75 Ceti
A red clump giant with two confirmed planets.
75 Ceti, an orange-hued star in the equatorial constellation Cetus, shines faintly enough to be seen without a telescope at magnitude +5.36. Based on parallax measurements, it lies 268 light-years from the Sun and is moving closer at a radial velocity of −6 km/s. This aging giant, classified as K1 III, has exhausted its core hydrogen and expanded to 10.6 times the Sun’s radius (about 0.05 AU). It is a red clump giant on the horizontal branch, fusing helium in its core. With 1.9 times the Sun’s mass and an age of 1.4 billion years, it radiates 56 times the Sun’s luminosity from a photosphere at 4,846 K. A companion star, just over half the Sun’s mass, orbits at a separation of 11.5 arcseconds (958 AU).
In Chinese astronomy, 75 Ceti is part of the asterism *Tiān Qūn* (Circular Celestial Granary), which includes α, κ1, λ, μ, ξ1, ξ2, ν, γ, δ, 70, 63, and 66 Ceti. It is known as the Tenth Star of Circular Celestial Granary.
The star hosts at least two planets. The first, 75 Ceti b, was detected in 2012 via Doppler measurements at the Okayama Astrophysical Observatory and is considered a typical gas giant. It receives more insolation than Jupiter or Earth. A second planet, 75 Ceti c, a Jupiter-mass world orbiting at about 4 AU, was confirmed in 2023; it too is more irradiated than Earth. An earlier signal suggesting a possible planet at 0.9 AU turned out to be an alias of planet c’s true orbital period.
- Apparent visual magnitude
- +5.36
- Distance
- 268 light-years (82 parsecs)
- Radial velocity
- −6 km/s
- Stellar classification
- K1 III
- Radius
- 10.6 times the Sun's radius (0.05 AU)
- Mass
- 1.9 times the Sun's mass
- Age
- 1.4 billion years
- Luminosity
- 56 times the Sun's luminosity
- Effective temperature
- 4,846 K
- Companion mass
- just over half the Sun's mass
Lore & Background
75 Ceti is an aging giant star with a stellar classification of K1 III, having exhausted the supply of hydrogen at its core and expanded to 10.6 times the Sun's radius, or 0.05 AU. It is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at the core. The star is 1.4 billion years old with 1.9 times the Sun's mass, radiating 56 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,846 K. There is a companion star just over half the Sun's mass at a separation of 11.5 arcseconds, corresponding to 958 AU.
In Chinese, 天囷 (Tiān Qūn), meaning Circular Celestial Granary, refers to an asterism consisting of α Ceti, κ1 Ceti, λ Ceti, μ Ceti, ξ1 Ceti, ξ2 Ceti, ν Ceti, γ Ceti, δ Ceti, 75 Ceti, 70 Ceti, 63 Ceti and 66 Ceti. Consequently, 75 Ceti itself is known as the Tenth Star of Circular Celestial Granary.
A planetary companion was discovered by Doppler measurements at the Okayama Astrophysical Observatory, and announced in 2012. The planet's discoverers consider the planet, designated 75 Ceti b, to be 'typical' of gas giants. Note that (like many recorded planets) b takes in much more insolation than does Jupiter and, indeed, Earth. There may be additional periodic factors in the data, corresponding to m sin i of around 0.4 MJ and 1 MJ, at distances of ~0.9 AU and ~4 AU, where i is the orbital inclination and m is the planet's actual mass. In 2023, the presence of a second, Jupiter-mass planet orbiting at 4 AU (75 Ceti c) was confirmed, which is more irradiated than Earth as well. The shorter period signal corresponding to a possible planet at 0.9 AU was found to be an alias of the true period of planet c.
Reader's Guide
75 Ceti is significant as a red clump giant star with a confirmed multi-planet system, providing an example of planetary formation and evolution around an evolved star. The discovery of 75 Ceti b in 2012 via Doppler measurements at the Okayama Astrophysical Observatory added to the growing census of gas giants around giant stars. The subsequent confirmation in 2023 of a second Jupiter-mass planet, 75 Ceti c, orbiting at 4 AU, further established the system as a benchmark for studying planetary architectures around post-main-sequence stars. The fact that both planets are more irradiated than Earth and Jupiter highlights the diversity of exoplanetary environments. The presence of a stellar companion at 958 AU also offers a context for understanding binary star influences on planetary system stability. The star's Chinese designation as the Tenth Star of Circular Celestial Granary ties it to traditional asterisms, enriching its cultural legacy.
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