* 1 Astronomical Unit (1 AU): This is the average distance between the Earth and the Sun. It's a very large distance, about 93 million miles.

* 1 Light-second: This is the distance light travels in one second. It's incredibly fast, but still a relatively short distance compared to astronomical units.

* A Star: Stars are massive balls of hot gas that generate their own light and heat. They are significantly larger than planets.

* A Planet: Planets are celestial bodies that orbit stars. They are smaller than stars.

Here's the order from smallest to largest:

1. 1 Light-second: The smallest unit of distance.

2. A Planet: Planets are much smaller than stars, even the smallest stars.

3. A Star: Stars are massive, but even the smallest stars are much larger than a light-second.

4. 1 Astronomical Unit (1 AU): This is the largest unit of distance in this comparison.

Therefore, the smallest is 1 light-second.