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Date September 12, 2011
Discoverers Mayor et al.
Detection method Radial velocity
Site La Silla Observatory
Name & designations
Pronunciation /'ër•pä/
Adjective Irpian
Planet numbers P588, 47 Lupi P1, Lupus P4,
Simianus P50, 2011 P92,
2011 Lup-2, 2011 Sim-4
Star designations Nu² Lupi b, 47 Lupi b,
334 Simiani b, P4 Lupi b,
P46 Simiani b, BF 2659 b,
PH 468 b, HD 136352 b,
HIP 75181 b, HR 5699 b,
Gliese 582 b, SAO 225697 b
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 0.100 032 AU (14.964 6 Gm)
Periastron 0.082 261 AU (12.306 1 Gm)
Apastron 0.117 804 AU (17.623 2 Gm)
Eccentricity 0.177 655 4
Orbital circumference 0.624 87 AU (93.479 Gm)
Orbital area 0.030 94 AU² (692.3 Gm²)
Orbital period 11.576 542 d (0.031 694 84 yr)
Avg. velocity 93.776 km/s (19.715 AU/yr)
Max. velocity 101.766 km/s (21.264 AU/yr)
Min. velocity 85.039 km/s (18.033 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 43.824° to ecliptic
33.112° to star's equator
57.412° to invariable plane
Argument of periastron 199.087°
Longitude of ascending node 198.785°
Longitude of periastron 37.872°
Angular separation 6.753 mas
Observing the parent star
Mean angular star size 5.560 76° (333.645')
Max. angular star size 6.762 08° (405.725')
Min. angular star size 4.721 89° (283.313')
Mean star magnitude −31.771
Max. star magnitude −32.195
Min. star magnitude −31.415
Bulk characteristics
Mean radius 2.237 2 R (14.253 Mm)
Equatorial radius 2.236 0 E (14.261 Mm)
Polar radius 2.239 6 P (14.237 Mm)
Mean circumference 89.555 Mm
Equatorial circumference 89.607 Mm
Polar circumference 89.451 Mm
Surface area 5.005 0 S (2 552.9 Mm²)
Volume 11.197 V (12 129 Mm³)
Flattening 0.001 74 (1:574.5)
Angular diameter 12.865 μas
Mass 7.597 5 M
Reciprocal mass
relative to star
43 667
Density 3.742 g/cm³
Gravitational influence
Surface gravity 1.518 g (14.89 m/s²)
Weight on Irpa
(150 lb (1 wa) on Earth)
228 lb (1.52 wa)
Standard gravitational parameter 3.028 × 106 km³/s²
Escape velocity 20.61 km/s
Hill radius 0.656 LD (0.252 0 Gm)
Roche limit
(3 g/cm3 satellite)
0.050 29 LD (19.331 Mm)
Stationary orbit 1.069 60 LD (411.152 Mm)
Stationary velocity 2.672 km/s (0.601 LD/d)
Rotation characteristics
Rotation period 277.837 0 h (11.576 542 d)
Rotation velocity 323 kph (1.30°/h)
Direction of rotation
relative to orbit
Axial tilt 3.633°
Longitude of vernal equinox 337.427°
North pole right ascension 20h 48m 45s (312.186°)
North pole declination +44° 44' 49" (+44.747°)
North polar constellation Cygnus
North polar caelregio Avis
South pole right ascension 08h 48m 45s (132.186°)
South pole declination −44° 44' 49" (−44.747°)
South polar constellation Vela
South polar caelregio Malus
Thermal characteristics
Surface temperature 868 K (595°C, 1103°F, 1563°R)
Mean irradiance 131 113 W/m² (95.871 I)
Irradiance at periastron 193 883 W/m² (141.769 I)
Irradiance at apastron 94 539 W/m² (69.128 I)
Albedo 0.245 (bond), 0.241 (geom.)
Scale height 31.97 km
Volume 19.527 ae (81.77 Mm³)
Total mass 1.417 atmu (7.28 Eg)
Surface pressure 0.005 20 atm (0.527 kPa,
0.076 4 psi)
Surface density 0.089 g/m³
Molar mass 43.40 g/mol
Composition 95.120% carbon dioxide (CO2)
4.242% nitrogen (N2)
0.550% carbon monoxide (CO)
0.250% sulfur dioxide (SO2)
415 ppm argon (Ar)
189 ppm neon (Ne)
126 ppm oxygen (O2)
71.0 ppm krypton (Kr)
4.71 ppm water (H2O)
106 ppb xenon (Xe)
Dipole strength 18.8 nT (188 μG)
Magnetic moment 6.45 × 1016 T•m³
Dipole tilt 0.22°
Satellite system
Number of moons 0
Number of rings 0

Irpa (47 Lupi b, P588) 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.

