The Solar System formed 4. 6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
Europa: Planetary scientists generally think that a layer of liquid water swirls around beneath Europa's surface, and that heat from tidal flexing causes the subsurface ocean to remain liquid. It is estimated that the outer crust of solid ice is about 6 to 19 miles thick, including a ductile "warm ice" layer that hints that the liquid ocean underneath may be 60 miles deep. This means that Europa's oceans would amount to slightly more than two times the volume of Earth's oceans.
Titan is certainly an exotic, fascinating, frigid oddball, that bears an eerie resemblance to the primordial Earth before life emerged and evolved on our planet (prebiotic). In fact, Titan is the only moon in our entire Solar System with a substantial atmosphere. It is also the only known world--other than Earth--where stable bodies of surface liquid exist.
Dr. Soderblom calculated the gravity signatures both in and around 1,200 craters that had been excavated by impacting objects on the lunar far side. He then went on to compare the gravity within each crater with the gravity of the surrounding terrain. Dr. Soderblom did this in order to determine whether an impact increased or decreased the local porosity.