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Alpha Centauri System: Our closest neighbor HA-12-3-05
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.
Sol's three closest stellar neighbors are located in the southeastern corner of Constellation Centaurus, the Centaur. Proxima Centauri (or Alpha Centauri C) is only 4.22 light-years (ly) away (14:29:42.95-62:40:46.14, ICRS 2000.0) but is too dim to be seen with the naked eye. The two bright stars, Alpha Centauri A and B (14:39:36.5-62:50:02.3 and 14:39:35.1-60:50:13.8, ICRS 2000.0), are a little farther away at about 4.36 ly, around 40,000,000,000,000 km. They form a close binary that is separated "on average" by only about 24 times the Earth-Sun distance -- 23.7 astronomical units (AUs) of an orbital semi-major axis -- which is only slightly greater than the distance between Uranus and the Sun ("Sol"). (See an animation of the orbits of Stars A and B and their potentially habitable zones in this system, with a table of basic orbital and physical characteristics.) Visible only from latitudes south of about 25° the star we call Alpha Centauri lies 4.35 light-years from the Sun. But it is actually a triple star system. The two brightest components Alpha Centauri A and B form a binary. They orbit each other in 80 years with a mean separation of 23 astronomical units (1 astronomical unit = 1 AU = distance between the Sun and Earth). The third member of the system Alpha Centauri C lies 13,000 AU from A and B, or 400 times the distance between the Sun and Neptune. This is so far that it is not known whether Alpha Centauri C is really bound to A and B, or if it will have left the system in some million years. Alpha Centauri C lies measurably closer to us than the other two: It is only 4.22 light-years away, and it is the nearest individual star to the Sun. Because of this proximity, Alpha Centauri C is also called Proxima (Centauri). Alpha Centauri A is a yellow star with a spectral type of G2, exactly the same as the Sun's. Therefore its temperature and color also match those of the Sun. Alpha Centauri B is an orange star with a spectral type of K1. Whereas Alpha Centauri A and B are stars like the Sun, Proxima is a dim red dwarf with a spectral type of M5 - much fainter, cooler, and smaller than the Sun. Proxima is so faint that astronomers did not discover it until 1915.
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 1.09 to 1.10 times Sol's mass and 1.23 its diameter (ESO; and Demarque et al, 1986), is about 52 to 60 percent brighter than Sol (ESO; and Demarque et al, 1986). It may may be only somewhat older than Sol at 4.85 billion years in age (ESO), or much older at around 7.6 (+/- around 10 percent) or 6.8 billion years if it does not have a convective core (Guenther and Demarque, 2000). Since Alpha Centauri A is very similar to our own Sun, however, many speculate whether it might contain planets that harbor life. According to Weigert and Holman (1997), 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), but more recent calculations based on Kasting et al (1993) allow for a wider "habitable zone." The distance separating Alpha Centauri A from its companion star B averages 23.7 AUs (semi-major axis of 17.59" 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= 0.519) 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 Dimitri Pourbaix, 2000 in the new Sixth Catalog of Orbits of Visual Binaries; Heintz Orbit Table, 12/1997; 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. (See an animation of the orbits of Stars A and B and their potentially habitable zones in this system, with a table of basic orbital and physical characteristics.)
Centauri B is a much dimmer companion star is a main sequence, reddish-orange dwarf (K0-1 V). It appears to have only 90.7 percent of Sol's mass, about 86.5 percent of its diameter, and 45 to 52 percent of its luminosity (ESO; and Johnson and Wright, 1983, page 681). Viewed from a planet at Earth's orbital distance around Alpha Centauri A, this companion B star 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. According to Weigert and Holman (1997), the distance from the star where an Earth-type planet would be comfortable with liquid water is centered around 0.73 to 0.74 AU -- somewhat beyond the orbital distance of Venus in the Solar System -- with an orbital period under an Earth year using calculations based on Hart (1979), but more recent calculations based on Kasting et al (1993) allow for a wider habitable zone. Useful catalogue numbers and designations for Alpha Centauri B include: Alp or Alf Cen B, HR 5460, Gl 559 B, Hip 71681, HD 128621, and LHS 51.
Proxima (Alpha Centauri C) is a very cool and very dim, main sequence red dwarf (M5.5Ve) that appears to have only 12.3 percent of Sol's mass and 14.5 percent of its diameter (ESO press releases of 3/15/03 and 2/22/02; and Doyle and Butler, 1990, page 337). With a visual luminosity that has reportedly varied between 0.000053 and 0.00012 of Sol's (based on a distance of 4.22 light-years)the star is as much as 19,000 times fainter than the Sun, and so if it was placed at the location of our Sun from Earth, the disk of the star would barely be visible. It is chromosperically active with a rotation period of 31.5 +/- 1.5 days and appears to be between five and six billion years old (Guinan and Morgan, 1996). The star is located roughly a fifth of a light-year from the AB binary pair and, if gravitationally bound to it, may have an orbital period of around half a million years. According to Anosova et al (1994), however, its motion with respect to the AB pair is hyperbolic. Accounting for infrared radiation, the distance from Proxima where an Earth-type planet would be "comfortable" with liquid water is around 0.02 to 0.06 AU (Endl et al, 2003, in pdf) -- much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with an orbital period of two to 16 days. Hence, the rotation of such a planet would probably be tidally locked so that one side would be in perpetual daylight and the other in darkness. Three star spots may have been observed recently with the Hubble Space Telescope (Benedict et al, 1998). Like many red dwarfs, Proxima is a "Flare Star" that can brighten suddenly to many times its normal luminosity. Its flares can roughly double the star's brightness and occur sporadically from hour to hour. Moreover, more than one flare may be emitting at a time. From May to August 1995, several flares were observed with changes within a time-scale of weeks, and archival data suggests that the star may have a long-term activity cycle (Guinan and Morgan, 1996). Its designated variable star name is V645 Centauri. Other useful catalogue numbers for Proxima include: Gl 551, Hip 70890, and LHS 49. Using data collected up to early 1994, astronomers using the Hubble Space Telescope discerned a 77-day variation in the proper motion of Proxima (Benedict et al, 1994). The astrometric perturbations found could be due to the gravitational pull of a large planet with about 80 percent of Jupiter's mass at a 1994 separation from Proxima of about 0.17 AUs -- 17 percent of Earth's orbital distance in the Solar System from the distance, or less than half Mercury's orbital distance. The Hubble astrometry team calculated that the chance of a false positive reading from their data -- same perturbations without a planet -- to be around 25 percent.
