Aita

Aita

Discovery
Date	December 19, 2012
Discoverers	Tuomi et al.
Detection method	Radial velocity (HARPS)
Site	La Silla Observatory
Name & designations
Pronunciation	/'ī•tä/
Adjective	Aitian
Planet numbers	P849, Tau Ceti P4, Cetus P33, Hippocampus P101, 2012 P160, 2012 Cet-11, 2012 Hip-18
Star designations	52 Ceti e, BF 1315 e, PH 629 e, Pi Hippocampi e, 376 Hippocampi e, P22 Ceti e, P73 Hippocampi e, HD 10700 e, HIP 8102 e, HR 509 e, Gliese 71 e, SAO 147986 e
Location
System	Tau Ceti
Constellation	Cetus
Caelregio	Hippocampus
Right ascension	01h 44m 04.08s (26.017 01°)
Declination	−15° 56' 14.9" (−15.937 48°)
Distance	3.650 pc (11.905 ly)
Orbital characteristics
Semimajor axis	0.549 513 AU (82.206 0 Gm)
Periastron	0.519 535 AU (77.721 3 Gm)
Apastron	0.579 491 AU (86.690 6 Gm)
Eccentricity	0.054 553 9
Orbital circumference	3.450 81 AU (516.234 3 Gm)
Orbital area	0.947 24 AU² (21 198.7 Gm²)
Orbital period	168.120 937 d (0.460 290 04 yr)
Avg. velocity	35.660 km/s (7.497 AU/yr)
Max. velocity	36.620 km/s (8.553 AU/yr)
Min. velocity	34.674 km/s (6.266 AU/yr)
Direction of orbit relative to star's rotation	Prograde
Inclination	72.941° to ecliptic 0.880° to star's equator −2.544° to invariable plane
Argument of periastron	313.413°
Longitude of ascending node	244.186°
Longitude of periastron	197.598°
Angular separation	150.543 mas
Observing the parent star
Mean angular star size	0.772 31° (46.338')
Max. angular star size	0.816 87° (49.012')
Min. angular star size	0.732 35° (43.941')
Mean star magnitude	−27.188
Max. star magnitude	−27.310
Min. star magnitude	−27.073
Classification
SDBEaC
Bulk characteristics
Mean radius	1.527 0 R⊕ (9.728 Mm)
Equatorial radius	1.525 7 E⊕ (9.731 Mm)
Polar radius	1.529 5 P⊕ (9.722 Mm)
Mean circumference	61.125 Mm
Equatorial circumference	61.143 Mm
Polar circumference	61.088 Mm
Surface area	2.331 6 S⊕ (1 189.3 Mm²)
Volume	3.560 3 V⊕ (3 856.5 Mm³)
Flattening	0.000 91 (1:1 103)
Angular diameter	35.631 μas
Mass	4.483 9 M⊕
Reciprocal mass relative to star	58 156
Density	6.946 g/cm³
Gravitational influence
Surface gravity	1.923 g (18.86 m/s²)
Weight on Aita (150 lb on Earth)	288 lb
Standard gravitational parameter	1.787 × 106 km³/s²
Escape velocity	19.17 km/s
Hill radius	3.763 LD (1.446 6 Gm)
Roche limit (3 g/cm3 satellite)	0.042 18 LD (16.215 Mm)
Stationary orbit	1.091 87 LD (419.712 Mm)
Stationary velocity	2.043 km/s (0.450 LD/d)
Rotation characteristics
Rotation period	366.809 3 h (15.283 722 d)
Rotation velocity	167 kph (0.98°/h)
Direction of rotation relative to orbit	Prograde
Axial tilt	2.895°
Longitude of vernal equinox	196.538°
North pole right ascension	09h 30m 09s (142.539°)
North pole declination	−06° 28' 56" (−6.482°)
North polar constellation	Hydra
North polar caelregio	Felis
South pole right ascension	21h 30m 09s (322.539°)
South pole declination	+06° 28' 56" (+6.482°)
South polar constellation	Pegasus
South polar caelregio	Testudo
Thermal characteristics
Surface temperature	334 K (61°C, 142°F, 601°R)
Mean irradiance	2 082 W/m² (1.522 I⊕)
Irradiance at periastron	2 329 W/m² (1.703 I⊕)
Irradiance at apastron	1 872 W/m² (1.369 I⊕)
Albedo	0.714 (bond), 0.749 (geom.)
Atmosphere
Scale height	10.39 km
Volume	2.866 ae (12.00 Mm³)
Total mass	0.598 atmu (3.08 Eg)
Surface pressure	24.769 atm (2 509.7 kPa, 364.00 psi)
Surface density	0.256 g/m³
Molar mass	38.00 g/mol
Composition	61.332% N2, 22.836% SO2, 8.767% CO2, 3.521% O2, 3.257% H2S, 0.102% Ar, 613 ppm CO, 315 ppm CH4, 143 ppm NO, 139 ppm H2SO4, 61.0 ppm H2O, 26.3 ppm NH3, 2.68 ppm Kr, 36.8 ppb PH3
Magnetosphere
Dipole strength	0.688 nT (6.88 μG)
Magnetic moment	8.01 × 1013 T•m³
Dipole tilt	15.477°
Satellite system
Number of moons	0
Number of rings	0

