Thanks to Blinkist for sponsoring today’s
video. 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. Now this sponsor is something useful you may
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This is the best video I've seen in a long time! Totally chilled me out! Thanks for sharing!
Are many of these planets in binary systems tidally locked?
This video deserves more views.