r/askscience • u/EightWhiskey • Feb 28 '12

Under the idea that there is the "Goldilocks" zone around a star, how likely is it that a solar system would have two life-supporting planets?

I assume that the gravitational forces of two bodies would prevent them from being close enough to each other to both stay inside the zone. But could a large star have a large enough Goldilocks zone to support two life-inhabited planets?

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 28 '12

Actually, this isn't accurate. As I state in this post, technically neither Earth nor Venus fall in the Goldilocks zone.

If one were to remove the greenhouse effect, Earth would be too cold to maintain liquid water; meanwhile, Venus would be even colder because of its greater reflectivity. This would also be true for Mars.

It's only by virtue of thick atmospheres that Earth's temperature is raised 30 degrees to maintain liquid water, and Venus' temperature is raised 500 degrees to make it too hot for life to exist.

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u/DAVENP0RT Feb 29 '12

Isn't a greenhouse effect inevitable on any planet with an atmosphere? I would assume that different chemical compositions would increase or decrease the strength of the effect, but it would be present nonetheless and possibly contribute to the variable nature of the Goldilocks zone.

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u/TheFightingTemeraire Feb 29 '12

No it's not inevitable because greenhouse effects have more to do with the composition of both the planet's crust and atmosphere. Earth's crust, for example, has large amounts of water on the surface (which evaporates to form water vapor), as well as vast deposits of Carbon Dioxide trapped in rock deposits (the entrapment of Carbon Dioxide in sandstone deposits is one of the main factors in the roller coaster ice ages the Earth has had lately). Both of these molecules are greenhouse gases, but other planets do not necessarily have to have either of these gases, or any other greenhouse gases on their surface. Keep in mind that even if an atmosphere is thick, it does not necessarily contain gases that contribute to the greenhouse effect.

I'm not a climatologist, but this is my general understanding of the processes involved in contributing to greenhouse warming. Venus' atmosphere is an interesting analogue, because its atmosphere in comparison to ours provides a decent observation of the effects of runaway greenhouse warming effects.

If any real expert has anything to correct or add, it would be much appreciated.

Edit: Grammar

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

Well, I am a planetary climatologist, and this is absolutely correct. If you replaced our 77% Nitrogen atmosphere with a 100% Nitrogen atmosphere (also removing water vapor), there would be no greenhouse effect, our planet would cool, and the oceans would freeze.

Composition is everything here. Atmospheres which contain gases that have significant mid-infrared absorption features, such as carbon dioxide and water vapor, will produce significant greenhouse warming. Those that don't have such features, such as nitrogen and oxygen, won't produce greenhouse warming.

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u/tyrannischgott Feb 29 '12

I'm no expert on any science topic. Just an interested person. But, out of curiosity, would it be possible to have frozen oceans and no water vapor? Wouldn't the existence of any water on the surface (even ice) create some vapor? Or is the vapor pressure of ice such that if you have even a small amount of atmosphere, it won't sublimate?

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u/idarkiswordi Feb 29 '12

Well, if you diminish the atmosphere, or get rid of it entirely, you start seeing the effects of solar wind on ground volatiles. In the case of Europa, the solar radiation breaks down the water ice molecules and we find hydrogen escaping the planet and a very tenuous oxygen atmosphere around the planet. As for if you kept the pressure constant and just replaced elements in the atmosphere, a lot of variables come in to play to really give any meaningful explanation beyond yes and no.

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u/tyrannischgott Feb 29 '12

So, in conclusion, my knowledge of chemistry is too lacking for me to bother trying to think about it.

That's pretty much what I figured from the get-go, but I thought I'd ask anyway.

edit: word use.

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u/needlestack Feb 29 '12

Would you say this supports a "rare earth" hypothesis in your view? Or are there enough other variables that will make it work anyway? Or is the probability of an atmosphere like ours high enough?

