Gliese 105 / HR 753 ABC

As a relatively bright star in Earth's night sky, Star A is catalogued as Harvard Revised (HR) 753, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- revised and expanded through the hard work of E. Dorrit Hoffleit and others. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)

David Golimowski, Palomar Observatory

Larger discovery image of Star C, with
dark circular coronagraphic stop at center
of Gl 105 A for suppressing light scatter.

Star C (protruding yellow spot at upper right)
was confirmed as Gl 105 A's fainter companion
with Palomar's 60" telescope (more).

Star A was found to have a distant, proper motion companion "B" by Adriaan van Maanen (1884-1946) before March 1938 (object Number 3 in Table 2 on page 33). However, a second, closer but unseen companion "C" to Star A had been on the astrometric program at Sproul Observatory (Swarthmore College) since 1937 (Sarah Lee Lippincott, 1973). In 1994, David Golimowski and collaborators at the Palomar Observatory in California visually detected the third stellar member to Gliese 105 (Golimowski et al, 1995; and May, 1995). While Star C is a much closer stellar companion of Star A than Star B, it apparently lies near the theoretical minimum mass limit of 0.75 to 0.80 Solar for core fusion of hydrogen (more from Palomar Adaptive Optics System Results). (See an animation of the orbits of Stars A and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

Gliese 105 A

© Torben Krogh & Mogens Winther,

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


Gliese 105 A is an orange-red dwarf
star, like Epsilon Eridani at left
center of meteor. (See a Digitized
Sky Survey image of Gl 105
(HR 753) from the Nearby Stars
Database.)

Star A's late spectral type and dim luminosity puts it possibly close to the lower limit of habitability for (multicellular) Earth-type plant and animal life, given the redness of its light and the increased risk of tidal locking from the closeness of the orbit necessary for liquid water on a planetary surface. The distance from Star A where an Earth-type planet would be "comfortable" with liquid water is centered around 0.45 AU -- within the orbital distance of Mercury in the Solar System. At that distance from the star, such a planet would have an orbital period of about 124 days, or around a third of an Earth year.

A 12-magnitude star "B" was discovered to share the same common proper motion with Star A around 1938 (Adriaan van Maanen, 1938). Stars A and B, however, have a wide separation of some 1,200 AUs -- 165" at a HIPPARCOS distance estimate of 23.5 ly (Golimowski et al, 2000, in ps; Heintz and Cantor, 1994; Phillip A. Ianna, 1992; Martin and Ianna, 1975; and Sarah Lee Lippincott, 1973). On the other hand, Star C is currently separated from the AB pair by only about 24 AUs (3.39"). The new Sixth Catalog of Orbits of Visual Binaries presents outdated 1994 orbit estimates for Stars A and C.

Relative astrometric measurements taken over three years of the period of Star A's orbital perturbation suggest that Star C's orbit must have a high eccentricity (e= 0.75) and a semi-major axis of 15 AUs, to satisfy both the observed orbital motion and Gl 105A's astrometric period. Measurements of Gl 105A's radial velocity over 12 years show a linear trend and slope which is consistent with these orbital constraints and a nearly face-on orbit. No other stellar or brown dwarf sized objects have been detected that can cause the observed perturbation. (Golimowski et al, 2000, in ps).

NASA -- larger image

Gliese 105 B and C are dim red dwarf stars, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.

Gliese 105 B or BX Ceti

This star is a red dwarf of spectral and luminosity type M3.5 Vn. It has about 21 percent of Sol's mass, 28 percent of its diameter, and 1/1000th of its brightness. The SIMBAD Astronomical Database identifies Star B as a variable star with the designation BX Ceti. Despite its variability, the star has "anomalously low" chromospheric and coronal activity (Doyle et al, 1998). Some alternative useful star catalogue numbers for the star are: BX Ceti, HR 5568 B, Gl 105 B, HD 16160 B, BD+06 398 B, G 73-71, G 76-12, LHS 16, LTT 10859, LFT 218, Vys or McC 396, and Rob 153.

For Star B, the water-zone orbit lies around 0.031 AU. As a result, a planet in such an orbit would have a period of only about 4.3 days. Tidal locking of such a closely orbiting planet would resulting in perpetual day on one side and perpetual night on the other.

Gliese 105 C

An extremely dim red dwarf, Star C is of spectral and luminosity type M7 V with only about 8.2 percent of Sol's mass, (Golimowski et al, 2000, in ps; and 1995). In addition, Star C may have a diameter not much larger than Jupiter's and only around 7.5 one-millionth of its visual luminosity. Some alternative useful star catalogue numbers for the star are: HR 753 C and Gl 105 C.

For Star C, the water zone orbit lies only around 0.0027 AU, even closer than for Star B. A planet in such an orbit would have a period of only around 14 hours. Again, tidal locking of such a closely orbiting planet would resulting in perpetual day on one side and perpetual night on the other. (See an animation of the orbits of Stars A and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

Hunt for Planetary Companions

Direct images of Gl 105A obtained with the Hubble Space Telescope (HST) and radial velocity analysis have failed to reveal substellar companions as faint and small as brown dwarfs lying within at least 10 AUs of the star (Golimowski et al, 2000, in ps; Schroeder et al, 2000; and Cummings et al, 1999). Hence, it is possible that small, terrestrial planets exist in inner orbits around each of the three stars of the Gliese 105 system. However, astronomers would find it very difficult to detect Earth-type planets around these stars using present methods.