This is a gas giant detected in 2021 and further identified in 2025 to orbit around Alpha Centauri A. The existence of this planet is not completely proven, it could be an artefact.
The planet was directly imaged by James Webb telescope, but some scientists argue that it might be an artefact. The planet orbits at roughly 2 AU on an eccentric orbit. Based on the data we know, it is a gas giant, possibly with a Saturn mass and with a size comparable with Jupiter. The planet is in the star's habitable zone.
A reason why this planet is very popular, especially on social media, is that the moon Pandora from Avatar movie is described as orbiting a gas giant around Alpha Centauri A.
The Star System[]
The host star belongs to the G-Type Stars along The H–R Diagram. It is slightly brighter than the Sun, with a slightly higher mass, radius, temperature and luminosity. The light and solar wind of this star are similar to what we have around the Sun.
However, this star is not alone but is part of a binary system. It orbits around a barycentre together with Alpha Centauri B, which belongs to the K-Type Stars group, being slightly smaller and redder than the Sun. The separation between the two stars varies greatly, as their orbit has an eccentricity of 0.5. The average distance between the two stars is like the orbit of Uranus, but varies from between Saturn and Neptune. As so, each star causes significant disruptions in the orbits of planets around the other star.
Stable orbits exist up to two AU around each star. However, given the continuous perturbations found inside the system, circular orbits are impossible. Any planet must be orbiting on eccentric orbits and most likely within a resonation with the other star.
Planetary Data[]
What we know about this planet comes with a big amount of uncertainty:
- Distance: 2 AU (error of 30%)
- Mass: 90 to 150 Earth masses
- Diameter: 1 to 1.1 Jupiter diameters
- Eccentricity: 0.4
From this, we can interfere a few more data. Please remember there is a high margin of error in all these measurements.
- Orbital period: 2.73 years (994 days)
- Apastron (furthest distance to the star): 418.6 million km (like main-belt asteroids)
- Periastron (closest approach to the star): 179.4 million km (close to Earth's orbit in the Solar System)
- Semi-major axis (average distance): 299 million km (between Mars and the Main Belt asteroids)
As seen here, the planet orbits its host star further than Earth orbits the Sun.
- Solar Constant at apastron: 0.416 (like between Mars and the Main Asteroid Belt)
- Solar constant at periastron: 2.267 (for Earth, it is 1.98)
- Solar constant at semi-major axis: 0.816 (like Mars)
From here, we can calculate the average temperature expected for a planet with an Earth-like atmosphere and greenhouse effect:
- Average temperature at apastron: -41.7 degrees C
- Average temperature at periastron: 80.4 degrees C
- Average temperature at semi-major axis: 0.73 degrees C
Even more, we can calculate the void temperature, the temperature of an object that is 25% grey and is exposed directly to sunlight:
- Apastron temperature: 31.5 degrees C
- Periastron temperature: 192 degrees C
- Semi-major axis temperature: 87.3 degrees C
As one can see, an Earth-Like Planet would experience significant climate variations, with hot summers and cold winters. In summer, life would be almost impossible at the equator, while in winter, it might snow even at the equator. The void temperature shows the daytime temperature on an airless moon, which proves that water ice would only remain in craters that are not exposed to sunlight at all.
Further data[]
The planet's mass cannot be determined properly. It might be something like Saturn or much smaller. The diameter is estimated to be Jupiter-like. If this is true, the planet must either have a very low density (which implies it is almost entirely made of hydrogen) or that it has a large ring that reflects light and makes it visible.
If the plant is very light, it must have a smaller density than Saturn, the planet with lowest known density in the Solar System (which is below 1 kg/l). As so, the planet must contain almost only hydrogen and have a very small core made of heavier elements. Young gas giants are known to be more fluffy thanks to their internal heat, but Alpha Centauri is considered to be older than the Solar System, so it is less probably that the planet is kept fluffier thanks to its internal heat. Hydrogen appears red-orange, so this must be the dominant colour on the planet. In addition, it is possible that bands of various other colours exist. Methane, ammonia, carbon dioxide and helium might also be in the planet's atmosphere. And given the average temperature, water vapours might also accumulate in the planet's clouds.
Such a model becomes less probable if the planet is close to its smaller estimated mass. In that case, even a hydrogen-based composition becomes unstable. A ring is needed to explain the extra brightness. If the planet has a large ring around it, then the planet itself can be far smaller and might have a completely different composition.
Possible Moons[]
The planet orbits at a distance where stable orbits do exist. Tidal forces are comparable to what we see at Earth's orbit from the Sun. However, there are a few other factors that influence the tidal dance of a possible moon:
Orbit[]
- Alpha Centauri is a binary (we can exclude Proxima, which is too far to influence with its gravity). In the Solar System, Jupiter is the strongest perturbator, but in Alpha Centauri we have planet B which is far more massive and has an eccentric orbit, strongly influencing any planet orbiting planet A. The fact that the AB system has eccentricities of 0.5, implies that gravitational influences on the planet are not the same. These gravity pulls distort the orbit of planet Ab over time, causing significant precession.
