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I'm dealing with a planet very similar to Earth, except that it has two suns. So, of course, it would be an extraordinary occasion when the inhabitants of the planet see only one sun in the sky. So the idea is that it is extremely rare for there to be a moment when the two stars and the planet align perfectly so that only one sun is visible in the sky. About once every 1000 years. Apparently, a prophecy made the last time the planet and the stars align, is to be fulfilled the next time it happens which (in the story) is about a few months away.

However, stars don't exist so close to each other for the simple reason that the gravitational pull between the two stars would be so great, they might very well end up colliding. Also, even if two stars could exist close together in space, a planet could not conceivably orbit around them both...

However, if we had a primary star that is maybe about the size of our current sun, and a secondary star in orbit who's radius is twice Jupiter's radius (by the way, the sun's radius is about 10 times Jupiter's radius), the gravitational pull would be stable enough that the secondary star, like our planets, would be able to remain in orbit. Instead of, you know, crashing into the primary star.

So basically, in this solar system, I have the primary star (the size of our sun), then the secondary star (with 2 times Jupiter's radius) is in the first orbital, then the home planet in the story (the size of earth) with at least one moon in the second orbital, and then the third orbital onwards may or may not be occupied by planets, it's not really relevant now.

Is this arrangement possible? Assuming the home planet is sufficiently far away enough from the two stars to not be baked to a crisp, could a star conceivably orbit another, and still support its system of planets?

I don't need super scientific answers. I'm not an expert in astronomy, and stuff, so errr... A slightly dumbed-down answer would be fine. :)

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    $\begingroup$ Here's what rainbows would look like for them $\endgroup$ – Rob Watts May 25 '16 at 19:04
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    $\begingroup$ Non-Scientific Dumbed Down Answer; Yes, Yes They Can. $\endgroup$ – codescape May 25 '16 at 19:28
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    $\begingroup$ Also a very god treatment of this in fiction can be found in goodreads.com/book/show/20518872-the-three-body-problem where just such a planetary system is discussed. $\endgroup$ – Ukko May 25 '16 at 20:43
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    $\begingroup$ You should read Nightfall. $\endgroup$ – Peregrine Rook May 26 '16 at 2:20
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    $\begingroup$ in fact, the most recent issue (May 2016) of New Scientist magazine has an article that quotes a NASA astronomer saying it is "more likely than not" that our closest neighbor, the Alpha Centauri system, has a potentially habitable planet around one of its two stars. (look up "Breakthrough Starshot" if you want to visit :) $\endgroup$ – sbeam May 26 '16 at 4:17

14 Answers 14

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I think you're missing some knowledge here, since what you are asking about is called a binary star system and they are extremely common.

Your planet can orbit the pair around their common center of gravity (the Barycenter), or you can have the binary pair further apart and have your planet orbit one of them. The second case sounds like what you want.

Note that nothing in space (even planets) orbits around something else without also causing it to wobble. The earth orbits the sun, but the sun also orbits the earth. The only thing is the huge mass difference between the two means that the earth moves much more than the sun does as a result.

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    $\begingroup$ IIRC the real point determining whether we say "A orbits B" and "A and B orbit each other" is where the barycenter is. In the Earth-Sun system, the barycenter is within the Sun, and in Earth-Moon it's within the Earth. Co-orbiting binary stars often have the barycenter in "empty space" between the two bodies. $\endgroup$ – Luaan May 26 '16 at 8:31
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    $\begingroup$ @Luaan Like Pluto and Charon. That system's barycenter is outside either body. Generally and very broadly speaking, this happens when the two bodies are of somewhat similar mass and size. When one of the bodies is significantly larger or more massive than the other, the barycenter falls inside the more massive body. $\endgroup$ – a CVn May 26 '16 at 8:34
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    $\begingroup$ Just to complete this answer... to achieve a 1 in 1000 years occurrence of a full eclipse of the other star, you just have to make sure the orbit is inclined. If they were all on the same plane, the eclipse would happen every year instead. The reason they don't is that the planet has to be in the middle 'height' in its orbit at the same time as being opposite of the other star. That way every year you see one above or below the other, but only rarely is one exactly behind. (and has to be small or far away enough to be completely blocked, like our moon blocking the far away sun) $\endgroup$ – Peter May 26 '16 at 11:30
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    $\begingroup$ @Bulrush Yes, the barycenter en.wikipedia.org/wiki/Barycenter $\endgroup$ – Tim B May 26 '16 at 12:21
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    $\begingroup$ @user2428118 No, always. See for example this table on Wikipedia - the barycenter of the Sun-Earth system is a mere 449km (less than 0.1% of its radius) from the Sun's center, very much inside the Sun. The image you linked is for the barycenter of the entire Solar system. $\endgroup$ – marcelm May 26 '16 at 20:22
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I give you Kepler 16b:

