65489 Ceto

65489 Ceto, as a binary also (65489) Ceto/Phorcys,^[4] is a binary trans-Neptunian object (TNO) discovered on March 22, 2003, by Chad A. Trujillo and Michael Brown at Palomar. It is named after the sea goddess Ceto from Greek mythology. It came to perihelion in 1989.^[2]

65489 Ceto

Hubble Space Telescope image of Ceto and Phorcys, taken in 2006
Discovery
Discovered by	C. A. Trujillo and M. Brown
Discovery site	Palomar
Discovery date	22 March 2003
Designations
MPC designation	(65489) Ceto
Pronunciation	/ˈsiːtoʊ/
Named after	Ceto
Alternative designations	2003 FX128
Minor planet category	TNO Centaur–extended[1]
Adjectives	Cetoan /siːˈtoʊən/ or Cetoian /siːˈtoʊ.iən/
Orbital characteristics[2]
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 2
Observation arc	9239 days (25.30 yr)
Aphelion	187.74 AU (28.086 Tm)
Perihelion	17.8498 AU (2.67029 Tm)
Semi-major axis	102.79 AU (15.377 Tm)
Eccentricity	0.82635
Orbital period (sidereal)	1042.22 yr (380669.7 d)
Mean anomaly	9.1219°
Mean motion	0.00094570°/day
Inclination	22.266°
Longitude of ascending node	171.85°
Argument of perihelion	320.086°
Known satellites	Phorcys /ˈfɔːrsɪs/ (171±10[3] ~ 132+6 −14 km[4] in diameter)
Earth MOID	16.895 AU (2.5275 Tm)
Jupiter MOID	12.7433 AU (1.90637 Tm)
Physical characteristics
Dimensions	223±10 km[3] 174+16 −18 km[4]
Mass	(5.4±0.4)×1018 kg (system)[4]
Mean density	1.37 g/cm3 (system)[4]
Equatorial surface gravity	3.3 cm/s2[4]
Synodic rotation period	4.43 h (0.185 d)
Geometric albedo	0.056±0.006[3] 0.084±0.02[4]
Absolute magnitude (H)	6.54±0.06,[3] 6.4[2]

Physical characteristics

Polar view of Ceto's orbit

65489 Ceto is an example of a close binary TNO system in which the components are of similar size. Combined observations with the infrared Spitzer Space Telescope and the Hubble Space Telescope allow the diameter of Ceto itself to be estimated at 174+16
−18 km and the diameter of Phorcys at 132+6
−14 km, assuming equal albedo for both components.^[4]

The binary nature of Ceto enables direct calculation of the system mass, allowing estimation of the masses of the components and providing additional constraints on their composition. The estimated density of Ceto is 1.37+0.66
−0.32 g/cm³, significantly less than that of the large TNOs (Haumea: 3.0 g/cm³, Eris: 2.26, Pluto: 2.03, Charon: 1.65) but significantly more than that of smaller TNOs (e.g. 0.7 g/cm³ for (26308) 1998 SM165). Phorcys has a mass of about 1.67×10¹⁸ kg.^[4] Unless the bodies are porous, the density is consistent with rock–ice composition, with rock content around 50%.^[4]

It has been suggested that tidal forces, together with other potential heat sources (e.g. collisions or ²⁶Al decay) might have raised the temperature sufficiently to crystallise amorphous ice and reduce the void space inside the object. The same tidal forces could be responsible for the quasi-circular orbits of the components of Ceto.^[4]

Ceto is listed on Michael Brown's website as possibly a dwarf planet.^[5]

Satellite

Phorcys

Discovery
Discovered by	Keith S. Noll et al.
Discovery date	2006 April 11
Designations
Pronunciation	/ˈfɔːrsɪs/
Named after	Phorcys
Alternative designations	(65489) Ceto I
Adjectives	Phorcynian /fɔːrˈsɪniən/,[6] Phorcyan /ˈfɔːrsiən/[7] [8]
Orbital characteristics
Satellite of	Ceto
Physical characteristics[9]
Dimensions	171±10[3] 132+6 −14 km.[4] primary/secondary ratio 0.76±0.01
Apparent magnitude	ΔM = 0.56±0.03

Ceto's satellite was identified as a binary on April 11, 2006, by K. Noll, H. Levison, W. Grundy and D. Stephens using the Hubble Space Telescope; the object was named Phorcys after the Greek sea god, formally (65489) Ceto I. Using an extended definition of a centaur as an object on a non-resonant (unstable) orbit with its perihelion inside the orbit of Neptune,^[10] the Ceto system can be considered the second known binary centaur.^[4]

Phorcys's diameter has been estimated to be 171±10^[3] and 132+6
−14 km.^[4]

See also

• List of Solar System objects by size