I want to have 3 different habitable BY HUMANS planets in a single system, I don't know much about physics so I bought Universe Sandbox2 and I'm trying to build solar system with 3 habitable planets in the goldilock zone. They all have separate orbits (no Lagrange points, orbiting gas giant things). From reading other threads most posters suggested they're unstable. I want their surface gravity to be from 0.8g to 1.6g and humans to be able to live there. I don't have any other requirements hot, cold, dry whatever.

I expected to have one dry planet on the beginning of green zone (Tattoine like), 1 Earth Like planets, and one planet with thick atmosphere near the end of the green zone and beginning of blue zone (liberal habitable zone estimate).

But from my experiments I'm only able to put:

  1. Earth with no CO2 on the border of the red & green zone
  2. Normal Earth on the beginning of the green zone
  3. Earth with 20 times more CO2 in the middle of green zone (Is it liveable?)
  4. Snowball Earth no matter how much CO2 I have

I tried playing with obliquity, mass, atmosphere density nothing helps.

Could someone more knowledgeable advise what kind of planets should I put?


From the comments I see that 4 is too many planets in separate orbits, so I changed it to 3. From my simulation Earth is on the beginning of the green zone which is weird because I expected a dry planet. What kind of planet should the next 2 be in order for humans to be able to live there.

Please note that I'm using Universe Sandbox 2 just to play around, I'm interested in the general type of the planets, something like :

  • in the middle of the green zone the put planet with 4 times more CO2 and less water surface

  • farthest planet should be larger then Earth, with thick atmosphere and high Volcanic activity.

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    $\begingroup$ You neglected a variable, atmospheric density. $\endgroup$
    – Feyre
    Nov 6, 2016 at 14:12
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    $\begingroup$ Your real problem here is that you want four distinct orbits within the habitable zone. I don't think the habitable zone is large enough to accommodate that in a stable system: the planets are simply too close together. At least one of the planets is going to crash into the others, and in an astronomical time scale that's going to happen very quickly. You could probably get away with two orbits, but not four. $\endgroup$
    – Palarran
    Nov 6, 2016 at 15:34
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    $\begingroup$ @Palarran: I agree with your statement, but the risk is not just the possibility of them colliding; the problem is that their orbits will perturb each other so greatly since they will pass so closely to the other orbits, resulting in unstable orbits. Unstable orbits may mean colliding planets, but more probably just changing orbital solar distances (like a comet orbit). Average orbit might remain the same, but if most of the time they are outside habitable zone it will always be freezing or frying any life. A possible partial solution: orbits not in the same plane. Less chance of perturbing. $\endgroup$ Nov 6, 2016 at 15:45
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    $\begingroup$ Seeing that you want precise values instead of general guidelines, I won't post an answer (as I don't have Universe Sandbox on my system). I would like to suggest that other than CO2, you might also want to invest in some methane. It is an order of magnitude more powerful than CO2 in its greenhouse effect. Also, you would want to have lots of volcanoes on your green-blue boundary planet. This would help warm the atmosphere with near-constant volcanic activity. You would need to put a lot of radioactive elements in this planet's core for this. $\endgroup$ Nov 6, 2016 at 16:34
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    $\begingroup$ @YoustayIgo I just use Universe Sandbox for playing around, and I don't know how good the simulation is, Answer that explains general type of the planets is much appreciated $\endgroup$
    – Soba
    Nov 6, 2016 at 16:37

4 Answers 4


Parent Star

Before we get to the orbits of planets, we first need to determine the most important variable in your system: what is the size and age of your star? Obviously, habitable zones around stars depend primarily on the heat emitted by the star. This, in turn is dependent on the size/mass of the star and also its age.

For the sake of convenience, lets say we have a main sequence star at the center of our hypothetical solar system. For those who don't know, a main sequence star is one which is fusing hydrogen into helium in its core. This means, we are excluding stars like red giants, white dwarfs, neutron stars and other such types.

