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The Inner Planets
The Inner Planets

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The Inner Planets

These worlds are mostly made up of metals or silicate rocks.

The Inner Planets

Released under Creative Commons CC0 on Pixabay.com.

Mercury

Diameter: 4,880km

Moons: 0

Distance from Sun: 58 million km

The closest planet to the Sun, Mercury is a place of extremes. It is the smallest and densest planet in the Solar System, barely larger than our Moon. It takes 59 Earth days to rotate once and 88 to orbit the Sun, meaning its parched surface experiences temperatures hot enough to melt lead on the sunward side, but is sub-Antarctic on the side in shadow.

This small world is a real challenge to observe for a variety of reasons. It's a fast mover, travelling around the Sun four times more quickly than Earth, so don't expect it to hang about in any part of the sky for very long. Mercury's orbit is a fairly eccentric oval shape, and it's on a bit of a tilt too, which means some times are better for viewing it than others: spring evenings and autumn mornings. If that's not tricky enough, you only have a relatively short observation window on any day you choose to look, as Mercury never strays very far from the Sun.

In spring, start looking 30 minutes after sunset, after which you'll have about another 45 minutes to see it. Autumn gives you a longer view, from about an hour and 45 minutes before sunrise, but that does mean getting up exceedingly early.

Venus

Diameter: 12,100km

Moons: 0

Distance from Sun: 108 million km

Venus is sometimes called 'Earth's evil twin'. It is similar in size and composition to our planet, but a dense carbon dioxide atmosphere and sulfuric acid clouds make its surface a hellish 470°C. The planet spins slowly, in the opposite direction to most planets, and takes about the same time to rotate on its axis (243 Earth days) as it does to travel around the Sun (225 days).

As Venus's orbit is slower than Mercury's, it can be visible for months on end, and sometimes for up to three hours after sunset or before sunrise. When Venus is at its brightest, it becomes the third brightest object in the sky, only beaten by the Moon and the Sun. This is caused by sunlight reflecting off its bright white carbon-dioxide clouds, and has led to Venus being called the 'Evening Star' or 'Morning Star' depending on when it appears. Venus can come very close to Earth, plus it's rather big, meaning that it's a good target for binoculars, through which you can easily see its larger phases.

Mars

Diameter: 6,800km

Moons: 2

Distance from Sun: 228 million km

The Red Planet is the most visited extraterrestrial destination in the Solar System. Dozens of missions have ventured there, and they have explored the Martian landscape in incredible detail. Smaller than Earth but with the same land area, Mars is like a cold, rocky desert, littered with canyons and volcanoes. The planet has polar caps and a thin atmosphere of mostly carbon dioxide. Although dry today, Mars's mineral salts and rock formations suggest that it was wet in the past, and could possibly have harbored life.

Mars differs from Mercury and Venus in that its position in the Solar System — on the other side of Earth — means it can be 'up' from sunset until sunrise. A small telescope can reveal lighter, pale-reddish areas, darker patches and the bright white of the ice caps.

The Dwarf Planets

Diameter range: 975km to 2,330km

A dwarf planet is, according to the International Astronomical Union, a body that orbits the Sun and is not a satellite, spherical in shape due to its own gravity and too small to have cleared its orbit of debris and so claim the title of a fully fledged planet. This classification was agreed after the 2005 discovery of Eris, an icy body in the outer Solar System very similar to Pluto, which was then considered a planet. In the fierce debate that followed Pluto was demoted into the newly created class, which also contains outer Solar System bodies Haumea and Makemake, and Ceres in the Asteroid Belt.

Ceres is the largest, but still comparatively small, so you will need binoculars to find it. Pluto is best seen by taking images of the region of sky it is in over consecutive nights and looking for the faint moving dot.

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Date Taken: 05/19/2017
Author: BBC Sky at Night Magazine
Category: Astronomy

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