HN Pegasi

The Star

HN Pegasi is a yellow-orange main sequence dwarf star of spectral and luminosity type G0 V. The star may have a mass a little greater than Sol's, about 94 percent of its diameter (Johnson and Wright, 1983, page 699), but around 1.2 times its visual luminosity. Still rotating at a fast rate of 4.67 to 5.5 days (J. You and R. Duemmler, 2005; and Bochanski et al, 2000) and moving through space as an apparent member of the Local Association of young star clusters that was first described by Olin J. Eggen in 1983, it appears to be only around 300 million years old (Eric J. Gaidos, 1998; and H.J. Rocha-Pinto and W.J. Maciel, 1997). HN Pegasi may be only around 76 percent as enriched than Sol in elements heavier than hydrogen ("metals") based on its abundance of iron (Beichman et al, 2006, page 37). Not surprising for its youth, it has been an active flare star and so has been given the variable star designation of HN Pegasi (Manuel Güdel, 1997). Useful catalogue numbers and designations for the star include: HN Peg, HR 8314, Gl 836.7, Hip 107350, HD 206860, BD+14 4668, SAO 107364, FK5 3737, Wo/GJ 9751, and LTT 16357.

Dust Disk

A circumstellar disk of cold dust beyond 10 AUs around HN Pegasi was recently detected with the Spitzer Space Telescope (Bryden et al, 2006; Beichman et al, 2006; and J.S. Greaves and M.C. Wyatt, 2003). Probably generated by collisions between icy Edgeworth-Kuiper Belt type objects, the dust disk is centered around 14 AUs and may only be a few AUs wide. The relative paucity of detectable dust with 10 AUs suggests that planets have already developed and swept away the bulk of dust and other planetesimal material.

HN Pegasi "b"

Luhman et al, PSU, CfA,
JPL, CalTech, NASA



Larger infrared image.



HN Pegasi "b" is a
T-type brown dwarf
with around 20 times
Jupiter's mass (more).

On September 18, 2006, a team of astronomers announced the discovery and direct imaging of a spectral type-T, methane brown dwarf companion (T2.5 +/- 0.5) to this star (press release; and Luhman et al, 2006). A comparison of its luminosity to the values predicted by theoretical evolutionary models suggests that the substellar object has a mass of 0.021 +/- 0.009 Solar-masses, or around 20 times Jupiter's mass (Luhman et al, 2006). The brown dwarf has a projected separation of around 795 AUs from HN Pegasi and was detected by direct imaging using NASA's Spitzer Space Telescope. It is one of the least massive and youngest methane brown dwarfs known.

Robert Hurt, SSC,
JPL, CalTech, NASA


Larger and jumbo illustrations.


Type-T, methane brown
dwarfs are the coolest
and smallest (more).

Hunt for Planetary Companions

Past radial velocity analysis suggests that giant planets of one tenth to 10 times the mass of Jupiter do not exist within 0.1 to four AUs of HN Pegasi (Cummings et al, 1999). The habitable zone around HN Pegasi where an Earth-type rocky planet could eventually support liquid water on its surface is centered around 1.1 AUs, where it would take somewhat more than an Earth-year to complete its orbit. Given the youth of the system and its still massive Edgeworth-Kuiper Belt (as suggested by the detected dusk disk), however, the surface of such an inner rocky planet would still be quite hot from planetary formation and relatively frequent impacts from asteroids and comets. Astronomers would find it very difficult to detect Earth-type planets around this star using present methods.