HD 217107 / HR 8734

HD 217107 is located about 64.3 light-years from Sol. It lies near the southwest corner of (22:58:15.5-02:23:43.4, ICRS 2000.0) of Constellation Pisces, the Fish -- southeast of Fum al Samakah (the "Fish's Mouth" or Beta Piscium), Gamma Piscium, and Kappa Piscium. In 1998, astronomers announced the discovery of a Jupiter-class planet around this Sun-like star (Fischer et al, 1999 -- details below). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

The Star

HD 217107 is a yellow-orange main sequence dwarf star of spectral and luminosity type G7 V, although it has also been classified as a G8 subgiant (IV). The star has about 98 percent the mass of Sol (HD 217107 at exoplanets.org) and 1.1 times its luminosity. The star is twice (2.0) as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (HD 217107 at exoplanets.org). Given that the star is brighter than Sol despite a similar mass, HD 217107 is likely to be more highly evolved because it is substantially older than Sol's 4.6 billion years. Useful catalogue numbers and designations for the star include: HR 8734*, Hip 113421, HD 217107, BD-03 5539, SAO 146412, and FK5 3836.

A Planetary System?

Greg Bacon, STScI, NASA


Larger image.


Despite its "torch" inner
orbit, planetary candidate
"b" is massive enough to
be a gas giant, like this
artist's rendition of
HD 209458 b and its
parent star.

In 1998, a team of astronomers (Debra A. Fischer, Geoffrey W. Marcy, R. Paul Butler, Steven S. Vogt, and Kevin Apps) announced the discovery of a Jupiter-class planet around HR 217017 using radial-velocity methods (Fischer et al, 1999). Planet "b" has at least 1.3 times Jupiter's mass, and, given its relatively short orbital period, the absence of tidal spin-up in its host star provides a theoretical maximum mass of less than 11 Jupiter masses The planet moves around HD 217107 at an average distance of only 0.07 AUs (a semi-major axis well within Mercury's orbital distance) in a highly circular orbit (e=0.14) that takes only 7.1 days to complete. Assuming a Jupiter-like composition, its radius may be about 1.2 times that of Jupiter, enlarged relative to Jupiter because of greater absorbed stellar radiation in its inner orbit.

A residual drift in the radial velocity data over several years suggest the presence of an even larger planet "c" in an outer orbit from HD 217107 (Multiple Planet Systems at exoplanets.org). Planet c may have more than four times the mass of Jupiter. Assuming an orbit that takes more than two years to complete and HD 217107's mass of 0.98 Solar, it may lie at an average orbital distance of 1.6 AUs.

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Planetary candidates "b" and "c" may
both be large gas giants like Jupiter
(shown here with Europa).

The orbit of an Earth-like planet (with liquid water) around HD 217107 may be centered around 1.1 AU -- between the orbital distances of Earth and Mars -- with an orbital period of just over 1.3 years. However, the development of an Earth-like planet in this zone could have been disrupted by the hypothesized inward migration of planet b and the presence of planet c. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)