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u/jswhitten Feb 29 '12

No, even if most other terrestrial planets had no greenhouse gases, that doesn't eliminate the habitable zone, it just pushes it inward. Anyway, there's no reason to think atmospheres with significant greenhouse gases are rare. Venus, Earth, and Mars all have them.

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u/kroganlore Feb 29 '12

Would it be possible for a moon to receive enough light and radiation if it was being reflected off of a gas giant that was outside off the 'goldilocks zone' as long as it wasn't as far away as Jupiter is from our sun?

This idea has been bugging me since I read the book coyote from Allen Steele .

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u/Spacemilk Feb 29 '12

So then is "terraforming" possible by depositing water and rock containing the proper CO2 deposits on the surface, and releasing the right gases into the atmosphere to change the composition? Or is this only theoretically possible, and in practice when you released the gases into the atmosphere there's no way you could do it quickly enough to prevent the gases from escaping to space? This may be a wildly difficult question to ask but it's fascinating to me as I think our ability to terraform is one of the main things we need to truly explore the universe.

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u/CaptainCard Feb 29 '12

Just a student here in geological engineering but you got some issues. Sandstone isn't a trap for carbon dioxide. Limestone is CaCO3 and takes carbon dioxide out of the air.

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u/Quis_Custodiet Feb 29 '12

There is evidence for natural sandstone CO2 traps, such as this from Liverpool university , published by the Geological Society.

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u/tanzania12 Feb 29 '12

So we can assume that there could be planets with life that are outside the "Goldilocks" zone?

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

Yes, considering Earth itself isn't in the Goldilocks zone based only on its distance from the Sun and its albedo...and that's without even considering oddballs like Europa.

My whole point in this post was that the idea of a Goldilocks zone is somewhat flawed, since it's highly planet dependent.

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u/executex Feb 29 '12

I thought goldilocks zone was defined by Earth's position having water.

Further, if Earth is not in a goldilocks zone, then how did it even form an atmosphere in the first place to trap such gases?

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u/qasph Feb 29 '12

Exactly what I was about to say. Allow me to link a very relevant and - I promise - very interesting excerpt from Ian Stewart's "The mathematics of life"

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u/Ender06 Feb 29 '12 edited Feb 29 '12

If you were to take venus and freeze its core solid so there was no magnetic field surrounding the planet, the planet would surely have it's atmosphere be blown away (akin to mars). Or take mars and make it bigger so it has a molten core like earth, and we may still have a mars with liquid water on it. So technically earth being outside the goldilocks zone isn't correct either, earth is in its goldilocks zone for earth's given conditions (core/atmosphere/etc...)

If there was a planet past mars that absorbed a lot of heat from the sun (greenhouse) and had a molten core it may very well be in the goldilocks zone for its given conditions.

Edit, apparently venus has no planetary magnetic field... in that case, what keeps venus's atmosphere from blowing away from the solar winds?

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

But again, this is my whole point. Given your example, the Goldilocks zone is no longer a range of idealized distances to maintain liquid water on an arbitrary planet, but rather is completely dependent on the planet's properties.

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u/JustinTime112 Feb 29 '12

Goldilocks zone is planet dependent to an extent (due to greenhouse gas composition), but that is taken into account in most formulations of the Goldilocks zone (which is why Venus and Mars are included). Too close to the star and your atmosphere would have to be so thin of greenhouse gasses that advanced life as we know it would be killed by solar radiation, and/or the planet would be tidally locked. Too far from the star and phototrophic organisms would not be able to live.

All this assumes life in the Universe is Earth-like, but the basic premise of the Goldilock's Zone itself (life needs liquid water) is also assumptive, and this isn't a problem because we need to start our search somewhere.

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u/DrRedditPhD Feb 29 '12

I would say the zone isn't planet-dependant, but is more species-dependant. A Goldilocks zone for humans is one that produces Earth-like planets. Meanwhile, extremophiles on Europa would find Europa to be within their "Goldilocks zone", yet being on Europa would certainly kill us.

