In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object, such as a planet or other object, in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo.

In the space environment close to a planetary body with a dipole magnetic field such as Earth, the field lines resemble a simple magnetic dipole. Farther out, field lines can be significantly distorted by the flow of electrically conducting plasma, as emitted from the Sun (i.e., the solar wind) or a nearby star. Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects of solar radiation or cosmic radiation. Interactions of particles and atmospheres with magnetospheres are studied under the specialized scientific subjects of plasma physics, space physics, and aeronomy.

Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that of a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core. Through the use of magnetometers, scientists were able to study the variations in Earth's magnetic field as functions of both time and latitude and longitude.

Beginning in the late 1940s, rockets were used to study cosmic rays. In 1958, Explorer 1, the first of the Explorer series of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of the Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the follow-up Explorer 3 later that year definitively proving its existence. Also during 1958, Eugene Parker proposed the idea of the solar wind, with the term 'magnetosphere' being proposed by Thomas Gold in 1959 to explain how the solar wind interacted with the Earth's magnetic field. The later mission of Explorer 12 in 1961 led by the Cahill and Amazeen observation in 1963 of a sudden decrease in magnetic field strength near the noon-time meridian, later was named the magnetopause. By 1983, the International Cometary Explorer observed the magnetotail, or the distant magnetic field.

The structure of magnetospheres are dependent on several factors: the type of astronomical object, the nature of sources of plasma and momentum, the period of the object's spin, the nature of the axis about which the object spins, the axis of the magnetic dipole, and the magnitude and direction of the flow of solar wind.

The planetary distance where the magnetosphere can withstand the solar wind pressure is called the Chapman–Ferraro distance. This is usefully modeled by the formula wherein ${\displaystyle R_{\rm {P}}}$ represents the radius of the planet, ${\displaystyle B_{\rm {surf}}}$ represents the magnetic field on the surface of the planet at the equator, ${\displaystyle V_{\rm {SW}}}$ represents the velocity of the solar wind, ${\displaystyle \rho }$ is the particle density of solar wind, and ${\displaystyle \mu _{0}}$ is the vacuum permeability constant:

A magnetosphere is classified as "intrinsic" when ${\displaystyle R_{\rm {CF}}\gg R_{\rm {P}}}$, or when the primary opposition to the flow of solar wind is the magnetic field of the object. Mercury, Earth, Jupiter, Ganymede, Saturn, Uranus, and Neptune, for example, exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when ${\displaystyle R_{\rm {CF}}\ll R_{\rm {P}}}$, or when the solar wind is not opposed by the object's magnetic field. In this case, the solar wind interacts with the atmosphere or ionosphere of the planet (or surface of the planet, if the planet has no atmosphere). Venus has an induced magnetic field, which means that because Venus appears to have no internal dynamo effect, the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see also Venus' induced magnetosphere). When ${\displaystyle R_{\rm {CF}}\approx R_{\rm {P}}}$, the planet itself and its magnetic field both contribute. It is possible that Mars is of this type.

When viewed from the Sun, a celestial body's orbital motion can compress its otherwise symmetrical magnetosphere slightly, and stretch it out in the direction opposite its motion (in Earth's example, from west to east). This is known as dawn-dusk asymmetry.