"Hot Jupiter" is the term that astronomers use to refer to massive extrasolar gas giants -- close to or larger than Jupiter -- that orbit within about 0.05 AU (astronomical units, or Earth-Sun distances) of their home star, about one-eighth the distance between Mercury and the Sun, or less than about 9 million kilometers (6 million miles). At such close distances to their sun, these planets can have surface temperatures of 1300 degrees F (700 degrees C) or greater, resulting in winds as fast as 6,000 mph (9,600 kph). Confirmed extrasolar planets given the Hot Jupiter status include HD 189733 b (63 light-years distant), HD 209458 b ("Osiris", 150 light-years distant), and 51 Pegasi b ("Bellerophon", 50 light-years distant, the prototype Hot Jupiter and the first planet discovered to orbit a Sun-like star).
Like other classes of planets such as rocky planets and gas giants, Hot Jupiter planets have a few characteristics in common. First of all, they have a high chance of transiting their star, that is, passing in front of it, making their observation easier from the perspective of astronomers on Earth. This transiting is observed as a momentary and systematic fluctuation in stellar output.
Secondly, because Hot Jupiter planets are so intensely hot, their density is lower and their size is larger than it would otherwise be if they were located further out, like our own planet Jupiter. This means that, unlike the Earth, which has a precise atmosphere, Hot Jupiters have a tenuous atmosphere that gradually fades away rather than having any sharp boundary. This can make determining their size from transit data difficult.
Hot Jupiter planets have two other common characteristics: likely formation much further from their stars than they currently are, and low eccentricity orbits. Like our Jupiter, these planets formed further away from their star, where there was more available matter, and slowly migrated inward due to unstable orbits over billions of years. Their location near their stars is one of the main reasons they are easier to spot -- large planets located far from their stars are harder to detect because their rarely transit their home star.
Hot Jupiters also have low eccentricity orbits, meaning their orbits are highly circularized. Furthermore, they are tidally locked to their home star, meaning that one face of the planet experiences eternal day and the other eternal night. These temperature differentials create immense winds that distribute some of the heat from the day side to the night side.
Though we have only discovered a few Hot Jupiters so far, many astronomers suspect we will discover dozens more over the coming decades, as our astronomical equipment becomes more sensitive.