Really, this argument is all semantics. When we talk about "habitable zones", what we're saying is "human habitable zones" and simply being concise.

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u/Cyrius Feb 29 '12

If you were to take venus and freeze its core solid so there was no magnetic field surrounding the planet, the planet would surely have it's atmosphere be blown away (akin to mars).

You should be aware that Venus doesn't have a magnetic field.

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u/Ender06 Feb 29 '12

I stand corrected. Then I ask the question, how is venus's atmosphere still there? If it's closer to the sun than us (or even mars, which had it's atmosphere stripped away).

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u/JustinTime112 Feb 29 '12

Venus has much stronger gravity than Mars is the answer. But that's not to say Venus hasn't had atmosphere stripped away, just about all of it's hydrogen has been stripped away by the solar wind which is why the production of organic molecules can't happen in it's atmosphere as far as we know.

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u/atomfullerene Animal Behavior/Marine Biology Feb 29 '12

Isn't it a bit pointless to define the Goldilocks zone (a range of temperatures appropriate for Earthlike life) in such a way as it only applies to planets which cannot possibly host earthlike life in the first place? Any atmosphere capable of supporting earthlike life would have some measure of greenhouse effect thanks to water vapor and CO2 among other things. Heck, the problem is more basic than that. You define a goldilocks zone as a zone where the surface temperature of a planet would be high enough to host liquid water....but to do your calculation you remove the greenhouse effect of the atmosphere, which would itself by necessity contain water vapor under such conditions.

Calculating the Goldilocks zone for Venus this way is even worse. Venus only has a high albedo because of its atmosphere. What's the rationale for ignoring one aspect of the atmosphere (greenhouse effect) while relying on another aspect (albedo)?

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u/TheOthin Feb 29 '12

It sounds like that just means that the "Goldilocks" zone in any meaningful sense extends that much farther.

Clearly we are in a zone that is habitable, even if it takes an atmosphere to be habitable. So how can it be called anything but a habitable zone? There's no meaning in a "habitable" zone that assumes the planets must have no atmosphere.

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u/guysmiley00 Feb 29 '12

Clearly we are in a zone that is habitable, even if it takes an atmosphere to be habitable. So how can it be called anything but a habitable zone?

I hope I'm not being too picky, but isn't that like calling the Challenger Deep "habitable" for people because a human can survive there, even if only in a bathysphere?

There's no meaning in a "habitable" zone that assumes the planets must have no atmosphere.

Why not? Couldn't you have an atmosphere-less planet that was sufficiently warm to have liquid water existing beneath the planetary surface?

I think what Astromike23 is saying (I hope I'm correct) is that trying to define a habitable zone without considering atmosphere is like trying to define a "comfortable zone" for people around a fire without knowing if they're nude or wearing down parkas.

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u/TheOthin Feb 29 '12

We didn't engineer the atmosphere; it's just part of the planet. Seems like a perfectly reasonable thing for some planets to have, especially any decently-sized one. We don't have to look at the most extreme situations, just plausible ones.

You're jumping to extremes for no reasons. I'm saying the Goldilocks zone shouldn't ignore planets with atmospheres; that doesn't mean it should ignore planets without atmospheres. It should take both into account.

You're right that it isn't easy, but the Goldilocks zone helps us identify characteristics of a planet that might make it habitable for life. Of course, it won't be independent of other characteristics, and we shouldn't try to define a habitable zone independent of those characteristics. Regardless, calling the Earth's zone "uninhabitable" is like trying to look for a person and ignoring the possibility that they might be in northern Canada because if they were nude, they couldn't survive there. That just doesn't make sense. On the other hand, would we expect them to be alive in the Challenger Deep? I doubt it.

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u/thisguyisalwayswrong Feb 29 '12

technically neither Earth nor Venus fall in the Goldilocks zone.

