Review

#01

(1) All planetary orbits in nearly same plane; (2) All planets orbit in same direction; (3) Planetary orbits in Sun's equatorial plane; (4) Sun rotates in same direction as planetary orbits; (5) Most planets rotate in same direction as the Sun; (6) Most planetary orbits are very nearly circular.

#04

A gas cloud that collapses to make a star also makes an accretion disk. Planets could form from this disk. This would imply that all the planets would orbit the central star in about the same plane, having the same sense of rotation, which would be the same as the star itself.

#05

Linear momentum depends only on mass and velocity, but angular momentum also depends on proximity to a point about which angular motion is measured.

#18

There are several techniques: direct imaging, eclipses, and indirect effects (e.g., due to gravity, such as Doppler effect or astrometric variations indicating a change of motion in star which could only occur if there were a companion object).

#19

We have found no Earth's, only gas giants. Some of the systems have gas giant objects, like Jupiter, in quite small orbits.