Mercury is the smallest and innermost planet in the Solar System. Its most defining characteristic is the extreme temperature variation on its surface, ranging from 430°C during the day to -180°C at night. This is due to the lack of a substantial atmosphere to retain heat.

Its surface is heavily cratered, resembling that of Earth's Moon, and is geologically inactive. Mercury possesses only a thin exosphere, composed of atoms blasted off its surface by solar radiation and micrometeoroid impacts, resulting in no appreciable weather.

The planet has an unusual orbital relationship with the Sun. It completes an orbit (Mercury Year) every 88 Earth days, the fastest revolution in the Solar System, yet its rotational period (Mercury Day) is much slower, taking 59 Earth days to complete a single rotation.

Interesting facts:

• The surface is marked by lots of wrinkles, massive cliffs formed by the global contraction of the planet as its large iron core cooled.
• Mercury has a disproportionately large metallic core, constituting approximately 85% of its radius. This suggests a unique formation history.
• Despite its proximity to the Sun, radar observations have confirmed the presence of water ice in permanently shadowed regions at the bottom of polar craters.

​Venus, the second planet from the Sun, is often termed Earth's "twin" due to its similar size and mass. However, its environmental conditions are profoundly different and hostile.

It is the hottest planet in the Solar System, with a constant surface temperature of approximately 465°C, a result of an extreme runaway greenhouse effect. The atmosphere is immensely dense, composed primarily of carbon dioxide with clouds of sulfuric acid, creating a surface pressure over 90 times that of Earth's.

The Venusian surface is a volcanic landscape, dominated by vast plains, shield volcanoes, and evidence of extensive past lava flows. Venus also exhibits unique orbital mechanics. It has a retrograde rotation, spinning clockwise on its axis, unlike the counterclockwise spin of most other planets. Consequently, a solar day on Venus (117 Earth days) is distinct from its sidereal rotation period of 243 Earth days, which is indeed longer than its orbital year of 225 Earth days.

Interesting facts:

• Venus rotates in the opposite direction to most planets, meaning the Sun would appear to rise in the west and set in the east.
• The thick atmosphere creates a supercritical state for carbon dioxide near the surface and causes any sulfuric acid rain to evaporate before reaching the ground.

Earth, the third planet from the Sun, is the largest and densest of the terrestrial planets. It is distinguished by its ability to sustain liquid water on its surface and is the only known astronomical object to host life.

This habitability stems from its location within the Sun's habitable zone and the presence of a stabilizing atmosphere composed primarily of nitrogen and oxygen. This atmosphere provides protection from solar radiation and frequent small-body impacts. Earth's surface is geologically active, with a crust divided into tectonic plates whose movement shapes continents, mountains, and ocean basins. It is also the only planet with persistent surface oceans.

Earth has one large natural satellite, the Moon, which stabilizes the planet's axial tilt and drives oceanic tides. The Earth-Moon system is dynamic; tidal interactions cause the Moon to recede from Earth at approximately 3.8 cm per year.

​Interesting facts:

• Earth is an oblate spheroid, with a slight equatorial bulge due to its rotation.
• Ongoing plate tectonics and a strong magnetic field generated by a liquid outer core are key drivers of its environmental evolution.
• Small asteroids, such as 2020 CD3, can be temporarily captured by Earth's gravity, becoming short-term natural satellites known as "minimoons."

Mars, the fourth planet from the Sun, is a cold, desert world characterized by its distinct reddish hue, a result of iron oxide (rust) prevalent on its surface. It possesses a thin atmosphere composed primarily of carbon dioxide, which provides negligible protection from solar radiation and offers minimal greenhouse warming, resulting in an average surface temperature of -63°C.

The Martian landscape is defined by extreme geological features, including Valles Marineris, a vast canyon system, and Olympus Mons, the largest volcano in the Solar System. Evidence of ancient riverbeds and deltas indicates that liquid water once flowed on its surface. The planet's polar caps consist of a mixture of water ice and frozen carbon dioxide ("dry ice").

Mars has two small, irregularly shaped natural satellites, Phobos and Deimos, which are thought to be captured asteroids. Phobos is gradually spiraling inward due to tidal forces and is predicted to either impact the Martian surface or disintegrate into a planetary ring system in tens of millions of years.

Interesting facts:

• The fine iron-rich dust in Mars's atmosphere scatters light differently than Earth's atmosphere, leading to a blue-grey hue around the Sun during sunsets.
• Olympus Mons is a shield volcano that dwarfs all terrestrial counterparts, reflecting a period of extensive volcanic activity on a planet without plate tectonics.
• The orbits of its moons, particularly the decaying orbit of Phobos, provide a visible record of gravitational interactions.