Question

A small satellite orbits Pluto. Eight satellites orbit Neptune, the closest planet to Pluto. Pluto is much smaller than Neptune. Why isn't Pluto a satellite of Neptune?

Answer 1

Despite that Pluto's perihelion is close to Neptune's aphelion, the orbital inclination of Pluto is as high as `17.2^o` compared to Neptune's `1.8^o`. It had been away from Neptune's gravity sphere.

Explanation:

Pluto's perihelion is 4436.8 million km. Neptune's aphelion is 4545.7

million km. If the orbits of both had been coplanar, probably, Pluto

could have become Neptune's satellite. The orbital inclination for

Pluto is as high as `17.2^o`, compared to Neptune's `1,8^o`. So, its

parent is the Sun and Neptune could not adopt Pluto.

It is a research problem to find when Pluto at perihelion would be

aligned with Neptune at aphelion, along with the Sun on the Pluto

side, enabling Neptune to attempt adoption of Pluto, as its satellite.

The Pluto-Neptune distance then would be about 109 million km.

. .