Alpha Centauri is the nearest solar system to us. It consists of three stars and has some confirmed and unconfirmed planets. It is likely that when humans will develop interstellar transport technology Alpha Centauri will be the first destination, first for unmanned probes and later for settlers.
The A and B stars orbit one another on an elliptical trajectory, with a semi-major axis of 17.5 AU and an eccentricity of 0.5. Both stars come occasionally close to each other, which would influence the orbits of any planets. They are often referred as the Alpha Cen AB system.
The third star of the system, Proxima Centauri, has a highly elliptical orbit and is located very far from the other two stars. Because of this, many scientists treat Proxima Cen as a distinct stellar system.
Alpha Centauri A
The main star of the system, Alpha Cen A, is a G2 main sequence G - type star (see G-Type Stars and Main Sequence Stars), similar to our sun, Sol, only slightly brighter. It releases 24% more visible light and 71% more UV radiation. Its mass is only 8% bigger then Sol, but its radius is 22% larger, even if both stars have the same temperature. The star produces a Stellar Wind similar to Sol's.
The habitable zone around this star is roughly similar to the one around our sun, Sol. The distance of an Earth-Like Planet would be 1.1 AU. Stable orbits around Alpha Cen A are as far as 2.8 AU.
Because of gravity interferences with the other star in the system, any planet orbiting the star would experience significant tidal stresses. Such a planet will tend to have an elliptical orbit and may occasionally be tied into an orbit resonance. Tidal stresses would make such a planet have a more intense volcanism and likely be a Low-Spinning Planet, with long days and lights. Since more stable orbits are located closer to the star, if there is a planet orbiting Alpha Cen A, it is more likely that it is an Inner Planet, something like Mercury in the Solar System.
Several unsuccessful attempts have been made to discover planets. While gas giants and super-Earths have been ruled out, small, terrestrial planets, could exist. The most interesting was an unconfirmed detection of a Super-Earth located at 1.1 AU, which is quite in the middle of the habitable zone.
Observations between 2007 and 2012 identified a circumstellar dust disk, which is a challenging aspect. It is not known if the disk exists around the A or the B star or around both. If this disk exists around Alpha Cen A, then a large planet like the Earth is ruled out, as it would clear its neighbourhood of dust. Also, there must be a source of dust, like a small planet or an asteroid belt.
Alpha Centauri B
This is the second star of the system. It is an orange, K1V star (see K-Type Stars and Main Sequence Stars for details). Its mass is 90% that of the Sun, its radius is 80% of Sol's, its surface temperature is slightly lower (5200 K) and its Solar Constant is 50% of Sol's. The star has its own Stellar Wind, less powerful then Sol's.
The star has stable orbits as far as 2.5 AU. The orbit of a hypothetical Habitable Zone Planet would be at 0.8 AU. Since Alpha Cen B is less bright then Alpha Cen A, its habitable zone is more inwards, where more stable orbits exist.
A planet orbiting this star would experience significant tidal stress from the other star in the system. This would stimulate volcanic and tectonic activity. The orbit of such a planet would be affected and forced to become elongated, elliptic. It is possible that such a planet will enter into an orbit resonance with the A star, at least for a period of time. Resonances are known to force elliptical orbits (as is the case of many asteroids locked in a 1/2 or 2/5 resonance with Jupiter). Another effect of tidal stress and elliptical orbit is that rotation period will decrease, but not completely. A planet will end-up like Mercury or Venus, never as Tidal Locked Planet, but more likely as a Low-Spinning Planet.
There have been many attempts to find planets around Alpha Cen B, with unconfirmed results, including a possible transit. The most known is of a close Inner Planet with an Earth-like radius, that would orbit the star in 12 days. If such a planet exists, it would have lakes of molten lava on its surface. Gas giants and super-Earths have been ruled out, but rocky planets, especially smaller then Earth, are likely to exist.
Observations between 2007 and 2012 identified a circumstellar dust disk, which is a challenging aspect. It is not known if the disk exists around the A or the B star or around both. If this disk exists around Alpha Cen B, then a large planet like the Earth is ruled out, as it would clear its neighbourhood of dust. Also, there must be a source of dust, like a small planet or an asteroid belt.
Around Alpha Centauri AB
Both stars orbit around a common barycenter. Their stellar winds interact one with each other in an interesting way. Each star dominates its surroundings and any planet orbiting a single star will lie within that star's magnetic bubble. However, somewhere close to the barycenter (actually closer to Alpha Cen B), both stellar winds hit each other, creating a shock wave. It is possible that a temporary radiation belt forms there too. Further away, both star's magnetic fields and solar winds merge and push forward, creating a common heliosphere.
It is possible that other planets orbit the AB system at some distance. Safe orbits start from 50 AU from the barycenter. Simulations show that any planet orbiting both stars would have an orbit perpendicular to the plane of rotation of the two stars. Brown Dwarfs have been ruled out completely. Gas giants have also been partially ruled out. If they exist, they must be far enough so that they reflect no detectable light and cold enough so they have no detectable infrared signature.
In one sci-fi novel, a fictional planet, Centarina, is described as orbiting both stars at 53 AU. The planet is protected by a thick layer of greenhouse gasses. The reality is that, as such a distance, even if huge amounts of greenhouse gasses might be able to keep temperatures within acceptable boundaries, there is not enough light for Earth-like plants to survive. Maybe some bacteria and some algae would survive, but not superior plants and certainly there will be no way without artificial sources of light to grow crops to feed a human population.
