Neptune is one of the forgotten giants of the solar system. It’s the fourth-largest planet — four times the diameter of Earth. But it’s so far away that it’s visible only with a telescope. And even through a telescope, it’s such a small target that it’s tough to study. It takes big telescopes and sensitive instruments to learn anything about it.
The planet is a little easier to study right now because it’s at opposition. That means it lines up opposite the Sun, so it’s closest to Earth for the entire year. It’s in the sky all night, too, near the border between Aquarius and Pisces.
Today, Neptune is almost three billion miles from the Sun. But when the solar system was born, it might have been much closer — perhaps just half as far as it is now.
Gravitational interactions with other bodies pushed it outward. At first, those interactions were with big iceballs left over from the birth of the planets. Such interplay changed the orbits of all the giant planets.
Later, interactions between the planets themselves altered their orbits even more. Neptune was given a swift “kick” by Jupiter, the largest planet. Before that kick, Neptune could have been the seventh planet from the Sun — inside the orbit of Uranus. But Jupiter pushed Neptune far beyond Uranus. Interactions with more small bodies then sculpted its orbit to the path it follows now — the edge of the realm of the planets.
We’ll have more about Neptune tomorrow.