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Alpha Centauri 3
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© Laro Schatzer (Artwork from Alpha Centauri, used with permission)
•
Breaking News
On January 15, 2010, a team of astronomers released the results of
computer simulations indicating that kilometer-size planetesimals can form
and accrete into rocky Earth-size planets around Alpha Centauri B despite
gravitational perturbations from Alpha Centauri A. The binary system,
however, does not offer "favorable" conditions for the formation of gas
giants, like Jupiter and Saturn. Star B was chosen for analysis because
efforts are underway to detect of an Earth-like planet in or near its habitable
zone (between 0.5 and 0.9 AU) using available technology -- more
discussion below (Xie et al, 2010; and Jessica Griggs, New Scientist,
January 29, 2010).
•
System Summary
© Akira Fujii / David Malin Images
(Used with permission from
Tim Bedding)
Larger image.
Alpha Centauri is the
brightest star in
Constellation Centaurus.
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. 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.)
La Silla Observatory, ESO
Larger image.
While Stars 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 A and
B. This is so far that Proxima may not be gravitationally bound to Stars 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
Larger and jumbo images.
Although Stars 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
Larger illustration.
All three members of the system
(including B, which has not been
depicted between A and Proxima
at left) are main-sequence stars.
Due to their proximity to Sol, the stars of this
system have been objects of intense interest
among astronomers. Stars A and B have been 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 (and so images
of Alpha Centauri A and -- basically the same images of -- Alpha Centauri B and
their position relative to the Milky Way in Earth's night sky are now available from
the TPF-C team). In addition, all three stars are 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). 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.
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; and Demarque et al, 1986), and is about
52 to 60 percent brighter than Sol (ESO; and Demarque et al, 1986). 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); however, recent interior
modeling with seismic constraints suggest that Stars A and B are 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. 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."
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.)
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. (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.)
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 are 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 currently
planned, the TPF will include two complementary observatory groups: a visiblelight coronagraph to launch around 2014; and a "formation-flying" infrared
interferometer to launch before 2020, while Darwin will launch a flotilla of three
mid-infrared telescopes and a fourth communications hub beginning in 2015.
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.
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 AB from WikiSky.org,
and at APOD.)
This much dimmer companion star is a main sequence, reddish-orange dwarf
(K0-1 V). It appears to have only 93.4 ± 0.7 percent of Sol's mass (Guedes et al,
2008; and Thévenin et al, 2002), 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.
Search for Planets around Stars A and B
In July 2008, astronomers (Michael Endl and Martin Kürster) analyzed used
seven years of differential radial velocity measurements for Proxima Centauri to
submit a paper indicating that large planets are unlikely to be orbiting Sol's
closest stellar neighbor within its habitable zone -- around 0.022 to 0.054 AU with
a corresponding orbital period of 3.6 to 13.8 days. A Super-Earth as small as two
to three Earth-masses was ruled out with a statistical significance exceeding 99
percent. In addition, their simulations also ruled out the presence of a planet of at
least Neptune-class in a circular orbit within one AU of Proxima (Endl and
Kürster, 2008).
Justin
Cantrell,
Todd J. Henry,
RECONS
Larger illustration.
In general, brighter
stars have wider
habitable zones than
dimmer ones (more).
In January 2008, astronomers working cooperatively within the Research
Consortium on Nearby Stars (RECONS) issued a press release about their
research project to estimate the size of the so-called habitable zone around
nearby stars (RECONS press release). Based on the latest data that RECONS
has collected on stars within 10 parsecs (32.6 light-years) of Sol, the
astronomers are carefully estimating what they call the "habitable real estate"
around each of the Sun's neighbors, where inner rocky planets like the Earth can
support liquid water on their surface. Confirming previous modelling of the Alpha
Centauri system, the RECONS astronomers found that Alpha Centauri A and B
orbit in such a way that when the light and heat of the two stars was combined,
neither star in the innermost AB system significantly changed the size of their
respective habitable zones, regardless of where each was currently located in its
orbit. Although stars A and B would be expected to interfere with each others'
habitable zones, the areas of the available good "habitable real estate" around
each star was affected by less than one percent. Not surprisingly, distant
Proxima was completely unaffected by the other two stars.
Unknown artist, Planet Quest,
JPL, Caltech, NASA
Larger illustration.
