Hades

Hades (Tau Ceti d, P848) is the third in orbit around, a star just 12 s away. It is one of the five planets discovered on December 19, 2012. This planet is a super-Earth which orbits in a Mercury-like distance from the parent star. It is also a with a year lasting 94 days.

was named after the of the, Greek equivalent of the.

Discovery and chronology
Hades 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 spectrograph mounted on the 3.6-meter telescope in  located in the Atacama Desert in Chile. Hades became the 840th exoplanet discovered since 1992 and is the 159th planet discovered in 2012. It is also the 32nd planet discovered in and 100th in Hippocampus.

Orbit
Hades orbits in the similar distance from the star as is from the Sun, thus the length of its year is similar to Mercury as well. Hades orbits at a of 55.84 s and distance varies by 15% throughout its orbit. The length of planet's year is 94.1 days, which is just over three months and one quarter of a year on Earth. Hades moves at an average speed of 43.25 km/s, 45% faster than the of Earth. Hades moves faster when it is closer to the star and slower when it is farther away. As seen from Earth, Hades maximum apparent separation from the parent star is about one-tenth of an arcsecond, which is very wide for the semimajor axis because the planet and the star is so near to us.

Rotation
Hades is locked into a 12:7 tide ratio, meaning one complete rotation is seven twelves of its orbital period. It takes 54.90 days or nearly eight weeks to complete one spin, similar to the rotation of Mercury (58.65 days). Long rotation periods mean it rotates slowly. In this case, the rotational velocity is just 43 kph (27 mph), just a bit faster than bike pedaling. The rotation rate is so slow that cars driving on a side street can outrun the apparent movement of the sky, thereby reversing the natural movement. For example, when the star rises and want to drive away from it, then the star would re-set and plunder back to night, whereas if the star sets and drive toward it, then the star would re-rise and relight the sky. The rotational axis tilts 1.8° to the orbital plane and is 208.1° with 0° denoting  and going eastward. When combining axial tilt, longitude of vernal equinox, inclination to line of sight, and coordinates seen from Earth would imply that the planet's north pole points to the constellation at right ascension 13:09:0 and declination −07°01′ while the south pole points to the constellation  at right ascension 01:09:0 and declination +07°01′.

Parent star observation and irradiance
As seen from the surface of Hades, the parent star would appear brighter than the Sun as seen from Earth because the planet orbits much closer according to the law of. The parent star would have a magnitude of −28.03 compared to −26.74 for Sun as seen from Earth. Sun appearing brighter usually means it is bigger. In this case, the angular diameter of the star is 1.14°, compared to about 0.5° as the angular diameter of the Sun as viewed from our homeworld.

Since the planet orbits nearly three times closer, irradiance would assume to be more than 8 times greater, but it should be noted that its parent star is less luminous than our Sun. Hades receives 3.3 times more insolation than Earth receives from our own star.

Mass and size
Hades' mean diameter is 18.24 s or 1.43 times Earth. From its size, Hades has a surface area just over two times greater and volume nearly 3 times greater than Earth's. It masses 3⅔ times greater than Earth's, classifying this as super-Earth since the mass is between 2 and 10 Earth masses. This results in the density of 6.9 g/cm³, denser than and other solar system planets.

Gravitational influence
Hades' surface gravity is 1.78, with 1 g is the surface gravity of Earth. Since 1 g acceleration is 9.8 m/s², then 1.78 g acceleration would 17.5 m/s². The planet's gravity influence the space in the vicinity of the planet, keeping any of its possible moons in orbit. This vicinity is called its with outer limit being equal in gravitational influence from planet and star, which is located 896 Mm or 98.2 planetary radii from the planet's center. Distance from the planet where an orbiting object has the same orbital period as the planet's rotation is just beyond the hill sphere and thus this orbit is not possible for long until it settles into the orbit around the star. If the planet's rotation is a bit faster, then such an orbit would be stable as it would lie within the hill sphere.

Interior
Like other terrestrial planets, Hades underwent, an event in which denser materials sink to form the core. The planet's core is made of and  with small amount of. Surrounding the core is mantle, where rocks are semisolid or molten and above it is the crust where the planet's surface lies at the top.

Surface
Hades' surface is mostly desert, but there are prominent terrains like mountains, canyons, ridges, volcanoes, and others. Because of the planet's surface feature, Hades is a.

Atmosphere
The most common gas in the Hadian atmosphere is, at 38.6%, followed by at 18.9%,  making up 18.6%,  at 12.5%,  making up 7.6%, and  at 2.7%. There are other gases making up less than 1% of the atmosphere by volume, the most abundant such gas is, making up 0.35%, with (0.29%) and  (0.25%) close behind. The atmospheric molar mass, based on their molecular weight of gases and their proportion, is about 37 g/mol.

Hades' atmospheric pressure is more than twice that of Earth's but it has total mass just that of Earth's. Hades' atmosphere making up about five cubic megameters, nearly 20% the Earth's atmospheric volume.

Magnetic field
The planet's strength is 4.38, which is weak due to its slow rotation.

Moons and rings
Hades has the only moon, but there were three moons in the past. The only moon is small, whose diameter is 153 km and orbits at 0.889 lunar distances from the planet. Two other moons were smaller and orbit beyond the only moon, 23 km and 29 km in order from the planet. Both moons escaped the planet's hill sphere as high eccentricities spend some of their time beyond the hill sphere during the orbit 3.1 and 2.7 billion years ago, respectively. The escaped moons are now s.

Future studies
Hades 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 Hades 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 very difficult given that planet orbits close to the glare of its star and is small, though future generations of technologies can make it whole lot easier. Direct imaging can be used to what planet appears like as well as if moons actually exist. Looking for signatures of volcanism can be done using reflected light or direct imaging.