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I'm trying to build as close to a 1:1 Earth as possible, but finding information on a non-jovian moon-based planet is extremely hard to find. I tend to ramble trying to explain things in a non-list form, and I don't have to level of comprehension in math to work out the equations I'm finding on my own, so I'm going to try to keep this small and concise.

  • Is an Earth like planet even possible around a binary star? I've seen a lot of discussion on Jovian planets and Super Earths, and this led me to the most plausible way for this to happen is a P-type orbit, but smaller planets aren't mentioned outside of moons.

I'd also like to take a second and thank everyone who reads this or posts here. Time and time again in my searches this place has popped up with the closest thing to an answer I could find for some of my more out there questions, and in quite a short amount of time has proved invaluable to learning about things theoretically possible in our universe; so thank you!

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    $\begingroup$ Welcome on Worldbuilding. What do you mean with 1:1 Earth? Can you clarify it? Also, can you limit your question to 1 problem per time? $\endgroup$
    – L.Dutch
    Jul 19, 2017 at 5:39
  • $\begingroup$ Sorry, I'm new here and not familiar with the formatting. I've fixed my post. And my question boils down to is it possible to make a planet nearly identical to Earth in terms of living on the planet. Obviously the time to complete an orbit around the binary stars would be longer than around our sun and that would have an impact; Im just not sure what kind. $\endgroup$
    – 1312412
    Jul 19, 2017 at 6:10
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    $\begingroup$ Welcome, 1312412, a nice question. I am going to suggest editing your tags by replacing the hard-science tag with the science-based tag. Here hard-science means answer with equations and citations to research papers. Science-based will provide all the information you need. Have fun! $\endgroup$
    – a4android
    Jul 19, 2017 at 7:12
  • $\begingroup$ There is a Wikipedia article. $\endgroup$
    – Alexander
    Jul 19, 2017 at 18:20
  • $\begingroup$ @Alexander Yeah, that was my initial source, but it only has information on giant planets versus the as close to earth as possible. Its definitely useful though, thank you for posting it here for posterity. $\endgroup$
    – 1312412
    Jul 19, 2017 at 20:44

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Yes, with no caveats. The stars in a binary star system can be extremely far apart, and if they are sufficiently far apart then an Earth-like planet in orbit around one of them will not be disturbed by the presence of the other. To pick a nearby example, if Alpha Centauri B did not exist then a planet around Alpha Centauri A would not be bothered by Proxima Centauri aka Alpha Centauri C, 13,000 AU away (about a fifth of a light year). It would just be a faint (magnitude 5) red star with a very fast proper motion.

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  • $\begingroup$ so there is a caveat, the distance between the stars. The mass of the planet of course also plays a role (but the mass of the earth is negligible compared to that of the stars so can probably be ignored, unless maybe the secondary star is a very small brown dwarf). $\endgroup$
    – jwenting
    Jul 19, 2017 at 9:07
  • $\begingroup$ well dude, that is precisely the caveat! heh! $\endgroup$
    – Fattie
    Jul 19, 2017 at 14:10
  • $\begingroup$ ... as jwt just said. the fact is the OP is thinking of a planet with "two suns in the sky" ("just like in Star Wars" sort of thing). Generally, you can't have that. $\endgroup$
    – Fattie
    Jul 19, 2017 at 14:11
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Yes with caveats. A binary Star System with an earth sized planet is an example of the three body problem, and the orbits are mostly unstable. However, since a planet's mass is negligible compared to the masses of the two stars, there will be stable regions known as Lagrangian Points. Of these the L4 and L5 Lagrangian points would be the best places to put a planet since they are the most stable.

If we assume a system of two 1 solar mass stars at distance of 1.41 AU from one another, then a Planet at L4 will be will receive the same luminosity from the two stars that the earth receives from the sun (L = k/d^2 -> L = 2*(L_sun/(1.41^2)) = L_sun).

I can't think of any serious issues for having two 1 solar mass stars being separated by 1.41 AU, but they would complete an orbit a bit faster than the earth orbits the sun - a back of the envelope math suggests a year is about 0.84 years so 10 months. Seasons are a function of the tilt.

Probably most dramatically, since the two stars are spaced out by 60 degrees, you basically have only 8 hours of night on the equinox (assuming a 24 hour period. The total luminosity reaching the planet should be the same, so I'm not sure how this will play out, but it would make the days 33% longer, and maybe mess with the weather in weird ways.

