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| The Outermost Planets |
1. All inside
3. Discovered in 1846, with a period of 165 years, it will complete one "Neptunian year" in 2011.
5. Saturn, because its tilt is too small (1 degree) and Uranus and Neptune because their tilts are too large (59 and 47 degrees respectively).
7. Pluto is 39.4 AU from the Sun on average, so
9. Taking a distance of about 30 AU and a diameter of about 50,000 km, it subtends about 2.3 arc sec.
11. Since all rings in the Solar System are inside the Roche limit, these moons lie inside the Roche limit for Neptune. Thus, they could not have formed by accretion in their present location and so must have formed elsewhere and then been captured by Neptune.
15. Pluto is almost twice the diameter of Charon, so from the solution to the preceding problem Pluto would appear to be about 8 degrees across in the sky of Charon, 16 times the size of the Moon in Earth's sky.
17. Since the system is completely tidally locked with both rotational periods equal to the orbital period, the description is symmetric: simply interchange "Pluto" and "Charon" in the answer to the preceding question.