Recent simulations suggest that
an Earth-life planet could have
formed within the habitable
zone around Alpha Centauri B,
which can be detected using
the radial-velocity "wobble"
method (more).
On February 25, 2008, a team
of astronomers released a
paper on simulation results which support the conclusions of previous studies
that multiple-planet systems could have formed in close orbits around both
heavy-element rich, Alpha Centauri A and B. Their simulations suggest that at
least one planet in the one to two Earth-mass range could have formed within
orbital distances of 0.5 to 1.5 AUs around either Star A or Star B; an important
finding was that the simulations frequently generated a Earth-like planet in or
near Star B's habitable zone (where liquid water could exist on the planet's
surface) which can be detected with three to five years of high cadence
observations (Javiera Guedes, 2009). Additional simulation work presented in the
paper also indicates that long-term telescopic observations may detect wobbles
from such planets using the radial velocity method. Star B, a orange-red dwarf
with a relatively calm chromosphere and acoustic p-wave mode oscillations, is an
easier target for detecting wobbles from terrestrial planets, possibly within only
three years of "high cadence" observations for a 1.8 Earth-mass planet (more
from New Scientist and Guedes et al, 2008).
Cassini-Huygens Mission,
SSI, JPL, NASA
Larger image.
The Cassini orbiter
imaged Alpha
Centauri A and B
over the horizon of
Saturn on May 17,
2008 (more).
In the latter half of 2008, two teams of astronomers began technically difficult
searches for small terrestrial planets around the two brightest stars of the Alpha
Centauri triple system. One group (including Debra Fisher, Bernie Walp, Howard
Isaacson, Greg Laughlin, Javiera Guedes, and Paul Butler) are hoping to find
planets as small as the Earth around both Alpha Centauri A and B within three to
five years, by assembling 100,000 radial-velocity observations using an unused
1.5-meter telescope and vintage equipment at the Cerro Tololo Inter-American
Observatory (CTIO) in Chile. They have nearly $100,000 from the U.S. National
Science Foundation for a one-year "design study" that was enough to pay for
telescope time through November 2009 and hope to obtain another year of funds
to seek promising results to justify additional grants. Since August 2008,
however, a competing team of astronomers (including Michel Mayor and
Stéphane Udry) has been collecting similar observations of just Alpha Centauri B
that can find a planet as small as 2.5 Earth-masses, using the High Accuracy
Radial velocity Planet Searcher on a telescope more than twice as large at the
European Southern Observatory, also located in Chile about 60 miles away from
the CTIO (Lee Billings, SEED, May 19, 2009; and Joel Achenbach, Washington
Post, June 1, 2009)
Proxima Centauri
© Steve Quirk (Views from Frog Rock; used with
permission)
Apparent motion of Proxima Centauri over 20 years.
A real-color, field image of Proxima by David Malin
is available at Astronomy Picture of the Day.
See close-up images of Proxima from Schultz et al, 1998.
Proxima (Alpha Centauri C) is a very cool and very dim, main sequence red
dwarf (M5.5Ve) that appears to have only 0.107 ± 0.021 percent of Sol's mass
(Pourbaix et al, 2002) 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 as short as 31.5 +/- 1.5 days but may possibly as long as 84 days
-- and an activity cycle from 442 to 1,100 days -- and appears to be between five
and six billion years old (Endl and Kürster, 2008; Benedict et al, 1998; Hunch et
al, 1998; Cincunegui et al 2007; and 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 0.022 to 0.054 AU (Endl and Kürster, 2008; and Endl et al, 2003, in pdf) - much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with a
corresponding orbital period of 3.6 to 13.8 days (Endl and Kürster, 2008). 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).
NASA -- larger image
Proxima is a dim red dwarf star, like Gliese
623 A (M2.5V) and B (M5.8Ve) at lower right.
(A Digitized Sky Survey image of Proxima may
become available at the Nearby Stars Database.)
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: Alp Cen B, Alp2 Cen, Gl 551, Hip 70890, and LHS 49.
Arnold O. Benz, Institute of Astronomy, ETH Zurich
High resolution and jumbo images (Benz et al, 1998).
