94 Ceti 2

© James B. Kaler, UIUC -- more information



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



On August 7, 2000, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (initial summary and exoplanets.org -- details below). (See an animation of the planetary and potentially habitable zone orbits of the 94 Ceti system, with a table of basic orbital and physical characteristics.)

94 Ceti A

94 Ceti A is a yellowish main sequence dwarf star of spectral and luminosity type F8 V, with about 1.3 times the mass of Sol (Smith et al, 2001), 1.05 times its diameter (Dodson-Hockey in Alan Hale, 1994, page 320), and 4.46 times its luminosity (Smith et al, 2001). The star may be 1.23 times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron -- 1.35 times as enriched based on an average of 22 elements (Smith et al, 2001). It may be around 4.3 billions years old, close to Sol in age (Smith et al, 2001).

The orbit of an Earth-like planet (with liquid water) around Star A may be centered around 2.11 AUs -- within the inner reaches of the Main Asteroid Belt in the Solar System -- with an orbital period of 2.69 years. However, the presence of planet b in an eccentric orbit with an average distance of 1.19 AUs would probably disrupt the orbital stability of an Earth-type planet in Star A's water zone. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods.

According to the new Sixth Catalog of Orbits of Visual Binaries, Stars A and B have an average separation of 151 AUs (6.77" at a HIPPARCOS distance estimate of 73.0 light-years -- 3.3" at 231° in 1958). Inclined at 114.10° (Sixth Catalog) or 66° (original source) to Earth's line of sight, their eccentric orbit (e= 0.26) takes about 1,470 years to complete (Alan Hale, 1994, pages 312 and 314). Star A was once thought to have a spectroscopic double but was discovered to have a giant planetary companion in 2000 (see below). Useful catalogue numbers and designations for the star include: 94 Cet, HR 962*, Gl 128 A, Hip 14954, HD 19994, BD-01 457, SAO 130355, FK5 116, LTT 1515, ADS 2406 A, and HJ 663 A.

Planet b

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Planetary candidate "b" appears to
be a gas giant somewhat larger than
Jupiter (shown here with Europa).

On August 7, 2000, a team of astronomers (Dominique Naef, Francisco Pepe, Michel Mayor, Nuno C. Santos, Didier Queloz, Stephane Udry, and M. Burnet) announced the discovery of a Jupiter-like planet around this Sun-like star (Observatoire de Genève and exoplanets.org). Planet "b" has at least 1.66 times of Jupiter's mass. It moves around Star A at an average distance of 1.19 AUs (a semi-major axis between the orbital distances of Earth and Mars) in an elliptical orbit (e=0.20) that takes 454 days or over 1.2 years to complete.

94 Ceti B

NASA -- larger image

Star B is a dim red dwarf star, like
Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.

Star "B" is a red main sequence dwarf star of spectral and luminosity type M3 V (Alan Hale, 1994, pages 312). It may have around 53 percent of Sol's mass based on a luminosity of 1.05 percent of Sol's (derived from a visual magnitude of 11.5). Useful catalogue numbers and designations for the star include: Gl 128 B, ADS 2406 B, and HJ 663 B.

The orbit of an Earth-like planet (with liquid water) around Star B may be centered around 0.102 AUs -- well within the orbital distance of Mercury in the Solar System -- with an orbital period of 16.5 days. Hence, such a planet is likely to be tidally locked with Star B with perpetual daylight on one side. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.