It is always hard for a small fish to live in a big pond. Pluto's recent departure from the classical planet family proves that size really matters.
Astronomers2 voted on August 24, 2006 to create the first scientific definition of the word “planet”, and Pluto obviously didn't make the cut. It turned out to be only a “dwarf3 planet”.
After weeks of heated debate, over 2,500 astronomers from 75 countries voted on the definition of a “planet” at a conference of the International Astronomical Union (IAU). According to the definition, a planet must have a clear neighbourhood around its orbit4. Pluto has widely been considered a planet since its discovery in 1930. Unfortunately, it has a special orbit which overlaps5 with Neptune's6. It is all because, compared with Neptune, Pluto is very small. It is attracted by Neptune's gravity when the two planets get closer.
According to IAU, a “dwarf planet” should have an orbit around the Sun. It will not have a clear neighborhood around its orbit, and must not be a satellite. The new classification means that the science textbooks will have to be updated7.
The solar system is now made up of the eight “classical planets,” together with a number of dwarf planets. The classical planets are: Mercury8, Venus9, Earth, Mars10, Jupiter11, Saturn12, Uranus13 and Neptune.
The debate over Pluto's status started in 1992. After an advanced telescope was sent into space, astronomers started to find a belt of objects, which lies beyond Neptune. The belt contains nearly 1,000 objects, including Pluto. As they continued the exploration, scientists recently discovered at least 41 dwarf planets at the edges of the solar system. There are at least two objects of a similar size to Pluto.
It seems that even if the IAU had kept Pluto's status as a planet, the classical theory of nine planets in the solar system would have had to be changed, but the other way around. More dwarf planets of a similar size to Pluto would join the family of planets. Scientists will probably find many more dwarf planets.
What Do the Planets' Names Mean?
Almost all the English names of the solar system come from the names of gods in either Greek or Roman mythology14.
Mercury is named after the Roman god of travel, because it appears to move so quickly across the sky.
Venus was the Roman name for the goddess of love. This planet was considered to be the most beautiful star in the sky.
Earth comes from the Greek word “eraze”, which means “on the ground”.
Mars was named by the Romans after their god of war, because of its red, bloodlike color.
Jupiter is the largest planet of the family. It was named after the most important god in Roman mythology.
Saturn was the Roman name for the god of farming, the father of Jupiter.
Uranus was named after the Greek god of the sky.
Neptune, a blue planet, was named after the Roman god of the sea.
Because Pluto is far away and hard to observe, it was named after the Roman god of the underworld, who was able to make himself invisible.
There is a good way to remember the order of the planets. It is to make up a memorable phrase in which each word begins with the first letter of the planets' names.
Here is an example:
My Very Educated Mother Just Served Us Nine Pizzas.
Since Pluto is off the list of planet now, you need to remember the updated version:
My Very Educated Mother Just Served Us Nuts.
对一条小鱼来说,生活在一个大池塘中往往是艰难的。冥王星最近被逐出传统的行星家族证明了体积大小的确关系重大。
2006年8月24日,天文学家投票确定了“行星”的第一个科学定义。冥王星显然不符合要求，它只不过是一颗“矮行星”。
经过几周的激烈争论，来自75个国家的2,500多名天文学家在国际天文学联合会的一次大会上投票决定了“行星”的定义。根据定义，一颗行星必须能够清除其轨道附近的其他天体。自从1930年发现以来，冥王星一直被认为是一颗行星。不幸的是，它有一条与海王星的轨道重叠的特殊轨道。这都是因为同海王星相比，冥王星太小，当两颗星靠近时，冥王星受海王星引力的吸引。
根据国际天文学联合会的定义，“矮行星”应该有一条环绕太阳的轨道，它不能清除其轨道附近的其他天体，而且不是一颗卫星。新的分类方法意味着科学教材应该进行修正。
太阳系现由八颗“经典行星”和一些矮行星构成。这些经典行星是：水星、金星、地球、火星、木星、土星、天王星和海王星。
关于冥王星身份的论证始于1992年。在一架先进的天文望远镜被送入太空以后，天文学家开始在比海王星还远的太空发现了一条带状区域。这一带状区域有近千个天体，其中包括冥王星。随着探索的进一步深入，近来科学家在太阳系边缘发现了至少41颗矮行星，其中至少有两个天体的体积与冥王星相仿。
看来，即使国际天文学联合会依然把冥王星视为一颗行星，太阳系有九颗行星的经典理论也必须改变，但那得是另一番情形。更多的、与冥王星体积相仿的矮行星将加入行星大家族。科学家将来可能会发现更多的矮行星。
行星的“名字”有什么含义？
太阳系所有行星的英语名称几乎都取自希腊或罗马神话中神的名字。
水星（Mercury）取自罗马人的旅行之神，因为它掠过天空如此迅速。
金星（Venus）是罗马人的爱神之名。这颗行星被认为是天空中最美丽的星星。
地球（Earth）来自希腊词“平地”，也就是“在地上”的意思。
火星（Mars）取自罗马战神，因为它的颜色是红色的，像血一样。
木星（Jupiter）是这个大家庭中最大的行星，取自罗马神话中最重要的神。
土星（Saturn）是罗马神话中的农神，朱庇特的父亲。
天王星（Uranus）取自希腊神话的天神。
海王星（Neptune）是一颗蓝色的行星，取自罗马神话的海神。
因为冥王星距离遥远，难于观察到，它的名字取自罗马神话中的地狱之神，它可以使自己隐身。
记住这些行星排列次序的一个好办法是编造一个难忘的短语，每个单词以行星名称的首字母开头。
举例说明：
我的博学母亲刚刚给我们吃九块比萨饼。
既然现在冥王星已经不在此列，你得记住更新的版本：
我的博学母亲刚刚给我们吃坚果。