Radio emission from the planets

The history of radio astronomy has been marked by a long series of surprises, many of which have changed the paths of advancement in the science. This has been especially true with the planets, probably the field in which fewest surprises were expected. Now, what kind of radio emission was expected from the planets? Optical studies of the planets have been made, of course, for centuries, and from these we felt that we understood well the physics of the planets. This knowledge suggested that we should expect to observe only thermal radio emission from planets, the Planck radiation due to the planets’ nonzero temperature. At radio wavelengths the Planck law becomes the simple Rayleigh‐Jeans approximation in which the brightness $$\mathrm{b\hspace{0.17em}=\hspace{0.17em}}\frac{\text{2kT}}{{\lambda }^2}$$ The temperature T refers to the level in the outer layers of the planet we are observing with the telescope. This level may be in the atmosphere or it may be at the very surface of the planet itself. From this formula, we expected the spectra of planetary radio emission to follow a ${\lambda }^2$ law. The observable quantity in radio astronomy is the radio flux density which, for the planets, is the brightness multiplied by the solid angle subtended bSSy the planet in the sky. Obviously, we would also expect this flux density to go as ${\lambda }^2.$

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