47 Ursae Majoris

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and 47 Ursae Majoris; used with permission)

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.

The Star

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.

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

Planetary candidate "b" depicted with rings and
an icy moon, as imagined by Whatmough.

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).

NASA, Cassini-Huygens Mission
to Saturn and Titan



Larger image.



Planetary candidates "b" and
"c" are massive enough to be
gas giants like Jupiter (shown
here with Europa) or Saturn.

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).

© Christoph Kulmann -- larger image


Artwork from Exoplaneten.de
(used with permission).


An inner moon of planet b
with tidally heated ice melt,
as imagined by Kulmann (more).

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, pdf),
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.