Date September 12, 2011
Discoverers Mayor et al.
Detection method Radial velocity
Site La Silla Observatory
Name & designations
Pronunciation /'prē•mä/
Adjective Primian
Planet numbers P590, 47 Lupi P3, Lupus P6,
Simianus P52, 2011 P94,
2011 Lup-4, 2011 Sim-6
Star designations Nu² Lupi d, 47 Lupi d,
334 Simiani d, P4 Lupi d,
P46 Simiani d, BF 2659 d,
PH 468 d, HD 136352 d,
HIP 75181 d, HR 5699 d,
Gliese 582 d, SAO 225697 d
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 0.439 803 AU (65.793 6 Gm)
Periastron 0.249 733 AU (37.359 6 Gm)
Apastron 0.629 873 AU (94.227 6 Gm)
Eccentricity 0.432 170 1
Orbital circumference 2.664 24 AU (398.565 Gm)
Orbital area 0.547 99 AU² (12 263.7 Gm²)
Orbital period 106.722 378 d (0.292 189 95 yr)
Avg. velocity 43.371 km/s (9.118 AU/yr)
Max. velocity 51.904 km/s (9.291 AU/yr)
Min. velocity 32.682 km/s (8.942 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 40.052° to ecliptic
8.997° to star's equator
7.441° to invariable plane
Argument of periastron 173.844°
Longitude of ascending node 44.145°
Longitude of periastron 217.988°
Angular separation 29.692 mas
Observing the parent star
Mean angular star size 1.264 78° (75.887')
Max. angular star size 2.227 40° (133.644')
Min. angular star size 0.883 12° (52.987')
Mean star magnitude −28.555
Max. star magnitude −29.784
Min. star magnitude −27.775
Bulk characteristics
Mean radius 3.638 8 R (23.183 Mm)
Equatorial radius 3.644 9 E (23.247 Mm)
Polar radius 3.626 8 P (23.055 Mm)
Mean circumference 145.664 Mm
Equatorial circumference 146.068 Mm
Polar circumference 144.856 Mm
Surface area 13.241 0 S (6 753.8 Mm²)
Volume 48.182 V (52 191 Mm³)
Flattening 0.008 32 (1:120.2)
Angular diameter 20.925 μas
Mass 14.943 0 M
Reciprocal mass
relative to star
22 201
Density 1.710 g/cm³
Gravitational influence
Surface gravity 1.129 g (11.07 m/s²)
Weight on Prima
(150 lb on Earth)
169 lb
Standard gravitational parameter 5.957 × 106 km³/s²
Escape velocity 22.67 km/s
Hill radius 2.494 LD (0.958 6 Gm)
Roche limit
(3 g/cm3 satellite)
0.063 01 LD (24.220 Mm)
Stationary orbit 3.109 75 LD (1 195.384 Mm)
Stationary velocity 2.214 km/s (0.498 LD/d)
Rotation characteristics
Rotation period 960.501 4 h (40.020 892 d)
Rotation velocity 152 kph (0.37°/h)
Direction of rotation
relative to orbit
Axial tilt 13.070°
Longitude of vernal equinox 225.433°
North pole right ascension 15h 21m 43s (230.427°)
North pole declination +54° 18' 19" (+54.305°)
North polar constellation Draco
North polar caelregio Avis
South pole right ascension 03h 21m 43s (50.427°)
South pole declination −54° 18' 19" (−54.305°)
South polar constellation Horologium
South polar caelregio Selachimorphus
Thermal characteristics
Surface temperature 420 K (147°C, 296°F, 756°R)
Mean irradiance 6 783 W/m² (4.960 I)
Irradiance at periastron 21 037 W/m² (15.382 I)
Irradiance at apastron 3 307 W/m² (2.418 I)
Albedo 0.160 (bond), 0.158 (geom.)
Scale height 7.13 km
Volume 12.145 ae (50.86 Mm³)
Total mass 1.533 atmu (7.88 Eg)
Surface pressure 0.247 atm (25.1 kPa, 3.64 psi)
Surface density 0.155 g/m³
Molar mass 11.89 g/mol
Composition 55.794% hydrogen (H2)
29.315% oxygen (O2)
10.759% helium (H2O)
3.118% water vapor (H2S)
1.724% argon (Ar)
0.276% carbon monoxide (CO)
845 ppm krypton (Kr)
403 ppm nitrogen (N2)
277 ppm methane (CH4)
37.0 ppm carbon dioxide (CO2
18.8 ppm octane (C8H18)
18.7 ppm ammonia (NH3)
572 ppb phosphorus pentachloride (PCl5)
376 ppb neon (Ne)
Dipole strength 4.21 nT (42.1 μG)
Magnetic moment 4.22 × 1014 T•m³
Dipole tilt 2.71°
Satellite system
Number of moons 0
Number of rings 0

