Fiction Science #1: The Planet of Blue and Red


(in memoriam: Stephen Hawking)



I originally got the idea for doing this from Halo Conversationalists.  If you have never listened to them, I strongly suggest you give them a try, they are great.  While they broke down each entry into the Halo Universe, they stumbled upon some questions regarding the feasibility of certain events that occurred, and I took it upon myself to answer to the best of my knowledge.  I believe the first science question posed was in regards to Contact Harvest and explosive decompression (I'll get to that one in time).  From that point, different science questions inevitably came up, including the feasibility of Element 121 (Halo: Nightfall), Biofoam (Halo: ODST), and this one, The Planet of Blue and Red (Halo: Broken Circle).  Each time I made my best attempt to determine if what was described in the books and games is feasible, or even remotely possible. It turns out, most of them are, or at least are in so much as is described in the media.  I do make a point to give as much leeway to the fiction, so long as it does not directly contradict with science, since this is, you know, fiction.  And I will do the same with other topics going forward.


So as this is a blog discussing both a fictional universe (Halo), and real science, there will inevitably be a point when the subject matter may get overly technical and complicated.  In that light, I will make a point to Keep is Simple, Stupid.  I don't want to dumb down the subject matter too much as I hope the readers might learn something new from this, but I will make a point to simplify as much as possible, and provide references for some topics so that the blog doesn't get bogged down with data.  Hopefully this will allow people who want to dive a bit deeper into the topics to do so, but let the more casual reader glean some good information out of this quickly.  If I ever skew too far in either direction, I hope the readers will let me know so that I can fix it.  


As I did in my Tin Foil Halo series, I am providing this disclaimer that I may spoil some aspects of different Halo games, books, or comics.  While I do not plan on going into great depth in the stories I cover as that is not the point of this series, I will inevitably mention some parts of the story that will be spoilers for those who are not up to date.  These spoilers should be minor, but if you want to catch up with your Haloing before you read this, I have provided links to any Halo media that might get spoiled.






This question was first featured on Halo Conversationalists Episode 37, and answered on Episode 39



To answer this question effectively, we first must first focus on what specifically is told to us about the Planet of Blue and Red (henceforth known as PBR to be concise and because beer) in the book.  Here is the list of seven items I was able to take out of the text:

  1. It is in a binary star system (planet orbits two stars)
  2. One star is much larger than the other (red giant vs blue dwarf)
  3. PBR is tidally locked to the center of mass of the star system (one side is always facing the suns)
  4. PBR experiences significant winds due to its orbit (it is windy)
  5. PBR is in a stable enough orbit to allow life to form/exist (there is some sort of atmosphere that is breathable)
  6. PBR is habitable (both the Sangheili and San'Shyuum seem to exist planet-side with little to no protection from the environment
  7. The stars are a 45 degree angle from each other in the sky (It literally says this in the book)

To determine whether it is feasible that PBR exists, I will break down each of these items separately and see whether they hold up to the facts.

 P-type and S-type orbits   (figure 1)     By Philip D. Hall - Own work, CC BY-SA 4.0

P-type and S-type orbits (figure 1)

By Philip D. Hall - Own work, CC BY-SA 4.0


It is estimated that possibly 50% of all observable star systems in the galaxy are binary systems.  Of those, it is suspected that most, if not all, star systems have one or more planets in orbit.  Calculations also suggest that 50-60% of binary star systems are capable of hosting planets in their habitable zone (ref).  There are also at least two known stable orbit types, P-type and S-type orbits.So given that, the fact that PBR is in a binary star system is completely feasible.



The binary star system consists of a red giant and a blue dwarf as is stated in the book, so one star is significantly larger than the other. This is also feasible, and depending on the masses of the stars, the orbit would potentially look a lot more like a planet orbiting a single star (center of mass of the pair is located within the larger star) rather than a binary system of equal mass stars (center of mass is between the two stars).  So again for #2, this is completely feasible.

 Binary system of two equal mass stars.   (figure 2)     By User:Zhatt (Own work) [Public domain], via Wikimedia Commons

Binary system of two equal mass stars. (figure 2)

By User:Zhatt (Own work) [Public domain], via Wikimedia Commons

 Binary system of two very unequal mass stars.   (figure 3)     By User:Zhatt (Own work) [Public domain], via Wikimedia Commons

Binary system of two very unequal mass stars. (figure 3)

By User:Zhatt (Own work) [Public domain], via Wikimedia Commons




We are told the suns in the sky are always stationary, meaning the planet is tidally locked.  Tidal locking is when one side of the orbiting body is always facing the object it is orbiting. The Moon is an example of an object tidally locked to what it is orbiting, which is why we only ever see one side of the moon.  The Earth, by contrast, is not tidally locked to the sun, since we have the day-night cycle everyone is used to.  Tidal locking is a lower energy configuration (ie more stable) than a planet rotating faster or slower than the orbital speed of the system. This is why the Moon is tidally locked to the Earth, and why, if given enough time, the Earth would become tidally locked to the Sun (though this will take longer than the life of the sun). Check out the video below for a better visual representation of why tidal locking exists.  The second half gets very mathy, but the first half is all animations.


The rate at which an orbiting body becomes tidally locked to its host body is inversely proportional to the orbital distance, i.e. the further an orbiting body is from the host, the longer it would take for it to become tidally locked. We aren't given any information on PBR that would make tidal locking unlikely, so I would assume this is completely feasible.



