86 Chapter 2 From the Solar System to the Milky Way
Size Approximately 11.2 times the size of Earth (equatorial radius of 44,423 miles,
or 71,492 km)
Mass Approximately 318 times the mass of Earth
Surface gravity Approximately 2.37 times the gravity of Earth
Satellites At least 63
Mean distance from the Sun 5.2 AU
Orbital period Approximately 12 years
Rotation period Approximately 9 hours, 50 minutes
What’s It Like There?
While Mercury, Venus, Earth, and Mars, which are composed of rocks or metals, are
referred to as the terrestrial planets, Jupiter and Saturn, whose primary component is
gas, are referred to as the Jovian planets (or the gas giants). The density of Jupiter is only
1/4 that of Earth, and since it is composed primarily of gaseous hydrogen and helium, it
does not have a “surface” like Earth does. Since the center (approximately 1/11 of the
radius) is a rocky core and the rest is just a collection of gaseous or liquid materials, Jupi-
ter differs from our ordinary image of a planet. Being on Jupiter would be like being in the
middle of a cloud.
Jupiter has a mass of more than 300 times that of Earth, and its gravity is more than
twice that of Earth. If she lived on Jupiter, Kaguya-hime would feel as if she were always
carrying around twice her own weight.
Jupiter’s most noticeable feature is the Great Red Spot—a massive, persistent storm on
the planet’s surface. It was discovered by the astronomer Robert Hooke in 1664 with one
of the first telescopes. The Great Red Spot has a larger diameter than Earth! Scientists still
don’t know exactly why the spot is red.
Some scientists believe that Europa, one of Jupiter’s moons, is one of the most likely
places to find extraterrestrial life in the solar system due to the heat-generating tidal forces
in the liquid water oceans believed to lie under Europa’s icy mantel. The conditions in Euro-
pa’s subsurface oceans may be similar to those that were the catalyst for life’s beginning on
Earth. Scientists hope to observe Europa and Jupiter’s other satellites more closely via space
missions set to launch in 2020.