Hot Jupiters are large gaseous planets that orbit their star closely. Unlike our Jupiter, which radiates more heat than it receives from the Sun, hot Jupiters receive more heat from their star than from their interior. As a result, they can have a surface temperature of 1,000 K instead of the 160 K that Jupiter has. They are among the most common types of exoplanets and are the easiest type of exoplanets to discover.
Most hot Jupiters orbit red dwarf stars, which means they receive plenty of infrared light to keep them warm. But that also means they can easily radiate heat to maintain a constant temperature. One of the most common side effects of a hot Jupiter is that its upper atmosphere tends to swell and vaporize, creating either a fluffy, low-density world or a dense world devoid of lighter elements, depending on the stage.
There are some of these worlds that have extremely hot surfaces that make them appear star-like rather than planet-like. In 2017, astronomers discovered hot Jupiter KELT-9b. It orbits a bright blue star so closely that its period is only 1.5 days. This means that the planet is tidally connected to its star and starside temperatures reach more than 4,600 K. At that temperature, the molecules in its atmosphere would dissolve, creating a high-pressure region that would propel atoms to the dark side of the planet, where they would recombine into molecules. The surface temperatures of most stars in our galaxy are below those on the stellar side of the planet. KELT-9b was thought to be the exoplanet with the highest surface temperature, but a new world even hotter has recently been discovered.
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This new world orbits a star called WD 0032?317, which is not a main sequence star. It’s a white dwarf with a mass about half that of the Sun and a surface temperature of 37,000 K. That’s hot even for a white dwarf, meaning the star is likely to have crossed the main sequence less than left a million years ago. Its companion is a brown dwarf about 75 times the mass of Jupiter, so technically not a planet, but not a star either. It orbits the white dwarf so closely that its orbital period is only 2.5 hours.
The average temperatures of different exoplanets. Photo credit: Hallakoun et al
Calculating the surface temperature of this companion isn’t easy, but we know it must be very hot. White dwarfs emit light primarily in the ultraviolet range, while brown dwarfs emit primarily in the infrared. Therefore, it is much easier for the white dwarf to heat up the brown dwarf than it is for the brown dwarf to shed its thermal energy. Even a rough calculation gives a stellar temperature about as hot as the Sun.
In new work, the team used different models to determine an accurate surface temperature. The end result depends on whether the white dwarf has a core composed mostly of helium, or whether it has a core with a mixture of other elements, such as carbon. This would affect the amount of heat transferred to the brown dwarf. Based on their calculations, the brown dwarf star side likely has a surface temperature of around 8,000 K – 9,000 K. The dark side temperature is likely around 2,000 K. Compare this to the Sun’s surface temperature, which is around 5,700 K.
Reference: Hallakoun, Na’ama et al. “An irradiated analogue of Jupiter, hotter than the Sun.” arXiv preprint arXiv:2306.08672 (2023).