Mid-Jupiter, also known coincidally as jovian planet, is a classification of planet with mass ranging from 0.5 to 2 Jupiter masses or 158.9 to 635.6 Earth masses. Mid-Jupiters are gaseous with no solid surface. Hence it is the second most massive mass class of planet, mid-Jupiter is rated M-Class II and has the symbol Jb.
Mid-Jupiters are gaseous with no solid surface, but there is a rocky core surrounded by liquid hydrogen-helium mixtures in upper mantles and liquid metallic H-He in lower mantles. Those planets have banded clouds with several featured storms powered by interiors or sunlight depending on the planet's distance from the star. Mid-Jupiters have radii ranging from 0.8 RJ to 1.4 RJ. Hot Jupiters are larger, up to 2 RJ, caused by intense heating from nearby stars causing thermal expansion.
There are an estimated 69 billion mid-Jupiters in our galaxy alone, making it the second least abundant mass class of planet after super-Jupiter. This corresponds that 84‰ of all 820 billion planets in our galaxy are mid-Jupiters.
Jupiter, the only mid-Jupiter in our solar system, and more than 300 around other stars are mid-Jupiters. The first planet found around an ordinary star, 51 Pegasi b (Bellerophon), is a mid-Jupiter. Few other notable exoplanets are mid-Jupiter as well: HD 209458 b (Osiris), (HD 189733 b (Teumesia), and PSR J1719-1438 b, (Tapio). Tapio, the only solid mid-Jupiter known, is an ultra-dense diamond planet that orbits around the pulsar. However Tapio was a white dwarf in the past that lost more than 99.9% of its original mass because of the proximity to its parent pulsar causing mass transfer.