# Multiple habitable planets in the same solar system: Theoretically possible? **URL:** \ **Category:** Factual Questions\ **Created:** [December 7, 2012, 7:01am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280 "2012-12-07T07:01:39Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![ekedolphin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ekedolphin/32/10003_2.png) [@ekedolphin](https://boards.straightdope.com/u/ekedolphin)\ **Post date:** [December 7, 2012, 7:01am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/1 "2012-12-07T07:01:39Z")
Is it theoretically possible to have more than one habitable planet in the same solar system, or would the necessary proximity of the two planets make that rather unlikely? (I mean, naturally habitable; not something created by futuristic terraforming, nor glass-enclosed domes). Does science have an answer for that yet, or is the fact that we haven’t discovered extraterrestrial life yet make the answer mere speculation? ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [December 7, 2012, 7:04am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/2 "2012-12-07T07:04:44Z")
As I understand it Mars is presently just barely outside the Goldilocks zone. It surely wouldn’t take too extreme a change to have it just inside instead; maybe no Venus, or Earth a little closer to the Sun, something of that nature. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [December 7, 2012, 7:35am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/3 "2012-12-07T07:35:21Z")
A related question is whether it’s possible for a Goldilocks planet to have a habitable moon. I would think this wouldn’t be TOO unlikely – obviously, relatively big moons aren’t impossible – but maybe the fact that Earth’s moon lacks an atmosphere points to some general principle that makes it unlikely after all. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [December 7, 2012, 9:09am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/4 "2012-12-07T09:09:19Z")
A gas giant with multiple habitable moons is another possibility. ---
**Author:** ![s0meguy](https://avatars.discourse-cdn.com/v4/letter/s/a3d4f5/32.png) [@s0meguy](https://boards.straightdope.com/u/s0meguy)\ **Post date:** [December 7, 2012, 9:18am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/5 "2012-12-07T09:18:58Z")
A large gas giant can have Earth sized moons. Far enough away from it, they will even rotate (so they’re not tidally locked, gives a day cycle). So if there’s a gas giant, there could be lots of habitable moons. But using the word moon here is misleading because they can be similar to Earth. There can also be a binary planet system in the habitable range, or planets in separate orbits. But it depends on how large the star’s habitable range is. It depends on the type of star. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [December 7, 2012, 10:07am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/6 "2012-12-07T10:07:16Z")
First you need to define ‘habitable’. Do you mean planets that are habitable by humans in their shirtsleeves, or habitable by any kind of life? Do you perhaps mean planets that are not currently habitable, but which could be terraformed very easily (like, for instance, a lifeless planet the same size and temperature as the Earth? [Martyn Fogg](http://en.wikipedia.org/wiki/Martyn_J._Fogg) calls these categories 'Habitable Planets (HP), Biocompatible Planets (BP) and Easily Terraformable planets (ETP). I would guess that the first category, ‘shirtsleeve planets’, are very rare indeed. It would be a remarkable coincidence for two shirtsleeve planets to co-exist in the same system, but (I think) not impossible. A planet with a high albedo could exist closer to the locan sun, while a planet with a low albedo could exist further out; if they were sufficiently far apart their orbits wouldn’t interfere with each other. Some results from the current studies of exoplanets show that planets can orbit quite surprisingly close to one another, although I suspect certain constraints need to be met. Lifebearing planets might, on the other hand, take many forms, and it could be possible to get numerous different types of life-bearing planet in a single system. Cold, Europa-type worlds might be the most common type of life-bearing world in the galaxy, or maybe helium-rich super-earths. Terraformable worlds might also exist in some quantity in the galaxy, and any particular system might have several examples. If one includes worlds which could in theory be terraformed with difficulty, we could include Mars and Venus; in this case a high proportion of systems probably have more than one terraformable world. ---
