Astronomy Basics: Planets and Stars

Ganymede, Jupiter's largest moon, is the largest natural satellite in our solar system with a diameter of about 5,268 km, even larger than the planet Mercury. It's one of the four Galilean moons of Jupiter. Earth's Moon is smaller (3,474 km), while Phobos and Deimos are tiny moons of Mars.

Mercury orbits the Sun at an average speed of 47.87 km/s, making it the fastest planet in our solar system. According to Kepler's laws, orbital speed decreases as distance from the Sun increases, so the innermost planet has the maximum orbital velocity.

Proxima Centauri is a red dwarf star 4.24 light-years from Earth, making it the nearest known star after the Sun. It is part of the Alpha Centauri triple star system but is physically closer than the Alpha Centauri AB pair.

Pluto has an average surface temperature of approximately -229°C, making it the coldest known planetary body in our solar system. Neptune is colder than the other major planets, but Pluto's extreme distance results in even lower temperatures.

Sinope (Jupiter XVIII) orbits at an average distance of 23.3 million km from Jupiter, making it the farthest known Jovian satellite and the farthest satellite from its parent planet in our solar system.

Sirius (the Dog Star) has an apparent magnitude of -1.46, making it the brightest star visible from Earth. Its brightness is due to its intrinsic luminosity and relatively close distance of 8.6 light-years.

Venus is the brightest planet as viewed from Earth because of its proximity and highly reflective thick atmosphere. At its brightest, it reaches an apparent magnitude of -4.6, far outshining Jupiter (magnitude -2.9) and Mars.