Passage V

Gas-giant planets have a solid core surrounded by a massive envelope of gases. Gas-giant planets form within stellar nebulae—large, flattened clouds of gas and dust that surround newly formed stars. Planet formation begins when some of the solid material within a stellar nebula clumps together to form a spherical body called a planet core. Two scientists discuss how gas-giant planets like Jupiter form from planet cores in stellar nebulae.

Scientist 1

A planet core continuously attracts additional solid particles from the stellar nebula, gradually increasing its mass. If a planet core reaches a minimum mass of 10 times the mass of Earth (10 M_E), its gravity is then strong enough to attract and hold gases from the surrounding stellar nebula. The formation of a 10 M_E planet core takes approximately 1 million years. The time required for a 10 M_E planet core to then attract a minimum 300 M_E of gases and thus become a gas-giant planet like Jupiter is 10 million to 100 million years more. Because it takes so long to form gas-giant planets like Jupiter, they are very rare in our galaxy.

Scientist 2

Gas-giant planets form in a stellar nebula if gravity variations cause the nebula to suddenly fragment into 2 or more massive spherical clumps of gas and dust called gas giant protoplanets (GGPP), each having a planet core at the center that is no larger than 6 M_E. A GGPP forms in less than 1,000 years. After formation, the GGPP contracts to form a gas-giant planet. The entire process from the start of GGPP formation to a new gas-giant planet takes only about 1 million years.

Gas-giant planets like Jupiter would not have time to form as described by Scientist 1. Observations indicate that stellar nebulae around newly formed stars do not remain in existence longer than 7 million years after the stars are formed. Also, planet cores that are not part of a GGPP do not have enough momentum to keep them from spiraling into the star at the center of the nebula within 100,000 years. Gas-giant planets like Jupiter are not rare. At least 130 gas-giant planets as large as or larger than Jupiter have been identified in our galaxy.