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Inner Planets of the Solar System and Their Properties

Grade 8
Jun 10, 2023

Inner Planets


Our planet Earth is a part of the solar system, which has 8 planets revolving around a star named the Sun. Additionally, there are natural satellites revolving around most planets in the solar system; there are other celestial bodies called asteroids, meteors, and comets which are all a part of the solar system. In this session, we are going to look at some of the planets called the inner planets and their properties.


The planets closest to the Sun – Mercury, Venus, Earth, and Mars are called the inner planets. And the rest of the planets are called the outer planets.

Inner and Outer planets


Astronomical Unit:

The average distance between the Earth and the Sun is called 1 astronomical unit. It is a unit of distance used to measure the distance between planets and the Sun in the solar system.

1 AU = 1.496 × 1011 m

Astronomical Unit

The inner planets are found to be in rocky composition, and all are found within 1.5 AUs from the Sun.

All the inner planets have a hard surface on which a space probe can land. All these planets have craters on their surfaces, created by meteor impacts.


Most of what is known about these planets has come from robotic spaceflight missions.


Mercury is the closest planet to the Sun, and it moves around the Sun in a highly elliptical orbit. As it is the closest planet to the Sun, it is the fastest moving planet. Its distance from the Sun varies from as close as 47 million km to as far as 70 million km. There was only one spacecraft named the Mariner to visit Mercury and take a picture in 1975.

The temperature on the surface of Mercury can reach as high as 4670C, as it is the closest to the Sun. As it lacks an atmosphere, the temperature at nighttime can fall to -1830C. The heat absorbed by its surface during the day escapes easily into the space at night as there is no atmosphere that would act like a blanket. The length of the day on Mercury is 59 Earth days, and the length of the year is 88 Earth days. ​



Venus is similar to the Earth in size, mass composition, and distance from the Sun. However, it has no oceans and is fully covered with thick clouds. It has an atmosphere mostly comprising of CO2, a greenhouse gas, that acts as a blanket to keep the heat in. It absorbs the infrared radiation emitted from the surface. That’s why its temperature is the highest among others, i.e., 4710C.

The thick clouds reflect so much sunlight that, whenever visible, Venus is the brightest object in the sky. On Venus, the length of the day is 243 Earth days, and the length of the year is 225 days. Its day is longer than that of its year. Scientists have found volcanic features and impact craters on Venus from radar images. The interior of Venus is believed to be similar to that of the Earth.



The Earth is the only planet that is believed to have life on it. The Earth’s atmosphere consists mostly of nitrogen (78%) and oxygen (21%), which affects its climate and weather. It acts as a shield for all the harmful radiation coming from the Sun and also meteors, most of which burn up before striking the surface.

The Earth looks blue due to the water content (70%), and hence it is called a blue planet. The length of the day on Earth is 23 hours, 56 minutes, 4 seconds, and the length of the year is 365.24 Earth days.



The atmosphere of Mars is composed of carbon dioxide, nitrogen, and argon. It is so thin that the average atmospheric pressure is less than 1% of that of the Earth. Its surface temperatures vary greatly from -1330C to 270C. It has two small moons, Phobos and Deimos.

In May 2002, the Mars Odyssey spacecraft detected large quantities of water ice close to the surface. The ice is believed to be mixed into the soil near the Martian south pole. Mars is too cold, and its atmosphere is too thin to allow liquid water to exist on the surface for long.

More water exists frozen in the polar ice caps, and enough water exists to form ice clouds. In 2004 and 2005, two land vehicles collected data and took many pictures. Mars is also called a red planet, as it appears to be reddish in color. This is because of the high iron content in its soil.


Questions and Answers:

Match the following:

iMercuryaThe distance between the Earth and the Sun
iiVenusbRed planet
iiiEarthcBlue planet
ivMarsdHottest planet
v1 Astronomical UniteThe nearest planet to the Sun


i—e, ii—d, iii—c, iv—b, v—a.

inner planets


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