# How to arrange three identical habitable planets in one solar system in similar or, if possible, same orbit?

I gather that we can't have three earth's around our sun. But would it be possible to have three earths around a sun that was much larger than our sun?

Would the gravity of such a massive sun neutralize the destabilizing effects of planets interacting with each other's gravity?

A larger sun would have a wider habitable zone farther away than ours. Will a larger orbit mean that the planets' do not affects each other even when in the same orbit given how far away they would be when at 120 degrees?

Alternatively, with a wider habitable zone, can we have three stable orbits with negligible difference in irradiance, such that their climate's mostly be same?

Ultimately, is there any way to fit three earth-identical planets in simple enough orbits such that making calendars does not require a full course in astronomy? And will having other uninhabitable planets have a negative impact on these arrangements?

PS: Naturally stable orbits without requiring magic or fictional technology

• Where do you see that the conclusion of those questions holds only for the Sun?
– L.Dutch
Commented Jul 7 at 7:07
– JBH
Commented Jul 7 at 19:19
• Of course you can: put the 3 planets at the vertices of an equilateral triangle, with initial velocities the same in amplitude and directed 120° apart on the common Ecliptic plane: they will run in similar elliptical orbits, with one of the focii their common center of mass and their main axes 120° apart. Put the Sun at this common CoM: this changes the excentricities of the orbits, leaving the other orbital elements unchanged.The configuration has questionable stability, though. Commented Jul 8 at 6:52
• @FrançoisJurain not questionable, it's completely unstable. If you want anything remotely stable you need two of the planets to be much smaller than the third, and to be +/- 60 degrees each at the Trojan and Greek Lagrangian points Commented Jul 8 at 8:55
• @FrançoisJurain Questionable stability meaning what? Planets ripping each other apart within a thousand years or Eventual collision in a couple million years? Can we squeeze out a million years of habitability from them before extinction event comes around?
– EMS
Commented Jul 8 at 10:21

Probably not, if the three Earth-sized planets are roughly the same size.

If you have just the Sun and the Earth, there are five points that orbit with the Earth where a satellite can be parked, and not have to use fuel to stay on station. These are the Lagrange points. Three of them are unstable: push the satellite off the point and it won't come back. Two (Lagrange points 4 and 5) are stable. These points are on the Earth's orbit, 60 degrees ahead and 60 degrees behind the Earth.

The potential well about Lagrange points 4 and 5 is a long, thin valley. It is not particularly good at trapping light bodies. The orbit of Cruithne is currently about one of these Lagrange points, but it can move its orbit from one point to the other, so its long-term orbit is confined to a horseshoe shape that orbits with the Earth.

It is tempting to try packing an orbit with planets at 60 degrees to each other, so that they are all sitting in each other's 4-5 Lagrange points. This site explores a number of geometries. A Klemperer Rosette is a similar arrangement but with large and small bodies about a sun. They are stable in the mathematical sense, but they would probably not be stable in a real orbit for long. The only naturally occurring configuration known is the Klemperer Rhombus, which is a planet with small bodies at Lagrange points 4 and 5, like Jupiter and the Greek and Trojan asteroids.

• What about different orbits in the habitable zone? How far apart will they have to be? Can all three planets have mostly similar climate and exact same year length?
– EMS
Commented Jul 7 at 11:37
• @EMS No. If the planets have the same length year, they have the same average orbital radius. The nearest I can come up with is some gas giant in the habitable zone, with a set of habitable moons. In this case, the 'year' would by the year of the giant planet. Commented Jul 7 at 12:14
• How about tilting the orbits? Three orbits of same average radius but tilted at different angles? How stable will that arrangement be?
– EMS
Commented Jul 7 at 16:19
• @EMS Probably not. Planets condense from clouds of matter. Random collisions will turn kinetic energy into heat, but cannot destroy angular momentum. A cloud of debris will turn into a flat disc, with the particles in nearly circular orbits. These then condense into planets with circular orbits in a common plane. Commented Jul 7 at 17:12

So you want to have three habitable planets orbiting a star and you want them to have orbits as similar as possible or even identical, so you don't have to do a lot of work making up different calendars for those worlds.

Maybe those worlds could have quite different orbits, but someone on this site will designed there different calendars for them for you.

