| Exoplanetary Scratchpad|
Alpha Centauri is a triple star system. It contains two stars similar to our Sun in mass and temperature. One a little larger than the Sun, Alpha Centauri A which is like our Sun a yellow dwarf star, and another star slightly smaller than the Sun, Alpha Centauri B, which is a very slightly cooler orange dwarf. They're in a close but eccentric orbit around a common centre of mass, and that orbit takes about 80 years to complete. Though relatively close to each other the distance between them varies greatly, between a maximum separation of about 36 au, which for comparison is a bit further out than the orbit of Neptune, down to a minimum separation of around 11 au, just a little bit further out than the planet Saturn's orbit. The star system’s third stellar member is much more distant at 15,000 au, which if it was in our solar system would place it out in the middle of the Oort cloud. It orbits the other two stars about every half a million years. It is a much colder and smaller star than our Sun, a Red Dwarf and its usual name is not Alpha Centauri C but Proxima Centauri. The triple star system also has one recently confirmed planet, it is nearly the same size as Earth but unlike Earth it is very tightly orbiting around its parent star, Alpha Centauri B. The planet, Alpha Centauri Bb, is in a torch orbit, just above the star, that is only about one tenth the distance out as Mercury's orbit is out from our Sun, and being so close it takes no more than a few days for the planet to complete a whole orbit.
On October the 16th 2012, astronomers announced to the world that an Earth mass planet was detected in orbit around Alpha Centauri B. The discovered Earth-like planet, with the designated name Alpha Centauri Bb, is slightly larger in mass than Earth with a minimum of 1.13 times the mass. It orbits extremely close to Alpha Centauri B at 0.04 au, much closer in than the star's habitable zone, and completes one orbit in just 3.236 days. Consequently, it is a tidally locked planet with a surface temperature, on the sun facing side, reaching in excess of 1000 degrees Celsius.
For an Earth-like planet the perfect temperature location, adjusted to account for Alpha Centauri B's relatively high infrared output when compared to the Sun's, would be around 0.75 au. The full extent of the habitable zone for a planet of Earth mass with an Earth similar atmosphere, again adjusted to account for the higher infrared heating from Alpha Centauri B, is between 0.71 au and 1.05 au. The habitable zone is slightly further out for a smaller mass planet or one with a thicker atmosphere and slightly closer in for a larger mass planet or one with a thinner atmosphere. For a desert planet the habitable zone would be wider.
To date, Alpha Centauri Bb is the closest known extrasolar planet, at 4.4 light years away. It is the lowest mass planet (Kepler-20 e has the smallest measured radius) yet detected around a Sun-like star. Like Gamma Cephei Ab but unlike Earth, Alpha Cenaturi Bb is orbiting one star in a close binary configuration where neither star is in orbit around the other, instead the stars orbit a common centre of mass in the empty space between them. The planet was detected by scientists using the HARPS (High Accuracy Radial Velocity Planet Searcher) high precision echelle spectrograph on ESO's (European Southern Observatory) La Silla Observatory telescope, located in Chile.
Alpha Centauri Bb has yet to be given a proper name. If it is given one, an Arabic name could well be chosen so as to be consistent with the name of its host star or perhaps in honour of the team that discovered it, led by Xavier Dumusque, it might be given a French, Portuguese or Swiss name.
There has been a long history of false detections of planets around the nearest stars, beginning in the middle of the 19th century and then unfortunately throughout the 20th century. In the 1990's, Lalande 21185, 8.3 light years away, was mistakenly claimed to host planets. Even as recently as 2012, a scientific paper was published that challenged Epsilon Eridani b's existence; the detection of Epsilon Eridani b back in the year 2000 has not yet been properly confirmed by HARPS despite many radial velocity measurements on Epsilon Eridani since 2002. The discovery of Alpha Centauri Bb required the most precise measurements ever recorded using the technique of radial velocity, pushing the HARPS instrument to very near the limit of its detection range. Hopefully this discovery will be the very first one of a planet within 10 light years that will stand the test of time.
Only a very marginal radial velocity peak, translating to a planet orbiting every 5.6 days, has been detected around the slightly closer Proxima Centauri, 4.2 light years away. This weaker signal was detected by the HARPS spectrograph, on the La Silla telescope.
No promising radial velocity signals or any other hints of a planet have yet been discovered for Alpha Centauri A.
Of course, with time and improving technology, it is very likely many more planets will be discovered in nearby star systems like Alpha Centauri.
Both Alpha Centauri A and B have been examined in the infrared band at 24µm by the Herschel and the Spitzer space telescopes and through combining their observations there appears to be a marginal infrared excess for both stars. If these infrared excesses are confirmed they could indicate a warm dusty disc inward from 4.0 au around each star, very similar to the zodiacal cloud around our Sun. The upper limit of the mass of these dust clouds is estimated to be around 10 to 100 times more than in the solar zodiacal cloud, a lower mass limit for these dust clouds has not yet been defined.
However, in contradiction to the possible dust cloud theory, it must be considered that zodiacal dust only survives for a few million years close to a star, so for it to exist, it must be constantly replenished from some outside source, beyond a star system's frost line. Alpha Centauri B's frost line lies so close to the outer edge of where the zodiacal dust could exist that this narrow zone would only provide a dust source for a few tens of millions of years. Alpha Centauri A's frost line lies beyond where any significant zodiacal dust collection could lie. 70-100 au from Alpha Centauri AB would be an ideal location for a Kuiper belt, that could provide a continuous source for both inner systems warm dust clouds but unfortunately, to date, there has been no infrared excess detected out there. Even if Proxima Centauri were indirectly contributing dust, by disturbing a much further out Oort Cloud, you would reasonably expect it to collect between 70-100 au from Alpha Centauri AB. The infrared excess at 24µm therefore might be caused by something other than dust, perhaps a phenomenon associated with both the stars' atmospheres, instead.
Planet Formation in Binary Systems
- The Role of Multiplicity in Disk Evolution and Planet Formation
- Planet Formation in Highly Inclined Binary Systems. I, Planetesimals Jump Inward and Pile Up