54 Piscium

The Star

© Torben Krogh & Mogens Winther,


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


54 Piscium is an orange-red
dwarf star, similar to Epsilon
Eridani at left center of meteor.
(See a Digitized Sky Survey
image of 54 Piscium from
the Nearby Stars Database.)

54 Piscium is an orange-red main-sequence dwarf star of spectral and luminosity type K0+ V. The star may have 79 percent of Sol's mass (exoplanets.org), 86 percent of its diameter (Johnson and Wright, 1983, page 645), and 46 to 49 percent of its luminosity. With a fairly long rotational period of 48 days compared with Sol that is suggestive of greater age (Noyes et al, 1984), 54 Piscium has been estimated to be from 5.1 to 8.2 +4.3/-5.2 billion years old based on chromospheric activity and isochronal analysis (Luhman et al, 2006; Saffe et al, 2005; Valenti and Fischer, 2005; and Wright et al, 2004). It appears to be around 1.1 times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (exoplanets.org). 54 Piscium is not chromospherically active (Fischer et al, 2003, in pdf) but has been catalogued as a New Suspected Variable star designated NSV 245. Useful catalogue numbers and designations for the star include: 54 Psc, HR 166*, Gl 27, Hip 3093, HD 3651, BD+20 85, SAO 74175, LHS 1116, LTT 10224, and LFT 59.

The orbit of an Earth-like planet with surface water would be centered within 0.68 AU -- around the orbital distance Venus in the Solar System -- and take around 240 days (two-thirds of a year) to complete. However, the inner, eccentric orbit of a giant planetary companion "b" recently discovered around 54 Piscium would probably disturb the orbit of such an Earth-type planet. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

Greg Bacon, STScI, NASA


Larger image.


The mass and orbit of planetary
candidate "b" is similar to 79
Ceti b, depicted at left with rings
and two moons, as imagined by
Bacon (more from NASA and
Astronomy Picture of the Day).

Planetary Companion "b"

On January 16, 2002, a team of astronomers (including (Debra A. Fischer, Geoffrey W. Marcy, R. Paul Butler, Steven S. Vogt, and Gregory W. Henry) announced the discovery of a planetary companion "b," whose mass has been estimated at a minimum of 20 percent that of Jupiter (exoplanets.org; and Fischer et al, 2003, in pdf) with a similar diameter. The planet moves around 54 Piscium at an average separation of 0.28 AUs, which would be within the orbit of Mercury in the Solar System. Its orbit around 54 Piscium has a high eccentricity (e= 0.63 +/- 0.04) but takes only around 62.2 days to complete (exoplanets.org). The highly elliptical orbit, however, suggested that the gravity of an unseen object farther away from the star was pulling the planet outward, which was later detected by direct imaging as a methane brown dwarf -- more below. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

NASA

Larger image.

The planet may be have only around
two-thirds the mass of Saturn.

54 Piscium "b"

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



Larger infrared image.



54 Piscium "b" is a
T-type brown dwarf
with around 50 times
Jupiter's mass (more).

Between August 24 and September 18, 2006, two teams of astronomers announced the discovery and direct imaging of a spectral type-T, methane brown dwarf companion (T7.5 +/- 0.5) to this star (PSU press release; Luhman et al, 2006; and Mugrauer 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 0.051 +/- 0.014 Solar-masses, or around 50 times Jupiter's mass in a range between 22 and 57 Jupiter-masses (Luhman et al, 2006; and Mugrauer et al 2006). Similar in spectral type and mass to Gl 570 d, the brown dwarf has a surface temperature between 500 and 600 degrees Celsius (932 to 1,112 degrees Fahrenheit), which places about ten times colder as well as 300,000 less luminous than the Sun (ESO press release).

ESO



Larger infrared image.



The discovery of 54 Piscium b
indicates that the highly
elliptical orbits of close-in
planets found around other stars
could be the result of orbital
perturbations by low-mass
companions at wide separations
from their host stars (more).

The brown dwarf has a projected separation of around 476 AUs from 54 Piscium and was detected by direct imaging using NASA's Spitzer Space Telescope and the United Kingdom Infrared Telescope. 54 Piscium b is the first brown dwarf to be detected around a star with a known close-in planetary companion based on radial-velocity surveys, and so its discovery indicates that the puzzling highly elliptical orbits of close-in planets found around other stars could also be the result of orbital perturbations by undiscovered, low-mass companions at wide separations from their host stars that are more easily found in infrared light. Previously, some stars hosting planets have been found to have low-mass stars and white dwarf stellar remnants, which can be detected in visual wavelengths.

Robert Hurt, SSC,
JPL, CalTech, NASA


Larger and jumbo illustrations.


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