Scylla appearing deep blue with few bands of noctilucent clouds
Date August 7, 2005
Discoverers Butler et al.
Detection method Doppler spectroscopy
Site Lick Observatory
Name & designations
Pronunciation /'skī•lä/
Adjective Scyllian
Planet numbers P152, HD 11964 P1,
Cetus P4, Hippocampus P19,
2005 P24, 2005 Cet-2,
2005 Hip-4
Star designations PH 119 c, P4 Ceti c,
P15 Hippocampi c, HD 11964 c,
HIP 9094 c, Gliese 81.1 Ac,
GJ 9063 Ac, SAO 148123 c
System HD 11964
Constellation Cetus
Caelregio Hippocampus
Right ascension 01h 57m 09.61s (29.290 03°)
Declination −10° 14' 32.7" (−10.242 43°)
Distance 32.850 pc (107.142 ly)
Orbital characteristics
Semimajor axis 0.229 721 AU (34.365 8 Gm)
Periastron 0.161 286 AU (24.128 0 Gm)
Apastron 0.298 156 AU (44.603 5 Gm)
Eccentricity 0.297 906 5
Orbital circumference 1.419 44 AU (212.346 Gm)
Orbital area 0.158 26 AU² (3 541.8 Gm²)
Orbital period 37.911 515 d)
Avg. velocity 65.047 km/s (13.675 AU/yr)
Max. velocity 74.106 km/s (13.785 AU/yr)
Min. velocity 54.504 km/s (13.565 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 173.402° to line of sight
−179.775° to star's equator
−135.742° to invariable plane
Argument of periastron 101.748°
Longitude of ascending node 250.030°
Longitude of periastron 351.778°
Angular separation 6.993 mas
Observing the parent star
Mean angular star size 2.251 39° (135.083')
Max. angular star size 3.206 68° (192.401')
Min. angular star size 1.734 63° (104.078')
Mean star magnitude −30.927
Max. star magnitude −31.695
Min. star magnitude −30.360
Bulk characteristics
Mean radius 1.037 21 RJ (72.545 Mm)
Equatorial radius 1.020 85 EJ (72.945 Mm)
Polar radius 1.072 15 PJ (71.746 Mm)
Mean circumference 455.815 Mm
Equatorial circumference 458.326 Mm
Polar circumference 450.794 Mm
Surface area 1.076 76 SJ (66 130 Mm²)
Volume 1.117 33 VJ (1.599 1 × 106 Mm³)
Flattening 0.016 52 (1:60.52)
Angular diameter 29.525 μas
Mass 0.967 672 MJ
Reciprocal mass
relative to star
1 218.1
Density 1.149 g/cm³
Gravitational influence
Surface gravity 2.372 g (23.26 m/s²)
Weight on Scylla
(150 lb on Earth)
356 lb
Standard gravitational parameter 1.226 × 108 km³/s²
Escape velocity 58.13 km/s
Hill radius 4.238 LD (1.629 2 Gm)
Roche limit
(3 g/cm3 satellite)
0.172 68 LD (66.378 Mm)
Stationary orbit 8.190 21 LD (3 148.307 Mm)
Stationary velocity 6.179 km/s (1.389 LD/d)
Rotation characteristics
Rotation period 909.876 4 h (37.911 515 d)
Rotation velocity 504 kph (0.40 °/h)
Direction of rotation
relative to orbit
Axial tilt 45.188°
Longitude of vernal equinox 155.604°
North pole right ascension 12h 18m 52s (184.715°)
North pole declination +25° 42' 00" (+25.700°)
North polar constellation Coma Berenices
North polar caelregio Noctua
South pole right ascension 00h 18m 52s (4.715°)
South pole declination −25° 42' 00" (−25.700°)
South polar constellation Sculptor
South polar caelregio Solarium
Thermal characteristics
Surface temperature 553 K (280°C, 536°F, 996°R)
Mean irradiance 20 177 W/m² (14.754 I)
Irradiance at periastron 40 932 W/m² (29.930 I)
Irradiance at apastron 11 978 W/m² (8.758 I)
Albedo 0.145 (bond), 0.163 (geom.)
Scale height 157.02 km
Surface density 0.333 g/m³
Molar mass 2.12 g/mol
Composition 95.807% hydrogen (H2)
4.143% helium (He)
375 ppm carbon dioxide (CO2)
117 ppm methane (CH4)
7.05 ppm water (H2O)
503 ppb carbon monoxide (CO)
502 ppb hydrogen chloride
322 ppb phosphorus pentachloride (PCl5)
267 ppb octane (C8H18)
532 ppt hydrogen sulfide (H2S)
237 ppt phosphine (PH3)
11.6 ppt disulfur dichloride (S2Cl2)
Dipole strength 47.1 μT (0.471 G)
Magnetic moment 6.35 × 1018 T•m³
Dipole tilt 0.91°
Satellite system
Number of moons 1
Number of rings 0