Prima (47 Lupi d, P590) 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.

Prima is the outermost of the three known planets in 47 Lupi system. Prima is an ocean planet nearly 3⅔ times the size of the Earth. It is a midplanet massing 14.9 Earth masses. The planet takes over 15 weeks to orbit the star in a 2:1 tidal lock ratio.

Prima is named after the goddess.

Discovery and chronology Edit

Prima 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 Prima. This wobble is relatively weak, which implies that all three orbiting planets are low-mass, either super-Earths or midplanets. Prima is one of 41 planets announced on September 12, 2011 and one of 84 found in that month, the monthly record.

Prima is the 582nd exoplanet discovered overall, 556th since 2000, 202nd since 2010, and 94th in 2011. It is the 6th exoplanet discovered in the constellation Lupus (4th in 2011) and 52nd exoplanet discovered in the caelregio Simianus (6th in 2011). Since Prima is the third planet discovered in the 47 Lupi system, the planet receives the designations 47 Lupi d (a is not used because the parent star uses this letter to reduce confusion) and 47 Lupi P3. Note that the chronology does not include minimum-mass planets that are speculatively brown dwarfs.

Orbit and rotation Edit

Orbit Edit

Prima orbits the star at an average distance of 2.13 microparsecs or 0.44 AU, slightly beyond the orbit of Mercury. Prima orbits eccentrically with twice the eccentricity of Mercury's. Prima's distance from the star ranges from 1.21 μpc (0.25 AU) to 3.05 μpc (0.63 AU). The planet takes 9.221 megaseconds or 106.72 days to orbit the star at an average velocity of 9.12 AU/yr (43.4 km/s, 26.9 mi/s). Prima is in a 4:1 resonance with the middle known planet Crom and 9:1 resonance with the innermost planet Irpa.

Parent star observation and irradiance Edit

Viewed from Prima, 47 Lupi would have a magnitude −28.55, over 1.8 times brighter than the Sun seen from Earth. However, observers on Prima would not see light from 47 Lupi the same time as it emits, but it takes about 3.6 minutes for light emitted from 47 Lupi to reach the planet. The parent star would have an angular diameter of 1.26° on average, which is 2½ times the angular diameter of the full moon we sometimes see at night.

Prima receives on average five times more energy from its star than Earth receives from the Sun, and it varies from over 3300 W/m² to over 21000 W/m².

Rotation Edit

Prima is in a 8:3 tidal lock ratio, meaning every time the planet orbits the star thrice, it completed its rotation eight times. Since the planet takes 106.72 days to orbit the star, then it would take 40.02 days to rotate once on its axis. So the year on Prima lasts exactly 2.6 days compared to 366 Earth days in an Earth year. The planet tilts 13.1° to the plane of its orbit, compared to the Earth's 23.4° tilt. The north pole points to the constellation Draco (subdivision of the caelregio Avis) while the south pole points to Horologium (in Selachimorphus).

