A ‘year’ on WASP-33b lasts just 29 hours (Image: NASA/ESA/G. Bacon/STScI)

Astronomers have found the hottest planet yet, a gas giant with a temperature of nearly 3200 °C, which is hotter than some stars.

A collaboration called the Super Wide Angle Search for Planets (SuperWASP) announced hints of the planet’s existence in 2006. The group had observed periodic dimmings of the parent star possibly caused by a planet about 1.4 times the size of Jupiter passing in front of the star once per orbit.

Follow-up measurements confirmed the planet’s presence in 2010, showing distortions of the star’s light spectrum that could only be due to a planet’s influence. The measurements showed the planet’s mass is less than 4.5 times that of Jupiter.


Called WASP-33b, the planet orbits its star at less than 7 per cent of Mercury’s distance from the sun, whipping around the star once every 29.5 hours.

Hot star, close planet

That is not the tightest orbit known, but WASP-33b’s parent star is one of the hottest known to host a planet. The star is a scorching 7160 °C, far hotter than the sun, whose visible surface is about 5600 °C.

The combination of its close orbit and its parent star’s temperature heat WASP-33b to nearly 3200 °C, according to new infrared measurements made with a camera on the William Herschel Telescope in the Canary Islands. Alexis Smith of Keele University in Staffordshire, UK, led the study.

WASP-33b’s incredible temperature is higher than some red dwarf stars. It is also about 900 °C hotter than another sizzlingly hot planet, WASP-12b, which was measured at around 2300 °C.

Puzzling planets

WASP-33b might help astronomers gain new insight on hot planets, whose properties have proven puzzling, says Drake Deming of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is not a member of Smith’s team.

For example, the outer atmospheric layers of some star-hugging planets appear to be colder than the deeper layers, which is surprising given that they are heated externally by intense radiation from their stars.

This might be due to complex carbon-based chemicals that change the way the planets’ atmospheres respond to radiation, Deming says. Such chemicals could be formed by the action of ultraviolet light from the parent stars, which would be especially intense in the case of WASP-33b, given how hot its parent star is, he says.

“This certainly would be a planet that you would want to look at,” he says. “It’s a really exceptional opportunity to study a planet around a really hot star.”

Journal reference: arxiv.org/abs/1101.2432

When this article was first posted, the sentence “It is also about 900 °C hotter than another sizzlingly hot planet, WASP-12b…” incorrectly stated “It is also about 700 °C hotter than another sizzlingly hot star, WASP-12b…”