1a) Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula – a spinning, swirling disk of material.
b) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
c)i) Inner planets have fewer moons, small, silicate surface, nickel-iron core, higher density and rotate more slowly compared to outer planets. Outer planets have a greater number of moons, no solid part; rotate faster, have a lower density as well as rings in some cases. It is likely that large rings couldn't form on inner planets (Mercury, Venus, Earth, Mars) because the rings are made of frozen icy dust, and the sun is too hot this close to it for the rings to form. Additional fact: Mars has two moons, one of which is very close to the planet.
ii) The four inner planets have shorter orbits, slower spin, no rings, and they are made of rock and metal. The four outer planets have longer orbits and spins, a composition of gases and liquids, numerous moons, and rings. The outer planets are made of hydrogen and helium, so they are called gas giants.
2) Scientists think planets, including the ones in our solar system, likely start off as grains of dust smaller than the width of a human hair. They emerge from the giant, donut-shaped disk of gas and dust that circles young stars. Gravity and other forces cause material within the disk to collide.
3) If gravity had a slightly greater intensity, the universe would not have expanded and would soon collapse. There would be no time to form stars, planets and even life, as occurs on Earth.