A view from a planet orbiting Alpha Centauri AB

Many solar systems are singular (composed of a single star with or without orbiting planets). However, there are many multiple solar systems. In each case, conditions on a terraformed planet are different.

Overall Edit

If we look at the nearby solar systems [1], we see that 17 of 57 systems are multiple (about 30%). The list includes UV Ceti, Alfa Centauri, Sirius, Procyon, Epsilon Indi and EZ Aquarii. The way these systems are formed is not well known, but certain mechanisms (distribution of the planetary nebula, passing nearby stars, a possible supernova) might have worked.

The stars Edit

Multiple systems include a high variety of objects: similar stars or very different types of stars, systems composed of a star and a stellar remnant, stars and black holes, stars and brown dwarfs and many others. Virtually, any possible combination has been identified.

One important aspect of a multiple system is that each star creates its own solar wind. Unlike our singular system, solar winds interact one with each other, forming complex structures, radiation belts and shock waves. Each star, also, has its own magnetic field and interacts with the wind generated by the other star. One star might shield a close planet from the wind of another star, but also, a planet can pass through the fighting zone between two solar winds. O and B type stars have some of the most powerful winds. On the other hand, white dwarfs have no or almost no winds, but very powerful magnetic fields.

Stable orbits and planets Edit

It was theorized that in a multiple system, the chance of a planet to exist is far smaller then in a singular system. However, the possible discoveries of planets in Alpha Centauri system might confirm that planets can exist around multiple stars [2].

Any simulation will show that around multiple star systems the area where safe orbits exist is limited to sharp disks around each stars and regions placed far away.

  1. Alpha Centauri model. This model works for stars which orbit at 5 to 50 AU one from the other. It works for majority of binary stars, like Alpha Centauri, Sirius or UV Ceti. In this model, there are limited safe orbits around each star, followed by a wide area of instability and a region with safe orbits around the barycenter, at high distance. Usually, around each stars, there are stable orbits only for rocky planets, the habitable zone being very close to the outer limit of safe orbits. Any planet orbiting at such a distance would be exposed to strong tidal forces, which will force the orbit to be elliptical, with an eccentricity of 0.1 to 0.3. Around Alpha Centauri, safe orbits can be found as far as 2 AU from each star. Compared to our solar system, all the four rocky planets can fit. At distances over 55 AU, a planet could orbit the Alpha Centauri binary. Such a planet, if it exists, will experience temperatures colder then Pluto. The same model can be applied for Sirius or Procyon. Please note that, compared to the Solar System, there are no safe orbits for all the four gas giants. There is only room for a distant Kuiper Belt.
  2. Epsilon Indi model. For this model, both stars must orbit at a distance higher then 50AU. Epsilon Indi is accompanied by a pair of brown dwarfs who orbit at very high distance, like in its Oort Cloud. Around the star, safe orbits can exist as far as its Kuiper Belt and further beyond. A planet orbiting the star would be completely unaffected by the brown dwarfs. It is theorized that the presence of a large body at high distance can disrupt many comets from the Kuiper Belt and the Oort Cloud, so that the risk of an impact would be far greater, but as yet we don't know how these stars formed and if they have an Oort Cloud at all or if the distant companion had cleared its orbit.
  3. Close binary model. This requires that the stars are much closer, at less then 5 AU one to each other. The best model is EZ Aquarii BC [3]. Around each star, safe orbits, if they exist at all, are located very close. A planet would be very close and scourged by extreme temperatures. However, safe orbits do occur at some distance from the binary. There spectroscopic binaries composed of stars that are very close (even below 0.01 AU). They offer many safe orbits for planets that orbit both of them. These stars might not offer safe orbits for a habitable zone planet (if they are not very close), but still have safe orbits for all gas giants in the Solar System.
  4. Combined models. There are also trinary, quaternary and even more complex solar systems.

An important observation is that in a multiple system stars usually have highly ecliptic orbits. This further diminishes the area with safe orbits, both close to each star and far away.

The planet Edit

The hosted planet will behave different based on what types of stars exist.

Close binary Edit

Hosted planet will orbit both stars around their barycenter. If the stars are different, the bright one will bring most of the light, the small one will count only for mass in the system. The sky will be illuminated by both stars, but the dimmer sun will not count too much. We will have a clear day-night cycle. Tidal forces will not be too strong. However, if the planet is close, it might be locked in a resonance with the small star and can have an elliptical orbit, with an eccentricity around 0.2. This will affect the seasons.

