It has to do with Bode's Law. This is from an email I received:
Bode's Law predicts that the distances of the planets from the Sun, should go up in the following sequence (measured in Astronomical units - AU)That explains the 2.8 spot.
0.4 - 0.7 - 1.0 - 1.6 - 2.8 - 5.2 - 10.0 - 19.6 - 38.8
At roughly those distances, there are.
0.4 - Mercury - at 0.4au
0.7 - Venus - at 0.7au
1.0 - Earth - at 1.0au
1.6 - Mars - at 1.5au
5.2 - Jupiter - at 5.2au
10.0 - Saturn - at 9.5au
19.6 - Uranus - at 19.2au
38.8 - Neptune at about 30au, Pluto at about 39au
As you can see, the planets actually do occupy positions at (or very near to) those predicted by Bode's Law.
Also - you'll see that there's nothing major at 2.8au (the 2.8 'spot' in the sequence).
Or at least that's what we used to think.
We have since discovered the asteroid belt, which averages at around 2.8au - including the orbit of Ceres (discovered 1801), which is by far the most massive asteroid.