In our Solar System, which planet orbits closest to the Sun and therefore has the highest orbital speed as it moves around the Sun?

Introduction / Context:
Planets move around the Sun in elliptical orbits, and their orbital speeds are not all the same. According to Kepler's laws and Newtonian gravity, planets that are closer to the Sun experience a stronger gravitational pull and must move faster to stay in orbit. This question asks which planet has the highest orbital speed, which is directly linked to which planet is closest to the Sun.

Given Data / Assumptions:
- The options list Saturn, Jupiter, Mercury, and Earth.
- Mercury is known to be the innermost planet, closest to the Sun.
- Jupiter and Saturn are gas giants much farther from the Sun.
- Earth is the third planet from the Sun.
- We assume standard orbital mechanics where closer planets move faster.

Concept / Approach:
Kepler's third law and Newton's law of gravitation together show that orbital speed decreases with increasing distance from the central body. In the Solar System, the planet nearest to the Sun, Mercury, must move very quickly to remain in orbit. In fact, Mercury's average orbital speed is much higher than that of Earth, Jupiter, or Saturn. Earth, being farther out, orbits more slowly, and the giant planets even more slowly. Therefore, identifying the closest planet to the Sun provides the answer: Mercury has the highest orbital speed.

Step-by-Step Solution:
Step 1: Recall the order of planets from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Step 2: Identify Mercury as the innermost planet and therefore the closest to the Sun. Step 3: Remember that planets closer to the Sun feel a stronger gravitational pull and must move faster to balance that pull. Step 4: Note that Earth is third from the Sun and moves slower than Mercury, and Jupiter and Saturn are far beyond, with even slower orbits. Step 5: Conclude that Mercury has the highest orbital speed around the Sun.

Verification / Alternative check:
Astronomy references list average orbital speeds for planets. Mercury's orbital speed is about 47 kilometres per second, Earth's about 30 kilometres per second, Jupiter's about 13 kilometres per second, and Saturn's even less. These figures directly confirm that Mercury moves fastest. The trend of decreasing orbital speed with increasing distance is consistent throughout the Solar System, reinforcing the idea that the innermost planet has the highest speed.

Why Other Options Are Wrong:
- Saturn, a gas giant with rings far from the Sun, is much farther away and therefore orbits more slowly than inner planets.
- Jupiter, the largest planet, has great mass but its distance from the Sun means its orbital speed is lower than that of the inner planets.
- Earth moves at a moderate speed compared with Mercury; being third from the Sun, it does not have the highest orbital speed.

Common Pitfalls:
Some learners may incorrectly associate size with speed and guess Jupiter because it is the largest planet. However, orbital speed depends mainly on distance from the Sun, not the mass of the planet itself. Others may think about day length or rotation speed, which are different from orbital speed. To avoid confusion, remember that the closer a planet is to the Sun, the faster it must move along its orbit. Since Mercury is the closest, it must also be the fastest.

Final Answer:
The planet with the highest orbital speed around the Sun is Mercury, the innermost planet nearest to the Sun.