**Author:** ![Peremensoe](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@Peremensoe](https://boards.straightdope.com/u/Peremensoe)\ **Post date:** [December 7, 2012, 10:08am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/7 "2012-12-07T10:08:49Z")
And there are rocky “super earths,” which could have atmosphere-keeping moons. There are a bunch of ways a system could have multiple habitable worlds. I bet they’re all out there. ---
**Author:** ![tdn](https://avatars.discourse-cdn.com/v4/letter/t/94ad74/32.png) [@tdn](https://boards.straightdope.com/u/tdn)\ **Post date:** [December 7, 2012, 2:56pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/8 "2012-12-07T14:56:26Z")
> [@Der\_Trihs](#): > > A gas giant with multiple habitable moons is another possibility. Sure, but wouldn’t the planet have a pretty huge magnetic field? Wouldn’t that make life unlikely? ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 7, 2012, 3:25pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/9 "2012-12-07T15:25:14Z")
The distribution of planets tends to be an interesting mathematical arrangement - which IIRC means they are in “non-harmonic” orbits - Titus Bode Law > **[Titius–Bode law](https://en.wikipedia.org/wiki/Titius%E2%80%93Bode_law)** > > The Titius–Bode law (sometimes termed simply Bode's law) is a formulaic prediction of spacing between planets in any given planetary system. The formula suggests that, extending outward, each planet should be approximately twice as far from the Sun as the one before. The hypothesis correctly anticipated the orbits of Ceres (in the asteroid belt) and Uranus, but failed as a predictor of Neptune's orbit. It is named after Johann Daniel Titius and Johann Elert Bode. > Later work by Mary Adela Blagg a... The relation goes something like 4,7,10,15… > [@](#): > > Orbital resonance from major orbiting bodies creates regions around the Sun that are free of long-term stable orbits. Results from simulations of planetary formation support the idea that a randomly chosen stable planetary system will likely satisfy a Titius–Bode law. basically, planets “sweep out” anything too close or in an an orbit that has a harmonic relationship. So basically, what you need is a system where the goldilocks zone covers 2 or more such orbits. Of course, what cosntitutes habitable is debatable. A planet more massive than earth might have a moon the size of Mars, with the ability to hold an atmosphere. Something the location of Venus, with the right Earth-type atmosphere and highly reflective clouds, might be habitable, especially at the poles. After all, a hundred-plus day and night does not appear to affect Venus’ current thermal distribution… Keep in mind, the current Earth atmosphere is a byproduct of life. Otherwise, our atmosphere might be mainly CO2 and N2. That pesky chlorophyl keeps splitting the CO2 into O2. ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [December 7, 2012, 3:49pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/10 "2012-12-07T15:49:06Z")
> [@ekedolphin](#): > > Is it theoretically possible to have more than one habitable planet in the same solar system, or would the necessary proximity of the two planets make that rather unlikely? (I mean, naturally habitable; not something created by futuristic terraforming, nor glass-enclosed domes). Does science have an answer for that yet, or is the fact that we haven’t discovered extraterrestrial life yet make the answer mere speculation? Yes, it’s certainly possible. “Goldilocks zone” is a fairly smudgy area, and it would be easy to change the size of our sun a bit and put Venus, Earth & Mars, and maybe a couple more in a Goldilocks zone. Nitpick for everyone: since the demotion of Pluto, planets and moons have pretty unambiguous definitions, so a habitable moon cannot be a habitable planet. 🙂 ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [December 7, 2012, 3:56pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/11 "2012-12-07T15:56:04Z")
Paper on “[Constraints on the habitability of extrasolar moons](http://arxiv.org/abs/1210.5172)” if anyone is interested. from the Abstract > [@](#): > > Detections of massive extrasolar moons are shown feasible with the Kepler space telescope. Kepler’s findings of about 50 exoplanets in the stellar habitable zone naturally make us wonder about the habitability of their hypothetical moons. Illumination from the planet, eclipses, tidal heating, and tidal locking distinguish remote characterization of exomoons from that of exoplanets. We show how evaluation of an exomoon’s habitability is possible based on the parameters accessible by current and near-future technology. ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [December 7, 2012, 4:12pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/12 "2012-12-07T16:12:06Z")