Anyway, there is a blog, PlanetPlanet, by astrophysicist Sean Raymond. which has a lot of discussions of various fictional solar systems and their degrees of plausibility.

It has a section called the Ultimate Solar System where Raymond designs solar systems with as many habitable planets as he can think of ways to make plausible.

https://planetplanet.net/the-ultimate-solar-system/

In the Ultimate Retrograde Solar System

Raymond mentions a scientific paper which shows that it is possible to have planetary orbits closer together in a solar system that thought before. This makes it possible to have more planets orbiting in the habitable zone of the system.

This is done by putting planetary orbits in alternating directions of movement. ONe planet moves in the prograde direction, the second in the retrograde direction, the third in the prograde direction, the fourth in the retrograde direction, and so on.

Raymond says, however, that a system of closely spaced planetary orbits with alternating directions could not form naturally (his occupation is modelling the formation of solar systems). Thus such a star system would have to have been created by super advanced beings millions or billions of years before your story.

I'm going to bed now my computer is acting up. But you really should read the following blogs. In the next blog post: The Ultimate Engineered Solar System.

https://planetplanet.net/2017/05/03/the-ultimate-engineered-solar-system/

Raymond mentioned a scientific paper:

Which claims that a bunch of planets, up to 42, could share a single planetary orbit if they were equally spaced. And Raymond says he has run similations showing that such a ring ofplanets could have stble orbits for billions of years.

And naturally, with his desire to create star systems with as many habitable planets as possible, Raymond speculated about several concentric rings of planets in the habitable zones of stars, each ring orbiting in the opposite direction to the rings in the next inner and outer orbits.

He created a system with six orbits of 42 Earths each for a total of 252 earths in the system, if all orbit in the same direction. With counter rotating rings of planets, Raymond got a system with 8 rings of 52 Earths each for a total of 416 planets in the habitable zone.

Of course the queston only asked about three habitable planets sharing a similar or idendical orbit.

Maybe EMS will happy to have even more than 3 habitable planets sharing the same orbit in their fictional star system.

But maybe the writer has a story planned that requires three and only three Earth like habitable planets sharing the same orbit. Since the minimum number of planets sharing the orbit has to be seven, that will leave at least four other planets in the orbit which have to be eliminated from the plot for some reason.

A molecule with chirality means that its mirror image is different from it. like a right human hand is different from a left human hand. Many ammino acids have chirality. And I think that animals can onlydigest organic materials with the proper "handedness".

If that is the case, there could be three planets in the orbital ring where the organic moleclules have the proper "handedness" for the lifeforms in your story, and four or more planets in the orbit where organic molecules have the opposite "handedness" and where life forms with opposite "handedness" biochemestry can live.

Or possibly three of the planet in the orbit have surface temperatures similar to Earth's. And possibly four or more planets in the orbit are "snowball Earths" covered by planet wide icecaps. Possibly there are liquid oceans below the icecaps, and possibly life, even intelligent life, lives in those oceans.

Or possibly the planets sharing the single orbit don't all have to have the same mass. Maybe three have a mass similar to Earth's and are habitable for humans and lifeforms with the same requirements. Maybe three are much smaller than Earth and are not habitable for humans, but do have lifeforms, possibly including intelligent life, on them. Maybe three are more massive that Earth and have surface gravities over 1.25 or 1.50 that of Earth, and no Earth lifeforms would want to live on them, but they do have life adapted to higher gravity and maybe intelligent life.

And there is another post, "Cohorts of co-orbital planets":

https://planetplanet.net/2020/11/19/cohorts/

where Raymond suggests that a ring of co-orbital planets doesn't have to be complete, there be a set of co-orbital planets in a arc segement of the orbit.

Raymond discusses the stability of various configurations, including cohorts of three co-orbital Earth like planets.

So unless you want to go with more than three habitable planets sharing the orbit, you should go with a set of three Earths sharing an orbit, spaced by 20 Hill radii along the orbit.

• A most of these configurations seem not only unnatural, but very unstable, especially in the presence of other inner and/or outer planets. Commented Jul 10 at 14:16

THE SAME ORBIT

You can have a Jupiter-like planet that traps Mars-sized objects at its Lagrange points L4 and L5 60 degrees ahead and behind it. But as has already been said, these points are not particularly good at trapping large bodies.

