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What Is the Nebular Theory?

Scientists have been asking the question "How can order come from chaos?" for centuries. In the 18th, they devised a possible answer: nebular theory. Initially only a hypothesis about how planets form around their suns, evidence has made it a full-fledged theory known by many names. Nebular theory explains the whirlwind formation of Earth's solar system and all other star systems in the universe.

Centuries of Searching
- In 1755, German academic Immanuel Kant furthered an idea by Swedish scientist Emanuel Swedenborg, which argued that gravity causes nebulae -- gaseous clouds -- to spin until they collapse and flatten. Gravity also causes debris from collapsed nebulae to whip into stars and planets. That same century, French mathematical astronomer Pierre-Simon Laplace proposed a similar hypothesis that a contracting and cooling nebular cloud flattens and flings debris that collapse into planets. These ideas became mainstream among scientists in the 1970s, when Soviet astronomer Victor Safronov came up with the Solar Nebular Disk Model, or modern-day nebular theory.
The Theory Explained
- According to nebular theory, solar systems form when giant molecular clouds collapse. The clouds rotate slowly at first, but gravity pressures them until they contract. Like spinning ice skaters pulling in their limbs to increase their angular momentum, the contraction causes the material to spin faster. This forms a star. Other debris -- mostly gas and dust -- that surrounds the star goes through a similar process, becoming planets. Eventually, the star's center heats until it blasts gases away from it. Only planets and heavy debris remain; the drifting materials sometimes become moons when caught by a planet's gravity.
Consequences for the Planets
- Besides explaining why the solar system is organized as moons orbiting planets orbiting a sun, nebular theory also explains the makeup of planets. Far-flung Jovian planets have large ice cores that drew hydrogen and helium to form thick, gaseous atmospheres. Meanwhile, planets closer to the sun could never hold onto such gases, thanks to the sun's heat. Instead, they are made of rock and metal -- hence their name, terrestrial planets. Stronger gravity close to the sun also means more collisions, which in the early days of the solar system prevented these planets from growing as large as the outer planets.
Sifting Through the Evidence
- Evidence in our solar system favors nebular theory. For instance, planets orbit in roughly the same plane around the sun -- like a pancake flattened by the collapse of the nebular cloud -- and in the same direction. The differences in the cosmic makeup of the terrestrial and Jovian planets also supports nebular theory.

Astronomy