The orderly nature of our solar system leads most astronomers to conclude that the planets formed at essentially the same time and from the same primordial (original) material as the Sun. This material formed a vast cloud of dust and gases called a nebula. The nebular hypothesis suggests that all bodes of the solar system formed from an enormous nebular cloud consisting mostly of hydrogen and helium as well as a small percent of all the other heavier elements known to exist. The heavier substances in this frigid cloud of dust and gases consisted mostly of such elements as silicon, aluminum, iron, and calcium—the substances of today’s common rocky materials. Also prevalent were other familiar elements, including oxygen, carbon, and nitrogen.

Nearly five billion years ago, some external influence, such as a shock wave traveling from a catastrophic explosion (supernova), may have triggered the collapse of this huge cloud of gases and minute grains of heavier elements, causing the cloud to begin to slowly contract due to the gravitational interactions among its particles. As this slowly spiraling nebula contracted, it rotated faster and faster for the same reason ice-skaters do when they draw their arms toward their bodies. Eventually, the inward pull of gravity came into balance with the outward force caused by the rotational motion of the nebula. By this time the once vast cloud had assumed a flat disk shape with a large concentration of material at its center, called the protosun (pre-Sun). Astronomers are fairly confident that the nebular cloud formed a disk because similar structures have been detected around other stars.

During the collapse, gravitational energy was converted to thermal energy (heat), causing the temperature of the inner portion of the nebula to dramatically rise. At such high temperatures, the dust grains broke up into molecules and energized atomic particles. However, at distances beyond the orbit of Mars, the temperatures probably remained quite low. At -200℃, the tiny particles in the outer portion of the nebula were likely covered with a thick layer of ices made of frozen water, carbon dioxide, ammonia, and methane. Some of this material still resides in the outermost reaches of the solar system in a region called the Oort cloud.

The formation of the Sun marked the end of the period of contraction and thus the end of gravitational heating. Temperatures in the region where the inner planets now reside began to decline. The decrease in temperature caused those substances with high melting points to condense into tiny particles that began to coalesce (join together). Such materials as iron and nickel and the elements of which the rock-forming minerals are composed—silicon, calcium, sodium, and so forth—formed metallic and rocky clumps that orbited the Sun. Repeated collisions caused these masses to coalesce into larger asteroid-size bodies, called protoplanets, which in a few tens of millions of years accumulated into the four inner planets we call Mercury, Venus, Earth, and Mars. Not all of these clumps of matter were incorporated into the protoplanets. Rocky and metallic pieces that still remain in orbit are called meteoroids.

As more and more material was swept up by the inner planets, the high-velocity impact of nebular debris caused the temperatures of these bodies to rise. Because of their relatively high temperatures and weak gravitational fields, the inner planets were unable to accumulate much of the lighter components of the nebular cloud. The lightest of these, hydrogen and helium, were eventually whisked from the inner solar system by the solar winds.

At the same time that the inner planets were forming, the larger, outer planets (Jupiter, Saturn, Uranus, and Neptune), along with their extensive satellite systems, were also developing. Because of low temperatures far from the Sun, the material from which these planets formed contained a high percentage of ices—water, carbon dioxide, ammonia, and methane—as well as rocky and metallic debris. The accumulation of ices partly accounts for the large sizes and low densities of the outer planets. The two most massive planets, Jupiter and Saturn, had surface gravities sufficient to attract and hold large quantities of even the lightest elements—hydrogen and helium.

太阳系的有序性使得大多数天文学家得出这样一个结论：即行星基本上是在同一时间形成的，并且形成行星原始（原始）物质与太阳形成的原始物质相同。这种材料形成了大量的尘埃和气体，称为星云。星云假说认为太阳系中的所有星系都由巨大的星云组成，而星云中大部分是氢和氦，还有一小部分已知存在的所有其他的重元素。这种灰尘和气体云层中的重物质主要由硅，铝，铁和钙等元素构成，这些元素是现在常见的岩石材料。氧气，碳和氮等其他一些熟悉的元素也很普遍。 大约五十亿年前，一些外部影响，如来自破坏性爆炸（超新星）所带来的冲击波可能引发了巨大的气体云和重元素的微小颗粒的瓦解，由于颗粒之间的重力相互作用，导致云开始慢慢收缩。由于漩涡星云在慢慢地收缩，它旋转的速度越来越快，这与滑冰者在滑冰时将手臂拉向身体的原理相同。最终，重力的向内拉力与由星云旋转运动引起的外向拉力相平衡。此时，曾经巨大的云朵已经呈现出一个平坦的圆盘形状，其中心处存有大量的物质，称为原太阳（前太阳）。天文学家非常确定星云已形成了一个圆盘，这是因为他们已在其他恒星周围检测到类似的结构。 在瓦解过程中，引力能量转化为热能（热量），导致星云内部温度急剧上升。在如此高的温度下，尘埃颗粒分解成分子，并激活原子颗粒。然而，在火星轨道以外的地方，温度可能仍然非常低。在零下200℃时，星云外部的微小颗粒很可能会覆盖上一层由冷冻水，二氧化碳，氨和甲烷构成的冰层。某些材料仍然存在于太阳系最外层的一个称为奥尔特云的地区。 太阳的形成标志着收缩期的结束，因此也结束了引力加热。内行星所在地区的温度开始下降。温度下降导致高熔点物质凝结成小颗粒，开始聚合（连接在一起）。诸如铁和镍之类的材料以及成岩矿物所组成的元素 - 硅，钙，钠等- 形成了围绕太阳轨道周围的金属和岩石团块。这些团块的多次碰撞使这些群体聚合成了众多小行星大小的天体，称为原行星，几千万年后，沉积到了我们称之为水星，金星，地球和火星的四大内行星中。并非所有这些团块都被纳入原行星中。仍然留在轨道上的岩石和金属物质被称为流星体。 随着内行星扫荡了越来越多的物质，星云碎片的高速撞击使得这些天体的温度升高。由于天体的温度相对较高和引力场相对较弱，因而内行星无法积聚宇宙云中大部分的轻质组分。其中最轻的成分是氢气和氦气，最终，太阳风将这两种成分从太阳系内部中吹走。 在内行星形成的时候，更大的外行星（木星，土星，天王星和海王星）以及它们广阔的卫星系统也处于发展中。由于距离太阳远，温度低，这些行星中形成的物质含有大量的冰 - 水，二氧化碳，氨和甲烷 - 以及岩石和金属碎片。冰块的堆积在一定程度上解释了外部行星的大尺寸和低密度。两颗最大的行星，木星和土星，其表面重力足以吸引和保存大量甚至最轻的元素 - 氢和氦。