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What is a hot Jupiter?

The classification of exoplanets is currently rather arbitrary and lacks concensus. While the concept of a hot Jupiter is quite clear, there are details that are only vaguely defined in the literature. In essence, a hot Jupiter is a planet that has a mass and radius of the order of that of our own Jupiter, but orbits the host star at distance that is less (often much less) than the Earth-Sun distance. (Our Jupiter has a mass and radius of 317.828 and 10.9733 times the corresponding quantities for Earth, respectively.) The term “hot” in this context means that the planet is expected to be much hotter than our own Jupiter by virtue of the fact that the planet is much closer to the host star than our Jupiter-Sun distance. (Be sure to undertsand the caveats about exoplanet temperatures though.) A short star-planet distance implies a correspondingly short orbital period (by virtue of Kepler's 3rd law), of the order of days. These massive, close-in, short-period planets were completely unexpected and one of the biggest surprises in exoplanet research. They have no counterpart in our solar system.

What is a hot jupiter? artists impression

The Wikipedia page on the definition of a hot Jupiter (on 3 August 2012) gives only a vague defintion: “...a class of extrasolar planets whose characteristics are similar to Jupiter..” The article goes on to say that hot Jupiters orbit very close to their host star, between 0.015 and 0.5 astronomical units (AU). Curiously, the citiation for these numbers is a physics forum (physorg.com) post from 2006. No scientific paper is referred to in any part of the definition. We can do better than this with more recent data. Below is a plot of orbital period versus minimum mass for 739 out 777 confirmed exoplanets that had measurements of both quantities in the Extrasolar Planets Encyclopedia.

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Exoplanets Period versus Mass

Orbital period versus mass. For comparison, the planets in our solar system are marked as Me, V, E, Ma, J, S, U, N corresponding to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune respectively (numerical values are from solarsystem.nasa.gov).

Immediately, we see in the diagram that there are, broadly speaking, three patches of data in the plot. The group of data points on the lower right-hand side obviously corresponds to the hot Jupiters (high mass, short periods). Obviously, there are planets that lie inbetween these groups so the boundaries are not clear-cut, and there could be selection effects at work. However, it is also clear that there is a very marked enhancement in the density of planets lying between 100 to about 700 Earth masses (roughly a third to twice the mass of our Jupiter), corresponding to orbital periods in the range of 1 to 6 days. The corresponding star-planet distances lie between 0.01 and 0.1 astronomical units (AU) (very roughly). So, although this is still vague, there are rough boundaries that we can actually see in the data. To put things into perspective, there are 185 out of the 739 exoplanets (25%) in the period versus mass diagram above that have a mass greater than 100 Earth masses and an orbital period less than 6 days. In reality a proper definition of a hot Jupiter will have to wait until we have a sound physical understanding of them. At the moment there is no concensus about their origin, how they got so close to the host star, and why there is a pile-up of hot Jupiters, collectively speaking. A more detailed discussion can be found in the book Exoplanets and Alien Solar Systems.

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File under: Exoplanet types; exoplanet taxonomy; exoplanet classification; What is a hot Jupiter? Definition of a hot Jupiter. Properies of hot Jupiters. Period versus mass diagram from confirmed exoplanets. Define hot Jupiter.

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