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  • $\begingroup$ Welcome to WorldBuilding Ben! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Jul 19, 2017 at 6:21
  • $\begingroup$ Would this mean that the planet doesn't actually revolve around the binary stars, but instead revolve around another spot in space? I've never looked into Lagrange points and picturing what it would look like is difficult. $\endgroup$
    – 1312412
    Jul 19, 2017 at 6:28
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    $\begingroup$ @1312412 - Yes. In any orbital configuration, all involved bodies orbit their collective center of mass. Even in our solar system, the sun itself orbits the center of mass for the whole system (sun + planets + moons + asteroids + ...). We just tend to ignore that because that center is practically the same point as the center of the sun. In a multiple-star system with stars of near-equal size, this would just be more obvious because the collective center of mass would be between them rather than within one star. $\endgroup$ Jul 19, 2017 at 7:10
  • $\begingroup$ Not really @DaveSherohman , it's quite possible for a body to orbit another much heavier body and that 2 body system to orbit another even heavier body. The orbit of the smallest body might be unstable, depending on the mass differences and distances, but that's basically how the moon can orbit the earth which orbits the sun. $\endgroup$
    – jwenting
    Jul 19, 2017 at 9:05
  • $\begingroup$ "Yes. In any orbital configuration, all involved bodies orbit their collective center of mass" Nah, in multi body problems you get weird looping orbits and so on. Here's one planetary orbits trojans $\endgroup$
    – Fattie
    Jul 19, 2017 at 14:13
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This is basically doable.


Put the Earth in a standard 1 AU near-circular orbit around a binary pair (a P-type orbit, which I think is what you're asking after).

As long as the binary pair is less than about 0.22–0.40 AU apart, this should be reasonably gravitationally stable. This is 2–3 orders of magnitude larger than the Sun's Roche limit, so the Sun-like stars comprising the binary should not destroy each other or the planet tidally.

Ideally, the stars you pick would each mass half as much, and output about half the Sun's total illumination (slightly more to compensate, on average, for eclipses). Unfortunately, a star with half the mass of the Sun is expected to output just 8.83% of the light. You can compensate in various ways:

  • Make the stars the same mass as the Sun, and move the orbit 41.4% outward to compensate (disadvantage: orbital period 18.9% longer; advantage: Sun-like light).
  • Make the stars more massive and keep orbit the same (disadvantages: your orbit will be faster, light is red).
  • Make the system about 30% the size, so you get enough light (disadvantages: shorter orbit, light is red).
  • Changing the star types: e.g. perhaps a Sun-like star, and a nearly insignificant white dwarf (advantage: pretty similar in most respects, disadvantage: same)
  • ...

You can generate innumerable other solutions (and similarly, if you use these numbers, you should check them, since I didn't). The main consideration is that irradiance (amount of power falling on surface) is 1/r^2 with distance. Formulae for orbital period, etc. are Google-able.


In short: there are lots of plausible ways to make an Earth-like planet around a binary. The main difficulty is finding star masses/luminosity/spectral class/orbital radius configurations that are similar to Earth's. It seems like either the year's length needs to change, or the amount of light or its spectral class needs to. On the other hand, this can also be a source of interesting difference in your story.

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We can have 3 distinct cases for binary systems:

  1. Close binaries (distance between stars is a fraction of AU)
  2. Medium-range binaries (1 AU to 10s of AU)
  3. Loose binaries (100+ AU)

For close binaries, as others have mentioned, there is no problem having an Earth-like planet orbiting both stars;

Medium range binaries (like a Centauri AB) would not allow an Earth-like planet in P-type orbit around both stars being in a goldilocks zone. S-type orbit (around one of the stars) is possible, but the orbit might be too much perturbed, and it is not even clear if such a binary system would allow planet formation in transient zone between the components;

Loose binaries (like Proxima Centauri orbiting in a range of 4,300 to 13,000 AU) should have no effect on inner planet formation (unless loose binaries are, actually, evolved medium range binaries). We can have Earth-like planets there without a problem. One factor to consider is star system's Oort's cloud. Distant component should affect this cloud in a major way, and I would expect that inner planets would suffer a much heavier bombardment throughout their history.

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  • $\begingroup$ This is a pretty good summation of answers to my question, in a concise reply. Thank you very much for this reply. What would your recommendation be on the Oort cloud? Ive heard that Jupiter and Saturn may have acted as a sort of bouncer for the earth in the distant past, and I'd figured I'd need more than a single planet in the system to help deal with various disturbances. $\endgroup$
    – 1312412
    Jul 21, 2017 at 7:40
  • $\begingroup$ You can do anything you like about Oort cloud, science can only speculate here. You can say that the cloud is stabilized, and all calamities are in the distant past. Or you can say your second component knocks out comets on every pass, and your inner planet is frequently hit by them. $\endgroup$
    – Alexander
    Jul 21, 2017 at 7:50

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