47 Ursae Majoris
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47 Ursae Majoris (UMa) is located about 45.9 light-years from Sol. It lies in the southcentral part (10:59:28.0+40:25:48.9, ICRS 2000.0) of Constellation Ursa Major, the Great Bear -- southeast of Omega UMa, southwest of Psi UMa, northeast of 46 UMa, and northwest of 55 UMa (see another chart with 47 UMa labeled). In 1996, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (Butler and Marcy, 1996 -- details below), with indications of an even larger planet in an outer orbit. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)
As 47 Ursae Majoris has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.
47 UMa is a yellow-orange main sequence dwarf star of spectral and luminosity type G0-1 V, with about 1.03 times the mass of Sol (47 UMa at exoplanets.org), 1.26 times its diameter (Henry et al, 2000), and (1.54 +/- 0.13) times its luminosity (Cuntz et al, 2003). The star may be less than or as enriched (83 to 102 percent) as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (exoplanets.org; and Guillermo Gonzalez, 1998). It may be around 6.32 (+1.2/-1.0) billion years old (Cuntz et al, 2003; and Henry et al, 1997). Useful catalogue numbers and designations for the star include: 47 UMa, HR 4277, Gl 407, Hip 53721, HD 95128, BD+41 2147, SAO 43557, FK5 1282, and LTT 12934.
In 1996, a team of astronomers (including Geoffrey W. Marcy and R. Paul Butler) announced the discovery of a Jupiter-class planet around 47 UMa using highly sensitive radial-velocity methods (Fischer et al, 2001 -- in pdf; and Butler and Marcy, 1996). Planet b has at least 2.54 times Jupiter's mass. It moves around 47 UMa at an average distance of only 2.09 AUs (a semi-major axis within the Main Asteroid Belt's orbital distance) in a slightly elliptical orbit (e=0.06) that takes 3.0 years to complete. Its orbit is inclined about 172° from the perspective of Earth (47 UMa at exoplanets.org).
A residual drift in the radial velocity data over several years suggest the presence of an even larger planet in an outer orbit, at about 3.73 AUs from 47 UMa (between the average orbital distances of Jupiter and the Main Asteroid Belt in the Solar System). Planet c may have at least 76 percent of the mass of Jupiter and an elliptical orbit (0 < e <0.2) that takes more than 7.1 years to complete (Fischer et al, 2001 -- in pdf).
Recently, astronomers at the University of Texas at Arlington performed refined calculations to determine that the habitable zone around 47 Ursae Majoris, where an inner rocky planet (with suitable mass and atmospheric gas composition and density) can have liquid water on its surface, lies between 1.05 and 1.83 AUs of the star. They found that the development of an Earth-like planet in the inner portion of this zone may survive disruption from the development of known planetary candidates planet b and c. If a small, rocky planet can develop without the interference of planet b, then stable orbits appear to be possible in the inner portion of the habitable zone (news release; and Noble et al, 2002, in pdf; and Jones and Sleep, 2003). Subsequent analysis suggests that the habitability of such an inner rocky planet would be boosted if the star was "relatively young" at six or less billion years old and has a "relatively small stellar luminosity" (Cuntz et al, 2003).
Noble, Musielak, and Cuntz (2002,
University of Texas at Arlington
Larger chart with terrestrial planet
in an inner orbit, located in the first
tenth of the habitable zone distance.
Stable orbits for a Earth-type,
rocky inner planet appear to exist
in the inner portion of the habitable
zone (1.1 to 1.8 AUs), where the
nearer planet b (labelled "A" in the
chart) less likely to disrupt its
orbit (news release).
A terrestrial planet orbiting 47 Ursae Majoris at around the first inner tenth of the orbital distance of the calculated habitable zone would have an average orbital distance just under 1.13 AU, between the orbits of Earth and Mars in the Solar System. It would complete this orbit within about 431 days or less than 1.2 years. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)
Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around 47 Ursae Majoris. As currently planned, the TPF will include two complementary observatory groups: a visible-light 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.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of 47 Ursae Majoris.
|Star System||Spectra &|
|BD+39 2376 AB||M2 Ve |
|G 199-62||M4 V||8.0|
|BD+46 1635||K7 V||8.0|
|AC+44 472-15 AB||M3-4 V |
|* plus bright stars *||. . .|
|20 Leonis Minoris||G1-3 Va||12|
|36 Ursae Majoris 3?||F8 V |
|Theta Ursae Majoris 3||F6 IV |
|SV Leonis Minoris AB||G8 V-IV |
|61 Ursae Majoris||G8 Ve||16|
|Groombridge 1830||G8 VIp||17|
|Talitha 4||A7 IV |
|HR 3881||G0.5-2 Va||19|
Up-to-date technical summaries on these stars can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS, the Nearby Stars Database, and the Research Consortium on Nearby Stars (RECONS). Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Ursa Major is only visible from the northern hemisphere. The seven stars of the Big Dipper in this constellation are famous as the traveller's guide to Polaris, the North Star. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Ursa Major. For another illustration, see David Haworth's Ursa Major.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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