Alpha Centauri

Often called "Rigil Kentaurus" (meaning "foot of the centaur"), Toliman from the Arabic meaning "Grape-vine shoot" (some claim that this is the most proper name because the Greeks couldn't see it - it is only visible south of the 29th parallel) Alpha Centauri is a three star system composed of α Centauri A, α Centauri B, and the red dwarf star α Centauri C. Of these stars, the two brightest ones (A and B) orbit each other as part of a binary system. Each orbit takes 80.089 years and the stars are separated by 23 AU (the distance from the sun to Uranus). Alpha Centauri A and B are the third and twenty-first brightest stars in the sky. These stars are 4.35 light years away (comparison: if the sun was 1" in diameter, the earth would be about 9 feet away, and Alpha Centauri would be 458 miles away.)

Alpha Centauri A is very sun-like in many respects - its the same spectral class and about the same size and mass. Alpha Centauri B is a slightly dimmer and oranger in color than Alpha Centauri A. While the format here is not that great for displaying color, the color of each star is: Sun #fff3ea; Alpha Centauri A #fff5f2; Alpha Centauri B #ffe0bc;. Feel free to enter these values into the E2 Color Toy.

Alpha Centauri C is often called Proxima Centauri because it is a bit closer to earth (only 4.22 light years away). Proxima Centauri orbits the binary pair 13,000 AU away (400 times the distance between the Sun and Neptune). At this distance, astronomers are uncertain about if Proxima is part of the system and gravitationly bound to the binary pair, or if it is just passing through and will leave the system in a few million years or so. From Earth, Proxima Centauri is two degrees away from the binary pair.

Proxima Centauri is a red dwarf (M5) - this means that it is very small, quite near the fusion boundary and also cool and faint. Proxima has a surface temperature of 2700 Kelvin (less than half that of the sun) and only is 0.00006 times as bright as the sun. Being so faint and small, Proxima was not discovered until 1915. The color of Proxima Centauri is #ffcc6f.

Besides being very close stars (which make them interesting), this system is interesting because it may harbor life as we know it. There are five tests a star must pass for it to harbor life that is familiar to us.

1. Maturity
   The question here is is the star on the main sequence? If it isn't (and off on some side group) it means that it isn't a stable star - either a red supergiant, a variable star or white dwarf. These stars are unsuitable for life.
   1. Main sequence: G2
   2. Main sequence: K1
   3. Main sequence: M5
   All stars in the Alpha Centauri pass this test.
2. Spectral type
   A bit stricter than the first test, the stars must be of the right spectral type. This is a measure of the energy a star emits. The hot stars (O, B, A, and early F), burn fuel too fast and dies in a big way. While the M and late K class stars don't produce enough energy to have an appreciable habital zone around the star without bringing it dangerously close. The stars must be either late F, G, or early K class stars.
   1. G2 - perfect (our sun is G2)
   2. K1 - early K, good
   3. M5 - fails... far too cold
3. Stability
   Does the star's brightness change too much - so that it would be hot one year and freezing the next?
   1. Not a variable star
   2. Not a variable star
   3. Red dwarfs are prone to huge flares that may double or triple the brightness of the star. Fail
   (Some mistake Alpha Centauri as a variable star because it is a binary system - there are times when one star is obscuring the other star as seen from earth and thus appears to be variable. Neither Alpha Centauri A nor B are any more variable than our own sun) The binary nature of this system also raises issues. The distance between A and B varies from 11 AU to 35 AU. From a planet that orbits either A or B, this means the brightness of one of the stars would change as it approaches and recedes. However, the distance from 11 AU (sun to Saturn) and 35 AU (sun to Neptune) would not cause significant variations within the habital zone of either A or B.
4. Stellar Age
   There must be time enough for life to develop on a planet about the star. The age of a star can be computed by looking at where it is on the main sequence and comparing this to the mass of the star. While the exact ages are not certain for minimum limit, the sun gives us a ball park of 4.6 billion years.
   1. 5-6 Billion years. Pass
   2. 5-6 Billion years. Pass
   3. unknown (maybe 1 billion years?). fail
5. More than Hydrogen
   For planets and life we need carbon, nitrogen, oxygen, and iron. The sun has about 2% of its weight from these heavy elements. This can be determined from the spectra of the star or the location of the star within the galaxy (compare population I stars with population II stars).
   1. Metal rich. pass
   2. Metal rich. pass
   3. unknown. fail