Aita (Tau Ceti e, P849) is the fourth exoplanet in orbit around Tau Ceti, a star just 12 light-years away. It is one of the five detected planets discovered on December 19, 2012 and is one of roughly thirteen in this planetary system. Aita orbits over half the distance between Earth and the Sun and is over half bigger than Earth. It is a 4.5-Earth mass sulfur planet with its year lasting 24 weeks.

Aita was named after the Etruscan god of the underworld, equivalent to the Greek god Hades.

Discovery and chronology

Aita was discovered on December 19, 2012, together with four other planets in this system. This discovery was made by carefully watching the wobble of Tau Ceti caused by gravitational tug of planets. It was successfully done using high resolution HARPS spectrograph mounted on the 3.6-meter telescope in La Silla Observatory located in the Atacama Desert in Chile. Aita became the 841st exoplanet discovered since 1992 and is the 160th planet discovered in 2012. It is also the 33rd planet discovered in Cetus and 101st in Hippocampus.

Orbit and rotation

Orbit

Aita lies halfway between the orbits of Mercury and Venus at 0.55 AU (82.2 gigameters). As it is typical for other planets orbiting Tau Ceti, its eccentricity is low. For Aita, the eccentricity value is 0.05455, which means that periastron distance from the star is 5.455% closer than average distance while apastron is 5.455% farther away. Aita's orbital circumference is 3.45 AU (516 Gm), calculated by multiplying average distance by 2π. Orbital area is defined as the area of space within the planet's orbit with star at the center. The value is 0.95 AU² (21200 Gm²), which is 30% the area within Earth's orbit.

Aita's year is defined as the period it takes to revolve around the star. It takes 168 days, 24 weeks, or 0.46 Earth year to make one orbit. Using orbital circumference and its year can be used to calculate orbital velocity as 35.66 km/s, or more appropriate in astronomical terms 7.5 AU/yr. Planets typically have orbital planes tilt just within few degrees of the plane of star's equator due to planetary formation mechanism where planets form from protoplanetary disk flattened and rotated with the rotation of the star.

Rotation

Like every other planet, Aita rotates on its axis, whether slow or fast. One 360° spin of this planet lasts 15²⁄₇ days or about 366.8 hours, which the rotation is slow. When taken from its rotation period and planet's circumference, the rotation velocity is just 167 kph or 104 mph. The rotation tilts 2.9° to the orbital plane, meaning that the parent star seen from the planet would reach its northernmost point at +2.9° and southernmost point at −2.9°.

Aita's angle of rotation, orbit, and inclination to line of sight would combine to have planet's poles pointing to different directions relative to Earth's, resulting in having different pole stars. On this planet, north pole points to the constellation Hydra, which is an equatorial constellation from Earth. The south pole points to the opposite side of the celestial sphere to north pole. The south pole points to Pegasus, which for Earth is a northern constellation.

Parent star observation and irradiance

According to the inverse square relation between light and distance, Aita receives 3⅓ times more sunlight than Earth receives since it orbits 55% of the distance between Earth and the Sun. But actually, since the different stars give off different amount of light, this is the case of Tau Ceti as being different from the Sun. Tau Ceti gives out less than half of the light Sun emits at any given time, meaning that Aita receives just 1½ times more sunlight than Earth receives instead. The apparent size of the star is inversely proportional to the distance, so Tau Ceti would appear 55% bigger than the Sun seen from Earth. But since Tau Ceti is just 80% of the Sun's size, then Tau Ceti as seen from Aita is just 15% bigger than the Sun seen from Earth. The amount of time taken for light to reach this planet depends on the same factor as apparent size but it does not depends on its physical size or even brightness. In this case, light takes 4.57 minutes for starlight to reach Aita, 45% shorter than sunlight to reach Earth.

Since the parent star is 1½ times brighter as seen from Aita than the brightness of Sun seen from Earth, then the insolation is 1½ times greater than Earth's. The Earth's insolation is 1368 W/m², while Aita's insolation is 2082 W/m².

Structure and composition

Mass and size

Aita has a mass 4½ times that of Earth's and the size is ⅓ the value of mass relative to Earth. The planet's surface area is 2⅓ times that of Earth's and volume 3⁴⁄₇ times that of Earth's. Volume is smaller in value relative to Earth than mass, resulting in matter being more compressed, thus making the planet denser. By dividing mass by volume relative to Earth, we can find planet's density relative to Earth. So we divide 4½ by 3⁴⁄₇ and we get approximately 1¼, which means that Aita is 1¼ times denser than Earth. The planet's density is 6.95 g/cm³, compared to 5.52 g/cm³ for the solar system's densest planet Earth.