The outer limit for stable orbits is imposed by the third star of the system, Proxima Cen. Its orbit comes as close as 4000 AU from the AB system. Safe orbits might exist around the AB system as far as 2500 or 3000 AU. However, there is a chance that planet-sized bodies or at least icy asteroids could orbit further, tied in an orbit resonance with Proxima.
The fact that no stable orbits exist below 50 AU means that no icy bodies like the moons of Solar System's gas giants or like Solar System's Kuiper Belt exists. If Alpha Cen has a Kuiper Belt, it must be further away, at a distance that is not perturbed by the A and B stars. For terraformers, this is a challenge, as water and volatiles needed for teraforming inner planets around A and B stars need to be brought from far away.
Proxima orbits quite where an Oort Cloud should be. Some scientists believe that there might be no Oort Cloud around Alpha Centauri system, while others speculate that Proxima continuously disturbs such a cloud and causes significant cometary bombardment. As for now, we have not detected any comets around the system. Detecting comets is not difficult, as their tails are wide and cause a gradual decrease in luminosity of a star if they pass between us and the star. We must also acknowledge that, in a binary system, comets passing too close will have their orbits affected and are likely to be ejected from the system. So, even if Proxima disturbs plenty of Oort Cloud objects, sending them inwards as comets, these comets are short lived.
Proxima Cen, also called Alpha Cen C, is a M5.5Ve star. It belongs to the dim M-Type Stars and to the V - Main Sequence Stars. The terminal e means it is an eruptive, flare star. Its mass is 12% of our un, Sol. Its radius is 15% of Sol's its Solar Constant is 0.17% and its luminosity is 0.005%.
Proxima is a highly active flare star. Flares are known to occur very frequent. While microflares erupt all the time, massive flares are not uncommon. So, any planet orbiting Proxima will experience massive blows of Stellar Wind if it has the misfortune to be hit by a flare, extreme heat and huge doses of UV and X rays. Proxima has a hotter corona then the Sun. Because of this, it glows in X rays as bright as the Sun does. This is a significant problem for close-orbiting planets.
The environment around Proxima is completely different then the environment around the Sun. Its stellar wind is about 20% as strong as Sol's, but highly irregular. There are plumes which correspond to flares and gaps, which allow cosmic radiation to move in very close to the star.
Proxima Cen has an elliptical orbit around Alpha Cen AB, its distance varying between 4000 and 500000 AU. Safe orbits around Proxima exist as far as 700 AU.
There are three planets known to orbit Proxima:
- Proxima Centauri b - orbits at 0.2885 AU,
- Proxima Centauri c - orbits at 1.489 AU,
- Proxima Centauri d (not proven) - orbits at 0.04857 AU.
Proxima Cen is an M5.5Ve star Its light is mainly in infrared (see Main Sequence M Type Objects - Habitable Simulation for details). If we exclude the flares from calculation, we can determine the following parameters:
- Solar Constant: 0.00347 (Sol = 1)
- Infrared radiation: 0.00894 (Sol = 1)
- Red light: 0.00215 (Sol = 1)
- Visible (yellow-green) light: 0.00111 (Sol = 1)
- Blue light: 0.000397 (Sol = 1)
Using greenhouse and anti-greenhouse technology, it might be possible to terraform a planet receiving up to 20 times more heat or down to 1000 times less then Earth does. For Proxima, this means planets located between 0.0131 and 1.863 AU can be made to have temperatures allowing liquid water to exist. However, at 0.0589 AU, water should be liquid on an Earth-like planet.
The biggest limitation for an outer planet around Proxima is the lack of blue light. Plants need both red and blue light in order to survive... and this light must not be below 1/1000 of what we have on Earth. Because of this, in the Solar System, the orbit of Neptune is basically the outer limit for plant life. On Proxima, the outer limit for blue light is at 0.630 AU.
The stellar wind around Proxima is 20% of Sol's. We can calculate its impact on the three planets:
- On Proxima b: 240 x Earth's.
- On Proxima c: 0.0902 x Earth's.
- On Proxima d: 84.8 x Earth's.
As one can see, Proxima c, which lies in the habitable zone, faces a powerful solar wind, doubled with Proxima's strong magnetic field. Unless the planet has an extremely powerful magnetic field, which seems unlikely, it has no chance of holding an atmosphere for long. It is unknown if it can hold an atmosphere for a few millennia, which is a good time interval for human civilizations to develop.
Proxima b is even more exposed to solar winds. This planet has almost no chance of holding a breathable atmosphere for millennia. On the other hand, Proxima c is safe from stellar winds, but is too far for plants to survive.
The biggest question for Proxima Cen is if a terraformed planet can survive its devastating flares. UV Ceti is the most known flare star and it was seen increasing its brightness 75 times within seconds. Proxima can produce similar flares. What would happen if, on Earth, the Sun suddenly increases its brightness 75 times for a minute? Temperature would rise to 500 C, plants will dry and maybe ignite, water on surface of ponds will boil, animals will have severe burns if they will survive. Tree trunks will still survive, so will do organisms living deep in water. However, the atmosphere will need time to cool down again after such an event. The thermal effect of a flare is like sitting in a fire for a minute.
Flares will cause massive auroras and will disrupt planet's climate equilibrium.
For an analysis of flare and a study if planets around such stars can support colonies, see Flare Stars.