On 15 September 2011, astronomers announced the first partial-eclipse-based discovery of a circumbinary planet. The planet, called Kepler-16b, is about 200 light years from Earth, in the constellation Cygnus, and is believed to be a frozen world of rock and gas, about the mass of Saturn. It orbits two stars that are also circling each other, one about two-thirds the size of our sun, the other about a fifth the size of our sun. Each orbit of the stars by the planet takes 229 days, while the planet orbits the system's center of mass every 225 days; the stars eclipse each other every three weeks or so.

See also here. The planet that was directly discovered is probably a gas giant (it's about the size of Saturn), but if it had a large rocky moon of some kind, that moon would be (barely) habitable.

Astronomers being astronomers, Kepler 16b has also gained the informal nickname "Tatooine".

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    $\begingroup$ An important note here is that if a planet is orbiting two stars that are orbiting each other, then those two stars will be fairly close together and their orbital periods would be relatively short (much less than 1000 years). A similar situation more useful for the world the OP proposes is one where a planet orbits only one star, but that star itself orbits a second star, which is much further away from the planet and its sun than the planet and the sun are from each other. $\endgroup$ – NeutronStar May 25 '16 at 18:40
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    $\begingroup$ If you're interested in visiting, there's a tourism poster for this planet: kepler.nasa.gov/education/resources/info $\endgroup$ – Will Vousden May 26 '16 at 7:40
  • $\begingroup$ @Joshua The orbital periods of the stars doesn't have to be that great, provided that the plane in which they orbit each other is sufficiently inclined to the plane of the planet's orbit. In that case, there are two times a year when the planet is in both planes; the two stars would have to align in their three-week cycle at just that time for an eclipse to occur. $\endgroup$ – Monty Harder May 26 '16 at 16:23
  • $\begingroup$ @MontyHarder, that is a good point. However, if the planet's orbital plane did not align with the stars' orbital plane, there will be some long-term effects on its orbit that may be disastrous for the planet (migrate inwards or outwards) or possibly even shift the planet's orbital plane to be aligned with that of the stars. $\endgroup$ – NeutronStar May 26 '16 at 21:11
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If your planet was on the same plane as both suns (most likely for any stable orbit), having only a single sun in the sky would be a yearly occurrence and will have a time frame measured in weeks.

If your planet is orbiting at a 90 degree angle to the outer sun, then there will be 2 spots where this sun passes through the orbital plane of the planet. For several years before and after this point, a lengthy eclipse of the outer sun will be a yearly event.

To give a 1000 year schedule for this cluster of yearly eclipses, simply place the outer sun in such an orbit as to take 2000 years to complete (about 159 AU away from the primary).

Note that each consecutive cluster would take place in the season opposite of the previous.

EDIT: I mis-read as the second star as being farther out than the planet. With the second star inside the orbit of the planet, the eclipse would be yearly.

Further EDIT: During the opposite season of the year during the cluster of eclipses, the sun and outer sun will be on opposite sides of the planet. So you would see the sun during the day and the outer sun during the night.

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  • $\begingroup$ If the second star is inside the orbit of the planet, and the orbital planes are at 90 degrees, the second sun passes through the orbital plane of the planet quickly, so it's not seen every year, only if the planet is at a certain point in its orbit. With some parameters, this might be a once in 1000 years event. $\endgroup$ – JiK May 26 '16 at 10:23
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When you have two stars of comparable mass, one does not orbit the other. They BOTH orbit their centre of gravity. You have 3 options.