Scylla (HD 11964 c, P152) is a planet which orbits the yellow G-type subgiant star HD 11964, meaning the star has just ran out of hydrogen fuel in its core. The star is considerably larger but cooler and lot more luminous than our Sun. It is approximately 107 light-years or 33 parsecs from Earth towards the constellation Cetus in the caelregio Hippocampus.

Scylla is a so-called blue Jupiter with no global cloud cover as it lacks suitable chemicals for clouds at a temperature of 509 K (536°C or 457°F), although there are few bands of high-altitude clouds near the equator and the poles. This planet takes 38 days to orbit the star.

Scylla is named after the monster in Greek mythology that lived on one side of a narrow channel of water.

Discovery and chronology Edit

Scylla was discovered on August 7, 2005 by a team of astronomers led by Paul Butler. The team used the spectrometer mounted on the telescope in Lick Observatory and found that this star wobble caused by planets. On that same day, the second planet Deino was also announced but reported unconfirmed.

Scylla is the 145th exoplanet discovered overall, 119th since 2000, and 24th in 2005. Scylla is also the 4th exoplanet discovered in the constellation Cetus (2nd in 2005) and 19th exoplanet discovered in the caelregio Hippocampus (4th in 2005). Scylla is the first planet discovered in the HD 11964 system, despite its designation HD 11964 c (a is not used because the parent star uses this letter to reduce confusion). More accurately, this planet is also designated HD 11964 P1. However, at the time of its discovery, this planet was designated HD 11964 b. Note that the chronology does not include speculative brown dwarfs (objects with minimum masses below 13 MJ but with speculative true masses above 13 MJ).

Orbit and rotation Edit

Orbit Edit

Scylla takes 3.28 megaseconds (37.9 days) to orbit the star at an average distance of 1.114 microparsecs (0.2297 astronomical units), which is 59% the average distance between Mercury and the Sun. Scylla orbits with an eccentricity of 0.298, which corresponds to the distances ranging from 0.782 to 1.446 μpc (0.1613 to 0.2982 AU). The planet moves at an average velocity of 65.05 km/s and it varies from 54.50 to 74.11 km/s during its orbit. Like some of the hot Jupiters known, Scylla orbits opposite to the rotation of the star, so-called retrograde orbit. The retrograde orbit maybe caused by the orbit being flipped by the gravitational interactions with three other planets in this system just after the planet formed. It's speculated inclination to line of sight is 173° (−179.78° to star's equator). The argument of periastron is 102°, which is the angle between periastron and ascending node. The longitude of ascending node is 250° counterclockwise from the origin of longitude at First Point of Aries as seen from north. Adding argument of periastron and longitude of ascensing node yields 352° in longitude of periastron.