Speaking of habitable moons, our own (Saturn’s) [Enceladus](http://www.space.com/18792-enceladus-sample-return-mission.html) is a life candidate. It’s believed to have a liquid ocean under an icy shell, warmed by tidal forces with another moon. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 7, 2012, 5:26pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/13 "2012-12-07T17:26:09Z")
What if Mars and Venus were simply swapped? The thicker Venusian atmosphere would make it warmer than Mars is at that distance. You might be able to have a shirtsleeves environment there. ---
**Author:** ![Peremensoe](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@Peremensoe](https://boards.straightdope.com/u/Peremensoe)\ **Post date:** [December 7, 2012, 5:58pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/14 "2012-12-07T17:58:49Z")
> [@levdrakon](#): > > Nitpick for everyone: since the demotion of Pluto, planets and moons have pretty unambiguous definitions, so a habitable moon cannot be a habitable planet. 🙂 That’s why I say “worlds.” ---
**Author:** ![Projammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/projammer/32/559_2.png) [@Projammer](https://boards.straightdope.com/u/Projammer)\ **Post date:** [December 7, 2012, 7:03pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/15 "2012-12-07T19:03:38Z")
Another thing to keep in mind is that the Goldilocks zone is relative to Goldilocks being Terran. While that’s pretty much the only scale we have to go by right now, there are examples of life in absurdly hostile environments right here on Earth. There are extremophiles which are quite content at temperatures above the boiling point of water. So with the ability to maintain liquid water being the simplest definition of the Goldilocks Zone we already know of exceptions. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [December 7, 2012, 8:06pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/16 "2012-12-07T20:06:07Z")
> [@ZenBeam](#): > > What if Mars and Venus were simply swapped? The thicker Venusian atmosphere would make it warmer than Mars is at that distance. You might be able to have a shirtsleeves environment there. If a planet in the outer part of the Goldilocks zone had a high concentration of CO2, then it could be warm enough for some kind of life; but too much CO2 would be deadly to humans, even if there is plenty of oxygen. The outer edge of the goldilocks zone is sometimes defined at the location where CO2 starts to freeze out, as frozen CO2 can’t trap heat like gaseous CO2 does. But a planet with a thick CO2 atmosphere wouldn’t really be a shirtsleeve environment, unless you are wearing shirtsleeves and scuba gear. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 7, 2012, 11:17pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/17 "2012-12-07T23:17:34Z")
That counts! ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [December 7, 2012, 11:48pm UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/18 "2012-12-07T23:48:01Z")
Double planets may be a possibility. Our own earth-moon system is approaching that. Even if not a double planet, one planet could host multiple habitable moons. One definition of a double planet would be that the center of gravity of the pair(which they both orbit) is outside the larger of the two. (it will always be outside the smaller of the pair) ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [December 10, 2012, 1:21am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/19 "2012-12-10T01:21:43Z")
> [@tdn](#): > > Sure, but wouldn’t the planet have a pretty huge magnetic field? Wouldn’t that make life unlikely? No, many planets have no significant magnetic field (Mars for instance). Earth has an unusually large one, supposedly due to having two planets’ worth of iron at our core (from the same collision that created the moon). Indeed our field probably helped life here, as it created the van Allen belts that shield us from much cosmic radiation. But why would an even larger magnetic field make life unlikely? We can live in an NMR scanner for an hour or more perfectly fine, and that has a field millions of times stronger than the earth’s. ---
**Author:** ![stui\_magpie](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stui_magpie/32/11010_2.png) [@stui\_magpie](https://boards.straightdope.com/u/stui_magpie)\ **Post date:** [December 10, 2012, 2:26am UTC](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280/20 "2012-12-10T02:26:32Z")
Why couldn’t you have 2 planets in the same orbit but on opposite sides of it? Like having another Planet Earth in exactly the same orbit but that we can never see because it’s always behind the Sun. That would make space travel a piece of cake, just kick out of Earths gravitational pull and wait 12 hours for it’s twin to arrive. [Next page](https://boards.straightdope.com/t/multiple-habitable-planets-in-the-same-solar-system-theoretically-possible/643280.md?page=2)