MLO 4 / HR 6426 / Gl 667 ABC

Guillem Anglada-Escudé, CIS


Larger illustration of planet "c" only.


MLO 4 C, orbiting Stars A and B,
appears to have at least two
super-Earths, and one ("c") in
the star's habitable zone (more).

Planetary candidates have been found around Star C. On October 19, 2009, a team of astronomers using the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph with the European Southern Observatory's (ESO) 3.6-meter telescope announced the discovery of a potential super-Earth "b" with at least 5.7 Earth-masses in a tight orbit (~0.05 AUs) around Star C at an ESO/CAUP conference on extra-Solar planets in Porto, Portugal -- ESO press release and more below). Beginning with a pre-print submitted on November 21, 2011 and a Carnegie Institution for Science news release on February 2, 2012, two teams of astronomers revealed the finding of a second, even smaller potential super-Earth "c" with around 4.5 (but possibly as little as 3.4) but as much as 9 Earth-masses in a potentially habitable-zone orbit (~0.12 AUs) completed in 28.15 days around Star C, with evidence of another super-Earth "d?" with a period of 75 days and a gas giant "e?" in outer orbits (Carnegie news release; UCSC news release; Anglada-Escudé et al, 2012; Lisa Grossman, New Scientist, February 2, 2012; Nancy Atkinson, Universe Today, February 2, 2012; Bonfils et al, 2011; and Delfosse et al, 2011, in prep -- more below). (See animations of the orbits of Stars A and B and their potentially habitable zones and of planetary and potentially habitable zone orbits of Star C, with tables of basic orbital and physical characteristics.)

Luis Calçada, ESO



Larger and jumbo illustrations.



Sunset on planetary candidate "c"
with binary stars A and B in the
darkening sky (more), as imagined
by Calçada.

As a relatively bright star in Earth's night sky, Star A also is catalogued as Harvard Revised (HR) 6426, 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 -- updated and expanded through the hard work of E. Dorrit Hoffleit (1907 - 2007) and others. In addition, HR 6426 is listed as HD 156384 in the Henry Draper (1837-82) Catalogue with extension (HDE), a massive photographic stellar spectrum survey carried out by Annie Jump Cannon (1863-1941) and Edward Charles Pickering (1846-1919) from 1911 to 1915 under the sponsorship of a memorial fund created by Henry's wife, Anna Mary Palmer. However, many astronomers now refer to this star by its designation as Gliese (or Gl or expanded GJ) 667 in the famous Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) of Wilhelm Gliese (1915-93), who was a longtime astronomer at the Astronomiches Rechen-Institut at Heidelberg (even when it was at Berlin). (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.)

Habitable Zones around the Stars

For an even cooler and dimmer red dwarf star like Star C, the water zone probably would be located between 0.1 and 0.28 AUs with a period of with less than two months Anglada-Escudé et al, 2012, Figure 3). At such a close distance, a planet would probably be tidally locked -- with one side in perpetual day. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently. (See animations of the orbits of Stars A and B and their potentially habitable zones and of the planetary and potentially habitable zone orbits of Star C, with tables of basic orbital and physical characteristics.)

MLO 4 A

This star is an orange-red dwarf star of spectral and luminosity type K3-4 V (NASA Star and Exoplanet Database). This star have around 71 to 75 percent of Sol's mass (NASA Star and Exoplanet Database, derived from David F. Gary, 1992), around 76 percent of its diameter (Pasinetti-Fracassini et al, 2001), and about 12 percent of its visual and over 32 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). It appears to be less enriched than Sol in elements heavier than hydrogen ("metals") with only about 26 percent of Sol's abundance of iron (Cayrel de Strobel et al, 1991, page 301). It is a New Suspected Variable star designated NSV 8482 (with an amplitude of 0.04). Useful star catalogue numbers for MLO 4 A include: HR 6426*, Gl 667 A, Hip 84709, HD 156384, CD-34 11626 A, CP-34 6803, SAO 208670, LHS 442, LTT 6888, LFT 1336, LPM 638, and UGPMF 433.

According to the Sixth Catalog of Visual Orbits of Binary Stars, Stars A and B have an average separation of 12.6 AUs (a semi-major axis of 1.81" at a HIPPARCOS distance estimate of 20.74 ly) in a highly eccentric orbit (e= 0.58). The orbital period takes 42.15 years to complete and is inclined 128° from the perspective of an observer on Earth (Staffan Soderhjelm, 1999; Baize and Petit, 1989, page 509; Roland Wielen, 1962, page 604; and W.P. Hirst, 1950, page 458; and 1947). In turn, Stars A and B have been separated from Star C by 56 to 213 AUs (8 to 30.5") from 1889 to 1948 at an orbital inclination of 139° -- to 215 AUs (30.8") at an inclination of 136°; calculations published in 2012 suggest a minimum separation of 230 AUs (Anglada-Escudé et al, 2012) (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

MLO 4 B

This star is an orange-red dwarf star of spectral and luminosity type K5 V (NASA Star and Exoplanet Database). This star may have around 65 percent of Sol's mass, 70 percent of its diameter (Pasinetti-Fracassini et al, 2001), and five percent of its visual luminosity. Useful star catalogue numbers for MLO 4 B include: Gl 667 B.

NASA -- larger image


MLO 4 C is a dim red dwarf star, like Gliese
623 A (M2.5V) and B (M5.8Ve) at lower right.

MLO 4 C

Currently separated from Stars A and B by at least 230 AUs, MLO 4 C is a red dwarf star of spectral and luminosity type M1.5-2.5 V (Anglada-Escudé et al, 2012; NASA Star and Exoplanet Database; and Hawley et al, 1996). This star may have around 31 to 38 percent of Sol's mass (Anglada-Escudé et al, 2012), 42 percent of its diameter (Pasinetti-Fracassini et al, 2001), and just over 3/1,000th of its visual luminosity. Assuming to have the same metallicity as Stars A and B with around 26 percent of Sol's metallicity, it appears to be a main sequence stars of two to 10 billion years old (Anglada-Escudé et al, 2012). Useful star catalogue numbers for MLO 4 C include: Gl 667 C, and LHS 443.