Parent star observation and irradiance Edit

When viewing HD 11964 from one of its moons since this planet is a gas giant with no solid surface, that sun would appear to be 47 times brighter than the Sun as seen from Earth, corresponding to its apparent magnitude −30.93. The angular diameter of HD 11964 as seen from Scylla is 2.251°, which is 412 times the angular diameter of the full moon and sun as seen from Earth, because Scylla orbits nearly 413 times closer to the star than Earth to the Sun. However that magnitude and angular diameter of a star change during its planetary orbit because the orbit is eccentric. During half of its year when the planet moves closer to the star, the sun would appear to get brighter and larger, while at the other half when the planet moves further away from the star, it would appear to get dimmer and smaller.

Scylla receives 20 kilowatts worth of stellar energy over every square meter, which is 14.75 I. However with its eccentric orbit, its irradiance varies from 12 to 41 kW/m² or from 8.76 to 29.93 I.

Rotation Edit

Scylla takes 909 hours, 52 minutes, and 35 seconds to rotate once on its axis, which is nearly 38 Earth days. The rotation velocity is 504 kph, or 313 mph. This planet rotates in the same direction as its revolution, but opposite to the rotation of the parent star. Scylla is tidally lock, because its period of rotation is identical to the period of its orbit. The tilt of this planet is such that its north pole points to the constellation Coma Berenices (in Noctua) while the south pole points to the constellation Sculptor (in Solarium).

Structure and composition Edit

Mass and size Edit

Using the speculated inclination, its speculated true mass for Scylla is 0.97 MJ or 308 M. This planet had a minimum mass 0.11 MJ or 35 M. It is classified as mid-Jupiter in the planetary mass classification scheme. This planet has a radius of 71.747 megameters, which is slightly bigger than Jupiter. Scylla has density 1.16 g/cm³, which is a little denser than water.

The radius mentioned does not mean it is measured from the center to anywhere on the planet's surface. There are really three types of radii, mean radius (average when measuring from the center to everywhere on its surface), equatorial radius (from the center to the surface at the equator), and polar radius (from the center to the surface at either poles). The radius mentioned is its mean radius. Its equatorial radius is 72.945 Mm whiles its polar radius is 71.746 Mm, a difference of 1.199 Mm. Using these values, circumferences can be calculated using the equation 2πr. The calculated circumferences are 455.815 Mm (mean), 458.326 Mm (equatorial), and 450.794 Mm (polar). Its aspect ratio is 0.98357, derived by dividing polar radius/circum. from equatorial radius/circum. Because of its equatorial bulge, the surface gravity at the equator is lower than at the poles since the gravity is inversely proportional to the square of its radius.

Gravitational influence Edit

Scylla has gravitational force nearly 2.4 times stronger than Earth's and 90% that of Jupiter's. The object falling to the planet accelerates at 23.4 m/s² or 76.7 ft/s². If you weigh 150 pounds on Earth, you would weigh 356 pounds on Scylla. So a person standing on Scylla would weigh as much as a lion, tiger, and black bear on Earth!

The roche limit, where a 3 g/cm³ moon tear apart by tidal forces, is just 0.173 lunar distances (66.4 megameters), which is 0.915 planetary radii. The hill radius, the boundary where the gravitational influence of the planet is identical to the star, is just 4.24 lunar distances (1.63 gigameters), which is 2212 times the radius of the planet. The stationary orbit, where the satellite's orbital period is identical to the rotation period of the planet, analogous to the Earth's geostationary orbit, is about 3148 Mm, which is 43.16 planetary radii and eight times the distance between Earth and the Moon. The stationary velocity, the orbital velocity at stationary orbit, is 6.2 km/s or 3.8 mi/s. Since the planet takes 30¼ days to rotate, then a moon would take 30¼ days to orbit the planet at stationary orbit, which is very similar to the Moon's orbital period around the Earth.

Interior Edit

Below Scylla's outer envelope (atmosphere), the weight of all the gases pressing down produce a tremendous pressure. That pressure allow hydrogen and helium to condense in the upper mantle despite the higher temperatures deeper down. In the lower mantle lies liquid metallic hydrogen where hydrogen can conduct electricity under even greater pressure heated beyond its critical point. In the lower mantle, the temperature is 11,400 K (11,200°C, 20,100°F) and a pressure 190 GPa. At the center lies a core of rock and metal with a mass 22 Earth masses, roughly 7.1% the total mass of the planet. The temperature of the core is estimated to be 20,200 K (19,900°C, 35,800°F) and an estimated pressure 4.2 TPa.

