The planets in the inner part of the Solar System are made primarily of refractory materials; the planets in the outer Solar System are made primarily of volatiles. The reason for the difference is that a. refractory materials are heavier than volatiles, so they sank farther into the nebula. b. there were no volatiles in the inner part of the accretion disk. c. the volatiles were lost soon after the planet formed. d. the outer Solar System has gained more volatiles from space since formation.

Refractory materials are substances that have high melting points and can withstand extreme temperatures. In the context of the Solar System, these materials include metals and silicates. They are fundamental components for the formation of inner planets like Mercury, Venus, Earth, and Mars. Because these regions were originally very hot due to their proximity to the Sun, only materials that could withstand such conditions were able to condense and form solid structures.
Examples of refractory materials in the Solar System include:

• Iron
• Nickel
• Silicon
• Magnesium

These materials combined to form the rocky surfaces and cores of terrestrial planets.

Volatile materials are substances that have low boiling points and tend to exist as gases at relatively low temperatures. In the cooler regions of the Solar System's accretion disk, these materials could condense into solid ice form. This is why planets in the outer Solar System, such as Jupiter, Saturn, Uranus, and Neptune, are rich in volatile compounds like water, methane, and ammonia.
Examples of volatile materials include:

• Water (H₂O)
• Methane (CH₄)
• Ammonia (NH₃)

These materials are crucial for creating the thick atmospheres and icy compositions of gas giants and their moons.

The temperature within the Solar System's accretion disk varied greatly, with much higher temperatures closer to the Sun and cooler conditions farther away. This temperature gradient was a determining factor in the composition of the forming planets.
The inner region, where temperatures were higher, caused volatile materials to vaporize and be blown away by solar winds. Only refractory materials that could withstand such heat stayed and coalesced into rocky planets.
Conversely, in the outer regions of the disk where temperatures were lower, volatile materials could condense. These materials then contributed significantly to the formation of the gas giants and their extensive systems of moons.
Understanding temperature variations in the accretion disk helps explain the stark contrasts between terrestrial and jovian planets.

Planet formation in the Solar System is a fascinating process that arose from the protoplanetary disk around the young Sun, known as the accretion disk. This disk contained a mixture of gas, dust, and ices, which eventually coalesced to form planets.
The process of planet formation can be broken down into several stages:

• Coagulation: Dust particles collided and stuck together to form larger aggregates.
• Planetesimal formation: These aggregates grew into kilometer-sized bodies called planetesimals.
• Protoplanet formation: Through further collisions and gravitational attraction, planetesimals merged to form protoplanets.
• Planet formation: Over time, protoplanets accrued enough mass to become full-fledged planets, clearing their orbital paths of debris.

The different types of materials that planets were able to accrete depended heavily on their location within the disk and the local temperature, dictating their eventual composition.
This radial gradient in temperature explains why rocky planets are found closer to the Sun, while gas giants with abundant volatile materials formed further out.