Date September 12, 2011
Discoverers Mayor et al.
Detection method Radial velocity
Site La Silla Observatory
Name & designations
Pronunciation /'krom/
Adjective Cromian
Planet numbers P589, 47 Lupi P2, Lupus P5,
Simianus P51, 2011 P93,
2011 Lup-3, 2011 Sim-5
Star designations Nu² Lupi c, 47 Lupi c,
334 Simiani c, P4 Lupi c,
P46 Simiani c, BF 2659 c,
PH 468 c, HD 136352 c,
HIP 75181 c, HR 5699 c,
Gliese 582 c, SAO 225697 c
System 47 Lupi
Constellation Lupus
Caelregio Simianus
Right ascension 15h 21m 48.15s (230.450 62°)
Declination −48° 19' 03.5" (−48.317 63°)
Distance 14.812 pc (48.310 ly)
Orbital characteristics
Semimajor axis 0.178 446 AU (26.695 1 Gm)
Periastron 0.150 325 AU (22.488 4 Gm)
Apastron 0.206 566 AU (30.901 8 Gm)
Eccentricity 0.157 584 2
Orbital circumference 1.116 09 AU (166.965 Gm)
Orbital area 0.098 79 AU² (2 210.8 Gm²)
Orbital period 27.582 105 d (0.075 515 69 yr)
Avg. velocity 70.300 km/s (14.780 AU/yr)
Max. velocity 75.636 km/s (16.902 AU/yr)
Min. velocity 64.523 km/s (12.296 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 52.115° to ecliptic
−16.538° to star's equator
−19.968° to invariable plane
Argument of periastron 230.081°
Longitude of ascending node 21.374°
Longitude of periastron 251.455°
Angular separation 12.047 mas
Observing the parent star
Mean angular star size 3.117 23° (187.034')
Max. angular star size 3.700 34° (222.020')
Min. angular star size 2.692 87° (161.572')
Mean star magnitude −30.514
Max. star magnitude −30.886
Min. star magnitude −30.196
Bulk characteristics
Mean radius 2.997 2 R (19.095 Mm)
Equatorial radius 2.999 2 E (19.130 Mm)
Polar radius 2.993 0 P (19.026 Mm)
Mean circumference 119.977 Mm
Equatorial circumference 120.194 Mm
Polar circumference 119.543 Mm
Surface area 8.983 0 S (4 581.9 Mm²)
Volume 26.923 V (29 164 Mm³)
Flattening 0.005 43 (1:184.1)
Angular diameter 17.235 μas
Mass 14.399 2 M
Reciprocal mass
relative to star
23 040
Density 2.950 g/cm³
Gravitational influence
Surface gravity 1.603 g (15.72 m/s²)
Weight on Crom
(150 lb on Earth)
240 lb
Standard gravitational parameter 5.740 × 106 km³/s²
Escape velocity 24.52 km/s
Hill radius 1.483 LD (0.569 9 Gm)
Roche limit
(3 g/cm3 satellite)
0.062 23 LD (23.923 Mm)
Stationary orbit 2.393 39 LD (920.017 Mm)
Stationary velocity 2.476 km/s (0.557 LD/d)
Rotation characteristics
Rotation period 661.970 5 h (27.582 105 d)
Rotation velocity 182 kph (0.54°/h)
Direction of rotation
relative to orbit
Axial tilt 19.578°
Longitude of vernal equinox 175.482°
North pole right ascension 20h 34m 37s (308.656°)
North pole declination −13° 52' 47" (−13.880°)
North polar constellation Capricornus
North polar caelregio Tarandus
South pole right ascension 08h 34m 37s (128.656°)
South pole declination +13° 52' 47" (+13.880°)
South polar constellation Cancer
South polar caelregio Felis
Thermal characteristics
Surface temperature 648 K (375°C, 707°F, 1166°R)
Mean irradiance 41 202 W/m² (30.127 I)
Irradiance at periastron 58 058 W/m² (42.453 I)
Irradiance at apastron 30 748 W/m² (22.483 I)
Albedo 0.132 (bond), 0.156 (geom.)
Scale height 8.70 km
Volume 9.617 ae (40.27 Mm³)
Total mass 1.156 atmu (5.94 Eg)
Surface pressure 0.135 atm (13.6 kPa, 1.98 psi)
Surface density 0.147 g/m³
Molar mass 10.10 g/mol
Composition 49.113% hydrogen (H2)
21.446% helium (He)
17.705% nitrogen (N2)
7.406% oxygen (O2)
3.512% water (H2O)
0.571% argon (Ar)
0.114% methane (CH4)
728 ppm nitric oxide (NO)
356 ppm carbon dioxide (CO2)
207 ppm ammonia (NH3)
71.5 ppm carbon monoxide (CO)
9.58 ppm krypton (Kr)
212 ppb xenon (Xe)
21.4 ppb hydrogen deuteride (HD)
Dipole strength 22.7 nT (227 μG)
Magnetic moment 8.41 × 1016 T•m³
Dipole tilt 0.47°
Satellite system
Number of moons 0
Number of rings 0

Crom (47 Lupi c, P589) is an exoplanet which orbits the yellow-white F-type main sequence star 47 Lupi, similar to our Sun. It is approximately 48 light-years or 15 parsecs from Earth towards the constellation Lupus in the caelregio Simianus.

