Alpha Centauri 3

Luis Calçada, Nick Risinger, ESO




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An Earth-mass planet has been
found in a very hot, tight orbit
around Alpha Centauri B (more).

On October 16, 2012, a team of astronomers announced the discovery of a planet with around 1.13 +/- 0.09 Earth-masses in a very hot and tight, circular orbit around Alpha Centauri B, using the European Southern Observatory's High Accuracy Radial velocity Planet Searcher (HARPS) instrument on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. Due to the close binary orbital interactions of the host star with Alpha Centauri A and Star B's own increased stellar activity during recent years, the astronomers were only able to detect the radial-velocity variations of host star B that were caused by the 3.236-day orbit of the planet (with a semi-major axis of 0.04 AU) only after more than three years of careful observation. Given that the presence of one close-orbiting planet usually indicates the presence of others, many astronomers are now expected to devote more resources to detecting such potential planets around Star B (ESO press release; Dumusque et al, 2012; Eric Hand, Nature, October 16, 2012; and Aron Jacob, New Scientist, October 17, 2012; and Astronomy Picture of the Day).

Sky & Telescope, IAU, ESO






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Our nearest known stellar neighbors lie
in a triple-star system that has been
measured at just over four light-years
from Earth, where Alpha Centauri A is
the brightest star in Constellation
Centaurus (more).

La Silla Observatory, ESO



Larger image.




While Star A and B form
a relatively close binary,
dim Star C (Proxima) is a
distant companion (more).

In contrast, Proxima (Star "C") is located around 15,000 +/- 700 AUs from Star A and B. This is so far that Proxima may not be gravitationally bound to Star A and B and so may leave the system after some million years, and according to Anosova et al (1994), all three stars may be part of a stellar moving group of nearby stars that includes: the triple ADS 10288 (Gl 649.1); the binaries, Gliese 140.1 and 676; and six single stars. A subsequent analysis using the most recent kinematic and radial velocity data available in the literature, however, found Proxima "is quitely likely" to be bound to to Stars A and B based on calculations of the binding energy of Proxima relative to the center of mass of the entire triple system, where its orbital semi-major axis exceeds 10,000 AUs and is "on order the same size as Alpha Centauri AB's Hill radius in the galactic potential" (Wertheimer and Laughlin, 2006). Hence, it is quite likely that all three stars formed together from the same nebula at roughly the same time and so should share a similar elemental composition.

VLTI, ESO




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Although Star A and B are
similar to Sol in size, Star C
(Proxima) is not a lot bigger
than the planet Jupiter (more).

Being visible to the naked eye, Alpha Centauri has been known for centuries, if not millennia, although perhaps not as a double star until the 1752 observation of the Abbé [Abbot] Nicholas Louis de La Caille (1713-1762) from the Cape of Good Hope, the southernmost point of Africa, where he was studying the stars of the southern hemisphere with just an half-inch (8x) refractor. Dim Proxima, however, was not discovered until 1915 by Robert Thorburn Ayton Innes (1861-1933) of Edinburgh, Scotland who also was observing from Cape Hope, probably with the 7-inch refractor at the Royal Observatory. If our own Sun, Sol, were viewed from the Alpha Centauri system, it would be located in Cassiopeia near the border with Perseus and about five degrees north of a double cluster near the nebula IC 1805/1848, visible as a bright yellow star that would be almost as bright as Capella (Alpha Aurigae) appears in Earth's night sky.

INES, LAEFF, ESA





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All three members of the system
(including B, which has not been
depicted between A and Proxima
at left) are main-sequence stars
(H-R diagram with Star B).

Due to their proximity to Sol, the stars of this system have been objects of intense interest among astronomers. Stars A and B were selected as two of the top 100 target stars for NASA's indefinitely postponed Terrestrial Planet Finder (TPF) to directly image small rocky planets in Earth-type habitable orbits. In addition, all three stars (including Proxima) were among the "Tier 1" target stars for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images on Stars A, B, and C are available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding. Astronomers are also hoping to use the ESA's Darwin group of infrared interferometers to analyze the atmospheres of rocky planets found in the "habitable zone" (HZ) around all three stars for evidence of Earth-type life (Lisa Kaltenegger, 2005).

JPL, CalTech, NASA


Larger illustration of the TPF.


Astronomers have identified
Stars A and B as prime targets
for NASA's TPF, while all three
stars are targets for NASA's
optical SIM and the ESA's
infrared Darwin missions.

AB Binary Star System

The distance separating Alpha Centauri A from its companion star B averages 23.7 AUs (semi-major axis of 17.57" with a HIPPARCOS distance estimate of 4.40 light-years). The stars swings between 11.4 and 36.0 AUs away in a highly elliptical orbit (e= 0.52) that takes almost 80 (79.90) years to complete and are inclined at an angle of 79.23° from the perspective of an observer on Earth (see Pourbaix et al, 2002, or 2000 in the Sixth Catalog of Orbits of Visual Binaries; and Worley and Heintz, 1983). As viewed from a hypothetical planet around either star, the brightness of the other increases as the two approach and decreases as they recede. However, the variation in brightness is considered to be insignificant for life on Earth-type planets around either star. At their closest approach, Stars A and B are almost two AUs farther apart than the average orbital distance of Saturn around the Sun, while their widest separation is still about six AUs farther the average orbital distance of Neptune.

