You have no doubt heard about the solar corona. It is the halo of plasma surrounding the Sun, most noticeable during a total solar eclipse. It extends millions of kilometers into space in all directions around the sun.
Every star has a halo. Solar is a well-studied phenomenon, and helps to reveal interesting dynamics of the solar magnetic field, solar wind, and even activity on the surface of the Sun.
Planets can have crowns too! Unlike stellar crowns, they are not made of plasma; instead, they are the upper levels of the planet's atmosphere, also known as the exosphere, where the density of neutral particles is extremely low.
The earth, for example, is called the geocorona, and it glows in the far ultraviolet light due to the scattering of sunlight from neutral hydrogen atoms. Mars also contains hydrogen in its corona, in addition to oxygen and carbon.
You can download a full size photo of Jupiter's optical halo, as well as a wallpaper version, here.
Sources: Photo: ESO / P. Horálek
