Mercury is the closest planet to the sun, the smallest and rocky planet. A year on Mercury is 88 Earth days. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system. Mercury's thin atmosphere or exosphere is composed mostly of oxygen (O2), hydrogen (H2), and other ingredients. Two missions have visited Mercury.

Here on Earth our home planet, we consider a year to be 365 days. Unless it’s a Leap Year, which takes place every four years (in which it is 366 days). Earth is a rocky planet with a stable and dynamic surface of mountains, canyons, plains, and more. Most of our planet is covered with water. Earth has one moon. Earth's atmosphere has 78 per cent nitrogen, 21 per cent oxygen, and other ingredients—the perfect balance to breathe and live. Earth is the ideal place for life as we know it. Humans stand on Earth.

Venus is a terrestrial planet, Earth-sized. A year on Venus is 225 earth days. Many scientists believe, water once existed on its surface. Venus and Mercury have too high temperatures and currently cannot support life as we know it on Earth. Future Venus explorers will search for evidence of an ancient ocean. Venus orbits our Sun, a star. Venus is the hottest planet in our solar system. Venus has many visitors, more than 40 spacecrafts have explored Venus.

Mars has an orbital period of 687 earth days which means a year on Mars is 687 earth days. Mars is known as the Red Planet because iron minerals in the Martian soil oxidise, causing the soil and atmosphere to look red. Mars has two moons, and a thin atmosphere made up of carbon dioxide (CO2), oxygen, and other ingredients. At the present time, the surface of Mars cannot support life as we know it on planet Earth. Several missions have visited Mars and the current missions are determining Mars' future potential for life. The radar data collected by Mars Express in July 2018, indicating a pond of liquid water can be interpreted as a positive sign.