Crom is the middermost of the three known planets in 47 Lupi system. Crom is an ocean planet three times the size of the Earth. It is a midplanet massing 14.4 Earth masses. The planet takes nearly four weeks to orbit the star in a 5:4 tidal lock ratio.

Crom is named after Crom Cruach, a deity in pre-Christian Ireland, reputedly propitiated with human sacrifice, whose worship is said to have been ended by St. Patrick.

Discovery and chronology Edit

Crom was discovered on September 12, 2011 by a team of astronomers led by Michel Mayor. The team used the HARPS spectrometer mounted on the 3.6m ESO Telescope in La Silla Observatory, located in the Atacama Desert in Chile. The team discovered that 47 Lupi is wobbling in three different cycles simultaneously caused by the presence of three orbiting planets, including Crom. This wobble is relatively weak, which implies that all three orbiting planets are low-mass, either super-Earths or midplanets. Crom is one of 41 planets announced on September 12, 2011 and one of 84 found in that month, the monthly record.

Crom is the 581st exoplanet discovered overall, 555th since 2000, 201st since 2010, and 93rd in 2011. It is the 5th exoplanet discovered in the constellation Lupus (3rd in 2011) and 51st exoplanet discovered in the caelregio Simianus (5th in 2011). Since Crom is the second planet discovered in the 47 Lupi system, the planet receives the designations 47 Lupi c (a is not used because the parent star uses this letter to reduce confusion) and 47 Lupi P2. Note that the chronology does not include minimum-mass planets that are speculatively brown dwarfs.

Orbit and rotation Edit

Orbit Edit

Crom orbits the star at an average distance of 0.865 microparsecs or 0.178 AU, about two times closer to the star than Mercury is to the Sun. Crom has a semi-circular orbit with an eccentricity of 0.158. The planet takes 2.383 megaseconds or 27.58 days to orbit the star at an average velocity of 14.78 AU/yr (70.3 km/s, 43.7 mi/s). Irpa is in a 5:2 resonance with the innermost known planet Irpa and 1:4 resonance with the outermost known planet Prima.

Parent star observation and irradiance Edit

Viewed from Crom, 47 Lupi would have a magnitude −30.51, over 32 times brighter than the Sun seen from Earth. However, observers on Crom would not see light from 47 Lupi the same time as it emits, but it takes 89 seconds for light emitted from 47 Lupi to reach the planet. The parent star would have an angular diameter of 3.12° on average, 614 times the angular diameter of the full moon we sometimes see at night.

Since the planet orbits at only about one-sixth the Earth-Sun distance, Crom receives about 30 times more energy from its star than Earth receives from the Sun. The planet absorbs 87% of the incoming energy.

Rotation Edit

Crom is tidally locked, meaning the planet rotates every time when the planet orbits the star. Since the planet takes 27.58 days to orbit the star, then it would take 27.58 days to rotate once on its axis. So the year on Crom lasts exactly 1 day compared to 366 Earth days in an Earth year. The planet tilts 19.6° to the plane of its orbit, which is three quarters of the Earth's tilt of 23.4°. None of the planet's poles point to bright star. The north pole points to the constellation Capricornus (subdivision of the caelregio Tarandus) while the south pole points to Cancer (in Felis).

Structure and composition Edit

Mass and size Edit

Crom is a midplanet, massing 14.4 Earth masses, more massive than Uranus but less massive than Neptune. Crom is three times the size of the Earth with the radius of nearly 20 megameters. Still Crom is a little bit smaller than Uranus and Neptune, corresponding that this planet is around twice the density of either ice giants.

Gravitational influence Edit

The acceleration due to gravity is 15.7 m/s², which is 60% stronger than 9.8 m/s² acceleration due to Earth's gravity. So if you weigh 150 pounds on Earth, you would weigh 240 pounds on Crom, similar to the weight of a professional football player on Earth!