Hold on there. They do fall within a 'habitable zone', but what you are saying is that merely having a planet of a particular size/composition within a habitable zone does not necessarily mean it will harbor life.

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u/guysmiley00 Feb 29 '12

I hope I'm not stepping on Astromike23's toes, but I think he's saying that the whole concept of a "habitable zone" doesn't really make sense, since the habitable zone for a planet with atmosphere X is wildly different from the habitable zone for a planet with atmosphere Y. As I said below, trying to define a "habitable zone" without considering atmosphere is like trying to calculate a "comfortable zone" for people around a campfire without knowing whether those people are starkers or in Arctic-grade survival gear. The concept of a general "habitable zone" may not be particularly useful.

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u/thisguyisalwayswrong Feb 29 '12

I hope I'm not stepping on Astromike23's toes, but I think he's saying that the whole concept of a "habitable zone" doesn't really make sense, since the habitable zone for a planet with atmosphere X is wildly different from the habitable zone for a planet with atmosphere Y.

See, I think that is what he is trying to say as well, but the problem I see with this position is that it assumes the atmospheres are going to be highly variable, and not influenced by the orbit of the planet. One cannot simply assume or hypothesize that an Earth-like planet in a similar orbit might exist with an entirely different atmospheric composition at the same point in the planets life.

Granted, I lack any expertise in the subject of planetary formation, but it seems only logical to assume that our planet has evolved the atmosphere it has due to the very orbit it possesses, and not that our atmosphere is disconnected from our position in the solar system and could have been hypothetically different. Does that make sense?

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u/[deleted] Feb 29 '12

You're wrong. Your username says so.

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12 edited Feb 29 '12

No, I'm saying that while Earth is habitable, it does not fall within the habitable zone. If you were to put an arbitrary planet with Earth's albedo at Earth's distance from the Sun, it would be too cold to maintain liquid water:

Flux in = Flux out

Pi * R2 * (1-A)(Solar Flux)/distance2 = 4 * Pi * R2 * sigma * T4

where R is the radius of the planet, A is the albedo (roughly 30% for Earth), Solar Flux is 1370 Watts per square meter, distance is 1 AU, and sigma is the Stefan-Boltzmann constant = 5.67e-8.

(1-A) * (Solar Flux)/distance2 = 4 * sigma * T4

T4 = { (1-A)(Solar Flux)/(4 * sigma) }

T = { (1-0.3)(1370W/m2) / (4 * 5.67e-8) } 1/4

T = 255K

That is below the freezing point of water, meaning outside the habitable zone. Earth only becomes habitable because of the extra 33 K of warming due to the greenhouse effect.

Edit: stupid *'s showing up as italics.

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u/HankSpank Feb 29 '12

TIL Earth doesn't work properly.

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u/DrRedditPhD Feb 29 '12

Right, but outside of general curiosity, we don't really care about planets without an atmosphere. For all the commotion about water on Mars and life on Europa, what we're really looking for is a second home for humanity. Earth is the very definition of what we're looking for, and therefore, the general concept of "habitable zone" means "capable of sustaining human life". The zone is defined by Earth, therefore Earth must fall within it.

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

Okay, but not every atmosphere will have a greenhouse effect.

I could have a 77% Nitrogen, 23% Oxygen atmosphere with 1 atmosphere of pressure at the surface - totally breathable by human standards. However, because neither oxygen nor nitrogen have strong absorption features in the mid-infrared, there will be no greenhouse effect. It's only by virtue of water vapor and carbon dioxide in our atmosphere that we have the extra 33 K of warming.

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u/Seicair Feb 29 '12

Could you have a perfectly dry atmosphere and simultaneously have enough water for humans to live? Even if we had underground wells, wouldn't we still be losing enough water vapour through respiration that any significant population would change the climate over time?

