The ninth planet is a hypothetical planet in the outer Solar System. Its gravitational influence may explain a statistical anomaly in the orbit distribution of isolated trans-Neptunian objects (TNOs) found mainly outside the Kuiper Belt in the scattered disk. An undiscovered planet the size of a minineptune should have a mass of ten Earth years, a diameter two to four times that of Earth, and an elongated orbit with a rotation period of approximately 15,000 earth years. To date, the search for the Ninth Planet has not been successful.
The suggestion that the clustering of the orbits of the most distant objects was due to the influence of a planet outside the orbit of Neptune arose in 2014, when astronomers Chadwick Trujillo and Scott Sheppard noted similarities in the orbits of Sedna, 2012 VP113, and several other objects. In early 2016, Konstantin Batygin and Michael Brown described how the similar orbits of six TNOs could be explained by the Ninth Planet, and suggested possible parameters for its orbit. This hypothesis can also explain the existence of TNOs with orbits perpendicular to the plane of rotation of the inner planets and others with an extreme inclination and with an inclination, as well as the inclination of the axis of rotation of the Sun.
Batygin and Brown suggest that the Ninth Planet is the core of a nascent gas giant that was ejected from its original orbit by Jupiter during the formation of the Solar System. It is also suggested that the planet may have been captured from another star, be a captured orphan planet, or that it formed in a distant orbit that was pulled out by a passing nearby star.

Planets orbiting other stars are sources of very weak light compared to the parent star, so direct observation and detection of exoplanets is quite a challenge. In addition to the considerable difficulty of detecting such a weak light source, there is an additional problem associated with the fact that the brightness of the parent star is many orders of magnitude greater than the stellar magnitude of the planet, which glows with light reflected from the parent star, and thus makes optical errors.
Interstellar objects are objects or comets that exist in interstellar space that are not bound by gravity to any star. The first known interstellar object discovered is 1I/Oumuamua. An interstellar object can only be detected if it passes through our Solar System close to the Sun, or if it has separated from the Oort cloud and started moving in a highly elongated hyperbolic orbit unrelated to the Sun’s gravity. Objects with weak hyperbolic trajectories.
Habitable zone, habitable zone, zone of life (English habitable zone, HZ) in astronomy — a conditional area in space, determined on the basis that the conditions on the surface of the planets located in it will be close to the conditions on Earth and will ensure the existence of water in the liquid phase. Accordingly, such planets (or their satellites) will be favorable for the emergence of life similar to Earth. The probability of the occurrence of life is greatest in the habitable zone in the vicinity of the star (circumstellar habitable.
According to modern data, in many respects the Moon is very different from the Earth, first of all, in its chemical composition: there is practically no water (although noticeable ice reserves have been found in the circumpolar regions), and a low content of volatile elements and compounds. Analysis of the lunar rocks suggests that the Moon has undergone a complete meltdown, unlike the Earth. The Moon’s density is comparable to that of the Earth’s mantle, but it has a very small iron-nickel core.
A solar-type star, a Sun-analog star, and a Sun-twin are three categories of stars that are more or less similar to the Sun. The study of these stars is very important for a better understanding of the properties of the Sun, its uniqueness or, conversely, its typicality among other stars, as well as the possibility of the existence of habitable planets in other solar-type stars.