Could earthlike planets exist?
This one, we don't know for certain. The very nature of the binary system confuses the issues of the planetary disk. Furthermore, such a system may cause planets to get kicked out with a gravitational nudge. Simulations indicate that bodies greater than 2 AU away from a star in this system will get ejected. This doesn't appear to be as much of a problem as it seems - the habital zone for our solar system is clearly within the orbit of Mars which has a orbit at 1.5 AU.

Whatever the case, we don't know if any planets exist within this system, however it is quite possible that small rocky planets within the area where liquid water can exist do.

                   |  Sun   | A      | B      | C       |
                   +--------+--------+--------+---------+
Color              | Yellow | Yellow | Orange | Red     |
Spectral Type      | G2     | G2     | K1     | M5      |
Surface Temp       | 5800 K | 5800 K | 5300 K | 2700 K  |
Mass (solar)       | 1.00   | 1.09   | 0.90   | 0.1     |
Radius (solar)     | 1.00   | 1.2    | 0.8    | 0.2     |
Brightness (solar) | 1.00   | 1.54   | 0.44   | 0.00006 |
Distance (ly)      | 0.00   | 4.35   | 4.35   | 4.22    |
Age (b years)      | 4.6    | 5 - 6  | 5 - 6  | ~1?     |

Alpha Centauri is located at right ascension 14h 39m 36s and declination -60^o 50' 2.308". It has a measurable proper motion of -0.50332 RA and +0.483067. The radial velocity is -26km/s (this means that the stars are heading in our direction)

The Alpha Centauri system has always been complex, and last month, it became more so. The two main stars in the system are Alpha Centauri A and Alpha Centauri B, a pair of stars of about solar mass, orbiting around each other at a distance of around 10 astronomical units. There is also Alpha Centauri C, a dim red dwarf star that may or may not be gravitationally bound to the larger two members, 10,000 astronmical units away.

And there is a newly discovered fourth member of this grouping: a light, small exoplanet orbiting very closely around Alpha Centauri B. In keeping with exoplanetary nomenclature, it is desginated Alpha Centauri Bb.

The technical skill needed to detect this planet is incredible. It was found by the European Southern Observatory, using ground-based telescopes in South America. Using the technique of observing doppler shift in Alpha Centauri B, they detected a slight wobble in its motion. This motion was only 1.8 kilometers an hour, which they helpfully rendered as "the speed of a baby crawling". From this, they calculated the probable size and orbit of Alpha Centauri Bb.

The good news is that the planet is around the mass of the earth. It is almost certainly a terrestrial world, with a solid surface. The bad news is that it is very close to its star, 4% of the distance from the sun to the earth. At that distance, its surface is probably hot enough to be perpetually molten, as well as being constantly bathed in ultra violet and X-Ray radiation. In other words, there is no way that life as we know it could possibly exist on this planet. For that matter, if we could figure out a way to get there, we couldn't even land a spacecraft on its surface.

But if there is one planet in the Alpha Centauri system, there may be more. Although the mechanics of keeping multiple planets orbiting in a dual (or triple) system are complicated, it could be possible that there are other earth-size planets in Alpha Centauri's habitable zone.

So while the existence of Alpha Centauri Bb might not immediately change much (since it is an uninhabitable planet orbiting a star we can't currently reach), it is still a heartening discovery due to the technical expertise used to discover it, and because there might be other, more hospitable planets, in the system.

The initial report of the ESO can be found here:
http://www.eso.org/public/news/eso1241/