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?
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.
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.
A gas giant with multiple habitable moons is another possibility.
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.
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 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.
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.
Sure, but wouldn’t the planet have a pretty huge magnetic field? Wouldn’t that make life unlikely?
The distribution of planets tends to be an interesting mathematical arrangement - which IIRC means they are in “non-harmonic” orbits - Titus Bode Law
The relation goes something like 4,7,10,15…
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.
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. 
Paper on “Constraints on the habitability of extrasolar moons” if anyone is interested.
from the Abstract
Speaking of habitable moons, our own (Saturn’s) Enceladus is a life candidate. It’s believed to have a liquid ocean under an icy shell, warmed by tidal forces with another moon.
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.
That’s why I say “worlds.”
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.
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.
That counts!
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)
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.
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.