蓝精灵粑粑
Our solar system consists of the sun, nine planets (and their moons), an asteroid belt, and many comets and meteors. The sun is the center of our solar system; the planets, their moons, the asteroids, comets, and other rocks and gas all orbit the sun. The nine planets that orbit the sun are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic). Easy ways to remember the order of the planets are the mnemonics: "My Very Excellent Mother Just Sent Us Nine Pizzas" and "My Very Easy Method Just Simplifies Us Naming Planets" The first letter of each of these words represents a planet - in the correct order. The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto. Jupiter is so big that all the other planets could fit inside it. The Inner Planets vs. the Outer Planets The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun). The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons. The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto. They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto, which is small, rocky, and has one large moon plus two tiny ones). Temperatures on the Planets Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere. The outer, gaseous planets are much less dense than the inner, rocky planets. The Earth is the densest planet. Saturn is the least dense planet; it would float on water. The Mass of the Planets Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, and Pluto are orders of magnitude less massive. Gravitational Forces on the Planets The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared. A Day on Each of the Planets A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours. The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days). The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change. The Average Orbital Speed of the Planets As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion). 链接的网址还有图片对照参考资料：
78海吃不胖
我们经常看到关于彗星、流星或小行星的新闻，但是在心中其实并不是很清楚它们的区别，那么，彗星、流星、小行星是什么样的天体，它们之间的差别又是什么呢？
让我们从小行星开始。小行星是围绕太阳运行的岩石物质组成的小型天体，“小而多”，被称为 Asteroids ；一般在 内太阳系 观察不到（此概念之前文章介绍过，具体指火星圈以内的太阳系区域），因此，它们也被称为 planetoids 。
在这里多一句嘴，讲一点英语，-这个 oid 后缀，意思是像什么形状，有什么特征的物体。
比如大名鼎鼎的安卓系统吧，英文是 Andr-oid ，其中Andr来自古希腊语，是“人，男人anthropos”的变体， Andr- 和 -oid 合在一起就是“类似人的一种什么物体”，即“人形机器人”的意思了。
所以 Aster-oids 就可以理解为“星状体”， planetoids 表示“行星状体”。
已经观测到的小行星的大小差别很大，目前已知的小行星，最大的直径超过500公里，最小的直径仅有10米。
科学家们认为，小行星是50亿年前太阳系内部各星体早期形成的过程中的遗留物。
太阳系中的大多数小行星都可以在所谓的“小行星带”内找到，该小行星带位于远边缘的木星和近边缘的火星之间。
与小行星类似的是，彗星具有围绕太阳运行的行为。但与小行星不同的是， 彗星主要由冰和尘埃组成 ，这些组成物质是彗星最显著特征之一——彗星尾巴背后的驱动力，下文将对此进行解释。
像小行星一样，彗星的大小也有很大的差异。它们的直径通常从大约1公里到最近刚刚观测确认的破纪录的彗星C/2014 UN271，其直径约为130公里。当然，目前有一些科学家假设冥王星实际上是一颗巨大的彗星（由10亿颗较小的彗星组成），如果此假说是真的，将使冥王星成为已知最大的彗星。
大多数彗星都可以在太阳系的外围观测到。其中一部分位于所谓的奥尔特云中，这是一个巨大而遥远的冰星球体，以2000个天文单位到200000个天文单位的距离包围着我们的太阳系。
其余的彗星位于 柯伊伯带 ，距离我们更近，在海王星轨道之外。更接近奥尔特云，也就是说，与上述小行星带相比，柯伊伯带仍然是一个非常遥远的区域。
在地球形成和生命的早期阶段，有大量小行星和彗星撞击了地球。许多科学家认为，这些彗星给我们的星球带来了大量的水。（尽管一些研究表明我们星球上的大部分水来自太阳）。彗星上大量有机分子的发现引出了一种假设，即彗星可能也给地球带来了生命（或生命的前身）。
让我们从流星体开始。 流星体是太阳系内颗粒状的碎片 ，它们 小至沙尘，大至巨砾， 大小从直径一米到一粒灰尘大小不等。最小的颗粒有时被归类为“微流星体”或太空尘埃。请注意，流星体一词仅适用于它仍在太空中时的情况。
流星体进入地球（或其他 行星 ）的大气层之后， 由于流星通常会加速到极高的速度，它们往往会加热并产生明亮的光带。它 在路径上发光并被看见的阶段则被称为流星。许多流星来自相同的方向，并在一段时间内相继出现，则称为 流星雨 。
如果一颗流星没有完全燃烧并撞到地面，它的名称就会变成陨石。有趣的是，研究人员估计，每天大约有2500万个流星体和微流星体撞击地球，相当于每天大约有41吨的物质飞入我们的大气层。
jackor57992
Asteroids：小行星。 Asteroids,also known as “minor planets”,are numerous in the outer space.any of numerous small celestial bodies composed of rock and metal that move around the sun (mainly between the orbits of Mars and Jupiter)comets: 彗星(astronomy) a relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbitNebulae:星云 the body exclude of the asteroid and comets ,all of the ductile astronomical body .the Solar System:太阳系the sun with the celestial bodies that revolve around it in its gravitational field。 以上回答供您参考！希望对您有所帮助 杭州图书馆
