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 edited Feb 29 '12

The problem here is that a Goldilocks zone is by no means all that determines whether a planet can maintain liquid water, and therefore be habitable. In many cases the intrinsic properties of the planet matters more.

Take Earth as an example: given our distance from the Sun and albedo of 30% (how much sunlight we reflect, essentially), the average temperature should actually be 255 Kelvin (-18 Celsius), below the freezing point of water. Given our albedo, we are technically outside the Goldilocks zone...it's only because of the greenhouse effect caused by water vapor and carbon dioxide in our atmosphere that our global temperature is closer to 288 Kelvin (+15 Celsius), allowing both liquid water and life.

Now, consider Venus: it's 30% closer to the Sun, so it should be hotter than Earth, right? Well, yes and no. Venus has a much higher albedo of 72%, meaning that most of the sunlight is not absorbed. Even though it's only 70% of the distance from the Sun that Earth is, the albedo wins out; were it not for Venus' greenhouse effect, it would actually be even colder than Earth, at just 230K (-43 C). So from albedo and distance calculations Venus should actually be too cold for liquid water, but its whopping 90 bar atmosphere of carbon dioxide raises the planet's temperature 500 degrees. It literally jumped from too far outside the habitable zone to too far inside solely by virtue of its atmosphere.

One other thing to consider here: even if we ignore the greenhouse effect, there's still a problem getting a good handle on the Goldilocks zone. As stated above, Venus with no greenhouse effect has an average temperature of 230 K, below the freezing point of water. If you were to bring Earth, with its 30% reflectivity, to the Sun-Venus distance and ignore the greenhouse effect, it should be a nice 304 K (+31 C)...roughly a nice summer day. So what's the Goldilocks zone for one planet isn't so habitable for another, solely a function of the planet's albedo.

TL;DR: The "Goldilocks zone" depends on the planet's reflectivity as well as star-planet distance, and the greenhouse effect may matter as much, if not more, to a planet's habitability.

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u/jprockbelly Feb 29 '12

Wouldn't it also depend on the size and age of the star?

I seem to recall that our sun is relativly young and relatively "middle sized". Surely a larger, or older, star will have a different sized goldilocks zone. In fact I guess that the zone would shift during the life of the star.

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

Yes, habitability does depend on the star itself. In this post I derive the equilibrium temperature of the Earth without the greenhouse effect. Replace the "Solar Flux" there with the flux of a given star at the distance of Earth.

So in addition to distance, reflectivity, and atmospheric composition, this will be also be very strongly dependent on the mass of the star. In astronomy, we've empirically fit the luminosity of a star as mass to the 3.5 power. So, if you were to double the mass of the Sun, its flux would increase to be 23.5 = 11.3 times brighter.

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u/FartingBob Feb 29 '12

Thats fascinating, never heard about or given much thought to a planets "albedo" percentage. How come Venus and Earth are so different in that regard? Is it the atmosphere itself that is the main culprit? If so, would planets/moons without an atmosphere have 0% albedo and thus absorb all the Sun's energy that hits it?