You can have two close stars that have one distant ring of planets around their centre of gravity. The people on the planet would see 2 suns held together in the sky during the day.

OR

You can have two more distant stars that each have their own tight ring of planets. The people on a planet would see 1 large sun and a smaller secondary sun that is out during the night for 6 months and out during the day for 6 months.

OR

You can have a more complex system where a planet does a figure 8 orbit that switches from one star to the other. Such an orbit would vary in proximity to the sun immensely and could never be inhabitable.

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  • $\begingroup$ The problem with figure-8 orbits is not the variation of distance to the sun, but that they're generally unstable. The atmosphere and oceans of an earthlike planet are quite effective at storing heat, and should be able to easily support life for a few months of low sunlight each year, provided that the average solar heating was appropriate for keeping water liquid and life happy. Sure, such a world (assuming you could somehow make it stay in its orbit) might have weird and harsh seasons, but that doesn't mean it wouldn't be inhabitable. $\endgroup$ – Ilmari Karonen May 25 '16 at 19:29
  • $\begingroup$ Well... I forgot to mention that they are rare. An inhabitable one would be rarer still. Pass too close to the sun and the surface is going to spontaneously combust. Maybe you could have life deep in the oceans, but I was thinking of humans. $\endgroup$ – Lorry Laurence mcLarry May 30 '16 at 13:07
  • $\begingroup$ Lorry Laurence mcLarry - stable figure 8 orbits should be impossible. If a planet has natural advanced lifeforms and/or is habitable for humans it should have had stable temperatures for at least 3,000,000,000 Earth years. And orbits gradually change over billions of years. Tidal forces increase or decrease the distances between 2 objects. So even if by one chance in 1,000,000,000 a figure 8 orbit works when a planet is only 10,000,000 years old it is not likely to work 10,000,000 years later. The planet should be ejected out of the figure 8 orbit. $\endgroup$ – M. A. Golding Apr 16 '17 at 2:06
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A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two, three, four, or even more stars are called multiple star systems.

They're called binary stars and yes it is possible for planets to be in these stars habitable zones. Although the habitable zone is partly deformed (due to 2 stars existing) life still should be able to sustain life although the combination may be very rare

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    $\begingroup$ Welcome to the site Albert. If you have questions about the site please check out the help center or feel free to visit us in Worldbuilding Chat $\endgroup$ – James May 25 '16 at 17:42
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Not only is this possible, but it is possible for each star to have a subsystem of planets orbiting each individual star and then for the combination of orbiting stars have more planets further out orbiting both of them, and for this to create really interesting habitable zones.

It is theoretically possible for a planet to sit at the LaGrange point between the stars, though this outrageously unlikely.

However, my favorite theoretical option in this regard are the butterfly planets that jump back and forth between two (or more!) stars. I want to write a story about a world like that...

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  • $\begingroup$ David - forget about butterfly planets or figure 8 orbits. Stable figure 8 orbits should be impossible. If a planet has natural advanced lifeforms and/or is habitable for humans it should have had stable temperatures for at least 3,000,000,000 Earth years. And orbits gradually change over billions of years. Tidal forces increase or decrease the distances between 2 objects. So even if by one chance in 1,000,000,000 a figure 8 orbit works when a planet is only 10,000,000 years old it is not likely to work 10,000,000 years later. The planet should be ejected out of the figure 8 orbit. $\endgroup$ – M. A. Golding Apr 16 '17 at 2:11
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Yes, binary star systems are common and can have planets.

The difficulty you have for your 1000-year plan, is that either:

  • the two stars are relatively close together and the planet orbits them both, in which case they'll always be close together in the sky like the Sun and Mercury. So you'll see only one of them briefly, during sunsrise and sunsset, and you'll also see them (partially or totally) eclipse each other, but you'll never have one of them up and the other one nowhere near it.
  • the planet orbits one of them inside the orbit of the other, like the Sun and Jupiter. Then the planet will pass roughly between them once a year, and when that happens they'll be nearly 180 degrees apart and so you'll see exactly one of them in the sky for much of that time.

