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In our Solar System, we like to keep things simple.
Just the one star found in the centre, with everything else orbiting around it, as is
the case for most planetary systems found in the universe.
However, there are some planetary systems out there where things can get a bit more
complicated, specifically multi star systems, where there are 2 or more stars that orbit
each other.
In such configurations, what happens to any planets orbiting them?
In fact, can planets orbit them at all with the gravitational tugs from different directions?
To answer the latter question, the simple answer is yes, planets can orbit in such situations,
although, to answer the former question, there is no one answer fits all rule about how a
planetary system in a multi star system might look.
What we can do, however, is explore some of the possibilities out there.
But before we look at planets, it would be good to understand how multi-star systems
work.
For the most part, stable star systems have organised themselves into hierarchical systems.
This is due to the proximity in which they formed with each other, which we will touch
on a little later.
Binary star systems are generally simple enough, binary stars orbit around a barycentre, or
in other words, their centre of mass.
If the masses of these two stars are similar, then nearly symmetrical, elliptical orbits
are often seen, although there can be occasions where they orbit in circles, in a similar
fashion to Pluto and Charon.
In the case that one object is more massive than the other, then the more massive object’s
orbit doesn’t take it as far out compared to the less massive object.
Beyond binary systems, you can have 3, 4, 5, 6, 7 or more stars in the same system,
and as you will see, there is a structure within these systems to keep them stable.
In the case of three stars, you’ll have two stars orbiting each other in a binary
configuration, with the third orbiting around a barycentre with the other two stars.
This keeps the system stable, because if three stars had their orbits cross, one would certainly
get ejected from the system at some point.
In a 3-star system, two of the stars are contained in their own enclosed little system, acting
as one star in the grand scheme of the whole system itself.
We group this binary configuration into a tier, with that tier acting together in its
association with the single star.
In a way, once you have grouped the binary configuration in the system, this upper tier
now acts like a 2-star system again, with the two stars and the one star orbiting each
other.
In the case of four stars, you’ll either have two binary configurations orbiting around
a barycentre, or one binary system orbiting a barycentre with a third star, and all three
of those stars orbiting around a barycentre with a fourth star.
From here on is where the hierarchical system really comes in handy.
With a chart like this, you can easily see how the system works.
In the two binary configuration case, you have the two binaries orbiting the system’s
centre of mass.
In the single binary and two single stars configuration, you’ll add another lower
tier in the configuration.
You have the binary here, which orbits with this third star.
And all three of these stars are together orbiting with the fourth star.
If there are more stars in the system, say 5, you can have an array of configurations,
with various binaries or single stars on a variety of tiers.
Yet with this chart, you can easily see where the mass lies.
You can do the same for systems with 6, 7 or more stars, but anything above 7 is exceptionally
rare and probably won’t remain stable, although maybe there are examples somewhere in the
universe where is exists.
So, let’s add planets into the mix.
Could there could a planet out there with 7 suns in their sky?
Well yes!
Let’s have a look at this hypothetical 7-star system again.
By here, there are 3 stars in a configuration.
Two in a binary configuration, and a single star.
We see that the single star has planets that specifically only orbit that star.
They have one sun.
Beyond that, there are planets that orbit both that star and its binary companions.
These planets could be said to have 3 suns, as they are orbiting 3 stars.
Going to the top tier, there is a planet here that orbits the entire system.
This planet could be said to have 7 suns.
Let’s see what that looks like outside the graph.
Interestingly, in a 7-star system, from this distance it only looks like the orbit of a
binary.
That’s because each set of stars is found in one of these two points.
Here is the planet with 7 suns.
Its orbit takes it so far out that these stars aren’t very bright in the sky, and so even
though it has 7 suns, they don’t really provide much in the way of warmth or light.
Zooming in on the set containing 3 stars, we see the planets which orbit all three of
these stars.
These stars would appear much larger in this planet’s sky compared to the planet’s
sky that was orbiting all 7.
In this example, the two in the binary configuration appear almost too close together to distinguish,
with the single star clearly separate.
The other 4 stars that are part of the other set in the system are dim in the sky, they
are very far away, and again don’t provide much in the way of light or warmth, meaning
there is a clear day and night on these planets.
Interestingly though, this other set of stars would get marginally brighter and dimmer during
their orbits, and the distance between their systems orbits would get more noticeable as
they get closer.
Lastly, let’s look at a planet orbiting just the one star in this system.
It orbits closely to the star and is tidally locked, although the night side would be disrupted
by the binary configuration this star orbits with as the planets orbit can take it between
the single star and the binary stars.
The binaries orbit together closely, and so wouldn’t easily be distinguished in the
sky without a filter.
Again, the other set of stars in the system would be dim and appear far away.
This may make you wonder, how many systems out there have more than one star?
Are we unusual, or is it the trend that there is only one star per system?
Well, if you look up into a clear night sky, it may surprise you to know that most of these
stars are binaries.
The brightest star in the sky, Sirius, is a binary.
Alpha Centauri, the closest star to us, is a binary, or maybe even a trinary if you include
Proxima.
Polaris, the North Star, is also a binary.
So, for the longest time, astronomers thought that we were the unusual ones.
However, it’s coming to light with the improvements in technology that this is not the case, and
that most star systems only contain one star.
Perhaps the reason why the brightest stars in our sky are binaries is because they are
giving off twice the light.
Single stars are just generally dimmer, and harder to see, in comparison.
It doesn’t help either that 85% of all the stars in existence are Red Dwarves, and it
seems that only about 25% of them have companions.
In fact, it’s interesting that binaries occur much more frequently with the really
massive stars, the blue and white giants.
This could be because of how these stars formed.
Red dwarves weren’t pumped with as much material as they were forming, hence why they
never attained the mass of a blue giant.
In an environment where forming a blue giant is possible, however, models suggest that
several stars could form at the same time.
At first, these systems would be chaotic, and stars would be ejected until order could
be found.
The ones that remained and survived were the ones that ended up absorbing the most interstellar
medium, thus becoming the most massive of stars.
The interstellar medium that ended up caught in protoplanetary disks around these stars
later form planets.
In such turbulent conditions, it is likely that it is harder for planets to form compared
to a single star, however it is certainly not impossible.
So, perhaps there is indeed somewhere out there where there is a planet with 7 suns.
What a cool sight that would be too, I just can’t help letting my imagination run with
what that would be like.
However, it probably wouldn’t be so great for a nice and consistent day and night cycle!
So, there we have it, how planets orbit in multi-star systems.
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