Alpha Centauri Bb
|Extrasolar planet||List of extrasolar planets|
|Star||Alpha Centauri B|
|Right ascension||(α)||14h 39m 35.0803s|
|Declination||(δ)||−60° 50′ 13.761″|
|Distance||4.366 ± 0.007 ly
(1.339 ± 0.002 pc)
|Spectral type||K1 V|
|Mass||(m)||0.934 ± 0.006 M☉|
|Temperature||(T)||5214 ± 33 K|
|Semimajor axis||(a)||0.04 AU|
|Orbital period||(P)||3.2357 ± 0.0008 d|
|Semi-amplitude||(K)||0.51 ± 0.04 m/s|
|Minimum mass||(m sin i)||1.13 ± 0.09 M⊕|
|Stellar flux||(F⊙)||308.66 ⊕|
|Discovery date||16 October 2012|
|Discoverer(s)||Dumusque et al.|
|Discovery method||Radial velocity|
|Discovery site||La Silla Observatory, Chile|
Alpha Centauri Bb is a possible extrasolar planet orbiting the K-type main-sequence star Alpha Centauri B, located 4.37 light-years from Earth in the southern constellation of Centaurus. If verified, it would be the closest extrasolar planet to Earth ever discovered, and the lowest-minimum-mass planet detected so far around a solar-type star. Its existence was announced in October 2012 by a team of European observers, and the finding received widespread media attention. However, the announcement was met with scepticism by some astronomers, who believed that the European team was over-interpreting its data.
On June 10, 2013, scientists reported that the earlier claims of an Earth-like exoplanet orbiting Alpha Centauri B may not be supported. The existence of the planet remains a subject of scientific debate, and research by at least three separate groups of astronomers is ongoing.
Starting in February 2008, and continuing through July 2011, a team of European astronomers, mainly from the Observatory of Geneva and from the Centre for Astrophysics of the University of Porto, recorded measurements of Alpha Centauri B's radial velocity with European Southern Observatory's HARPS echelle spectrograph at the La Silla Observatory in Chile. The team made 459 observations of Alpha Centauri B's color spectrum over a four-year period, then used statistical filters to remove known sources of variance.
On 16 October 2012, the team announced they had detected an Earth-mass planet in orbit around Alpha Centauri B. The discovery was presented in the scientific journal Nature, with lead author credit going to Xavier Dumusque, a graduate student at the University of Porto. He called his findings "a major step towards the detection of a twin Earth in the immediate vicinity of the Sun".
Alpha Centauri Bb was discovered using Doppler spectroscopy. As it orbits Alpha Centauri B, its gravity causes extremely small (semi-amplitude of about 0.5 m/s) periodic shifts in the host star's velocity. Variations of the line-of-sight velocity component cause tiny shifts in the star's spectrum. Using an extremely sensitive spectrometer, the team was able to infer variations of 0.51 m/s in Alpha Centauri B's radial velocity. The ESO called the findings the most precise measurements ever recorded using the technique. The team estimated the probability of a spurious detection at 0.02%.
Alpha Centauri Bb is not in the habitable zone. At just 0.04 AU, the distance between the planet and its parent star is roughly one tenth of the average distance between Mercury and the Sun. It completes one orbit in just 3.2357 ± 0.0008 days (3 days, 5 hours, 39 minutes, 20 ± 70 seconds), and at such a close range is likely to be tidally locked. It has a mass of at least 1.13 times that of Earth; only the lower limit is currently known, as the planet's orbital inclination has not been determined.
As the planet has not been observed to transit its parent star, its size, composition, and atmospheric conditions are unknown, although its mass is indicative of a terrestrial planet. The surface temperature is estimated to be 1,200 °C, which is above the melting temperatures of many silicate magmas. (By comparison, the surface of Venus, the hottest planetary surface in the Solar System, is 460 °C.) At such high temperatures large areas of the planet's surface are likely to be molten, leading to popular descriptions of the planet as a "lava world".
The orbit of the planet, as well as orbits of hypothetical planets out to the outer edge of Alpha Centauri B's habitable zone (about 0.9 AU), are not believed to be destabilized by the influence of companion star Alpha Centauri A, which has a periastron (closest approach) distance of 11.3 AU, a bit larger than the radius of Saturn's orbit.
The planet's parent star would subtend an angle of about 11 degrees in its sky and be about 300 times brighter than the Sun appears from Earth. At periastron, Alpha Centauri A would have an apparent visual magnitude of −21.9, and therefore be about 1% as bright as the Sun appears from Earth. Viewed from the planet, our own Sun would appear as a bright star of apparent magnitude +0.47, slightly dimmer than Procyon appears from Earth.