Research Consortium on Nearby Stars (RECONS)




Cerro Tololo Inter-American Observatory




Infrared image of Alpha Centauri AB, with
diffraction effects from partially closing the
mirror covers of the 1.5-m telescope. (See
Sloan Digital Sky Survey field images of
Alpha Centauri AB from WikiSky.org, and
and at Astronomy Picture of the Day.)

In a binary system, a planet must not be located too far away from its "home" star or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. Recent numerical integrations, however, suggest that stable planetary orbits exist: within three AUs (four AUs for retrograde orbits) of either Alpha Centauri A or B in the plane of the binary's orbit; only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for planets circling both stars (Weigert and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like the Solar System: Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AUs).

Indeed, the AB system is significantly more enriched (1.7 to 1.8 times) in elements heavier than hydrogen ("high metallicity") than our own Solar System (Chmielewski et al, 1992; Cayrel de Strobel et al, 1991, page 297; Furenlid and Meylan, 1984; and Flannery and Ayres, 1978). Hence, either stars A or B could have one or two "rocky" planets in orbital zones where liquid water is possible. Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for rocky inner planets in the so-called "habitable zone" (HZ) around both Stars A and B. As originally proposed, the TPF will include two complementary observatory groups, a visible-light coronagraph and a "formation-flying" infrared interferometer, while Darwin would launch a flotilla of three mid-infrared telescopes and a fourth communications hub.

---------------------------------------------- [Guide] -- [Larger] ----------------------------------------------

	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Solar)	Diameter (Solar)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
Alpha Centauri A	10.9	79.9	0.519	79.23	1.10	1.23	...	...	1.3-2.3
Inner H.Z.-A Edge?	1.17	1.20	0	79.23	...	...	...	...	...
Outer H.Z.-A Edge?	2.33	3.39	0	79.23	...	...	...	...	...
Alpha Centauri B	12.8	79.9	0.519	79.23	0.93	0.86	...	...	1.3-2.3
Alpha Centauri Bb	0.04	0.0089	0.519	79.23	1.13	~1	...	...	...
Inner H.Z.-B Edge?	0.56	0.43	0	79.23	...	...	...	...	...
Outer H.Z.-B Edge?	1.10	1.20	0	79.23	...	...	...	...	...

Alpha Centauri A

Rigil Kentaurus ("Foot of the Centaur" in Arabic) is the fourth brightest star in the night sky as well as the brightest star in Constellation Centaurus. Like Sol, it is a yellow-orange main sequence dwarf star of spectral and luminosity type G2 V. It has about 1.105 ± 0.007 times Sol's mass (Guedes et al, 2008; and Thévenin et al, 2002) and 1.23 its diameter (ESO science release; and Demarque et al, 1986), and is about 52 to 60 percent brighter than Sol (ESO science release; and Demarque et al, 1986).

NASA





Alpha Centauri A is a yellow-orange
star like our Sun, Sol.

Without consideration of interior seismic constraints, Star A (and B) has been estimated to be older than Sol, from 4.85 billion years in age (ESO) to around 7.6 (+/- around 10 percent) billion years or more -- or 6.8 billion years if it does not have a convective core (Guenther and Demarque, 2000). Recent recent interior modeling with seismic constraints, however, suggest that Stars A and B could be 5.6 to 5.9 billion years old (Mutlu Yildiz, 2007). Since Alpha Centauri A is very similar to our own Sun, however, many speculate whether it might contain planets that harbor life. Useful star catalogue numbers and designations for Alpha Centauri A include: Alp or Alf Cen A, Alp1 Cen, HR 5459, Gl 559 A, Hip 71683, HD 128620, CP(D)-60 5483, SAO 252838, FK5 538, and LHS 50.

Habitable Zone around Star A

Calculations by to Weigert and Holman (1997) indicated that the distance from the star where an Earth-type planet would be "comfortable" with liquid water is centered around 1.25 AUs (1.2 to 1.3 AUs) -- about midway between the orbits of the Earth and Mars in the Solar System -- with an orbital period of 1.34 years using calculations based on Hart (1979). More recent calculations based on Kasting et al (1993), however, allow for a wider "habitable zone." According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Star A's habitable zone should be located around 1.17 AU from the star, while the outer edge edge lies around 2.33 AUs. Viewed from a planet at Earth's orbital distance around Alpha Centauri A, stellar companion B would provide more light than the full Moon does on Earth as its brightest night sky object, but the additional light at a distance greater than Saturn's orbital distance in the Solar System would not be significant for the growth of Earth-type life.

Alpha Centauri B

© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)





Alp Cen B is an orange-red dwarf star,
like Epsilon Eridani at left center of
meteor. (See a Sloan Digital Sky Survey
field images of Alpha Centauri A and B
from WikiSky.org, and at APOD.)