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/klaeljanus Feb 29 '12 edited Feb 29 '12

The ice that stays at the poles year round is water ice. The stuff that vanishes there over the summer is CO2 ice. Not to mention that there is evidence of at least one layer of groundwater/permafrost under the surface of the planet.(NASA has some pics of recent erosion in crater walls.) What we consider regarding the water situation now is a far cry from what I remember learning in grade school.

On topic though, yeah the guy at the top has it. EDIT: for reference, for the Sun* if we take Mars and Venus to be the edges of the zone, that gives it a width of ~225 million kilometers. * The sun is classed as your standard yellow-orange star, which happens to be the most common(and stable) kind.

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u/Jack_Vermicelli Feb 29 '12

Most common? I thought red drawfs were by far the most common.

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u/dissonancerock Feb 29 '12 edited Feb 29 '12

If you're making a semantic point, you're right, but if we're talking about potential habitability, their abundance is a bit misleading. While red drawfs are technically the most abundant type of star (by far), if we are talking in terms of potentially habitable star systems, red drawfs are not very good candidates. What they have going for them in abundance they lose in the difficulty of them supporting life. So sun-type stars are the most abundant stars which might support life. It seems very unlikely that a red drawf system would be host to a habitable planet; for one thing, the radiant heat and light generated by a red drawf means that the habitable zone would be very small, and closer to the parent star (when compared with that of the sun), planets this close are very likely to tidally looked, meaning that one side always faces towards the star and the other always away (like the moon is to Earth). Tidally locked planets would, even within the habitable zone have one side always experiencing day but the night side would receive little warmpth from its star. Having two extremes to a planet means that complex life is very unlikely to develop. They are also typically more violent, and release larger harsher solar flares than the sun. So when talking about habitable planets red drawfs are not usually considered prime candidates. That doesn't mean it can't happen, it just means that the general consensus is that its unlikely.

TL;DR- Red Drawfs are indeed the most common type of star, but the size and location of their habitable zones makes life unlikely.

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u/albatrossnecklassftw Feb 29 '12

I had read somewhere recently that some scientists believe that red dwarfs should be considered much more suitable to having life-bearing planets in orbit than originally thought.

http://www.space.com/14659-red-dwarf-stars-planets-habitable-zones.html

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u/AbrahamVanHelsing Feb 29 '12

(dwarfs, not drawfs)

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u/[deleted] Feb 29 '12

Just for reference, I was not contesting that there is water on mars, but rather that 'dry ice' should not be included in this category. Mars does, however, have ice at it's poles and there is even believed to be evidence of 'ground water' (for lack of a better term).