Estimates of the age of the Alpha Centauri system based on asteroseismology, chromospheric activity, and gyrochronology show that it is likely to be slightly older than the Solar System, with values ranging from 4.5 to 7 billion years. According to widely accepted models of planetary formation, Alpha Centauri Bb and any other planets in the system are expected to be nearly as old.
Skepticism and confirmation efforts
Dumusque used a variety of mathematical transformations to isolate the planet's gravitational signal and remove the surrounding stellar noise. The transformations filtered out other sources of radial-velocity variance, including effects of starspots, photospheric granulation and rotation, as well as interference from the neighboring star Alpha Centauri A. The final, derived radial-velocity measurements showed the periodic influence of the planet on the star, but were near the threshold of the HARPS instrument's sensory capabilities. (The long-term precision of HARPS is about 0.8 m/s.)
In a commentary piece that ran in Nature alongside the original paper, American exoplanet hunter Artie Hatzes praised the team's technical achievements and admitted he observed a planetlike signal amongst the data, but called it a "a weak signal in the presence of a larger, more complicated signal". He stressed the need for further empirical confirmation, and opined that the existence of the planet was "still open to debate". (Hatzes had previously expressed scepticism about unproven methods of data analysis during the dispute over the unconfirmed planet Gliese 581 g.)
American Astronomer Debra Fischer was more enthusiastic, and endorsed Dumusque's methodology. Nevertheless she cautioned that the statistical filters "essentially constitute a new approach", and also emphasized the need for further observational confirmation. Immediately after the announcement, Fischer and her team of exoplanet hunters at the Cerro Tololo Inter-American Observatory began analyzing a separate set of spectrometer observations of Alpha Centauri B to confirm the planet's existence. This analysis is ongoing, with further observations scheduled to begin when Centaurus reappears in the observatory's night sky in January 2013.
The European team that made the discovery will now attempt to detect a planetary transit of the host star, and has applied for time on the Hubble Space Telescope to make these observations. A successful transit observation would provide information about the planet's size, composition, and atmospheric conditions. However, given the lines of sight, the relatively small size of the planet, and the binary nature of the star system, the chances of a transit of Alpha Centauri Bb being observable from Earth are estimated at between only 10 and 30%.
Possible additional planets in the Alpha Centauri system
Astronomers have already ruled out the existence of Neptune-sized planets or larger in the Alpha Centauri system. However, because of its proximity, stability and lower mass than the Sun's, astronomers believe that Alpha Centauri B is one of the best possible candidates for the detection of an Earth-like planet by Doppler spectroscopy. Statistical analysis of results from NASA's Kepler Mission indicates that low-mass planets tend to form as members of multi-planet systems, so the discovery of Alpha Centauri Bb means that it is likely that there are additional low-mass planets in orbit around Alpha Centauri B.
These hypothetical companions are likely to have wider orbits, and would be difficult to find with current instruments. The HARPS spectrometer can only detect changes in radial velocity of about 30 centimeters per second, whereas Earth's gravitational influence on the radial velocity of the Sun is a mere 9 centimeters per second. An astronomer in the Alpha Centauri system looking toward the Sun with this equipment could not find Earth. Detecting additional planets in the system will become easier when the ESO's next-generation spectrometer ESPRESSO comes online in 2017. ESPRESSO is specifically designed to look for Earth-like planets, and will provide radial-velocity measurements several times more precise than those used to find Alpha Centauri Bb.
Of particular interest are planets within the habitable zone of Alpha Centauri B, which one estimation determines to lie between 0.5 and 0.9 AU. In 2009, computer simulations showed that planets were most likely to form toward the inner edge of that zone. Special assumptions are required to obtain an accretion-friendly environment farther from the star. For example, if Alpha Centauri A and B initially formed with a wider separation and later moved closer to each other, as might be possible if they formed in a dense star cluster, the region favorable to planet formation might extend farther.
Possibility as target for interstellar probe
The announcement of the planet's existence sparked renewed interest in Alpha Centauri as a potential target for an interstellar probe, including enthusiasm from mainstream astronomers. Debra Fischer of Yale opined that "If you were going to send a spacecraft anywhere, or a probe anywhere, that's where you'd go first." A representative from NASA's Innovative Advanced Concepts suggested several theoretical technologies that could make the voyage to Alpha Centauri in a few hundred years, including antimatter rockets and explosive nuclear propulsion.
Planetary scientist Greg Laughlin admitted that such approaches were "speculative and farfetched so far", and noted that with current technology a probe sent to Alpha Centauri would take 40,000 years to arrive, but expressed hope for new technology to be developed to make the trip within a human lifetime.
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