|Date||October 16, 2012|
|Discoverers||Dumusque et al.|
|Detection method||Radial velocity (HARPS)|
|Site||La Silla Observatory|
|Name & designations|
|Planet numbers|| P832, α² Centauri P1,|
Centauri P16, Simianus P63,
2012 P143, 2012 Cen-1,
|Star designations|| Rigil Kentaurus Bb, α² Centauri b,|
175 Centauri Bb, α² Simiani b,
278 Simiani Bb, BF 1213 b,
PH 620 b, P14 Centauri b,
P49 Simiani b, HD 128621 b,
HIP 71681 b, HR 5460 b,
Gliese 559 Bb, SAO 252838 b
|System||Alpha Centauri B|
|Right ascension||14h 39m 35.08s (219.896 17°)|
|Declination||−60° 50' 13.8" (−60.836 94°)|
|Distance||1.348 pc (4.395 ly)|
|Semimajor axis||0.041 852 AU (6.261 0 Gm)|
|Periastron||0.040 052 AU (5.991 7 Gm)|
|Apastron||0.043 653 AU (6.530 3 Gm)|
|Eccentricity||0.043 018 2|
|Orbital circumference||0.262 88 AU (39.325 7 Gm)|
|Orbital area||0.005 497 7 AU² (123.037 Gm²)|
|Orbital period||3.235 674 d (0.008 858 79 yr)|
|Avg. velocity||141.146 km/s (29.674 AU/yr)|
|Max. velocity||144.150 km/s (30.078 AU/yr)|
|Min. velocity||138.077 km/s (29.264 AU/yr)|
| Direction of orbit|
relative to star's rotation
|Inclination|| 62.193° to ecliptic|
−9.578° to star's equator
−2.754° to invariable plane
|Argument of periastron||91.882°|
|Longitude of ascending node||250.134°|
|Longitude of periastron||342.017°|
|Angular separation||31.057 mas|
|Observing the parent star|
|Mean angular star size||11.047 50° (662.850')|
|Max. angular star size||11.544 11° (692.647')|
|Min. angular star size||10.591 86° (635.512')|
|Mean star magnitude||−32.766|
|Max. star magnitude||−32.862|
|Min. star magnitude||−32.675|
|Mean radius||1.178 1 R⊕ (7.506 Mm)|
|Equatorial radius||1.177 9 E⊕ (7.513 Mm)|
|Polar radius||1.178 5 P⊕ (7.492 Mm)|
|Mean circumference||47.161 Mm|
|Equatorial circumference||47.205 Mm|
|Polar circumference||47.072 Mm|
|Surface area||1.388 0 S⊕ (707.96 Mm²)|
|Volume||1.635 2 V⊕ (1 771.3 Mm³)|
|Flattening||0.002 81 (1:355.5)|
|Angular diameter||74.464 μas|
|Mass||1.279 9 M⊕|
| Reciprocal mass|
relative to star
|Surface gravity||0.922 g (9.04 m/s²)|
| Weight on Ixionidae|
(150 lb (1 wa) on Earth)
|138 lb (0.92 wa)|
|Standard gravitational parameter||5.102 × 105 km³/s²|
|Escape velocity||11.66 km/s|
|Hill radius||0.180 LD (0.069 2 Gm)|
| Roche limit|
(3 g/cm3 satellite)
|0.027 77 LD (10.676 Mm)|
|Stationary orbit||0.241 74 LD (92.925 Mm)|
|Stationary velocity||2.257 km/s (0.507 LD/d)|
|Rotation period||77.656 2 h (3.235 674 d)|
|Rotation velocity||608 kph (4.64°/h)|
| Direction of rotation|
relative to orbit
|Longitude of vernal equinox||180.048°|
|North pole right ascension||22h 01m 55s (330.479°)|
|North pole declination||+00° 51' 50" (+0.864°)|
|North polar constellation||Aquarius|
|North polar caelregio||Hippocampus|
|South pole right ascension||10h 01m 55s (150.479°)|
|South pole declination||−00° 51' 50" (−0.864°)|
|South polar constellation||Sextans|
|South polar caelregio||Felis|
|Surface temperature||1033 K (760°C, 1400°F, 1860°R)|
|Mean irradiance||386 171 W/m² (282.371 I⊕)|
|Irradiance at periastron||421 670 W/m² (308.328 I⊕)|
|Irradiance at apastron||354 974 W/m² (259.560 I⊕)|
|Albedo||0.189 (bond), 0.161 (geom.)|
|Scale height||77.56 km|
|Volume||13.269 ae (55.56 Mm³)|
|Total mass||0.034 2 atmu (0.176 Eg)|
|Surface pressure|| 4.38 × 10−6 atm (444 mPa,|
6.44 × 10−5 psi)
|Surface density||0.000 317 g/m³|
|Molar mass||37.68 g/mol|
|Composition|| 51.332% CO2, 26.511% Na,|
21.663% K, 0.265% Kr,
0.223% Xe, 38.8 ppm Hg2,
23.1 ppm CO, 854 ppb SO2,
65.0 ppb H2O
|Dipole strength||0.231 μT (2.31 mG)|
|Magnetic moment||8.77 × 1017 T•m³|
|Number of moons||0|
|Number of rings||0|
