Of the roughly 4,300 exoplanets confirmed to date, about ten percent of them are classified as “hot Jupiters.” These are planets with masses between about 0.4 and 12 Jupiter-masses and orbital periods less than about 110 days (implying that they orbit close to their star—usually much closer than Mercury is to the Sun—and have hot surface temperatures). A “hot Neptune” has a smaller mass, closer to that of Neptune which is about twenty times less than Jupiter, and which also orbits close to its star. Astronomers study not only the properties of exoplanets but also how they evolved within their planetary systems. Hot Jupiters and hot Neptunes are puzzles. They are expected to have formed much farther out in the cold reaches of their systems as did the giant planets in our Solar System and then to have migrated inward to their current, close locations. Evidence supporting this evolutionary history should be found in the planets’ orbital eccentricities and other clues, but is difficult to obtain.
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Source: Phys.org