While popular science tends to favor extragalactic astronomical research that emphasizes current challenges to physics, such as the existence of dark matter, dark energy, and Cosmic inflation, significant research continues to take place in the field of planetary astronomy on the formation of our own solar system. In early attempts to explain this phenomenon, astronomers believed in the encounter, or “rogue star,” hypothesis, which suggests that matter was tidally stripped away from our sun as a larger star passed within a gravitationally-significant distance some billions of years ago.
The encounter hypothesis postulates that after being stripped away, the matter cooled as it spun farther from the sun, and formed planets with their own centers of gravity. This hypothesis conveniently accounts for the fact that all planets in the solar system revolve in the same direction around the sun; it is also consistent with the denser planets remaining closer to the sun, and the more gaseous planets traveling further away.
The encounter hypothesis explained the phenomenon sufficiently enough that it allowed scientists to focus on more immediately rewarding topics in physics and astronomy for most of the first half of the 20th century. Closer investigation, however, found several significant problems with the encounter hypothesis, most notably that the hot gas pulled from the sun would not condense to form dense planets, but rather would expand in the absence of a central, gravitational force. Furthermore, the statistical unlikelihood of a star passing in the (astronomically speaking) short time of the sun’s existence required scientists to abandon the encounter hypothesis in search of a new explanation. Soon after, astronomers formed a second theory, the nebular hypothesis, which submits that the solar system began as a large cloud of gas containing the matter that would form the sun and its orbiting planets. The nebular hypothesis suggests that when the cloud reached a critical mass, it collapsed under its own gravity. The resulting angular momentum would have morphed the nebula into a protoplanetary disc, with a dense center that generated intense heat and pressure, and a cooler, thinner mass that revolved around it. The central mass would have continued to build in density and heat, forming the sun, while the centrifugal force around the disc’s edge kept smaller masses from being pulled in to the sun; those masses, upon cooling, would break off to become planets held in orbit by the competing gravitational force of the sun and centrifugal force of their orbital inertia.
The nebular hypothesis, however well it explained the sun’s formation, remained problematic in its ability to account for the formation of several planets with differing physical and chemical properties. Encouraged by their advance toward a provable hypothesis for the solar system, scientists have recently come to adopt a third hypothesis, the protoplanet hypothesis. This currently accepted theory holds that the gaseous cloud that would form the solar system was composed of particles so cold that even the heat of the forming sun could not significantly impact the temperature of the outer reaches of the cloud. Gas in the inner region, within what scientists refer to as the frost line, was quickly either burned or dispersed, leaving a small amount of metallic matter, such as nickel and iron, to form the inner planets. Such matter would need to have an extremely high melting point to avoid becoming liquefied, ensuring that Mercury, Venus, Earth, and Mars would remain small and dense. Outside the frost line, however, gas was kept cool enough to remain in solid, icy states. Over time, planets such as Jupiter and Saturn would amass large quantities of frozen gas, enough to grow to hundreds of times the size of the Earth.
According to the nebular hypothesis, a protoplanetary disc formed in the early stages of the solar system because
(A) Cold gases in the outer reaches of the nebula were repelled from the hot center of the spiraling mass.
(B) Gravity forced the nebular cloud to contract upon itself, creating significant angular momentum.
(C) Cooling matter held safely from the center of the mass could eventually form planets.
(D) Matter with a high melting point could not be consumed by the heat in the center of the disc.
(E) Gravity from a passing star pulled matter away from the sun, allowing planets to form around it.
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