*2.2.3.2.1 Pluto*

Perturbations to Neptune's orbit suggested the existence of a new planet that led to the discovery of the dwarf planet Pluto. However, Pluto's mass is too small to have perturbed Neptune's orbit. The orbital perturbation issue involving other Kuiper Belt Objects has led to the speculation of additional Solar System planets [15, 29].

Pluto is likely composed of rock, and it has an icy surface mixture of frozen nitrogen with smaller amounts of carbon monoxide and methane. As such, Pluto's surface is similar to Triton [15]. It is believed to have a rocky core. Pluto has a very thin atmosphere that includes nitrogen, methane, and carbon monoxide [15]. This composition is consistent with the sublimation of surface material [15].

Pluto has a diameter of about 2300 km and has a rotation period of about 6.4 days. It has five moons [28]. Pluto's largest moon (Charon) and its characteristics are summarized in **Table 6** [15].

## *2.2.3.2.2 Eris*

Eris is an additional dwarf planet that is slightly larger than Pluto. It has a radius about 20% of Earth's. The internal structure of Eris is unknown [27]. Ref. [15] suggests that its surface is similar to Pluto that incorporates a composition of frozen methane, rock, and ices of various elements including nitrogen. Eris' surface temperature is about 33 K [27]. This dwarf planet also has a small moon Dysnomia that has a circular orbit with a rotational period of about 16 days.

#### *2.2.3.2.3 Planet Nine*

Periodically, predictions of additional planets beyond Pluto have been made. One of the more recent developments is an assertion by Batygin and Brown [12]. Ref. [12] suggests the existence of an additional planet with a mass of approximately 10 Earth masses, and an orbit at about 20 times farther from the Sun than Neptune. Their estimates for Planet Nine were inferred from observed orbital characteristics of a number of Kuiper Belt Objects.

Ref. [12] performed numerical simulations to fit the observed orbits. A perturbing body (Planet Nine) was incorporated in the simulation to reproduce the observed orbital characteristics of a number of Kuiper Belt Objects. The simulations [12] suggest that the object perturbing the orbits is a "primordial giant planet core that was ejected during the nebular epoch of the Solar System's evolution". At the present time, the existence of Planet Nine is unconfirmed.