Atmosphere Edit

Scylla's atmosphere composes of 95.8% hydrogen and 4.1% helium. Scylla's composition other than hydrogen and helium is considerably different compared to the four giant solar system planets because this planet orbits much closer to its star. Scylla contains no ammonia in the atmosphere. However, this planet has gases that are not found in our solar system, such as hydrogen chloride, phosphorus pentachloride, octane, and disulfur dichloride. Also there are no ice crystals in the atmosphere because it is too hot.

Scylla doesn't have a global cloud cover and this planet appears deep blue, although it has bands of noctilucent clouds around 5° north and south of the equator, another bands at 65° north and south of the equator, and significant cloud cover around the poles. These clouds are made of phosphorus pentachloride. These cloud bands are shaped by the planet's fast rotation and zonal jets. These clouds form from a localized concentrations of PCl5, which are produced by the chemical reactions between hydrogen chloride and phosphine using cosmic rays and ionized particles from the parent star as their catalysts.

5HCl + PH3 → PCl5 + 4H2

This jovian planet is cloudless as the mean temperature is 553 K (280°C or 536°F). Because there are no clouds over most of the planet, it may have only a light wind and no storms.

Magnetic field Edit

This planet has a strong magnetic field, one-and-a-half times stronger than Earth's. Even though this planet has no clouds, it can still produce auroras when the stellar activity is high.

Moons and rings Edit

Scylla has one moon and no rings. The moon, designated as Scylla I or HD 11964 c1 is a volcanic moon that orbits close to the planet, at a distance of 122,341 miles or 196,889 kilometers, which is two times closer to the planet than Moon's distance from Earth. This moon has mass 1.27 Lunar masses and has diameter 0.867 Lunar diameters (1,872 miles, 3,013 kilometers), corresponding to its density 6.54 g/cm³. This moon has gravity 0.280 g (2.75 m/s²). If you weigh 150 lbs on Earth, you'll weigh just 42 lbs on Scylla I. So the average weight for adults on Scylla I is as light as the average weight for 5 year olds on Earth!

The volcanism on the moon is caused by the strong tidal bulges exerted by the parent planet, just like Jupiter exerting a lot of tidal bulges on Io which causes volcanism.

Future studies Edit

The method will use to study Scylla might be direct imaging to see what this planet actually looks like. But direct imaging of this planet can be difficult to achieve because this planet orbits real close to its star, only 23% the distance between Earth and the Sun. Maybe looking for transits across the star would be a better idea, but it is speculated that this planet will not transit since the speculated inclination is 173°, which is almost face-on. Even with the difficulty of direct imaging and low probability of transit, the inclination of Scylla's orbit can still be constrained using astrometry from Gaia, James Webb Space Telescope (JWST), or Space Interoferometry Mission (SIM). Constraining the inclination is important for measuring its true mass.

Perhaps in about two decades, direct imaging of this planet can be achieved using the Advanced Technology Large-Aperture Space Telescope (ATLAST). The direct imaging can then constrain the size of this planet. After constraining its size, density and surface gravity can be calculated. Using the density of the planet, astronomers can probe the interior and estimate the mass and size of the core. Astronomers will also study the mantle and its temperature of the core using astroseismology. Using the spectrometer mounted on the ATLAST, it can constrain its temperature and study the chemical makeup of the atmosphere. Using the same method, the rotation rate can also be constrained using Doppler shifts. Using the rotation rate and circumference of the planet (calculated using 2π radius), rotation period can then be calculated. In orbit around the planet, moons can be detected using the transit across the planet, detecting the wobble of the planet, or even direct imaging. Rings can also be detected using just two methods: transit or direct imaging.

Related links Edit

  • Deino (HD 11964 b, P153)