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Given that some exoplanets, particular "Hot Jupiters", orbit very closely to their parent star, how hot can these planets become?

What is the hottest exoplanet discovered so far?

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The temperature of an exoplanet is dependent on a number of factors, including:

  • The distance between the exoplanet and the parent star.

  • The type (energy output) of the parent star.

  • The ability of the exoplanet's atmosphere to distribute the heat energy.

Recent discoveries, such as reported in the article "Sizzling Planet Makes Some Stars Look Cool" (Than, 2007), have found that some exoplanets are hotter than some types of stars.

For example, the exoplanet HD 149026b has a star-facing side of over 2000C (3700F), this planet reflects so little light that it would appear like a "glowing piece of charcoal". It is believed (from the article) that the planet is tidally locked and the 'dark side' of the planet is believed to be substantially cooler.

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What is the hottest exoplanet discovered so far?

As far as I can tell, that honor goes to Kepler-70b. Estimates for its surface temperature range up to 7,143 K. The other confirmed planet in the system, Kepler-70c, may also be extremely hot. The two orbit their parent star - a hot b-type subdwarf - at distances of 0.0060 and 0.0076 AU, respectively, and thus receive quite a lot of radiation. The discovery paper (Charpinet et al. (2011)) reported that the two are likely chthonian planets, having been engulfed in the star's envelope during its red giant phase.

The Habitable Planets Catalog has lists of top exoplanet extremes. Kepler-70b and Kepler-70c top the list of hottest exoplanets, and are listed as having surface temperatures of 7,143 K and 6,351 K, respectively. I would imagine that planets close in would have much greater surface temperatures, as they are heated more; these two may also have residual heat from the engulfment.

There are several other exoplanets on the lists that top 2,000 K (there are likely more):

  • Kepler-13 b (3206 K)
  • PSR 1719-14 b (3118 K)
  • WASP-33 b (2505 K)
  • WASP-12 b (2368 K)
  • WASP-103 b (2342 K)
  • KELT-1 b (2296 K)
  • WASP-18 b (2258 K)
  • OGLE2-TR-L9 b (2213 K)
  • WASP-121 b (2202 K)
  • WASP-76 b (2039 K)

The page is a year or two old, and there are clearly other hot exoplanets that have since been discovered. However, it seems that Kepler-70b and Kepler-70c are still the hottest.

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