85 Pegasi 3

According to Robert Burnham, Jr. (1931-93), 85 Pegasi was found to be a fairly close binary by noted double-star observer Sherburne Wesley Burnham (1838-1921) with the 18-inch refractor at Dearborn in 1878, who listed it in his catalogue as BU 733. By 1949, however, the fainter companion itself was suspected to be a close double of dimmer objects (e.g., red dwarf stars) as well by R.G. Hall, Jr., as a possible explanation of its computed large mass -- which is close to that of the bright primary (Wulff Dietz Heintz, 1993; Wallerstein and Helper, 1959; Arne A. Wyller, 1956; and R.G. Hall, 1949). Subsequently, one speckle interferometric observation of Star A suggested that the bright primary also might have a close companion, but this analysis was not confirmed and is now considered to be "obviously" spurious (Wulff Dietz Heintz, 1993; and the Yale Bright Star Catalogue, 1991 5th Revised Edition's notes entry for HR 9088). Two nearby faint stars in the system's field are believed to be optical companions, as they do not share the system's large proper motion.

85 Pegasi exhibits a large proper motion of 1.29" annually in PA 140°, with a radial velocity of 21.5 miles per second in approach. Deficient in heavy elements, the system also a high space velocity and appears to be relatively old at around 9.3 +/- 0.5 billion years old based on these and other considerations (Fernandes et al, 2002; Perrin et al, 1977; and Wallerstein and Helper, 1959). (See an animation of the orbits of Star A and and tight binary stars Bab and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

85 Pegasi A

The primary is now believed to be a yellow-orange main sequence dwarf star of spectral and luminosity type G5 Vb, although it was once classed as yellow as G2 (ten Brummelaar et al, 2000). This star may have as much as 88 percent of Sol's mass (Fernandes et al, 2002), 91 percent of its diameter (Johnson and Wright, 1983, page 705), and about 61 percent of its luminosity. Based on measurements of the star's abundance of iron, 85 Pegasi A may be 58 to 75 percent as enriched as Sol with elements heavier than hydrogen ("metallicity") (Fernandes et al, 2002; and Cayrel de Strobel et al, 1991, page 312; and Wallerstein and Helper, 1958).

According to a recent analysis, the star may be 9.3 +/- 0.5 billion years old (Fernandes et al, 2002). Although speckle interferometric analysis once suggested that 85 Pegasi A itself might have a close stellar companion, the observation was not confirmed and is now considered to be spurious (Wulff Dietz Heintz, 1993; and the Yale Bright Star Catalogue, 1991 5th Revised Edition's notes entry for HR 9088). The star has been designated as a New Suspected Variable (NSV 14809). Useful star catalogue numbers for the star include: 85 Peg, HR 9088*, Gl 914 A, Hip 171, HD 224930 A, BD+26 4734 A, SAO 91669, LHS 101, LTT 17088, LFT 1848, ADS 17175 A, and BU 733.

According to relatively recent measurements (Staffan Soderhjelm, 1999) found in the Sixth Catalog of Orbits of Visual Binaries, Star A and the close binary pair Bab are separated by an "average" distance of about 10.3 AUs (of a semi-major axis of 0.83" at 40.5 ly) -- just beyond the orbit of Saturn in the Solar System -- in an elliptical orbit (e= 0.38) of 26.28 years. Hence, Star A and the Bab pair get as close as 6.4 AUs and as far away as 14.2 AUs. When observed from Earth, the inclination of their orbit is about 49 degrees. (See an animation of the orbits of stars A and Bab and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

The orbit of an Earth-like planet with surface water around this ancient star liquid water zone would have to be centered around 0.78 AU -- between the orbits of Venus and Earth in the Solar System. At that distance from Star A, it would have an orbital period around 270 days or roughly three-fourths of an Earth year. Useful catalogue numbers for this star include: 85 Peg B, Gl 914 B, BD+26 4734 B, and ADS 17175 B.

© Torben Krogh & Mogens Winther,

(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)



85 Pegasi Ba is an orange-red
dwarf star, like Epsilon Eridani
at left center of meteor.

85 Pegasi Ba

This star is now believed to be a orange-red main sequence dwarf of spectral and luminosity type K7 V (ten Brummelaar et al, 2000). It may have about 55 percent of Sol's mass (Fernandes et al, 2002), 67 percent of its diameter (Johnson and Wright, 1983, page 705), and around 0.5 percent of its luminosity. Astronomers are now reasonably certain that Star Ba has a very dim, low-mass, red dwarf companion Bb in a tight orbit -- possibly within two AUs (Fernandes et al, 2002).

The orbit of an Earth-like planet around Star Ba in the liquid water zone would have to be centered around 0.2 AU -- around half of Mercury's orbital distance around Sol -- with an orbital period around 52 days. However, if the existence of a relatively close, stellar companion (see Star Bb below) around Ba is confirmed, then a planetary orbit in Star Ba's water zone may not be stable if Star Bb lies substantially less than one AU of Star Ba. Useful catalogue numbers for this star include: 85 Peg B, Gl 914 B, BD+26 4734 B, and ADS 17175 B.

NASA -- larger image

85 Pegasi Bb may be a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.

85 Pegasi Bb

This close companion of Star Ba is probably a main sequence, red dwarf star of spectral and luminosity type M V. It may have about 11 percent of Sol's mass (Fernandes et al, 2002). A stable orbit for this dim companion star may exist within two AUs of Ba.