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 may be more than twice (1.3 to 2.3 times) as enriched in elements heavier than hydrogen ("high metallicity") than our own Solar System (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. Useful star catalogue numbers and designations for Alpha Centauri A include: Alp or Alf Cen A, HR 5459, Gl 559 A, Hip 71683, HD 128620, CP(D)-60 5483, SAO 252838, FK5 538, and LHS 50.
Comet Swift-Tuttle orbits the sun once every 134 years and last visited our region of the solar system in 1992. Its orbit stretches from near Neptune to inside the orbit of Earth. The velocity of the Comet of Swift Tuttle is estimated 60,000 m/sec, 60 km/sec, 3,600 km/min, 216,000 km/hr, 5,184,000 km/day, 1,892,200,000 km/yr. A light year represents the 9,500,000,000,000 kilometers that light travels in one year. Impact with the Comet of Swift Tuttle is estimated at 225,000 gigatons of TNT. The question that we hope to answer is does the Comet of Swift-Tuttle reach the next solar system similar to an electron in a double bond in organic chemistry? After 134 years traveling at 1,892,200,000 the Comet of Swift Tuttle would only travel 253,554,800,000 km in the entire round trip voyage, therefore on the basis of the facts presented by the authors in the bibliography we must concur that the trajectory of comets do not leave the Solar System.
Alpha Centauri is a special place, because it may offer life conditions similar to our solar system. A star must pass five tests before we can call it a promising place for terrestrial life as we know it. Most stars in the Galaxy would fail. In the case of Alpha Centauri, however, we see that Alpha Centauri A passes all five tests, Alpha Centauri B passes either all but one, and only Proxima Centauri flunks out.
The first criterion is to ensure a star's maturity and stability, which means it has to be on the main sequence. Main-sequence stars fuse hydrogen into helium at their cores, generating light and heat. Because hydrogen is so abundant in stars, most of them stay on the main sequence a long time, giving life a chance to evolve. The Sun and all three components of Alpha Centauri pass this test.
The second test is much tougher, however, we want the star to have the right spectral type, because this determines how much energy a star emits. The hotter stars - those with spectral types O, B, A, and early F - are no good because they burn out fast and die quickly. The cooler stars - those with spectral types M and late K - may not produce enough energy to sustain life, for instance they may not permit the existance of liquid water on their planets. Between the stars that are too hot and those that are too cool, we find the stars that are just right. As our existance proves, yellow G-type stars like the Sun can give rise to life. Late (cool) F stars and early (hot) K stars may be fine too. Luckily, Alpha Centauri A passes this test with bravour, as it is of the same class as our Sun. Alpha Centauri B is a K1 star, so it is hotter and brighter than most K stars, therefore it may pass this test or it may not. And the red dwarf Proxima Centauri seems to be a hopeless case.
For the third test, a system must demonstrate stable conditions. The star's brightness must not vary so much that the star would alternately freeze and fry any life that does manage to develop around it. But because Alpha Centauri A and B form a binary pair there's a further issue. How much does the light received by the planets of one star vary as the other star revolves around it ? During their 80-year orbit, the separation between A and B changes from 11 AU to 35 AU. As viewed from the planets of one star, the brightness of the other increases as the stars approach and decreases as the stars recede. Fortunately, the variation is too small to matter, and Alpha Centauri A and B pass this test. However, Proxima fails this test, too. Like many red dwarfs it is a flare star, prone to outbursts that cause its light to double or triple in just a few minutes.
The fourth condition concerns the stars' ages. The Sun is about 4.6 billion years old, so on Earth life had enough time to develop. A star must be old enough to give life a chance to evolve. Remarkably, Alpha Centauri A and B are even older than the Sun, they have an age of 5 to 6 billion years, therefore they pass this test with glamour, too. Proxima, however, may be only a billion years or so old, then it fails this test, too.
And the fifth and final test: Do the stars have enough heavy elements - such as carbon, nitrogen, oxygen and iron - that biological life needs ? Like most stars, the Sun is primarily hydrogen and helium, but 2 percent of the Sun's weight is metals. (Astronomers call all elements heavier than helium "metals".) Although 2 percent may not sound a lot, it is enough to build rocky planets and to give rise to us. And again, fortunately, Alpha Centauri A and B pass this test. They are metal-rich stars.
Now to the final question. Do we find at Alpha Centauri warm, rocky planets like Earth, full of liquid water ? Unfortunately, we don't know yet whether Alpha Centauri even has planets or not. What we know is that in a binary system the planets must not be too far away from a particular star, or else their orbits become unstable. If the distance exceeds about one fifth of the closest approach of the two stars then the second member of the binary star fatally disturbes the orbit of the planet. For the binary Alpha Centauri A and B, their closest approach is 11 AU, so the limit for planetary orbits is at about 2 astronomical units. Comparing with our system, we see that both Alpha Centauri A and B might hold four inner planets like we have Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AU). Therefore, both Alpha Centauri A and B might have one or two planets in the life zone where liquid water is possible.