The novel is the story of a young Earthling named Don Harvey who leaves his ranch school on Earth to be with his scientist parents on Mars. He gets caught up in the Venusian war of independence when his ship is captured by Venusian colonials. He is nearly blown up with the captured ship, but released at a space station because of his determination to go to Mars.
This novel marks Heinlein's first benign depiction of extraterrestrials. A Venusian dragon by the name of Sir Isaac Newton befriends Don on board the ship from the space station to Venus. He turns out to be a genius physicist collaborating with Don's parents on advanced scientific principles that the colonials use to win the war with the government on Earth.
In astrology, the planets, sun, moon and other movable points (such as the moon's nodes) are still all described as "planets", the wanderers of the zodiac.
Planets are located symbolically in the chart: the signs and houses filter their energies through the planets, much as a coloured lens filters the image thrown by a stage-light, or received by a camera.
Should a planet have difficult aspects, or be in an unsympathetic position in the zodiac, it is said to be afflicted.
While the inner planets became mostly rocky, the outer planets were able to accrete much more gas and to evolve voluminous atmospheres.
Pluto, the "planet" beyond Neptune, is not gaseous like the other planets of the outer solar system, nor is it rocky like those of the inner; it is actually most like a giant comet.
Between the fourth and fifth was a belt of asteroids that were tiny bodies significantly smaller than any planet.