Here's the order of planets in our solar system from slowest to fastest orbital speed around the Sun:

1. Neptune

2. Uranus

3. Saturn

4. Jupiter

5. Mars

6. Earth

7. Venus

8. Mercury

Why does this order occur?

The farther a planet is from the Sun, the slower its orbital speed. This is due to Kepler's Laws of Planetary Motion, specifically Kepler's Third Law. It states that the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit (basically, the average distance from the Sun).

* Longer orbital period = slower orbital speed

Let me know if you'd like more details about Kepler's Laws!