54 Piscium A

Lore & Background

The Flamsteed designation 54 Piscium originated in the star catalogue of the British astronomer John Flamsteed, first published in 1712. The star has an apparent magnitude of 5.86, allowing it to be seen with the unaided eye under suitable viewing conditions. It has a classification of K0V, with the luminosity class V indicating this is a main sequence star that is generating energy at its core through the thermonuclear fusion of hydrogen into helium. The effective temperature of the photosphere is about 5,267 K, giving it the characteristic orange hue of a K-type star. As a typical K-type dwarf, 54 Piscium is smaller and fainter than the Sun, at 85% of the Sun's mass and half of the luminosity. The angular diameter has been directly determined by interferometry using the CHARA array, yielding a radius 86.4% of the Sun's radius. The rotational period of 54 Piscium is about 40.2 days. The age of the star is about 6.4 billion years, based on chromospheric activity and isochronal analysis, though another study suggests a higher age of ten billion years. There is some uncertainty in the scientific press concerning the higher ratio of elements heavier than hydrogen compared to those found in the Sun; Santos et al. (2004) report the logarithm of the abundance ratio of iron to hydrogen, [Fe/H], to be 0.12 dex, whereas Cenarro et al. (2007) published a value of –0.15 dex. Long term observation of this star's magnetic activity levels suggests that it is entering a Maunder minimum period, which means it may undergo an extended period of low starspot numbers. It has a Sun-like activity cycle that has been decreasing in magnitude. As of 2010, the most recent period of peak activity was 1992–1996, which showed a lower level of activity than the previous peak in 1976–1980. The star rotates at an inclination of 83+7−56 degrees relative to Earth.

Reader's Guide

54 Piscium A is significant as the first star known to host both an extrasolar planet and a brown dwarf, with the brown dwarf discovered via direct imaging in 2006 after the planet was confirmed in 2003. The planet, 54 Piscium b, has a mass of about 20% that of Jupiter and orbits at 0.28 AU with a high eccentricity of 0.65, which was initially puzzling until the brown dwarf's gravity was identified as the cause. The brown dwarf, 54 Piscium B, is a methane brown dwarf of spectral type T7.5V with a mass about 50 times Jupiter's, a radius 80% of Jupiter's, and a surface temperature of about 810 K. Its projected separation from the primary is about 476 AU. The system's architecture—a close-in eccentric planet and a distant brown dwarf—provides a natural laboratory for studying planetary system formation and dynamics. The star's uncertain metallicity and age, along with its declining magnetic activity cycle, add to its interest for stellar evolution studies. The system also constrains the possibility of Earth-like planets: simulations show that the planet's orbit sweeps clean most test particles within 0.5 AU, and observations rule out Neptune-class planets with periods of one year or less, but Earth-sized planets at 0.6 AU or more remain possible.

Did You Know?

• 54 Piscium B was the first brown dwarf detected around a star with an already known extrasolar planet.
• The planet 54 Piscium b has a high orbital eccentricity of about 0.65, caused by the gravity of the brown dwarf.
• The star's age is uncertain, with estimates of 6.4 billion years and 10 billion years from different studies.
• The brown dwarf 54 Piscium B is smaller than Jupiter despite having 50 times its mass.