Ixionidae (Alpha Centauri Bb, P832) is the nearest known exoplanet, located just 4.4 light-years from Earth in orbit around Alpha Centauri B. With the minimum mass of 1.13 Earth masses, it is the lightest confirmed exoplanet discovered by RV. Because planet orbits so close to the heat and tidal forces of the star, much of the planet's surface are covered in lava spewed out by many volcanoes erupting at the same time. Planet orbiting the star next door would be among the first to be explored by interstellar probes.
Ixionidae was named after the synonym for mythological creature centaur, because it is the most interesting planet in Centaurus to date. On April 26, 2013, voters submitting to Uwingu won Albertus Alauda as the name. However, I will not use that name since planets are traditionally named after gods and other aspects in mythology.
Ixionidae was discovered on October 16, 2012 after four years and 459 observations of Alpha Centauri B. The observations was carried out by using HARPS echelle spectrograph in La Silla Observatory located in Atacama Desert, Chile. Astronomers determine that the star caused by the tug of planet move at only 51 cm/s, which is the lowest semi-amplitude ever measured. For comparison, Earth exerting on the Sun has a semi-amplitude of 9 cm/s, about one-sixth of that value. Stellar wobbling has a revolution period of 3.236 days, corresponding to the planet's orbital period. Combining with the orbital period, parent star's mass, and rate of wobble, it was determined that Ixionidae has a minimum mass of only 1.13 Earth masses, the lowest measured mass of any planet detected by radial velocity. To date, all but three planets with lower mass than Ixionidae are detected by Kepler, including one orbiting a pulsar.
With the discovery of Ixionidae, it is one step closer to finding twin-Earths in the solar neighborhood. Also it represents that there could be planets in the Alpha Centauri B system way beyond the orbit of Ixionidae, including one in the habitable zone, as well as around the stellar component Alpha Centauri A.
Ixionidae became the 832nd planet known, including eight solar system planet, hence the planet number 832. Subtracting eight and is the 824th exoplanet identified. It is also the 143rd planet detected in 2012 hence the year-planet number 2012 P143. Ixionidae is the 16th planet identified in the constellation Centaurus and 63rd in Simianus, a caelregio where Centaurus is part of. However, other team of astronomers casted doubt on the planet's existence.
Orbit and rotation Edit
Ixionidae's orbital period given from radial velocity method is 3.236 days (about 0.28 megaseconds), implying that it this planet is in B orbit around the parent star. It orbit at a semimajor axis of just 6.26 gigameters or just 10.42 stellar radii. The orbit is so small that it has an area of just 27 times greater than the surface area of the parent star, just 22% of the proportion of the ratio between surface areas of Sun and Earth. Current planetary formation models say that this planet cannot form this close to the star due to tidal forces by the parent star and thus not enough material, but it formed farther away and then migrated in.
Planets with such small orbits tend to move in circular trips due to libration. This is the case for this planet which has a eccentricity of 0.043, low enough to be qualified as circular. Since eccentricity is non-zero, the star-planet distance varies throughout its orbit with the closest approach (periastron) and farthest approach (apastron) 180° from each other. Using the eccentricity value, periastron would be 4.3% closer to the star than orbital average while apastron would be 4.3% farther than average.
