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“Hot Jupiters” are gas giants close to their parent star, resulting in high surface temperatures and fast winds. They have low eccentricity orbits and a high probability of transiting their star. Their density is lower and size larger than Jupiter due to their intense heat. They formed farther from their star and migrated inward over billions of years. Astronomers expect to discover more in the future.
“Hot Jupiter” is the term astronomers use to refer to huge extrasolar gas giants – close to or larger than Jupiter – that orbit within about 0.05 AU (astronomical units, or Earth-Sun distances) of their parent star, about one-eighth the distance between Mercury and the Sun, which is less than about 9 million kilometers (6 million miles). At such close distances from their sun, these planets can have surface temperatures of 1300 degrees F (700 degrees C) or higher, resulting in fast winds of up to 6,000 mph (9,600 kph). Extrasolar planets confirmed given Hot Jupiter status include HD 189733 b (63 light years away), HD 209458 b (“Osiris”, 150 light years away) and 51 Pegasi b (“Bellerophon”, 50 light years away , the Hot Jupiter prototype and the first planet discovered orbiting a Sun-like star).
Like other classes of planets such as rocky planets and gas giants, the Hot Jupiter planets have some characteristics in common. First, they have a high probability of transiting their star, that is, passing in front of it, making it easier for astronomers to observe them on Earth. This transit is observed as a momentary and systematic fluctuation of stellar production.
Second, because the Hot Jupiter planets are so intensely hot, their density is lower and their size is larger than they would otherwise be if they were located further away, like our own planet Jupiter. This means that unlike Earth, which has a definite atmosphere, Hot Jupiters have a tenuous atmosphere that gradually fades away rather than having a sharp boundary. This can make it difficult to determine their size from transit data.
Jupiter’s hot planets have two other common characteristics: probable formation much farther from their stars than they currently are, and low eccentricity orbits. Like our own Jupiter, these planets formed farther from their star, where there was more matter available, and slowly migrated inward due to unstable orbits over billions of years. Their location close to 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 they rarely transit their home star.
Hot Jupiters also have low eccentricity orbits, meaning their orbits are highly circular. Additionally, they are tied to their home star, which means that one side of the planet experiences eternal day and the other side experiences eternal night. These temperature differences create immense winds which distribute part of the heat from the day side to the night side.
While we’ve only discovered a few hot Jupiters so far, many astronomers suspect we’ll discover dozens more over the next few decades as our astronomy equipment becomes more sensitive.
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