There is one particular situation: of n M - type star and a white dwarf. The first star will give most of its light in infrared and red, while the second one will produce most of light in blue. This will be just what is needed for plant life. However, if the M - type star is a flare star, its flares can send matter to the white dwarf, where it will accumulate and from time to time will explode as novas, killing all life on any planet.

Average distance binary Edit

Alpha Centauri and Sirius fit into this model.

A planet can orbit at a small distance around each star. The planet will be exposed to strong tidal forces, but still will have a safe orbit, with a high probability to be in the habitable zone. Given the tidal stress, we expect the orbit to be slightly elliptical (eccentricity around 0.25). On the sky, the classical concept of day-night cycle will no longer apply. Each star will create its own day and night cycle. When both stars are seen in the same area of the sky, the planet will experience a day with two suns and a complete night. But, when the planet is between the stars, there will be no night. When one star will not be visible, the other one will. Around the year, there will be times when only one star will be seen, times when only the other star is on the sky, times when both stars are visible and times when no star illuminates the surface.

If we take Alpha Centauri, both main stars shine with enough light. A planet that orbits only one star, will receive almost all its heat from that star, the other one will not influence the climate too much. However, it will bring a significant amount of visible light.

If we think of a binary system composed of a bright and a dim star, a planet orbiting the dim star might receive more light and heat from the bright one.

Since majority of binary stars have elliptical orbits, the amount of light and heat received from a planet varies, based on both stars' position. If the planet orbits one star, but the second star brings a significant amount of light, we will have a complex climate pattern. It can be described as an year-in-year. The planet will experience a short-term year, caused by its rotation around the first star. A long-term year, lasting at least 10 short-term years, will be influenced by the distance to the second star. If we look at Sirius, a planet can have a safe orbit around Sirius B, circling it at every 0.5 Earth years. However, Sirius B orbits around Sirius A at 50 Earth years, on an elliptical orbit.

If the system is composed of a large and a small star, with circular orbits, a planet can be found in the Lagrangean positions L4 and L5 [4]. Such a planet will be always at nearly the same distance from both stars and will receive a similar amount of light. However, since solar luminosity increases exponential with mass, most of the light will come from the main star. The second star will be seen on the day sky and will bring some light after main sunset (or before main sunrise). The day pattern will have the following time sequences: complete night, main sunrise, second sunrise, main sunset and second sunset. The second star will not contribute significantly to heating the planet, but it might bring enough light so that you can see.

Also, there is room for a planet to orbit at high distance around both stars. Such a planet will be a frigid, ice covered world. Terraforming it with greenhouse gasses might be possible, but at least for Alpha Centauri, at over 55 AU, there will not be enough light for plants to survive. For Sirius, such a high distance planet can hardly support plant life. However, there is a high probability that such a planet will have an elliptic orbit. If this is the case, at apogee it can be too far for plants to survive, while at perigee, temperatures will increase too much and we will need to get rid of some greenhouse gasses.

High distance binary Edit

If the stars composing a binary system are at high distance, there will be many stable orbits around them. The influence of the second star will be very slow. Alpha Centauri is in fact a trinary system, with small Proxima at high distance from the AB binary. From Proxima, the Alpha binary will be seen at a magnitude of -6.8. They might be visible in the day sky, but still too dim to see your way on the ground. From Alpha AB, Proxima will be of magnitude +4, so very dim. A non-experimented eye will miss it on the night sky.

There are high distance binaries where from a planet orbiting a star, the second star would shine like the full Moon on Earth or even brighter, but still too dim to influence the local climate. Also, there are binaries where one star cannot be seen at all from a planet orbiting the other.

Trinary and other complex systems Edit

There are many multiple systems, some of them with over 6 stars. However, in most of cases, the stars fit into categories listed above. For example, we can imagine a system made of 8 stars. Each two of them form a close binary, with a distance of 1 AU. So, we have 4 close binaries and each two of them orbit around each other at 30 AU, forming two quadruple subsystems. Both subsystems orbit one around each other at 1000 AU.

Most simulations conclude that around multiple stars planets are more rare then around single stars. Conditions on a planet will be different from what we would encounter around a singular star. However, seeing many suns different suns, each one illuminating the planet and the sky in a different color, would probably be the most amazing landscapes settlers will one day see on a terraformed world.

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