Facts collected about Jupiter are known to be true because the scientific method is used to arrive at logical, rational conclusions about the universe. Because of the scientific method, experts have a high degree of confidence that science's conclusions about the planet are correct.
Every method used to study Jupiter has also been used to study phenomena on Earth, where scientists can directly confirm the accuracy of the results. Jupiter is known to have at least 63 moons. These have been observed using telescopes. Telescope observations of objects in space can be trusted, because scientists have demonstrated the trustworthiness of telescope observation of objects on Earth.
The atmosphere of Jupiter is made mostly of hydrogen. Scientists discover the composition of another planet's atmosphere by observing the light spectrum as light travels through the planet's atmosphere. Each chemical element has a unique response to light. That response, known as the element's emission spectrum, shows that element is present. Hydrogen's abundant emission spectrum in analyses of Jupiter's atmosphere reveals it is by far the most common element in the gas giant's atmosphere.
Jupiter's mass is equal to approximately 318 Earths. Scientists are able to gauge an object's mass by its gravitational effects on other objects per Newton's Law of Gravitation, which has been tested by Earth-based experiments.