Proxima is a flare star, like UV Ceti (Luyten 726-8 B)
shown flaring at left. UV Ceti is an extreme example
of a flare star that can boost its brightness by five times
in less than a minute, then fall somewhat slower back
down to normal luminosity within two or three minutes
before flaring suddenly again after several hours.
Proxima Centauri B?
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.
© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)
Glowing red through gravitational contraction, the candidate brown dwarf
companion
to Proxima Centauri is depicted with two moons (one eclipsing the flare star)
with
distant Alpha Centauri A and B at upper right, as imagined by Whatmough.
In 1996, another group of astronomers using the Hubble Space Telescope
discovered that they might have directly observed a companion to Proxima with
the implied brightness of a brown dwarf and an apparent visual separation of only
about half the Earth-Sun distance -- 0.5 AU (Schultz et al, 1998). A substellar
companion at that distance would imply an orbital period of around a year, or it
could be in a highly eccentric orbit with a much greater average distance from
Proxima. However, later observations by other astronomers using interferometric
astrometry and recent radial velocity data found no evidence to support the
existence of a companion greater than 0.8 Jupiter mass with an orbital period
around Proxima Centauri of between one and about 2.7 years (Benedict et al,
1999). Proxima has been selected to be one of the Tier 1 target stars for NASA's
Space Interferometry Mission (SIM) -- which is planned for launch as early as
2011 -- to detect a planet as small as three Earth-masses within two AUs of its
host star.
Life Around a Flare Star
Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size
and brightness. These flare stars are actually common because red dwarfs make
up more than half of all stars in our galaxy. Although flares do occur on our Sun
every so often, the amount of energy released in a solar flare is small compared
to the total amount of energy that Sol produces. However, a flare the size of a
solar flare occurring on a red dwarf star (such as Proxima Centauri) that is more
than ten thousand times dimmer than our Sun would emit about as much or more
light as the red dwarf does normally.
Flare stars erupt sporadically, with successive flares spaced anywhere from an
hour to a few days apart. A flare only takes a few minutes to reach peak
brightness, and more than one flare can occur at a time. Moreover, in addition to
bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000
times as many X-rays as a comparably-sized solar flare on our own Sun, and so
flares would be lethal to Earth-type life on planets near the flare star. Hence,
Earth-type life around flare stars may be unlikely because their planets must be
located very close to dim red dwarfs to be warmed sufficiently by star light to
have liquid water (about 0.007 AU for Proxima), which makes flares even more
dangerous around such stars. In any case, the light emitted by red dwarfs may
be too red in color for Earth-type plant life to perform photosynthesis efficiently.
•
Closest Neighbors
The following star systems are located within 10 light-years of Alpha Centauri
AB.
•
Star System
Spectra & Distance
Luminosity (light-years)
Sol
G2 V
4.4
Barnard's Star M3.8 V
6.5
Ross 154
M3.5 Ve
8.1
Wolf 359
M5.8 Ve
8.3
Sirius 2
A0-1 V
DA2-5 /VII
9.5
Epsilon Indi
K3-5 Ve
9.7
Other Information
o Try Professor Jim Kaler's Stars site for other information about Rigil
Kentaurus (Alpha Centauri A) at the University of Illinois' Department of
Astronomy.
o Up-to-date technical summaries on these stars can be found at: the
Astronomiches Rechen-Institut at Heidelberg's ARCNS pages on stars A
and B and star C; the NASA Stars and Exoplanet Database for stars A, B,
and C; and the Research Consortium on Nearby Stars (RECONS) list of
o
o
o
the 100 Nearest Star Systems. Additional information may be available at
Roger Wilcox's Internet Stellar Database.
For more discussion about the suitability of this star system for terrestial
life, go to Laro Schatzer's website on Alpha Centauri.
Alpha Centauri is not visible in much of the Northern Hemisphere, as
Constellation Centaurus cannot be viewed from middle northern latitudes
of around 40 degrees, but should become more easily visible to observers
that travel south of the equator. For more information about the stars and
other objects in this constellation, go to Christine Kronberg's Centaurus.
For an illustration, see David Haworth's Centaurus.
For more information about stars including spectral and luminosity class
codes, go to ChView's webpage on The Stars of the Milky Way.
Note: Thanks to Andrew James for notifying us of updated orbit information for Stars A and B and to Aaron Freed for new calculations of
the apparent brightness of Stars A and B on planets orbiting in the water zone of each star.
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