You can also have several Mars-sized moons orbiting the Jupiter-like planet. The most plausible arrangement is perhaps two Mars-sized habitable moons orbiting the Jupiter-like planet and a third Mars-sized body at one of the Lagrange points.

All bodies in this system would be subject to the same orbital period, hence fulfilling your requirement of having the same year. The moons could be locked in orbital resonance with each other (as several of Jupiter's moons are) giving them a common reference for months. They also possibly be tidally locked so that they always have the same face toward the primary planet, giving them a common reference for days.

Jupiter's 4 largest moons are known as Galilean moons (after their discoverer.) The inner 3 Galilean moons Io, Europa and Ganymede have orbital periods in a resonance of 1:2:4 (the longest of these is approximately 7 earth days.) The orbital period of the outer one, Callisto, does not have an integer relation to the other three. All four bodies are however tidally locked (have the same face toward Jupiter at all times.)

The tidal influence of Jupiter on these bodies is very significant. Tidal heating is is a major cause of geologic activity on Io and Europa, which have sulphurous and H2O volcanoes respectively, though the mean surface temperature is not significantly affected.

It is certainly possible to have a Jupiter-like world a little warmer / closer to the sun (it might need to be even slightly more massive to compensate for the increased tendency for atmospheric boiloff) and such a body might have numerous moons of habitable temperature. As can be observed from Jupiter's moons there are several factors (tidal heating leading to excessive geological activity, radiation belts, etc.) that make having more than 2 moons which are truly habitable unlikely. Hence we can consider the option of a third habitable body at one of the Lagrange points, which is also more interesting.

SIMILAR ORBITS

In terms of the size of the star, larger stars have wider habitable zones but burn faster and die quicker. Planets can't be too closely spaced so it's unlikely there would be more than two planetary orbits in the habitable zone. For example in our solar system, Venus's orbit is about 75% the diameter of Earth's, and Mars's orbit varies in the range 138% to 169% of Earth's. It's unlikely that planets would be closer to each other than that. Earth's gravity does affect the rotation of Venus (it rotates extremely slowly and is tidally locked to Earth.) Mars's orbital period is approximately twice that of Earth's and is already fairly eccentric. It would not take too much modification of Mars's orbit (in a simulation) to bring it into a 2:1 resonance with Earth.

• This doesn't have to be limited to Jupiter-sized planets. A super-Jupiter up to 20x more massive than Jupiter would still not quite be a brown dwarf yet, and could easily accommodate Earth sized satellites and at it's L4 and L5 points. "Naturalness" is still a problem, but this would easily address the instability problems of most of the other answers here, Commented Jul 10 at 14:14

## A dense Asteroid Belt with especially large "Dwarf" planets

True planets by definition have cleared thier orbital; so, to begin you should start by looking at dwarf planets which are planet like asterial bodies (large enough to become spherical under thier own gravity), but share thier orbitals with other similar bodies. Ceres, Vesta, and Pallas are 3 dwarf planets in the asteroid belt which are each as large as a mid-sized moon.

The reason the Asteroid belt exists is because the gravity of Jupiter causes a wave pattern in the orbital paths of asteroids that disrupts any attempt for the asteroid belt to coalesce under its own gravity... but this has not stopped several large, mostly permanent dwarf planets from forming. If you were to form a solar system in an overall denser area of space with a large gas giant closer to the sun than Jupiter, then you would get a dense asteroid belt inside of the Goldilocks zone that could form several Earth sized worlds but not be able to merge into one planet because the gas giant would cause them to harmonize with its own orbit so each dwarf planet follows a repeating wave pattern that never intersects with the other harmonic paths.

