While popular science tends to favor extragalactic

While popular science tends to favor extragalactic astronomical research that emphasizes current challenges to physics, such ass 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 father 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 large cloud of gas containing the matter that would form the sun and its orbiting planets. The nebular hypothesis suggests that angular momentum would have morphed the nebular 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.





1. Which of the following discoveries, if true, would best support the protoplanet hypothesis that the temperature difference is responsible for the different sizes of planets on either side of the frost line?

A. The core of Saturn and the core of Mercury are found to be 98% composed of the same materials.
B. The cores of Saturn and Jupiter are found to each contain at least five chemical elements not found in the other.
C. The core of the Earth and the core of Mars are found to be comprised of the same mix of chemical elements.
D. A nearby star is found to be orbited by six planets, and the size of each is inversely proportional to its distance form the star.
E. The Earth;s moon is found to have a vastly different composition from that of the moons of Jupiter.