 This would be an effect of a planet that is tidally locked, since wind is generated from the temperature differences in the atmosphere by the sun heating the planet unevenly. Since all the energy from the stars would be hitting the same side of the planet, there would likely be a constant wind generated blowing from one side to the other, then back in some sort of a planet-wide current. A strong wind would be expected on a tidally locked planet. (ref) This one again seems completely feasible.



I don't think the book specifies whether there was life on the planet before the Sangheili colonized it, but there is definitely an atmosphere, and presumably oxygen present since I think the Sangheili and San'Shyuum can breathe on PBR without masks. The existence of oxygen would mean there is life on the planet generating it, since free oxygen is very reactive and will naturally react with the other elements in the planetary crust and get scrubbed out of the atmosphere (oxidation/rust). This would then mean the planet has to have been in some sort of stable orbit to be able to support life for at least several million years, if not billions.

This is where things get a little tricky. In a "standard" binary star system with two equally sized stars (figure 2), the stars would never remain in the same position and angle in the sky. A planet orbiting around both stars (P-type, figure 1) has to be far enough away to maintain a stable orbit.

Orbital velocities get slower the further you get from the center of mass (known as the barycenter), so the orbital velocity of the stars themselves would be faster than the planet.  For the stars to stay in the same position in the sky, the orbital speed of a planet at that distance would actually have to be even faster than the stars, which is impossible. (If this is confusing, consider that both the star, which is close to the center, and the planet, which is further from the center, have to travel one rotation for one orbit.  The star is travelling in a tighter circle, which means it has to travel less distance for one orbit than the planet does.  For the planet to keep up with the star, it has to go faster, which would fling the planet out of its orbit.)

Lagrange Points orbiting the Earth-Moon system.  The PBR system could be similar, with the red giant being Earth, the blue dwarf being the Moon, and PBR lying in one of the Lagrange points.

However, we know one star is very large, and the other is relatively small, meaning the stellar orbit could look like a planet orbiting a star (figure 3). In this configuration, there would be 5 points around the orbit that would be stable or semi-stable, known as Lagrange points. These are points in the orbit where another object could sit and experience no net gravitational force from either star.

The easiest to imagine are the first three, L1, L2, and L3. L1 sits between the stars, at a point where the force of gravity from each body are equal. L2 sits on the far side of the smaller object, where the force of gravity from the smaller star allows the planet to orbit at a rotational speed equivalent to the small star (normally the further you get from the large object, the slower the orbit, but the extra gravity from the second object keep the planet from shooting off into space). Lastly, L3 sits on the far side of the large star exactly opposite the small star and follows the same orbit as the small star, except on the other side of the large star. All three do us no good though, since the stars would either appear in opposite sides of the planet, or in the exact same location in the sky, which is not what is described. These orbits are also only semi-stable, since any nudge in any direction would push the planet out of that orbit, and the book describes the stars sitting in the same place in the sky at all times.  Additionally, for life to exist, the planet would have to be in a stable orbit for a long time, something L1-L3 do not provide.

 Lagrange points: Stability region of L4 and L5 points.  Imagine the red giant as Earth and the blue dwarf as the moon.   (figure 5)

Lagrange points: Stability region of L4 and L5 points.  Imagine the red giant as Earth and the blue dwarf as the moon. (figure 5)

There are, however, two other Lagrange points, L4 and L5 that could potentially work. L4 sits in the same orbital path as the smaller star, but "ahead" of the small star, while L5 is the same but "behind" the small star. Both of these orbits are stable, meaning a nudge wouldn't push the planet out of orbit, and they sit on one point of an equilateral triangle, where the other two points would be the other two stars. This would allow for the stars to sit in the same place in the sky at all times, and still put the planet in an orbital configuration that could potentially last long enough for life to form, generate an atmosphere, and create a habital planet. The only caveat to this is that the L4 and L5 points are only stable when the larger object is at least 25 times larger than the smaller object (in mass). Based on the book description, I don't see why this is not possible. So this too, is completely feasible.


6. PBR IS HABITABLE (feasible)

Habitability is based on many factors, most importantly the distance the planet is from the sun. If you were to imagine the Earth and Sun, the L4 and L5 Lagrange points also sit in the habitable zone since they are at the same distance from the sun that the Earth is. In the case of PBR, both objects are stars, so the orbital radius would have to be outside the habitability zone of the large star, but still warm enough due to the extra heat from the second star. There are no physics I can think of that would specifically prohibit this configuration, so I am again going with completely feasible.



This one is the one I have the most trouble with, but I think it can be somewhat explained away. As I said earlier, the only orbital configuration that meets all the criteria described in the book is at the L4 or L5 points. The main issue I have is that L4 and L5 sit at one point of an equilateral triangle, which is famous for having all three angles be 60 degrees. This would mean the stars would have to sit 60 degrees apart in the sky, not 45 degrees. The stars are, of course, not just points in the sky but large objects, and red giants are less dense, and therefor take up more space in the sky. This would mean the angle from the edge of each star to the other COULD be 45 degrees, though why someone would decide to measure that angle and not from the center of the objects is beyond me. So I would say this is close enough, though pointless, since everything else is completely feasible based on everything I looked at. My vote would be for a retcon of the 45 degree angle to 60 degrees, though that is probably not the most pressing retcon in the Halo universe.



Based on the seven criteria I came up with, the Planet of Blue and Red could, conceivably, exist.  It is a very unlikely configuration, but as there is no science I can come up with that specifically disproves it, PBR could exist in real life.  I don't think it would be a particularly great place to live, but it might be habitable enough.

If you have any questions regarding this analysis, or maybe think of something that I didn't, let me know, and we can discuss.  Who knows, I could very well be wrong.  I always like a good challenge.



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