• And would these 'dwarf' planets would be stable long term? i.e. would they have fixed orbital periods and no gravitational disruption from other asteroids? Basically can they support life similarly to our planet? (I suppose meteor falls would be regular phenomenon.)
– EMS
Commented Jul 10 at 3:39
• @EMS Fixed orbital periods, not exactly, but it would follow a repeating pattern, and the pattern could be stable enough to make sure the planets never hit each other even though they may occasionally pass rather close. Commented Jul 10 at 15:37

In response to the inquiry regarding the possibility of three celestial bodies orbiting one another in a habitable configuration, as depicted in the provided image, without the aid of advanced technology or supernatural forces, the answer is negative. Such a scenario, closely resembling the illustrated arrangement, cannot support habitability or even maintain stability. While it may be theoretically feasible for a brief cosmological period during the formation of a planetary system, practical examples, such as Mars and its moons, demonstrate the eventual collision or ejection of celestial bodies due to gravitational interactions. In the specific case presented, involving Earth, Venus, and Mercury, their relative proximity compared to the depicted arrangement further emphasizes the improbability of such a configuration supporting habitability or stability. There will be points in time where the gravity of 2 plants will combine to pull on the 3rd. And when it's 2 biggest pulling on the smallest would be immense. If earth was the smallest the oceans would all be pulled to the side of the planet closest to the other 2 flooding the land on half the planet shifting Glaciers that would now become ice burgsthe size of continents, then when orbit changed all that water would be sent rushing back with giant chunks of ice to Scour and destroy everything in its path. Destroying anything that may had been built in it's path. Not to mention the extreme change in gravity could rip trees out of the ground only to cone crashing down. If you want to see something in the same general idea as this watch 3 body problem (place where aliens tested humans if they could come up with solutions shows this.

As for you question about multiple planets in the same orbit around the sun answer is yes very possible. No issues with 2 at all as long as travel at same speed and mass is the same as long as they were on opposite sides of the star there be no issues whatsoever. 3 most likely can be done also 4 going say be most and be a lot trickier to have them truly be in stable orbits. Also anything more the 2 would be temporary since over the course of the life of the star it changes it's size and energy output but that still give plenty of time for Civilization to develop and find a new home which be much easier since you could get to your closest plant behind you in orbit in just under 4m without any use of propulsion besides the speed left over escaping earth gravitational field and with simple technology like a solar sail could cut that time by alot weeks instead of months.

• "No issues with 2 at all" Please show how you determined that the configuration would be stable. Commented Jul 7 at 10:00

My answer here is rather more supplementary than anything - but a lot of people are pointing out that three planets is a lot... which is fair, for sharing the same orbit. I know lagrange points were mentioned, and I think that if you have a pretty broad habitable zone it wouldn't be inconceivable to have, say, two orbits in the habitable zone. My suggestion being here that, if you want, you could put two of these planets in a binary orbit with each other. Binary planetary systems are definitely real (although, to be fair, this would mean that those two planets would not be "identical" as you said. This being especially so if you're putting the other planet closer or further away from the star)... But in any case I think this would make things easier, and would allow you to have three habitable planets but to treat it like two planets (at a larger scale) if you wanted to avoid some of the potential extra work that comes of this situation. If you want to test out your ideas, I would recommend playing around a little with this gravity simulator.

The following is likely to be gravitationally stable for geological time periods, although unlikely to occur naturally. Have earth-sized planets A and B mutually orbiting each other as close as possible to each other (but a bit outside their mutual Roche limit) around a sun like ours. So they might be separated by 40-50,000 km. To avoid ridiculous tides and enormous earthquakes it is best to assume that they are tidally locked to each other, but at 40-50,000 km separation the orbital period is about 25-35 hours, so thay can both have reasonable 'days' relative to the sun. If the mutual orbit is significantly tilted relative to the orbit round the sun, they won't spend too much time in each other's shadow.

Now you just need to add a third non tidally locked earth C at about twice the earth-moon separation from the centre of mass of A and B. It will happily orbit around the other pair with an orbital period of 6-8 weeks or so (although you mat want to pick a separation so C's orbital period is either in exact resonance or anti-resonance with the AB orbital period,to ensur elong term stability of the orbits). It can rotate at whatever rate you want, no need to be tidally locked. The whole system should be stable for megayears. Because twice (or even three times) the earth-moon distance is small compared with the earth-sun distance, the variation in distance from the sun resulting from the local orbits wouldn't ahve any significant effect on solar radiance of earths A, B, or C, so they could all have an earth-like climate.

That avoids the need to locate the planets at Lagrange points, and the instabilities that would likely occur over long time periods for Lagrange-point orbits.