Irpa is the innermost of the three known planets in 47 Lupi system. Irpa is a lava planet 2¼ times the size of the Earth. It is a super-Earth massing 7.6 times that of the Earth. The planet takes just 1½ weeks to orbit the star and it is tidally locked.

Irpa is named after the Norse goddess who is the sister of Þorgerðr Hölgabrúðr.

Discovery and chronology Edit

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

Irpa is the 580th exoplanet discovered overall, 554th since 2000, 200th since 2010, and 92nd in 2011. Irpa is the 4th exoplanet discovered in the constellation Lupus (2nd in 2011) and 50th exoplanet discovered in the caelregio Simianus (4th in 2011). Since Irpa is the first planet discovered in the 47 Lupi system, the planet receives the designations 47 Lupi b (a is not used because the parent star uses this letter to reduce confusion) and 47 Lupi P1. Note that the chronology does not include minimum-mass planets that are speculatively brown dwarfs.

Orbit and rotation Edit

Orbit Edit

Irpa orbits the star at an average distance of 0.485 microparsecs or 0.1 AU. Irpa has a semi-circular orbit with an eccentricity of 0.1777. The planet takes 11.58 days or almost exactly one megasecond to make one complete trip around the star at an average velocity of 19.72 AU/yr (93.8 km/s, 58.3 mi/s). Irpa is in a 2:5 resonance with the middle known planet Crom and 1:9 resonance with the outermost known planet Prima.

Parent star observation and irradiance Edit

Viewed from Irpa, 47 Lupi would have a magnitude −31.77, over a hundred times brighter than the Sun seen from Earth. However, observers on Irpa would not see light from 47 Lupi the same time as it emits, but it takes 50 seconds for light emitted from 47 Lupi to reach the planet. The parent star would have an angular diameter of 5.56° on average, which is 11 times the angular diameter of the full moon we sometimes see at night.

Irpa receives 96 times more energy from its star than Earth receives from the Sun, because it orbits at one-tenth the Earth-Sun distance from the energy source. The amount of energy received from the star is inversely proportional to the square of the star-planet distance. Every square meter of the surface, 131 kilowatts worth of energy is received.

Rotation Edit

Irpa is tidally locked, meaning the planet's rotation is synchronized to its orbital period caused by tidal forces of its nearby sun. Since the planet takes 11.58 days to orbit the star, then it would also take 11.58 days to rotate once on its axis. So the year on Irpa lasts exactly one day compared to 366 Earth days in an Earth year. The planet tilts 3.6° to the plane of its orbit, much less than the Earth's 23.4° tilt. The north pole points to the constellation Cygnus (subdivision of the caelregio Avis) while the south pole points to Vela (in Malus).

Structure and composition Edit

Mass and size Edit

Irpa is a super-Earth, massing 7.6 Earth masses and radius 14,253 kilometers or 2.2 Earth radii. Irpa has a density of 3.74 g/cm³, which is relatively low for a rocky planet, slightly less dense than Mars. Based on its density, Irpa would have a small core.

Gravitational influence Edit

The gravitational force of Irpa is 52% stronger than Earth's. So if you weigh 150 pounds (1 wame) on Earth, you would weigh 228 pounds (1.52 wames) on Irpa.

