To determine which two planets should be the closest in diameter on the model, we can compare the diameters of the planets listed:
- Mercury: 4,800 km
- Venus: 12,100 km
- Earth: 12,800 km
- Mars: 6,700 km
- Jupiter: 143,000 km
- Saturn: 120,000 km
- Uranus: 51,100 km
- Neptune: 50,000 km
Looking at the diameters, the two planets with the closest sizes are:
- Uranus: 51,100 km
- Neptune: 50,000 km
Their diameters differ by only 1,100 km, which is significantly closer than the differences between the other planet pairs.
Thus, the answer is Uranus and Neptune.