Out of several different types of main sequence stars, I would prefer keeping a B type star at the center of our solar system. They are 2 to 6 times larger than the sun and have surface temperatures ranging from 10,000 to 30,000 kelvin (as compared to 5000 kelvin for the sun).

The advantage of this excessive heat generation is the habitable zone will be all the more wider than the sun's habitable zone. For convenience sake, we would pick a star which is at the lower end of B type. That is, one that has a mass of nearly 2 solar masses and a surface temperature of 10,000 K.

Habitable Zone Of Our Sun

Before we get to the habitable zone of our B type star, a word about the habitable zone of our own sun would be helpful. This wikipedia article (which is quite interesting btw, unlike most wiki articles) states that the habitable zone around our sun extends from Venus' outer limit of orbit (109 million km) to as far as the inner side of the asteroid belt!

Habitable Zone Of A B-Type Star

While I don't know with absolute certainty at which distance from our B type star, its habitable zone will begin, it should be safe to assume that distances of 1 AU to 4 AU will fall in the habitable disc.

Planetary Distances And Composition

Of course you would want to keep the planet at the inner edge (1.2 AU) to be a small, Mars-mass object with relatively thin atmosphere rich in oxygen. Greenhouse gases as less as possible and lesser water content on the planet so that a runaway greenhouse effect has a chance to be reversed naturally. Also, make sure to keep the planet's core active (for a strong magnetic field) but make sure that there is almost no volcanism on the planet, as that may likely trigger runaway greenhouse effect.

The main risks for losing habitability for this planet would be the loss of atmosphere through solar wind. A strong magnetic field would be critical in keeping this from happening. Also, considering that this planet is the innermost of all the habitable planets in this system and the stellar type is B, you would want a thick layer of ozone to shield it from the deadly UV rays. Also, solar flares and magnetic storms would be potentially deadly.

In the middle (2.1 AU) you would want to place an Earth-sized planet with earthly atmospheric density and composition. A small proportion of greenhouse gases. Some volcanism and 55-65% water covered area.

This planet, being in the center of the habitable zone, will be the one with least risks. Even if it temporarily loses its habitability status due to a gigantic meteorite collision, it will naturally regain its habitability within a couple million years or so.

On the outer edge (3.8 AU) you would want to keep a super-Earth. A planet with 3 Earth-masses, thick atmosphere, rich in carbon dioxide and lots of volcanic activity. You would also want to have many organisms producing methane, as it would help in the greenhouse effect.

The hazards for this planet would be any perturbation in its methane production chain or too much increase of flora on the planet, that could trap most of the carbon dioxide from the atmosphere. Also, any snowball events would have a dangerous tendency to be perpetual, ending its habitability status.


I strongly recommend you to read this article. I have found it to be very knowledgeable and written in very simple language.


Let's start with the number of planets. As you've been told, 4 orbits is almost certainly too many - their interactions will almost certainly cause instability. In the worst case you get collisions, of course, but that is wildly unlikely. Much more likely is the ejection of one or more planet from the Goldilocks zone for at least part of its year.

However, as the Navy Seals like to say, "If you ain't cheating you ain't trying." So how about finessing the problem. There is no rule saying that the planets have to be singletons. So make the planets dual systems, like the earth-moon but with equal sizes. You could have one pair close in, and one pair farther out. I wouldn't even try justifying the creation of two such pairs in a single system, but there's nothing I can think of that prohibits it. Assuming formation by collision (as is the current thinking about the Earth/Moon system) you could invoke an asymmetric distribution of volatiles as the impactor strips away the crust from the other, and the resulting water-rich debris settling preferentially on one of the two resulting objects. This will give you a wet/dry warm pair and a wet/dry cold pair.

Please keep in mind that cold/dry is going to have a lot of trouble supporting life. You are focusing on CO2 as a dominant greenhouse gas. In the case of the Earth, it's not. It's water that does the job. CO2 is much more persistent than water vapor, but there's a whole lot more water vapor in the atmosphere. Likewise, hot/wet runs the risk of runaway greenhouse as the increased temperature causes greater evaporation, leading to a closed loop with positive feedback.