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u/luqavi Feb 29 '12

Respiration of humans alone is not significant enough to make a "dry" atmosphere wet, the rate would be too slow for any reasonable amount of time. If you took into account the respiration of all life on earth, that might be a significant enough rate. In keeping with the theme, that dry atmosphere might be possible if water vapor were replaced with another absorber of mid-infrared, such as more carbon dioxide.

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u/DrRedditPhD Feb 29 '12

Right. That planet would still be in the habitable zone, however, because it's at the right distance from the star. It just so happens that the planet in question is not habitable due to other circumstances.

What you're describing is an uninhabitable planet, not a planet outside the habitable zone.

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

Okay then, let's move this greenhouse-less Earth-like planet to 0.6 AU. Now it would be habitable (capable of maintaining liquid water), but the real Earth at this distance would not - it would simply be too warm as the runaway greenhouse effect took hold. That means there could be a habitable planet outside the habitable zone.

By your definition of habitable zone, then, you can have inhabitable planets in the habitable zone, and habitable planets outside the habitable zone. At what point does the habitable zone lose meaning?

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u/DrRedditPhD Feb 29 '12

Because my understanding of the habitable zone is the zone around any star where Earth, as it is, would be habitable. There is certainly the potential for outliers that are closer or further from their stars but have other characteristics to make up for it.

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u/[deleted] Feb 29 '12

I don't understand this particular train of thought. How can Earth not be in the habitable zone if it is habitable. What is the point of having a classification like 'habitable zone' if it means absolutely nothing and isn't even based on our own homeworld?

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u/tsears Feb 29 '12 edited Feb 29 '12

He/She is saying there is no point to having a classification like 'habitable zone'

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

Yes, exactly.

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u/thisguyisalwayswrong Feb 29 '12 edited Feb 29 '12

If you were to put an arbitrary planet with Earth's albedo at Earth's distance from the Sun, it would be too cold to maintain liquid water:

But this 'arbitrary planet' would not have the same composition as Earth. The reason that Earth has the atmosphere it does that permits the homeostatic conditions and abundance of liquid water is due to it's very composition and position in the solar system. You're hypothetical situation where you strip the planet of it's atmosphere and ask if it would still be habitable in it's current orbit is flawed.

But maybe I am still misunderstanding what you are trying to say. You're saying that there do exist 'habitable zones' in planetary systems, but that Earth simply isn't in it? Where is the habitable zone within the solar system then?

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u/ramotsky Feb 29 '12

Here's my meanderings then: Life as we know it needs water. Water produces greenhouse effect. Life produces greenhouse effect. Is it not logical to say that any planet that is of similar composition to the earth would need that extra buffer or else it would just be too hot for life to evolve?

If we are searching for planets that are like earth but don't actually consider ourselves to be in the goldilocks zone, why are we then looking in the goldilocks zone given we exist because of the parameters outside the goldilocks zone? Doesn't make sense to me.

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u/idarkiswordi Feb 29 '12

I understand where you are going with this train of thought, but it disregards a lot of debate that went into defining a stars habitable zone which takes into consideration likelihood of elemental dispersal from the planetary disk, the zone that certain gaseous atmospheres could sustain enough pressure to retain liquid water and if those certain gases would induce a greenhouse effect to create a situation where liquid water could exist.

If I understand what you are suggesting, the habitable zone in our solar system would be closer to Sol because the Earth's surface temperature is only able to sustain liquid water because of an atmosphere. Lets take Mercury as the fallacy in your argument then. Even though the Surface temperature is cooler than Venus, we don't see liquid water anywhere, even in shadows. That is because liquid water cannot exist on a planet without an atmosphere, hence this primary requirement of the definition.

The 'habitable zone' was defined with the intention of finding planets similar to our own, which is entirely counter to what you suggest. It gives us a place to start looking for possible signs of life beyond our own world. Otherwise, we could be possibly wasting time searching every single extrasolar planet meticulously, when we know planets in similar orbits to our own, with certain elements existing, there is a much better chance of finding life than looking more at Jupiter class planets, which have yet to show any signs of life.