- The planet has a significant eccentricity (0.4), which brings it further and closer to its home star. This strongly influences the orbit of any potential moons in the same way star B influences planet Ab in its orbit.
- When parent bodies have elliptical orbits, there is a high chance their moons also have elliptical orbits, sometimes greater than their parents.
So, there are stable orbits around the planet. The Earth can host a moon at 1 million km away. A planet with a greater mass can host a moon even further. However, the orbit of such a moon would most certainly be elliptical. We can expect an orbit with a similar eccentricity (0.4 or 0.5) for any moons. Earth's Moon orbits at 0.384 million km. A possible moon orbiting planet Ab would have a stable orbit even at greater distances.
Assuming an orbit at 1 million km and an eccentricity of 0.4, like its host planet, the moon would be at between 0.6 and 1.4 million km from its host planet. Eccentric orbits like this one require more space, which also implies that there is not much space left for many other stable orbits. So, the planet is likely to host less moons than it would host in the Solar System.
Number of moons[]
As moons are likely to have eccentric orbits and eccentric orbits occupy more space than concentric ones, there is not much space left for many moons.
As we can see in the Solar System, there are two factors which determine how many moons orbit a planet. One factor is the Hill Sphere (the gravity well surrounding a planet, where its gravity overcomes the pull from its parent star). Stable orbits exist only within the center of the Hill sphere. A planet that is more massive and placed further from its host star will have a larger Hill sphere. Instability (as seen here from perturbations from elliptical orbits) dramatically reduce the chances for moons to exist.
Larger planets are known to host more and larger moons than smaller planets. It can be seen in the Solar System, where the combined mass of Jupiter's moons is greater than the combined mass of all other moons. On the other hand, exceptions also exist. Earth's Moon is greater than would be expected around a planet the size of Earth.
The gas giants of the Solar System host a few small inner moons located very close, larger moons at a greater distance and outer moons on even higher orbits. Planet Ab is likely to have inner moons at close distance, which would be on stable orbits. Large moons might exist too. Given the elliptic orbits that are likely to exist, there might be a maximum of two possible safe orbits with an eccentricity of 0.4. Outer, irregular moons, are unlikely to exist, as they would be pulled away by perturbations.
This is a scheme of a possible moon system:
- Moon Aba: semi-major orbit of 300 000 km (apastron of 420 000 km, periastron of 180 000 km)
- Moon Abb: semi-major orbit of 1 000 000 km (apastron of 1 400 000 km, periastron of 600 000 km)
In such a system, there would be very strong tidal friction, caused by the elliptical orbits of these moons. Also, they would not be tidal locked but would rotate slowly. From the surface, depending on position along orbit, the planet would appear sometimes moving from East to West and sometimes from West to East.
In addition to this, there would be a number of inner moons, with a radius below 50 km, orbiting closer to the planet, on nearly circular orbits. There could also be rings. Such moons would orbit below 200 000 km. At such orbits, a large moon approaches the Roche Limit and would break into a ring.
Size[]
In the Solar System, there is a correlation between the mass of a planet and the mass of its moons. This can be seen clearly for Jupiter, Saturn and Uranus. However, in case of Neptune and Earth, one can see that the mass of moons is greater. So, there is not an universal rule. Given the tendency seen for Jupiter, Saturn and Uranus, the mass of moons orbiting planet Ab would be something similar to the mass of moons orbiting Saturn. If that is the case, we can expect two moons, one the size of Jovian moons (or the size of Titan) and one smaller, the size of Saturn's larger moons (like Rhea for example).
The Solar System shows that it is possible for a planet to have larger moons than the average, as one can see with Neptune. It is possible to even have a double planet. So, there is no way to know through theory what kind of moons orbit planet Ab. We can theorise that a system with a large moon would require a more circular orbit and that the larger the moon, the smaller the chances for other moons to orbit.
Environment[]
Whatever moons orbit (if they exist) are likely protected, at least partially, by planet Ab with its magnetosphere. A gas giant is likely to rotate fast enough and maintain a strong dynamo, protecting its closer moons from stellar wind.
Structure of possible moons[]
The following material is speculation. As there is, for now, no way to know if moons do orbit the planet, all we can do is speculate about the possible conditions and ways for colonisation and terraforming.
Small Irregular Moons[]
This is what is expected for moons with a diameter smaller than 400 km. Such moons don't produce enough heat from their formation to melt their interiors. Heat from decay of radioactive elements is also not enough to melt their cores. Such moons would look probably like the inner moons of Jupiter, Saturn, Uranus and Neptune, only that they won't be made of water ice but of solid rock. They would not contain water. These small moons can be used for Industrial Colonization, for mining (if they contain important ores) and can be partially made habitable through Paraterraforming. Domes can be built on their surfaces. protected from stellar wind by the planet's own magnetosphere.