Gravitational influence

Aita's surface gravity is 1.92 g, meaning it is almost twice the gravity of Earth. Falling objects would accelerate at 18.86 m/s², compared to 9.81 m/s² for Earth. Due to planet having mass, it is at the center of the gravitational sphere, called hill sphere. It extends to 3.76 lunar distances with boundary where gravitational influence from the planet is equal to influence from the parent star. For a satellite to have an orbital period identical to the planet's rotation period, which is about 37.6 days, it would orbit at a shade over two lunar distances and would have a velocity of just over 2.0 km/s. In addition to the outer limit allowing for the moon's existence at hill sphere boundary, there is also an inner boundary. If it orbits within that boundary, which is 0.042 LD or 16.2 megameters, a moon would tear apart by the tidal forces assuming a density of 3 g/cm³.

Interior

Like other terrestrial planets, Aita underwent differentiation, an event in which denser materials sink to form the core. The planet's core is made of 82% iron, 13% sulfur, and 5% nickel, plus trace amounts of mercury. Surrounding the core and underneath the crust is mantle, where molten rocks are that causes volcanism. There is strong convection currents in the mantle in which hotter, molten rocks rise while cooler, solid or semisolid rocks sink. The convection current is what causes crust to break apart, however strong gravity acting on convection current would make crust stronger, and thus more resistent to breaking. So Aita would be a "one-plated planet" like Mercury, Venus, and Mars.

Aita's abundance of sulfur don't confine to its core, but also layer-by-layer. Molten rocks in the mantle are sulfur-rich, which is responsible for large amounts of sulfur gases in the atmosphere through volcanism. In the crust, sulfur is the second most abundant element after silicon, then oxygen then carbon.

Surface

Aita's surface is littered with varied terrains. There are mountain ranges connected to another, as well as other terrain like plateaus, canyons, valleys, and ridges. Due to its high surface gravity, mountains don't rise that particularly high. The highest peaks have heights just over a mile. Aita's highest point is located at 43°N latitude whose height is 2.2 miles (3.6 km). The deepest canyon is 3.8 miles (6.1 km). In addition to all the forementioned terrains, there are volcanoes and craters. But there are very few impact craters due to atmosphere being so thick that it shields meteors so effeciently. There are numerous volcanoes on Aita due to its active interior but no plates to subduct away volcanoes.

On the surface, there are sulfur-rich rocks and pure sulfur deposits can be found near volcanoes. Due to sulfur richness both on its surface and in the interior, Aita is classified as a sulfur planet. When viewed from space, much of the planet's surface would appear yellow or yellowish and would have high reflectivity because yellow is a light color.

Atmosphere

Since Aita is a sulfur planet, then it atmosphere surrounding the planet should be sulfur-rich of course in the form of gases. Nature does follow the rules as chief sulfur gas, sulfur dioxide, makes up 23% of the planet's atmosphere, with the other sulfur gas, hydrogen sulfide, makes up 3%. SO₂ is in the shadow of nitrogen, making up 61% of the atmosphere by volume. The remaining gases that make up between 1–10% are carbon dioxide (9%) and oxygen (4%). Aita's atmosphere also contains trace amount of other gases with volumetric proportion well under 1%, including argon, carbon monoxide, methane, nitric oxide, sulfuric acid, and water. Of all the mentioned gases, only sulfuric acid and water are capable of forming clouds. Since the concentrations are low, clouds would be rare and thin on Aita.

Aita's atmosphere is dense, having the pressure 25 times greater than Earth's and 27% the pressure on Venus, measuring at 364 psi compared to just 14 for Earth. The mean atmospheric molecular weight is 38 g/mol and has atmospheric volume of 12 Mm³.

Magnetic field

Aita has a weak magnetic field due to its slow rotation. The strength is measured at 0.7 nanotesla, compared to over 300,000 for Earth. When the parent star emits ionizing radiation and charged particles, they hit Aita's atmosphere quite easily and would readily form bright planet-wide auroras. Stellar winds is responsible for washing away the two lightest gases, hydrogen and helium, and much of water vapor.

Moons and rings

Aita has no moons nor rings. The planet never had a moon but it formed faint temporary rings a couple times after bolide impacts.

Future studies

Aita poses a challenge since it does not transit its star. An alternative is to observe reflected light, which is difficult as it only been done for Jupiter-size planets. Future generations of telescopes can pick up reflected light from Aita and study its atmosphere as well as physical characteristics such as its actual mass and size. In addition to reflected light, this planet can be studied using direct imaging, which is difficult given that planet orbits close to the glare of its star and is small, though future generations of technologies can make it lot easier. Direct imaging can be used to what planet appears like as well as if moons actually exist. Spectroscopy can be carried out to see if Aita is actually a sulfur-rich planet.