> Another thing is that Earth is close to the upper limit of size to be practical to build rockets to get off of. We have low orbital speed just below 8 km/s.

> Another thing is that Earth is close to the upper limit of size to be practical to build rockets to get off of. We have low orbital speed just below 8 km/s. A planet with another km/s extra required to get to orbit is quite likely still able to launch rockets. Then, we launch probes with total speed well above 11 km/s. While that speed is achieved over some time, which incurs gravitational losses, we can still assume that even higher low orbital speed can be achievable by chemical rockets.