Small Round Moons[]
This is what one would expect if there are moons like the large moons of Uranus or like the spherical moons of Saturn (except Titan). Such moons produce enough heat from their formation and from radioactive decay to partially melt their cores of lighter elements, but not enough for iron to melt. While some volcanism might have existed, it is likely extinct. So, there is a chance that some ancient tectonic faults exist, but most of the moon's surface would be covered by craters. Without an atmosphere and given the expected temperatures, water has long been lost to space. The moons would be made of various rocks.
These moons would have many crater impacts.
These moons can be very useful for industrial colonisation, as material from meteorite impacts might be on the surface. Since the inner structure is only partially separated, important ores like heavy metals might be found in larger amounts on the surface or close to the surface. With such an abundance of minerals, industrial facilities are likely to develop.
Jovian Sized Moons[]
There is a chance that some moons might be the size of Jovian moons, Titan or Earth's Moon. In this case, internal heat from their formation was enough to melt the core. As so, through tidal friction, the core is likely maintained liquid. Volcanism is likely to be intense. We could see tortured moons like Jupiter's moon Io, with massive volcanoes and with rivers of lava. Such a moon would have its own magnetic field and would release into space matter that would feed a strong magnetosphere around the planet, something similar to what we see around Jupiter.
A moon of this size is unable to hold a stable atmosphere. Still, through outgassing, it can replenish its atmosphere. So, an atmosphere like the one that exists on Mars, is likely to exist. Volcanism would bring heavier elements to the crust, which is important for industrial colonisation.
Such a moon can be terraformed. However, there would be significant challenges. First of all, the atmosphere is unstable and one would need to replenish the losses of gas and water. Second, experiencing massive volcanism, the moon would threaten to destroy ecosystems built on its surface. And thirdly, the climate would be wild, with extremely hot summers and very cold winters.
Earth - Sized Moons[]
Although somehow unlikely, given the planet's mass, such moons are still possible. If such a moon exists, it will move into a more circular orbit and it will be more stable, less exposed to tidal stress. A large moon will also make the possibility of other moons less likely.
The moon will be far more active than Earth, with more volcanism. It might host oceans and would have a cooler climate. The moon would be tidal locked to its planet (although some Libration is likely). Being tidal locked, it will experience longer days than on Earth, which will result in extreme heat during day and low temperatures during night.
Through terraforming, an Earth-like atmosphere might be created. However, conditions on such a moon would be extreme, compared to Earth. During summer, temperatures would rise to extreme, especially during the day. Water would boil at the equator and good conditions for living would be expected closer to the poles. Any ice caps would melt. During winter, the equator would be the only place where water would not freeze.
Such extreme temperature variations will force settlers to periodically migrate. There will be summer and winter cities. People will migrate depending on climate.
Flora and fauna will also have to adapt. There will be different climate zones:
- Polar regions will be covered with Arctic, Siberian vegetation, able to hibernate during the long winter. During summer, they will come to life and cover the land. It is questionable if trees can survive this long. Most mammals and birds will not survive the long winter, which will last for at least one Earth year. While some trees can survive this, the animals that will resist and adapt are mostly invertebrates. Some animals would migrate periodically.
- Temperate regions will experience harsh winters and warm summers. In these areas, more trees can survive. It is likely that plants will blossom in spring and give their fruits before summer. In summer, most plants will hardly survive. During autumn, plants will blossom again and there is even a chance that some fruits can be harvested. Grain crops could be harvested twice an year. It would be hard, but not impossible, for humans to survive the winter and the summer. Many migrating animals would temporarily reside in here.
- Equatorial regions will experience mild sinters and extreme summers. The ground would probably be sterilised by extreme temperatures during summer. Here, humans would probably plant crops in winter, when conditions allow. It is questionable what kind of plants can survive the extreme temperatures of summer, so they can grow again once temperatures allow.
A similar migration would happen in water, where fish and algae would move to areas where they can survive.
Double Gas Giant[]
There is also a possibility that the planet is actually a binary, with two gas giants orbiting close one to another. If this is the case, there would be no stable orbits for large moons, although small moons might exist on low orbits. This would be bad news for future settlers.
In case of a double gas giant, the orbits are expected to be more circular.
Pandora?[]
In the movie Avatar action takes place on Pandora, a moon orbiting a gas giant around Alpha Centauri A. The planet Ab is indeed a gas giant. But can it host a moon like Pandora? The possibility still exists, but it is not so big. Statistically speaking, the planet Ab is less likely to host a moon large enough to host a stable atmosphere and a stable ecosystem.
One thing can be said with a high degree of certainty. If a moon with atmosphere does exist around planet Ab, it experiences a very unstable climate, with extreme summers and winters and with strong volcanism. It certainly is not the paradise-like world that we have imagined and dreamed about.