How is the information about the properties of planets deduced?
Astronomers have used a variety of methods. Once the scale of the solar system is determined, the angular diameters of the planets can be converted into linear diameters in kilometers. The mass of a planet is determined from the orbital period and distance of a planet’s satellite. In the case of Venus and Mercury which do not have moons, you use the gravitational deflection of man-made probes, or estimates of their mean density based on their likely chemical composition. The chemistry of planetary atmospheres comes from direct spectroscopic analysis in which the various elements and compounds can be read-off from the sunlight reflected from these planets like fingerprints. The presence or absence of planetary magnetic fields can only be determined from space probes equipped with magnetometers and particle detectors. Planetary surface temperatures can be deduced from infrared observations in which the actual heat radiation from these warm bodies can be measured.
