Irpa

Irpa (47 Lupi b, P588) is an which orbits the yellow-white   , similar to our. It is approximately 48 s or 15 s from towards the   in the caelregio Simianus.

Irpa is the innermost of the three known planets in 47 Lupi system. Irpa is a 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.

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

Discovery and chronology
Irpa was discovered on September 12, 2011 by a team of astronomers led by. The team used the  mounted on the  in, located in the  in. 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 580$⊕$ exoplanet discovered overall, 554$⊕$ since 2000, 200$⊕$ since 2010, and 92$⊕$ in 2011. Irpa is the 4$⊕$ exoplanet discovered in the constellation Lupus (2$⊕$ in 2011) and 50$⊕$ exoplanet discovered in the caelregio Simianus (4$⊕$ 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 planets that are speculatively s.

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

Parent star observation and irradiance
Viewed from Irpa, 47 Lupi would have a −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 of 5.56° on average, which is 11 times the angular diameter of the  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
Irpa is, 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 (subdivision of the caelregio Avis) while the south pole points to  (in Malus).

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

Gravitational influence
The gravitational force of Irpa is 52% stronger than Earth's. So if you weigh 150 on Earth, you would weigh 228 pounds on Irpa.

Based on its periastron distance and the mass ratio between planet and star, Irpa's radius is calculated to be about 0.66. 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, 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, where its orbital period is 11.58 days, is calculated to be 1.07 LD, slightly beyond the orbit of the around the Earth and 11 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
Like many s, Irpa has the crust, mantle, and core. The crust is made out of rocks while mantle is made of molten rocks or. 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. 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 with small amounts of, , and.

Surface
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
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 on Irpa with many es constantly erupting. The tidal forces of the star exerted on Irpa causing intense volcanism is analogous to the tidal forces of exerted on its moon, which causes Io to have the most volcanically-active world in our. 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 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 (, P311), discovered in early 2009 by, which spectroscopically revealed it to be a volcanically-active world.

Atmosphere
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 s, which is 192 times thinner than and slightly thinner than.

Like and, Irpa's atmosphere is made mostly of  (CO$⊕$), making up merely 95.1% of the atmosphere. All of the CO$2$ are given off by active volcanoes. Most of the remaining atmosphere is made of (N$2$), making up 87% of the remaining atmosphere. This atmosphere does contain small amounts of life-giving (O$2$) at 126  and  (H$2$O) at 5 ppm. The atmosphere also contains 0.25% given off by volcanoes, even though SO$2$ can quickly decomposes by intense radiation from the nearby star, but thick clouds of  shield SO$th$ from radiation. Volcanoes erupting continuously over much of the planet replenish SO$th$ at the same rate as SO$th$ decomposing.

Magnetic field
Irpa has an extremely weak, about 190 millionths of a or 19 s, which is about 1600 times weaker than. 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
Because Irpa orbits so close to its star, keeping the small, Irpa has no moons nor rings. But if moons actually exist, they have to orbit within 0. LD from the planet or they'll flung off into space.

Transits
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
If Irpa does not transit, as speculated, then this planet can still be studied using different methods, such as. This method can be used to study this planet using (launched in December 2013) and  (JWST, to be launched around 2018), or even the current  (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
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 between the planet and the star is 6.7 s.  (to be launched between 2025–35) may be able to image Irpa and other planets in the 47 Lupi system.

Astroseismology and spectroscopy
Astronomers may eventually use to study the interior, including the extent, features and compositions by layers. Using the 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 s, which in turn can then be calculated.

Detecting moons and rings
Moons transiting Irpa can reliably be detected while the planet transit its star if it does so.

Related links

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