Journals Online - International Journal of Astrobiology - Abstract - Earth-like worlds on eccentric orbits: excursions beyond the habitable zone (also http://courses.ncssm.edu/kolena/life/eccentricorbitplanets.pdf (PDF))

That last one shows some maps of calculated temperatures for Earthlike continents and oceans for eccentricity 0.7 and incoming light 0.71 * Earth's. At periastron, the equatorial continent interiors go up to 80 C, and the tropical and subtropical parts of the oceans go up to 40 C. At apastron, those interiors go down to 0 to 10 C, while those ocean parts go down to 20 C.

So even if a planet has an orbit with a sizable eccentricity, it may still be at least partially habitable.

 

[willstaruss22]

So basically its possible to go from an ice age at aphelion to a hot house stage at perihelion.

 

[Torbjorn_L]

"Wait so it would get 20% hotter which is an extra 56k at perhilion and 12% colder which is an extra 33.6 k at aphelion? That scary."

Why scary? Remember that the faint young Hadean/Archean Sun was 25 % fainter, and life survived that. This week's paper on Iceland vs Jack Hill zircons support that, Hadean/Archean Earth had relatively cold surface waters but was wet, not icy. Yes, with known and unknown effects that ameliorated the early climate, but that is what we are discussing here.

And it gets better: As Venus with its near locked rotation shows us, radiative extremes can be evened out smoothly without having oceans or too strong surface winds, given that an atmosphere is dense enough. If you have problems with Earth analogs, in most cases you simply push up habitability range towards superEarths/dense atmospheres AFAIK. And there are likely _more_ of these!

To add to what has already been said on the problems, we can't yet predict the young "alien" Archean climate with consensus acceptance. (But recently, with going from 1D to 3D models, the problem has become tractable. E.g. it is enough to, say, have negative cloud formation feedback at the water/ice interface to predict a non-snowball Archean Earth.) Until we can do that, with much less eccentricity, the eccentrics are hard to grasp.

 

[willstaruss22]

Its just the temperature extreams sound like a lot from perihelion to aphelion. However it could be good as rink of a runaway greenhouse and icehouse would be unlikely.

 

[lpetrich]

Greenhouse gas - Wikipedia The Earth's greenhouse effect raises its average surface temperature by about 33 C to 14 C. Without it, the Earth's average temperature would be - 19 C.

For an eccentric orbit, the average received luminosity is (1 - e²)^-1/2 times the circular-orbit value. For e = 0.3, that is about 5% greater.

If the Earth's orbit's eccentricity was changed to 0.3, and the Earth's atmosphere's behavior stayed the same, then the average temperature at perihelion would be 70 C and the average temperature at aphelion would be -21 C. Adjusting to give our average sunlight over the year changes the limits from 62 C and -27 C.

 

[lpetrich]

Oceans are a moderating influence. Here's a good illustration. Three inland cities and a city on a small oceanic island.

The inland cities:
Calgary, Canada
Kiev, Ukraine
Astana, Kazakhstan

The island city:
Port-aux-Français, Kerguelen Islands, a.k.a. Desolation Islands

Coordinates:
Calgary: 51°03′N 114°04′W / 51.050°N 114.067°W / 51.050; -114.067
Kiev: 50°27′00″N 30°31′24″E / 50.45000°N 30.52333°E / 50.45000; 30.52333
Astana: 51°10′0″N 71°26′0″E / 51.16667°N 71.43333°E / 51.16667; 71.43333
PaF: 49°21′S 70°13′E / 49.350°S 70.217°E / -49.350; 70.217

I selected the three cities so that their latitude would be the same as that of the Kerguelen Islands, to within a reversed sign. It was hard for me to find small inhabited oceanic islands in temperate-climate latitudes.

Climate:

Temperatures are record low, average low, average, average high, record high

Calgary:
Summer (July): -0.6 C, 9.8 C, 16.5 C, 23.2 C, 36.1 C
Winter (January): -44.4 C, -13.2 C, -7.1 C, -0.9 C, 17.6 C

Kiev:
Summer (July): 5.8 C, 16.1 C, 20.5 C, 25.6 C, 39.4 C
Winter (January): -31.1 C, -5.8 C, -3.5 C, -0.9 C, 11.1 C

Astana:
Summer (July): 2.3 C, 15.0 C, 20.8 C, 26.8 C, 41.6 C
Winter (January): -51.6 C, -18.3 C, -14.2 C, -9.9 C, 3.4 C

PaF:
Summer (January): -1.5 C, 4.1 C, 7.2 C, 11.2 C, 23.0 C
Winter (July): -8.0 C, -0.8 C, 2.0 C, 4.8 C, 13.4 C

So you can see that oceans moderate climates by being heat buffers. Though all four cities have similar daily temperature changes, the three inland ones have much hotter summers and colder winters than the island one.

Sources: Calgary, Kiev, Astana, Kerguelen Islands, Port-aux-Français at Wikipedia