The orbital inclination relative to line of sight does not allow for transits across its parent star. The value is 62.2° to line of sight or −9.6° to the direction of star's rotation. Its argument of periastron is 92°, which is the angle between periastron and its ascending node, which is 250°, resulting in the longitude of periastron of 342°. The apparent angular separation between Ixionidae and Alpha Centauri B seen from Earth is 31.06 mas, determined by dividing distance from Earth in parsecs and the planet's semimajor axis in astronomical units.
Parent star appearance Edit
Since Ixionidae orbits 24 times closer to the star than Earth is to the Sun, then the parent star would appear lot brighter and lot bigger than the Sun seen from Earth. Apparent brightness is inversely proportional to the square of its distance, so the parent star would appear around 576 times brighter than the Sun seen from Earth since 576 is 24². However, since the parent star is only half the Sun's brightness, Alpha Cen B appears just 257 times brighter at a apparent magnitude of −32.78, compared to −26.74 for Sun seen from Earth. The angular star diameter from Ixionidae is over 11°, 22 times the apparent size of the Sun we see.
Ixionidae is tidally-locked to the star, meaning the rotation is synchronized with orbit. For this planet, rotation period can be taken directly from its orbital period of 3.236 days. Due to this, the parent star seen from the surface of Ixionidae would be motionless and thus have no day-night cycle we experience every day. While the parent star stays motionless, stars on the perpetual night side would still transverse across the sky due to orbit-induced rotation. The stars appear to move east 4.64° per hour.
The planet rotates at 10.2° to the plane of its orbit, compared to 23.4° for Earth. The longitude of vernal equinox is 180°, opposite side of the celestial sphere to that on Earth. The planet's north pole points to the constellation Aquarius while the south pole points to the constellation Sextans.
Physical characteristics Edit
Mass and size Edit
Ixionidae is only slightly bigger than Earth with a diameter of about 15 megameters. From Earth, the apparent (angular) diameter of Ixionidae is about 74½ microarcseconds, roughly four billionth the size of the full moon and one thousandth the apparent size of Pluto at current position in orbit. Like other planets, Ixionidae is not a perfect sphere due to gravitational tides from the parent star and planets beyond Ixionidae. Diameter along its equator is larger in value than from pole to pole due to planet's rotation. The equatorial diameter is 15.03 Mm while the polar diameter is 14.98 Mm. When using diameters, it can use geometrical formula to calculate planet's circumference, even equatorial and polar. The equatorial circumference is 47.20 Mm while the polar circumference is 47.07 Mm. Ixionidae has a volume of 1771 Mm³, which is 1.635 times that of Earth, while the surface area is 708 Mm², 1.388 times Earth.
Another important physical property of the planet in addition to its size and derivatives is mass, a quantity about how much stuff it contains. Ixionidae contains 7.645 yottagrams or 1.28 Earth masses. In the planetary mass classification scheme, Ixionidae is classified as a mid-Earth, defining as a planet with mass between 0.5 and 2 Earth masses.
Density is the derivative of mass and size. Ixionidae's density is 4.32 g/cm³, 78% that of Earth. Lower density tell that this planet contains lower proportion of heavy materials than Earth contains, such as percious metals like gold. But due to same value of percentage of Earth's mass relative to Ixionidae, the exoplanet contains just as much heavy materials as Earth contains. Another mass/size derivative is its surface gravity. It is proportional to mass but inversely proportional to square of its radius or surface area. Because surface area is slightly larger in value than mass relative to Earth, the resulting surface gravity of Ixionidae would be 8% weaker than Earth's despite its greater mass. Ixionidae has a gravitational acceleration of slightly over 9 m/s² or approaching 30 ft/s².
Composition and structure Edit
Ixionidae is a rocky planet, meaning it is comprised of rock and metal with at least thin envelope of gases that make up the atmosphere. Most rocky planets have layers in their interiors, including Ixionidae. Like Earth, Ixionidae has crust, mantle, and core. Also like Earth, Ixionidae's crust is thin and broken into plates, caused by not just convention in the mantle, but also driven by parent star's tidal forces. Because plates is caused by two driving forces instead of one for Earth, then plates would be smaller and move faster. Rocks in the mantle are melted due to tidal waves and radioactive materials. The planet's core is its center and is the source of heat. Ixionidae's core measures 1.6 megameters across and has temperature about 7200°C (13000°F).
