The correct option is They are small-rounded objects formed by the process of cohesion..
Explanation
The formation of planets is generally explained through the process of accretion within a solar nebula. As the gas cloud condenses, matter aggregates to form larger bodies. This stage is critical in the transition from dust and gas to fully formed planetary bodies.
Option Analysis
- They are large gas giants formed in the outer solar system. is incorrect: Gas giants (like Jupiter and Saturn) are fully formed planets located in the outer solar system. Planetesimals are the smaller, precursor bodies that collide and merge to eventually form such planets.
- They are small-rounded objects formed by the process of cohesion. is correct: According to standard theories of planetary evolution (such as the accretion hypothesis), the gas within a nebula condenses into lumps. Through the process of cohesion, matter within these lumps develops into small-rounded objects known as planetesimals. Larger bodies eventually form as these planetesimals collide and stick together due to gravity.
- They are the final stage of star formation. is incorrect: Planetesimals are associated with the formation of planets, not the final stage of star formation. The final stages of star formation involve nuclear fusion and the stabilization of the star, whereas planetesimals form in the accretion disk surrounding the star.
- They are fragments of the primordial atmosphere. is incorrect: Planetesimals are solid bodies formed from the dust and gas of the solar nebula, not fragments of a primordial atmosphere. The primordial atmosphere refers to the early gaseous envelope surrounding a planet.
Key Takeaway
Planetesimals are the early building blocks of planets, formed by the cohesion and accretion of dust and gas particles in the solar nebula.