Besides Earth, Mars, and Mercury, the other planet in our solar system that has a solid surface where astronauts could potentially land is the Moon. The Moon is Earth's natural satellite and is the only celestial body beyond Earth that humans have walked upon during the Apollo missions.
The gas giants in our solar system, such as Jupiter and Saturn, do not have solid surfaces. They are primarily composed of gas and have no well-defined solid ground to land on. Instead, they have dense atmospheres and a transition from gas to liquid or solid deep within their interiors.
Other planets like Venus, Uranus, and Neptune have solid surfaces beneath their thick atmospheres, but they present significant challenges for human exploration due to extreme temperatures, high atmospheric pressure, and other harsh conditions. Venus, for example, has a thick, toxic atmosphere with a surface temperature that exceeds 800 degrees Fahrenheit (427 degrees Celsius), making it inhospitable for humans.
In summary, besides Earth, Mars, and Mercury, the Moon is the only celestial body in our solar system with a solid surface where astronauts have landed.