タイトル:The Early Solar System(初期の太陽系)


The origin of our solar system has long been the subject of speculation. The most widely accepted explanation is the nebular hypothesis, which argues that the solar system began as a massive cloud of interstellar gas and debris that collapsed under immense gravitational forces. The resulting mass of swirling matter eventually formed into pockets of material that then came together to become the sun and planets. While this accounts well for some aspects of the solar system’s formation, other predictions derived from the hypothesis do not line up with the solar system’s current state. For example, in computer simulations based on the hypothesis, the masses of certain planets and the regions in which those planets reside were inconsistent with current astronomical observations. The discrepancies, however, could be due to the fact that the nebular hypothesis was originally based on the assumption that planets in our solar system formed in their current locations and have mostly remained there.




Some of these mysteries may be close to being solved. The smaller planets of the inner solar system are believed to have formed from matter able to withstand the sun’s tremendous heat, such as rock and metal. Conversely, the gas giants Jupiter and Saturn are thought to have formed in regions farther from the sun, beyond what is referred to as  the frost line, as temperatures at this distance are low enough for the matter that makes up gas giants to solidify. Scientists, however, have observed Jupiter­like gas planets in distant solar systems nicknamed “hot Jupiters” orbiting their suns at surprisingly close distances. To explain this, scientists proposed the concept of planetary migration, and a scenario named the “grand tack” was modeled, using Jupiter’s formation as a basis, to test the concept. In the scenario, Jupiter takes shape beyond the frost line, as is expected, but migrates toward the inner solar system, imitating hot Jupiters. However, as a result of gravitational forces from our solar system’s other gas giant, Saturn, Jupiter is drawn back out to its current location farther from the sun. Despite sounding far­fetched, the scenario  has been praised for plausibly explaining why hot Jupiters exist.



The grand tack scenario may also provide clarity for other apparent inconsistencies in our solar system. When the computer simulations based on the nebular hypothesis projected that Mars should be much larger than it currently is, the proposed grand tack migration of Jupiter suggested an explanation. By factoring in Jupiter’s trajectory in the grand tack scenario, scientists realized the gas giant was likely scattering matter close to the sun that would otherwise have greatly increased Mars’s mass. Furthermore, had Jupiter not made its migration, planets in the inner solar system, which includes the area where Earth is now, could very well have become larger, uninhabitable worlds shrouded by gas. The vast majority of the inner regions of other known solar systems are populated by such planets, so scientists believe our solar system may be extremely unique in this regard. If the grand tack scenario proves to be correct, humankind owes a great deal of gratitude to Jupiter for the planet’s journey through the solar system.












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