Depending on vulcanism for long-term CO2 production is not a good idea. On earth, it's a mark of plate tectonics, and this will periodically produce eras of little activity. Furthermore, volcanic emission of CO2 is also accompanied by all sorts of nasties like sulfur dioxide and ash, which will make the surface not real hospitable to life. An alternative to plate tectonics is tidal friction, such as occurs on the Jovian moon Io. However, this again is not a good idea in the long run, as in the long run the two planets will become tidally locked and the volcanic activity cease.

Methane is a bad choice as a greenhouse gas. The problem is not that it doesn't work. It does. It's a good deal better at trapping heat than CO2. The problem is that it doesn't persist in the atmosphere in the presence of a biosphere. It oxidizes in very short order, decades or less compared to centuries for CO2. Worse, its effectiveness invites wild swings in concentration and resulting temperature. See the potential problems of oceanic clathrates.

You can specify whatever surface gravity you like. For a given density the surface gravity is proportional to the radius of the planet, but running in the range of 0.5 to 1.5 gs only produces planetary masses with a mass range of 27 to 1, which are probably not a real problem. Certainly the result would be nothing comparable to Jupiter and Saturn, and the solar system seems pretty stable with them in the house.


Remembering that the setting has to match what you're trying to accomplish:

Inhabited by whom?

You want three or four habitable worlds orbiting the same star. Do they all have to be inhabitable by humans? If so, then the bounds are much tighter; your planet has to have water in its liquid phase on the surface at some point during the year, and have an atmosphere breathable by humans and a gravity that humans can withstand.

Will ab-humans work for your story (allows you to discuss the inter-racial issues as a metaphor for real-life racial issues)? If so, then you can get away with a somewhat broader gravity range, allowing for more variability in the other parameters.

Will completely unrelated intelligent species work? If you can have races that don't breathe oxygen, or don't breathe at all, then the goldilock zone extends out much farther. You can simply hand-wave an alien metabolism (which breaths ammonia and for whom oxygen gas is horribly poisonous), necessitating environmental suits for anyone visiting a planet with a different environment.

This last option can be adapted to serve your narrative. Maybe a system in which race A (humans), race B (an insect-like species that prefers a 55C environment), race C (ammonia breathers), and race D (non-breathing frigid species that prefer temperatures in the single-digit Kelvin range) can all live at once in relative comfort, makes it an ideal location for the headquarters of your galactic empire (in which these four races are predominant).

How fast is travel from place to place?

If you only want to have three or four planets in the same system because you want humans to travel from one planet to another in the space of time measured by hours or days instead of by years or generations, remember that you don't really need this for a fictional tale. You can put the planets in three star systems that are close to each other, and simply hand-wave a drive system that gets from one star to the other as quickly as you need for your narrative.

And added bonus is that by author's fiat you can establish that three closely-located star systems, with an inhabitable planet or two in each, is a rare occurrence in the galaxy, and that this makes the three systems strategically important. More grist for your plot.

  • $\begingroup$ Inhabited by Humans. I want one to be mother planet and two other which people colonized. I don't want to hand wave the physics behind the drive, that's the big part of my story. $\endgroup$
    – Soba
    Nov 7, 2016 at 21:03

There can be no oxygen without life

Note that a copy of Earth without indigenous life would have a CO2 and nitrogen dominated atmosphere with almost no free oxygen, just as Earth had before life formed, and a long time after that. A lifeless planet really cannot have a human-breathable atmosphere because free oxygen at that concentration is highly reactive and will simply react with the surrounding minerals.

The existence of e.g. 20% oxygen atmosphere implies the following:

  • Something is producing oxygen at very large quantities, so the planet is full with life - possibly but necessarily microbial, likely but necessarily using energy from the star, but definitely a lot of it;
  • That life has been there for a long time - it took many millions of years to saturate the eagerly oxidizing minerals (in part, rusting all the iron) until the oxygen could accumulate without being absorbed as quickly as it can be produced.

A planet with human-breathable atmosphere is not only habitable, but already inhabited.


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