79 Ceti

© James B. Kaler, UIUC -- more information



Photo from Stars, Planet Project, and
79 Ceti (used with permission).



On March 29, 2000, astronomers announced the discovery of a Saturn-sized planet around this Sun-like star (NASA announcement and exoplanets.org -- details below). (See an animation of the planetary and potentially habitable zone orbits of the 79 Ceti system, with a table of basic orbital and physical characteristics.)

The Star

79 Ceti is a yellow-orange dwarf star of spectral and luminosity type G5 IV, with about the same mass (1.00) as Sol, about the same diameter -- 1.01 Solar (Johnson and Wright, 1983, page 651), and twice (2.0) its luminosity (exoplanets.org). As the star is unusually bright for its spectral type despite a cooler surface temperature, it is classified as a subgiant star that has begun to evolve off the main sequence as it may soon be unable to sustain hydrogen burning at its core. The star may be 1.66 times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Klaus Fuhrmann, 1998). Given its slow rotation and weak chromospheric activity, 79 Ceti appears to be a relatively old star. Useful catalogue numbers and designations for this star include: 79 Cet, GJ 9085, Hip 12048, HD 16141, BD-04 426, SAO 129992, FK5 4237, Wo 9085, and LTT 1267.

"Planet b?"

Greg Bacon, STScI, NASA


Larger image.



Planetary candidate "b"
with rings and two moons,
as imagined by Bacon (more
from NASA and Astronomy
Picture of the Day).

On March 29, 2000, a team of astronomers (Geoffrey W. Marcy, R. Paul Butler, and Steven S. Vogt) announced the discovery of a Saturn-sized planet around this Sun-like star using the Keck telescope in Mauna Kea, Hawaii (NASA announcement; exoplanets.org; and Marcy et al, 2000, in ps). The latest radial velocity measurements suggest that 79 Ceti has a companion "b" with at least 77 percent of Saturn's (23 percent of Jupiter's) mass (exoplanets.org). It moves around Star A at an average distance of 0.35 AUs (a semi-major axis inside the orbital distance of Mercury) in an elliptical orbit (e=0.21) that takes about 75.6 days to complete. (See an animation of the planetary and potentially habitable zone orbits of the 79 Ceti system, with a table of basic orbital and physical characteristics.)

NASA

Larger image.

The planet may be smaller
than Saturn.

Subsequent astrometric analysis, however, suggests that "planet b" may have as much as 111 times the mass of Jupiter with an inclination of 0.1° from Earth's line of sight (Han et al, 2001, in pdf). Thus, the "planet" could be a very dim red (M) dwarf stellar companion of 79 Ceti. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

NASA -- larger image

Astrometry, however, suggests that 79 Ceti b may a very dim red
dwarf star, like Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.

The orbit of an Earth-like planet (with liquid water) around 79 Ceti may be centered around 1.41 AUs -- within the inner reaches of the Main Asteroid Belt in the Solar System -- with an orbital period of 611 days (or 1.67 years). However, the eccentric orbit of "planet b" in with an average distance of 0.35 AUs could disrupt the orbital stability of an Earth-type planet in 79 Ceti's water zone, depending on the mass of this planetary or stellar companion. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods.

Geoffrey W. Marcy, University
of California at Berkeley



Larger image.



The orbit of object "b" around
79 Ceti is only mildly eccentric,
but may still be more elliptical
than that of any planet in the
Solar System other than Pluto.