Astronomy picture of the day

Frequently asked questions about the Solar system

How many planets in our solar system?

Our solar system consists of nine planets and one star – the Sun. We list the planets in order of distance from the Sun is mercury. Venus. Earth. Mars. Jupiter. Saturn. Uranium. Neptune and Pluto. In order to remember which planet is farther from the Sun, and what is closer to him (the order of the planets) often use the English mnemonic: M y V ery E ducated M other J ust S erved U s N ine P izzas. If You have Russian analogue of such a mnemonic, then we are happy to publish it, putting in the authorship.

3. What is the shape of the Solar system?

The planets, asteroids and comets in the Solar System are loose particles left over from the formation of the Sun. Originally the gas and dust which would become the Sun was the core of a cloud much larger than the Solar System, probably several light years across; where 1 light year is equal to approximately 10,000,000,000,000 kilometers (or 6 trillion miles). The core was slowly rotating at first, but as it collapsed it spun faster, like a spinning ice-skater pulling in their arms. The rotation prevented the core’s equator from collapsing as fast as the poles, so the core became a spinning disc.

Gas and dust in the disc spiralled gradually in to the center, where it accumulated to form the Sun. But because dust is denser than gas, some of the dust settled to the mid-plane of the disc. These dust particles stuck together to make clumps, then clumps stuck together to make rocks, then rocks collided to make planets. In the case of the `gas giant’ planets, Jupiter, Saturn, Uranus and Neptune, the rocky cores were massive enough to also attract some of the gas. The outer layers of these planets are made up of hydrogen and other gases.

So the Sun is the collapsed core of an interstellar gas cloud, and the planets, asteroids and comets are small lumps of dust which stayed in orbit instead of spiralling into the Sun. The planets all formed within a very short period, probably a few million years, about five billion years ago. Within that short time, no-one knows for sure which of them formed first. Maybe the inner planets formed first and were dragged by the spiralling gas so they are now closest to the Sun; or maybe the outer planets formed first, and the inner ones are small because they didn’t have a long time to grow.

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