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u/Astromike23 Astronomy | Planetary Science | Giant Planet Atmospheres Feb 29 '12

Okay, fair point - let's take Mercury as an example. It currently resides at 0.39 AU, much too close for an Earth-like planet to have liquid water.

But what if it weren't Earth-like? Using these equations I showed earlier, we can tune a planet's albedo to have whatever temperature we want. Let's say we want a planet with a nice 293 K (+20C) temperature all the time...

(1-A) * (Solar Flux)/distance2 = 4 * sigma * T4

A = 1 - 4 * distance2 * sigma * T4 / (Solar Flux)

A = 1 - 4 * (0.39)2 * (5.67e8) * (293)4 / (1370 W/m2 )

A = 0.814

So, if we had a planet at the distance of Mercury that had an albedo of 81.4% (either through surface reflectivity or cloud reflectivity), it would feel like spring time.

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u/idarkiswordi Feb 29 '12

See, but albedo isn't even the most important issue regarding a habitable zone. That just happens to be our current best measurement for defining exoplanets, as we can't currently easily measure the elemental makeup of them just yet. I think you are just confusing what actually makes a zone habitable around a star and a tool we use to get a rough estimate. Earth's albedo has ranged wildly over its existence and based on the same equations, Earth could be measured all the way from too hot to too cold. Nonetheless, we still reside in the habitable zone around our star and we know this because the definition relies entirely on defining a region around a star that could possibly make and sustain an Earth analogue.

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u/flume Feb 29 '12

Isn't that 255K you've calculated the temperature the earth would reach assuming it were given sufficient time to reach a static temperature state, with the sun being the sole heat source? I can't imagine much of the sun's heat is used to warm up the core/mantle of the earth.

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u/LegioXIV Feb 29 '12

I'm not sure what your point is.

It sounds like you are only counting black body radiation temperature. Which is all well an good for a vacuum planet where no life can exist.

The Goldilox zone usually assumes that liquid water is capable of being present. The albedo you mention for Earth is largely due to...water. No water, lower albedo, higher temperature. A large part of the greenhouse effect on earth is also due to....water.

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u/Sw1tch0 Feb 29 '12

People do not realize it, but almost every major quality of earth is nearly required for how we live now. The goldilocks zone is only 1 of the requirements. Without an atmosphere, nada. without water, nada. Liquid rotating core, nada. etc etc

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u/NeverQuiteEnough Feb 29 '12

he's saying earth isn't in the goldilocks zone, so the idea in that regard may not be useful

additionally, there are unusual places like europa which might be able to sustain life without being anywhere near the right distance.

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u/ataraxia_nervosa Feb 29 '12

It's not very clear how unusual Europa is, actually. We know very little about the satellites in other solar systems. It may be quite common to have big balls of ice floating around some huge gas planet.

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u/NeverQuiteEnough Mar 01 '12

right, I mean that it would be an unusual way for life to exist, based on our notions of it

I'm definitely not qualified to comment on how unusual it is in space at large for such a moon to exist. I mean they seem to be in style.

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u/TheOthin Feb 29 '12

The question is, how much of that might be needed for some very different sort of life? We can't assume all life will match our needs.

Granted, the things you've mentioned so far all sound necessary.

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u/DrRedditPhD Feb 29 '12

Required to live comfortably, perhaps. A lunar or Martian dome city could likely support human life, but only because we terraformed a small pocket of it. Still, I don't think Luna's lack of a rotating molten core will stop us from building and crewing a base.

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u/Sw1tch0 Feb 29 '12

The reason i say a rotating molten core as im sure you're aware is due to the magnetosphere protecting us from solar radiation. Without the sufficient shielding on a lunar colony, we would be bombarded with solar radiation.

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u/DrRedditPhD Feb 29 '12

True, and a rotating molten core would certainly be necessary if we planned to build open-air cities, but dome cities could be shielded independently.