Figure 4-16 Terrestrial Versus Jovian Planet Formation The precise details of how planets form are complicated, but terrestrial and Jovian planets might have formed somewhat differently. (a) Planetesimals about 0.5 mi (<1 km) in size most likely form in the solar nebula from small dust grains colliding and sticking together. (b) Whereas planetesimals in the inner solar system grouped together to form the terrestrial planets, the outer Jovian planets could have begun as terrestrial-like planets that accumulated massive envelopes of hydrogen and helium, or, alternatively, the Jovian planets could have formed directly from the gas of the solar nebula.