Based on its periastron distance and the mass ratio between planet and star, Irpa's hill sphere radius is calculated to be about 0.66 LD. If a moon orbits inside the hill sphere, the orbit would be stable; if outside, orbit around the planet would be unstable and eventually end up in orbit around the star. 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 0.050 LD. Denser moons would be required to orbit closer to the planet in order to break up, and vice versa. A 5 g/cm³ moon would have to orbit within 0.042 LD while a 1 g/cm³ moon orbit within 0.072 LD in order to tear apart by tidal forces. Because Irpa rotates so slowly because it is tidally locked to its parent star, 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 11.58 days, is calculated to be 1.07 LD, slightly beyond the orbit of the Moon around the Earth and 1112 times further out than Earth's geostationary orbit. If a satellite orbits at that distance, it would eventually escape the planet's orbit into the orbit around the star.

Interior Edit

Like many terrestrial planets, Irpa has the crust, mantle, and core. The crust is made out of rocks while mantle is made of molten rocks or magma. At the center of this planet is a hot solid pressurized core with a temperature of 8900 K (8600°C, 15500°F) and a pressure 5.79 GPa. Based on the planet's density of 3.73 g/cm³, the radius of the core would estimated to be 6.2% the radius of the planet or about 884 kilometers. The core is made dominantly of iron with small amounts of sulfur, magnesium, and nickel.

Surface Edit

Like all other terrestrial planets, Irpa has a solid surface, but much of it is covered in lava. There are few terrains like hills, mountains, canyons, ridges, and plateaus.

Volcanism Edit

Since Irpa is several times more massive than Earth and orbiting very close to the star, the tidal forces of the star would cause planet to stretch and squeeze constantly, causing friction, releasing heat in the process. The released heat would melt rocks into magma. The magma melted by tidal forces causes intense volcanism on Irpa with many volcanoes constantly erupting. The tidal forces of the star exerted on Irpa causing intense volcanism is analogous to the tidal forces of Jupiter exerted on its moon Io, which causes Io to have the most volcanically-active world in our solar system. So Irpa would be the massive version of Io, known as "Super-Io." Perhaps, Irpa would be even more volcanically-active world than Io since the tidal forces of the star is lot stronger than Jupiter. With the surface temperature of 868 K (595°C, 1103°F, 1563°R), which is hotter than Venus, it is hot enough for lava to cover much of the surface for long-periods of time.

The only known example of a Super-Io is Icarus (COROT-7b, P311), discovered in early 2009 by transit, which spectroscopically revealed it to be a volcanically-active world.

Atmosphere Edit

Since this is a low-mass planet orbiting very close to the star, Irpa has very little atmosphere because gases are constantly stripping away by the radiation from the star. Irpa has the atmospheric pressure of just 527 pascals, which is 192 times thinner than Earth's and slightly thinner than Mars'.

Like Venus and Mars, Irpa's atmosphere is made mostly of carbon dioxide (CO2), making up merely 95.1% of the atmosphere. All of the CO2 are given off by active volcanoes. Most of the remaining atmosphere is made of nitrogen (N2), making up 87% of the remaining atmosphere. This atmosphere does contain small amounts of life-giving oxygen (O2) at 126 ppm and water vapor (H2O) at 5 ppm. The atmosphere also contains 0.25% sulfur dioxide given off by volcanoes, even though SO2 can quickly decomposes by intense radiation from the nearby star, but thick clouds of sulfuric acid shield SO2 from radiation. Volcanoes erupting continuously over much of the planet replenish SO2 at the same rate as SO2 decomposing.

Magnetic field Edit

Irpa has an extremely weak magnetic field, about 190 millionths of a gauss or 19 nanoteslas, which is about 1600 times weaker than Earth's. The reason for its weakness is because the planet rotates so slowly because it is tidally 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

Because Irpa orbits so close to its star, keeping the hill sphere small, Irpa has no moons nor rings. But if moons actually exist, they have to orbit within 0.6 LD from the planet or they'll flung off into space.

Future studies Edit

Transits Edit

Speculatively, Irpa will not transit since Irpa's orbit is slanted diagonally. If Irpa does transit, its signal can be found with little effort as Irpa only takes 11.6 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 Irpa 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 incredibly difficult because it orbits only 0.1 AU (within the glare of its star). The angular separation between the planet and the star is 6.7 milliarcseconds. ATLAST (to be launched between 2025–35) may be able to image Irpa 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 Irpa can reliably be detected while the planet transit its star if it does so.

Related links Edit

  • Crom (47 Lupi c, P589)
  • Prima (47 Lupi d, P590)
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