109 Piscium

NASA

109 Piscium is a yellow-orange star
like our Sun, Sol. (See a Digitized
Sky Survey image of 109 Piscium at
John Whatmough's web site,
"Extrasolar Visions.")

System Summary

This star is located around 106 light-years (ly) away (HIPPARCOS Plx=30.71 +/- 0.81 mas) from our Sun, Sol. It lies at the eastern edge (1:44:55.8+20:4:59.3, ICRS 2000.0) of Constellation Pisces, the Fish -- northeast of Kullat Nunu (Eta Piscium), northwest of Omicron and Alrescha (Alpha Piscium), southwest of Sheratan (Beta Arietis) and Hamal (Alpha Arietis), west of Chi Piscium, and southeast of Phi, Nu, and Tau Piscium. It is probably not visible with the naked eye for most Humans on Earth. (Note - While 109 Piscium is not included in the Bright Star Map available on-line, it is available on the "150ly-h.zip" map file for the PC version of ChView that contains stars known to be located within 150 ly of Sol.)

In 1999, astronomers announced the discovery of a substellar companion object around this star (see: exoplanets.org; and Vogt et al, 2000, more details below). See an animation of the orbit of this substellar object around 109 Piscium, with a table of basic orbital and physical characteristics.

The Star

Evolving off the main sequence, 109 Piscium is a yellow-orange star of spectral and luminosity type G3-5 Va-IV, reflecting a difference of opinion between the Yale Bright Star Catalogue, 1991 5th Revised Edition and the Gliese Catalogue of Nearby Stars (now ARICNS). The star may have about 1.1 times Sol's mass (Vogt et al, 2000, page 904), 1.1 times its diameter (Johnson and Wright, 1983, page 649), and more than 2.7 times its visual luminosity. It appears to be about 1.41 times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron. 109 Piscium is a slow rotator and chromospherically inactive (Vogt et al, 2000, page 904). The star may be around eight billion years old and may have already ceased hydrogen fusion at its core. Useful star catalogue numbers for 109 Piscium include: 109 Psc, HR 508, Gl 72, Hip 8159, HD 10697, BD+19 282, and SAO 92611.

Planet or Brown Dwarf "b"

In 1999, a team of astronomers (including Steven S. Vogt, Geoffrey W. Marcy, R. Paul Butler, and Kevin Epps) announced the discovery of a giant planet companion "b" to 109 Piscium (exoplanets.org; and Vogt et al, 2000). The object has at least 6.05 times the mass of Jupiter, with a similar diameter -- about 11 times that of Earth. However, because the method of detection relies on radial velocities using the Doppler technique only determines the object's minimum mass, companion b may actually be a brown dwarf.

Subsequent astrometric analysis, however, suggests that planet b may have as much as 38 +/- 13 times the mass of Jupiter with an inclination of 170.3° from Earth's line of sight (Han et al, 2000; and Zucker and Mazeh, 2000). Thus, the "planet" could be an extremely dim brown dwarf, substellar companion of 109 Piscium. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

The companion object b is separated from 109 Piscium on average by 2.12 AUs. It moves around the star in a mildly eccentric orbit (e= 0.13) that takes just over 2.9 years to complete. Given the great mass and slightly eccentric, inner orbit of companion b, it is highly unlikely that an Earth-type planet could have developed in a stable orbit within the star's water zone (centered around 1.7 AUs out from 109 Piscium) before the star left the main sequence. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

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.

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.

Closest Neighbors

The following star systems are located within 10 light-years of 109 Piscium.

Star System	Spectra & Luminosity	Distance (light-years)
BD+20 254	?	4.5
BD+22 287	G5 V	6.0
BD+23 204	G0 V	9.1
BD+17 202	K0-2 V	9.5
* plus bright stars *		. . .
BD+20 341	G5 V	12
BD+21 298	G5 ?	13
BD+23 296	K1 IV-III	14
Eta Arietis	F5 V	14
BD+19 342	F8 V	15
102 Pi Piscium 2	F0 V ?	16
BD+27 262	G8 V	16
BD+21 150	G5 V	16

Other Information

Try Professor Jim Kaler's Stars site for other information about 109 Piscium at the University of Illinois' Department of Astronomy. The late John Whatmough has developed illustrated web pages on this system in Extrasolar Visions.

Up-to-date technical summaries on these stars can be found at: Jean Schneider's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS; the Yale Bright Star Catalogue, 1991 5th Revised Edition; the HIPPARCOS Catalogue using the VizieR Search Service mirrored from the Centre de Données astronomiques de Strasbourg (CDS); NASA's ADS Abstract Service for the Astrophysics Data System; and the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access.

Constellation Pisces (the Fish) is faint but can be found along the celestial equator swimming north of Aquarius (the Water Bearer) and Cetus (the Whale or Sea Monster) and surrounded by Pegasus, Andromeda, Triangulum, and Aries. According to Greek mythology, Aphrodite and her son Eros turned into fish (commemorated as the Northern and Southern Fishes of Pisces) and jumped into a river in Egypt to escape from Typhon, who sought to overthrow Zeus and his group of gods. For more information on the constellation and an illustration, go to Christine Kronberg's Pisces. For another illustration, see David Haworth's Pisces.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.