ECHO-1237-4

ECHO-1237-4, officially named Nereidos, is the fourth planet orbiting the star Aldhahabi in the Orion Constellation. Nereidos is an extremely rare Hycean-world, a supermassive planet with a deep ocean of liquid water spanning its entire surface, along with a hydrogen-rich atmosphere. In 2021, researchers at the University of Cambridge in England coined the term "Hycean" as a portmanteau of "hydrogen" and "ocean." They hypothesized that these types of planets are promising candidates for extraterrestrial life. This hypothesis turned out to be true for Nereidos.

In 2098, the Ulrich-Barfuss Exoplanetary Research Telescope, located in the Mare Moscoviense Moon Station, revealed methane in the planet's atmosphere, along with high concentrations of carbon dioxide. This led researchers to believe that this planet was an incredibly strong candidate for life. The colonization of the star system 17 years later revealed that, although an incredibly hostile planet for human life, it contained its own simple, single-celled organisms dubbed the Nereites, marking the third time humanity has come into contact with alien life.

## Discovery

Nereidos was discovered in June of 2019 by the Kepler Space Telescope while studying the orbit of Cronus, which was previously discovered in 2017. Little was known about both planets, but the Aldhahabi was considered a prime target for exoplanet research long before the Kepler Telescope ever laid eyes on it. In 2027, the James Webb Telescope studied the Aldhahabi system for 8 months, starting in Febuary and ending In early October, leading to the discovery of 2 more planets, Epimetheus and heliades.

### Name

The name Nereidos is derived from the ancient Greek sea-nymphs called the Nereid's. The name was chosen because of the planets massive world-spanning ocean. Three of its moons, Ceto, Arethusa, and Dynamene, are all named after individual Nereids in the ancient Greek mythos.

## Physical characteristics

Nereidos features an extensive ocean that covers nearly the entire planetary surface, plunging to depths of almost 126 kilometers in certain areas. Scattered across this oceanic world are several island chains, with the largest spanning 1,712 kilometers in length and 652 kilometers in width. These islands appear to have formed through volcanic activity in the planet's early history. It's worth noting that, despite the substantial size of these land masses, Nereidos is a significantly more massive than planet Earth, being 4.5 times its mass and having a diameter of nearly 18 thousand kilometers, making it over 40% larger than Earth. In the context of the planet's overall dimensions, these islands appear relatively small.

Beneath the planet's surface, a variety of geological features thrive, including thermal vents, underwater volcanoes, and modest-scale plate tectonics. These geological processes play a crucial role in regulating the planet's temperature and providing a sustainable habitat for native life forms, which exclusively inhabit the ocean regions surrounding the warm, chemically rich thermal vents.

### Rings

Nereidos has an extensive ring system that formed during a collision with a large sub-planetary object, now believed to be the moon Dynamene. This ring system has a large impact on the planets environment, blocking out sunlight on the daylight side and dropping global temperatures by as much as 4° C annually. On the nightside of the planet the ring system is incredibly bright, making it easily visible through the thick cloud layer that is ever-present on the planet. When the planet was much younger, the ring bombarded its surface with meteorites daily and likely contributed to its chemical abundance.

### ECHO-1237-4a - Ceto

Ceto is the smallest and closest major moon in orbit around Nereidos. It has a mass of 0.009 M☾ (lunar masses), a diameter of 635.95 km, a gravitational influence of 0.04 g, and orbits at an average distance of 67,000 km. Its surface is heavily cratered and features a prominent equatorial ridge that was likely formed when Nereidos's rings were significantly larger. This moon, along with Arethusa, most likely formed during a collision event between a large sub-planetary body that is now Dynamene, the largest moon in orbit around Nereidos.

### ECHO-1237-4b - Arethusa

Arethusa is the second largest and second closest major moon in orbit around Nereidos, It has a mass of 0.06 M☾, a diameter of 1,194 km, a gravitational influence of 0.09 g, and orbits at an average distance of 110,000 km. Its surface is heavily cratered like Ceto but lacks an equatorial ridge, indicating that the rings of Nereidos did not extend to or past the orbit of Arethusa.

### ECHO-1237-4c - Dynamene

Dynamene is the largest and furthest of the major moons, and is the only moon in orbit around Nereidos that had an atmosphere at one point. It has a mass of 0.24 M☾, a diameter of 1,910 km, a gravitational influence of 0.13 g, and orbits at an average distance of 180,500 km. Its surface is less cratered than its neighboring moons, but displays features of atmospheric weathering in the form of canyons and erosion around crater rims and mountains. Based on chemical deposits on and below the moons surface, its atmosphere likely consisted of a majority of SO2 (sulfur dioxide), N2 (nitrogen), and trace elements of Kr (krypton) and Xe (xenon).

### Minor moons of Nereidos

Nereidos has 9 minor moons, two of which exist within the planets ring system, the remaining 7 all orbit beyond Dynamene, with the largest being 22 km in diameter and orbiting at a distance of 1,268,000 km. As of today none of the minor moons have proper names.