It's impossible to say for sure which two planets are exactly 856,000,000 miles apart without more information. Here's why:

* Planetary Distances Vary: The distance between planets is constantly changing as they orbit the Sun.

* No Precise "Average" Distance: While we can calculate average distances, these are just estimates. The actual distance between two planets will depend on their specific positions in their orbits at any given moment.

To find the answer, you would need:

1. Specify the time: Do you want the distance at a specific point in time?

2. Specify the reference point: Are you measuring from the center of each planet, or from their surfaces?

Let me know if you can give me more information about what you're looking for, and I can help you figure out which planets are close to 856 million miles apart at a specific point in time!