Geology of solar terrestrial planets billions of years ago in the cores of massive stars. These elements constituted only 0.6% of the material in the solar nebula. That is why the terrestrial planets could not grow very large and could not exert large pull on hydrogen and helium gas. Also, the faster collisions among particles close to the Sun were more destructive on average. Even if the terrestrial planets had had hydrogen and helium, the Sun would have heated the gases and caused them to escape. Hence, solar terrestrial planets such as Mercury, Venus, Earth, and Mars are dense small worlds composed mostly from 2%
Ice giant the terrestrial and gas giants is relatively straightforward and uncontroversial. The terrestrial planets of the Solar System are widely understood to have formed through collisional accumulation of planetesimals within the protoplanetary disc. The gas giants—Jupiter, Saturn, and their extrasolar counterpart planets—are thought to have formed after solid cores around 10 Earth masses () formed through the same process, while accreting gaseous envelopes from the surrounding solar nebula over the course of a few to several million years (Ma), although alternative models of core formation based on pebble accretion have recently been proposed. Some extrasolar giant planets may instead have formed