One of Earth's defining features is its magnetic field, which acts as a shield against high-energy particles from the Sun, providing a safer environment for life to evolve.
Auroras, colorful light displays near the poles during high solar activity, and the fact that a compass points north, are visible signs of Earth's magnetic field.
To determine if other planets or moons have magnetic fields, scientists use spacecraft equipped with magnetometers or observe phenomena such as radio emissions from auroras.
Gas giants like Jupiter and Saturn, and ice giants like Uranus and Neptune, possess strong intrinsic magnetic fields, whereas it's more complex for terrestrial planets and moons.
Earth, Mercury, and Jupiter's moon Ganymede have internally generated magnetic fields, while Mars and Earth's moon have old rocks preserving remnants of past magnetic fields.
Venus shows no intrinsic magnetism, but instruments haven't been placed close enough to its surface to search for magnetized crust.
For a magnetic field to exist, a planet or moon needs a large volume of conductive liquid in motion. If these materials stop moving or there's insufficient temperature difference to drive convection, the magnetic field can disappear.
Possible reasons for Venus' lack of a magnetosphere include a slowly cooling core, a completely solid interior, absence of an inner core, or a chemically stratified core. Upcoming missions aim to explore these possibilities.