Structure and composition Edit

Mass and size Edit

Prima is a midplanet, massing 14.9 Earth masses, more massive than Uranus but less massive than Neptune. Prima is over 3.64 times the size of the Earth with the radius of over 23 megameters, slightly smaller than Uranus and Neptune, corresponding that this planet is slightly denser than either ice giants.

Gravitational influence Edit

The acceleration due to gravity is 11.1 m/s², which is 13% stronger than 9.8 m/s² acceleration due to Earth's gravity. So if you weigh 150 pounds on Earth, you would weigh just 19 more pounds on Prima than your weight on Earth at 169 pounds.

Based on its periastron distance and the mass ratio between planet and star, Prima's hill sphere radius is calculated to be about 2.5 LD. Within the hill sphere is where the orbit of satellites would be stable, outside of 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 0.063 LD. Denser moons would be required to orbit closer to the planet in order to break up. A 5 g/cm³ moon would have to orbit within 0.053 LD in order to tear apart by tidal forces. Because Prima rotates so slowly caused by tidal forces of the parent star, taking 53.36 days to complete a rotation, 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 53.36 days, is calculated to be 319 LD, compared to nearly one-tenth of a lunar distance 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 1.71 g/cm³, Prima has no crust but deep oceans of water, mantle of exotic forms of water ice, lower mantle of diamond, and rocky core. Its structure is similar to Tuonetar orbiting around 61 Virginis because these planets have similar densities. Despite the surface temperature of 381 K (108°C, 226°F, 686°R), slightly above the boiling point of ordinary water at 373 K (100°C, 212°F, 672°R), all of the planet's surface is covered by liquid water with an estimated depth of 1303 kilometers or 810 miles, approximately two 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 8377 km or 5205 mi under the pressure of 43.8 MPa. The lower mantle composes of diamond with the radius of 4708 km or 2925 mi under the pressure of 74.0 MPa. At the center of this planet is a hot rocky core with a temperature of 6900 K and a pressure 1.23 GPa. The core has a radius of 8793 km or 5464 mi.

Diamond abundance Edit

In its lower mantle, Prima has an estimated 133 million times more diamond than Earth has! There would be enough diamond to build cities covering about 80% the surface area of this planet with every building and appliances made of diamond!

Atmosphere Edit

Prima has one-quarter the thickness of the atmosphere as Earth's and about 41 times thicker than Mars'. More than half of all gases in the atmosphere is hydrogen with considerable amounts of helium. The second most abundant gas is oxygen, making up 29% of the atmosphere, compared to 21% of the Earth's atmosphere. Water vapor makes up 3.1% of the atmosphere, compared to 1% for Earth's depending on climate. Carbon dioxide makes up 37 ppm, less than 110 the proportion in the Earth's atmosphere. The atmosphere contains merely 0.276% carbon monoxide. This gas must be produced when a lot of oxygen reacts with methane incompletely caused by the presence of water vapor.

The planet is virtually cloudless most of the times except near apastron when there are few hydrogen sulfide clouds.

Magnetic field Edit

Prima has an extremely weak magnetic field, about 42 millionths of a gauss or 4.2 nanoteslas, which is about 350 times weaker than Earth's. The reason for its weakness is because the planet rotates so slowly because it orbits the star in a 2:1 tidal lock ratio. Because the magnetic field is so weak, stellar radiation and cosmic rays bombard the surface almost constantly.

Moons and rings Edit

Prima has no moons nor rings.

Future studies Edit

Transits Edit

It is speculated that Prima will not transit since the inclination of Prima is about 40°. If Prima does transit, its signal can be found with considerable effort because Prima only transits once every 107 days, which is the planet's orbital period. 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 Prima 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 super difficult because it orbits only 0.44 AU, well within the glare of its star. The angular separation between the planet and the star is 29.7 milliarcseconds. ATLAST (to be launched between 2025–35) may be able to image Prima 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 Prima can reliably be detected while the planet transit its star if it does so.

Related links Edit

  • Irpa (47 Lupi b, P588)
  • Crom (47 Lupi c, P589)