Solar System – Odds & Ends

So what was that “retrograde motion” stuff all about? Why do the planets appear to move backwards sometimes? It’s an ILLUSION caused by the earth “passing” other planets as we all orbit the sun. We would not see this if earth was at the center.

Dwarf Planets Eris and its moon 1500 miles wide (drawing) Ceres – the largest object in the asteroid belt; 600 miles wide (photo) Pluto and its moon 1430 miles wide (drawing)

The Kuiper Belt – billions of icy objects orbiting the sun, beyond Neptune’s orbit. Pluto and Eris are both part of the Kuiper Belt.

The Oort Cloud – even farther out – billions of icy objects orbiting the sun. Home of comets that occasionally visit the inner Solar System

The orbit of Sedna is extremely elliptical. It takes 10,500 years to circle the sun! ssc v1-Orbit-of-Sedna What about something like Sedna – outside even the Kuiper Belt. Is Sedna a planet? Rest of Solar System!

What makes something a planet? Why did Pluto get kicked out? Possible definitions of “planet”: (from Mike Brown – discoverer of Eris, Quaoar, and Sedna - and the “killer” of Pluto - “ )“ 1) Purely historical: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are planets. No scientific basis. 2) Historical plus: Mercury through Pluto are planets, as is any newly discovered object larger than Pluto. But why is Pluto’s size the cutoff? No scientific basis.

Possible definitions of “planet”: 3) Gravitational rounding: Any object which is round and which directly orbits the sun is a planet. Scientifically based. Pluto would be a planet, but so would Ceres, Eris, Quaoar, Sedna, and perhaps a dozen other Kuiper belt objects. But what’s so special about roundness?

Possible definitions of “planet”: 4) Population classification: If the object is a “solitary individual” it is a planet. If it’s part of a whole population of objects ranging in size, it is not. So: Ceres – just part of the asteroid belt. Pluto, Eris, Quaoar – just part of the Kuiper Belt Sedna – just part of the “inner Oort Cloud” (they think) But Mercury, Venus, Earth, etc……. No other objects anywhere near their size in their region of the solar system. They are definitely PLANETS. (This one seems pretty good, according to Mike Brown.)

SO: Our Solar System has 8 planets and 3 “dwarf planets” (But why isn’t Sedna a dwarf planet then??)