Like solar system terrestrial planets, Ixionidae's rocks are chiefly made of silicates instead of carbon-rich that is the characteristic of carbon planets. Like Earth, oxygen is the most abundant element in the crust, followed by silicon, boron, and magnesium. Ixionidae's core is made of 78% iron, 18% nickel, 2% chromium, and 2% sulfur.
Surface and volcanism Edit
The same driving forces causing surface to break into plates also cause volcanism. The volcanism is intense on Ixionidae because the planet orbits so close to the star that tidal forces causes planet to stretch and squeeze due to its elliptical orbit, thus heating the interior due to friction. It is analogous to Jupiter's tidal forces exerting on Io, making it the most volcanically active world in the Solar System. Ixionidae is even more volcanically active than Io since the parent star is far more massive than Jupiter. Many volcanoes would be erupting simultaneously at all times. Volcanoes on Ixionidae spew out lava just like volcanoes on Earth, and because the world is so hot, lava would stay molten on its surface for a great amount time. The constant eruption would build up lava over much of the surface, becoming the so-called "lava-ocean planet." Lava can cover as much as 150 meters (500 feet) deep.
Ixionidae has only a tenuous atmosphere with a pressure of only 1⁄230000 that of Earth's, measuring at 444 mPa. It is due to intense heating and stellar winds from the nearby star causing hydrodynamic escape. Almost all the gases that make up atmosphere were outgassed by volcanoes. Carbon dioxide makes up about half of the atmosphere, while most of the remaining are alkali metals sodium and potassium vapors. The atmosphere also contains small amount of mercury vapor, carbon monoxide, sulfur dioxide, and water vapor. Mercury came from mercury amalgams in crustal rocks via volcanism while sodium and potassium vapors came from decomposed salt.
Moons and rings Edit
Ixionidae has only a small hill sphere, because of its low mass and in torch orbit around the star. Hill sphere is a region within where moons are in stable orbit around the planet, if they orbit outside it, they may end up orbiting the star instead. The radius of the hill sphere is just 18% of the Earth-Moon distance or nine planetary radii. In reality, Ixionidae has no moon, like the two closest planets in the Solar System Mercury and Venus. Like most rocky planets, Ixionidae has no rings unlike the Solar System's giant planets.
Ixionidae orbits only 6¼ gigameters, which is too close to the star for this planet to form as tidal influences prevented planets to form at this distance. Instead Ixionidae formed considerably farther away where tidal influences are much less and more available materials. After the planet form, pressure differences in protoplanetary disk as well as gravity interplay between planets and the parent star caused this planet to orbit eccentrically. During close periastron, star's tidal exert forces on the planet, thus slowing down its top speed so that next apastron would not be as far, and thus decreasing it semimajor axis while circulizing its orbit. When the planet first formed, it orbited at distance similar to Mercury from the star, 83⁄7 times the present distance.
Future studies Edit
Ixionidae is the nearest known exoplanet, so it is very good candidate for future studies. Ixionidae is speculated not to transit across the face of Alpha Centauri B, however, it can be studied using reflected light. When using this technique, spectra can be carried out to study the chemical makeup of the atmosphere, and can alternatively be done using polarimetry. Evidence of volcanoes can be found using infrared spectra.
Direct imaging of Ixionidae is extremely difficult since it orbits very close to the star, where the star outshines the planet by the factor of millions. But since it is the nearest known exoplanet at just 4.4 light-years from Earth, this close-in planet is easiest to directly image than planets of similar distances around more distant stars, because it widens the angular separation with decrease in stellar distance from the observer. This technique can be used to study the planetary characteristics, including size, rotation, actual mass, atmosphere, surface and composition, and finding any moons.
Because Ixionidae is the nearest exoplanet to us, it would be one of the first destinations of interstellar probes. The probes can study this planet close-up, just like space probes visiting planets in our solar system besides Earth. It can discover something that even direct imaging from Earth can't find.