I can't off-hand think of an arrangement in which the two stars are usually close together from the POV of the planet, but very rarely move so far apart that only one is in the sky (other than close to sunsrise and sunsset). You might be able to arrange it so that they only very rarely eclipse, though: perhaps the orbit of the planet is at a substantial angle to the plane containing the two stars, so that eclipses are only possible at the two times a year the planet passes through that plane. Then, if the stars actually align with this point only 1/2000th of the time, you'd observe an eclipse and in this sense "only one sun" about once every 1000 years. Or something.

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  • $\begingroup$ This is perfect! There are lots of variables to play with—the inclination of the planet's orbit, the masses and orbital radii of the planet and stars, and the radii of the stars—and the eclipse frequency should be quite sensitive to all of them. The inclination can plausibly go pretty high, too: in our solar system, Mercury, Ceres, Pluto, and Eris have orbital inclinations of something like 5°, 10°, 15°, and 40° from the invariable plane. With all that freedom, it should be possible to tune the eclipse frequency over a very wide range. $\endgroup$ – Vectornaut May 27 '16 at 6:43
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There is no reason conceiveably (that I am aware of) in which this would not be possible. For example, the Alpha Centauri System, which is composed of a three star system, and many planets.

Earth-Sized Planet in Alpha Centauri System

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    $\begingroup$ Unfortunately, subsequent analysis of the data indicated that the Earth-like planet around Alpha Centauri B probably doesn't exist. $\endgroup$ – Michael Seifert May 25 '16 at 16:27
  • $\begingroup$ "We are not 100 percent sure..." $\endgroup$ – spectre-d May 26 '16 at 12:57
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YES

not only can it happen it does happen; https://en.wikipedia.org/wiki/Binary_system

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YES!

One of the coolest things in space is VV Cephei.

Not only is it a binary star system but one of them is a freaking vampire star that siphons off the other star whenever they get too close.

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Something to consider:

I suspect that any secondary star in orbit who's radius is twice Jupiter's radius (by the way, the sun's radius is about 10 times Jupiter's radius) would be very dim compared to the more massive star, and a dim red in color.

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I already see a lot of good answers, so I just want to add 2 things:

  1. Extra-solar planets have been found orbiting isolated stars (like the Sun) as well as in a range of multiple-star systems. A planet can either orbit around a close binary (or "Tatooine" binary -- this is called a P-type orbit) or orbit one star that has a more distant orbiting companion star ("S-type") enter image description here

We know planets orbiting in both categories. The extra star can be bad or neutral for planetary life, depending on its orbit (see here: https://planetplanet.net/2013/06/06/binary-stars-friends-or-foes/)

There are also planets known to exist in systems with 3 and even 4 stars. Those get more complicated.

  1. We can imagine a range of planets orbiting in physically-plausible, stable systems with many stars. Here are two examples that might interest you: https://planetplanet.net/2016/03/22/an-earth-with-five-suns-in-the-sky/ https://planetplanet.net/2016/04/13/building-the-ultimate-solar-system-part-6-multiple-star-systems/
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Yes even the star system that is closest to the solar system: Alpha Centauri contains tree stars of which two (alpha and beta centauri) revolve around each other and according to Wikipedia 2012 and 2015 reports were that there may be planets in their neighborhood.

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Possibly you could make it a triple star system.

If all the stars and planets orbit in almost exactly the same plane the time between the an eclipse of one star by another would be a multiple of several synodic periods.

No matter how you arrange the orbits, there would often be periods when only one star was visible and periods when no star was visible.

If the planet orbited about one single star the star would appear as a sun in its sky during the daytime and would not be seen at night.

And in every year of that planet it would be between its own star and the two farther stars for part of the year and on the other side of its sun for the other part of the year.

At one extreme of the orbit the planet would be exactly between its sun and the other pair of stars. The other two stars would rise when the sun set, and would set when the sun rose.

At the other extreme of the orbit the planet would be exactly on the opposite side of its sun from the other pair of stars, that would thus appear close to its sun in the sky. The other two stars would rise when the sun rose, and would set when the sun set.

And for most of the year the alignment would be in between.