Based on its periastron distance and the mass ratio between planet and star, Crom's hill sphere radius is calculated to be about 1.5 LD. Within the hill sphere is where the orbit of satellites would be stable, outside it would be unstable. The region of orbit closest to the planet is the roche limit, where satellites break up via tidal forces. Moons with a density 3 g/cm³ would tear apart if it orbit within 0.062 LD. Denser moons would be required to orbit closer to the planet in order to break up, because denser moons are stronger and often have stronger gravity. A moon with the same density as Earth's would have to orbit within 0.051 LD in order to tear apart by tidal forces. Because Crom rotates so slowly, taking 22.07 days to complete a rotation because of the tidal forces of the star confining it to a 5:4 tidal lock ratio, the satellite would have to orbit far from the planet beyond the stable zone of its hill sphere for orbital period to be synchronized with the planet's rotation, called stationary orbit. The stationary orbit, analogous to the Earth's geostationary orbit, where its orbital period is 22.07 days, is calculated to be 2.4 LD, compared to nearly 0.1 LD for Earth's. If a satellite orbits at that distance, it would eventually escape the planet's orbit into the orbit around the star.

Interior Edit

With the density of 2.91 g/cm³, Crom has crust underneath deep oceans of water, mantle of exotic forms of water ice, lower mantle of diamond, and rocky core. Crom has similar structure to Tamar orbiting around 61 Virginis because these planets have similar densities. With the surface temperature of 598 K (325°C, 617°F, 1077°R), it is too hot to have normal liquid water on its surface but cool enough to have exotic form of liquid water under heavy pressure. All of the planet's surface is covered by liquid water with an estimated depth of 677 kilometers or 421 miles, a hundred times deeper than average ocean depth of Earth. The upper mantle composes of the exotic form of water ice called ice VII or "hot ice" with the radius of 6152 km or 3823 mi and a pressure 32.1 MPa. The lower mantle composes of diamond with the radius of 4125 km or 2563 mi and a pressure 61.7 MPa. At the center of this planet is a hot rocky core with a temperature of 6600 K and a pressure 1.15 GPa. The core has a radius of 8141 km or 5059 mi.

Diamond abundance Edit

In its lower mantle, Crom has an estimated 76 million times more diamond than Earth has! There would be enough diamond to build cities covering about 75% the surface area of this planet with every building and appliances made of diamond!

Atmosphere Edit

Crom has atmosphere about 17 the thickness of the Earth's and about 22 times thicker than Mars'. About half of all gases in the atmosphere is hydrogen. Helium makes up the same portion of the atmosphere as oxygen on Earth, while there are 13 as much oxygen as Earth's. Nitrogen makes up one-quarter the proportion of Earth's. Water vapor makes up 3.5% of the atmosphere, compared to 1% for Earth's depending on climate. Carbon dioxide makes up 356 ppm, identical to the concentration of CO2 in Earth's atmosphere in 1993 and slightly less than approximately 400 ppm for Earth's atmosphere at present.

Clouds are virtually nonexistent on Crom because there are no chemicals suitable for cloud formation at the temperature of around 600 K.

Magnetic field Edit

Crom has an extremely weak magnetic field, about 230 millionths of a gauss or 23 nanoteslas, which is about 1300 times weaker than Earth's. The reason for its weakness is because the planet rotates so slowly because it is partially locked to its star. Because the magnetic field is so weak, stellar radiation and cosmic rays bombard the surface almost constantly.

Moons and rings Edit

Crom has no moons nor rings.

Future studies Edit

Transits Edit

It is speculated that Crom will not transit since Crom's orbit is slanted diagonally. If Crom does transit, its signal can be found with some effort as Crom takes 27.6 days to orbit the star. Transit is useful for determining its size and inclination of this planet. The derivative parameters, including density and surface gravity, can then be calculated using the radius constrained from transit and true mass calculated by inclination. Using the calculated density, astronomers can model the interior of this planet.

Astrometry Edit

If Crom does not transit, as speculated, then this planet can still be studied using different methods, such as astrometry. This method can be used to study this planet using Gaia (launched in December 2013) and James Webb Space Telescope (JWST, to be launched around 2018), or even the current Hubble Space Telescope (HST) guidance sensor. However, this planet would be too small and orbits too close to the star for even Gaia and JWST to be studied astrometrically.

Direct imaging Edit

The direct imaging can see what the planet may really look like. But directly imaging this planet would be extremely difficult because it orbits only 0.18 AU (within the glare of its star). The angular separation between the planet and the star is 12 milliarcseconds. ATLAST (to be launched between 2025–35) may be able to image Crom and other planets in the 47 Lupi system.

Astroseismology and spectroscopy Edit

Astronomers may eventually use astroseismology to study the interior, including the extent, features and compositions by layers. Using the spectrometer mounted on the JWST, the atmosphere can be studied, including temperatures, chemical makeup, and features. Using the same method, the rotation rate can be constrained using Doppler shifts, which in turn rotation period can then be calculated.

Detecting moons and rings Edit

Moons transiting Crom can reliably be detected while the planet transit its star if it does so.

Related links Edit

  • Irpa (47 Lupi b, P588)
  • Prima (47 Lupi d, P590)