So the natives of the planet would be used to the idea that sometimes the sun and the two other stars would be the sky together, sometimes only the two other stars would be in the sky, sometimes only the sun would be in the sky, and sometimes no sun or double star would be in the sky and it would be dark night. And they would experience all four conditions during the course of the year.

And if the planet has one or more large moons that can be bright in the sky that can complicate matters.

Your idea that it would be vary rare to see only one star in the sky is very naive. Because if two or more stars are seen together in the day sky on one side of the planet, then they can't be seen on the opposite side of the planet when it is night. Many people have tried on these boards to imagine how to make a planet have eternal day on both sides and have failed.

No matter how many stars they add to the system, they can have the planet surrounded by stars on all sides and have eternal day on all sides for periods of years, decades, centuries or millennia. But sooner or later all the orbits of the various stars will put them in the same direction as seen from the planet, and thus there will be night on the side of the planet opposite the stars, perhaps for years, decades, centuries, or millennia.

So the natives of the planet will be used to dark night.

A) Unless there are at least three stars in the system and their orbits are not in the same plane that the planet orbits around its star. Thus the planet would have day and night as it rotated so that each spot faced the star part of the day and faced away from the star part of the day, just like Earth.

Bu the other two stars, farther away, would not heat it up much but might light it up with several times the light of the full moon, for example.

If the other two stars orbited around the center of mass of the system at a right angle to the plane of the planet's orbit around its star, and happened to be at opposite sides of their long orbits for the centuries or millennia of the planet's recent history, they could have illuminated opposite sides of the planet constantly for all of recorded history.

Or there could be only two stars in the system, the star the planet orbits and that gives it day and night and heats it up, and another star that orbits at right angles to the planet's orbital plane and lights one side of the planet, the side where the protagonist's civilization is located, and the other side is unknown and unexplored and nobody knows they have much darker nights there.

And maybe once in a thousand years the planet's moon will eclipse the farther star during the night of the brighter star and bring on true darkness for the first time in a thousand years.

In a different type of system where all the planets and stars orbit in the same plane there could be rare periodic celestial events, periodic eclipses or occultations of the stars in the system..

1) a double eclipse of the farther two stars by the nearer star or sun.

a) One of the farther stars eclipses the other one.

The planet will orbit its star or sun with an orbital period or year, the planet's sun or star and the other two stars will orbit each other around their common center of mass with a second orbital period, and the two other stars will orbit around their common center of mass with a third orbital period.

As the two other stars orbit their common center of mass and the planet orbits its star which orbits around a common center of mass with the two other stars, there will periodically be alignments between the two other stars and the planet so that as seen from the planet one of the two other stars will eclipse or occult the other one.

b) The nearer star or sun eclipses the two farther stars one at a time.

And as the planet orbits its star or sun and its star or sun orbits around the center of mass with the two other stars the sun will periodically eclipse or occult one of the other two stars and then pass into the space between them and then eclipse or occult the other one.

c) Combination of a) and b).

And on rare periodic occasions both events will happen at the same time. One of the two other stars will eclipse or occult the other one while the nearer star or sun will eclipse or occult both of them.

2) the planet's moon (if any) eclipses the near star or sun.

Similarly to the Earth/Moon/Sun in our solar system, the planet's moon (if any) may periodically eclipse or occult the nearer star or sun.

3) Combination of 1) and 2)

At rare periodic intervals the planet, its moon, the nearer star or sun, and the two farther stars may all line up in a line so that one of the two farther stars eclipses or occults the other one while the nearer star or sun eclipses or occults both of them and the moon eclipses or occults all three of the stars.

4) simplified super eclipse.

The situation can be simplified by turning the two farther stars that orbit their common center of mass into one single star. Thus one of the orbital periods can be eliminated.

And on rare periodical occasions the two stars, the moon, and the planet can be aligned so that the near star or sun eclipses the farther star while the moon eclipses them both.

5) Make the planet a double planet or a moon of a much larger giant planet. That way the other planet will appear much larger in the sky of your planet and will have a much better chance of eclipsing the other stars.

Anyway, I think you should have someone else calculate the orbits and orbital periods so that your story is plausible.

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