About the image. Drawn as a pale, swollen gas giant close to its hot white star, with high, hazy clouds, and from just above those clouds, where JWST found tiny quartz crystals. WASP-17b has never been seen; it is known from the dip in its star's light as it passes in front, and the cloud patterns are imagined.
29×as wide as our Sun looks from Earth
1 400×the light Earth gets
No ground: the view from the cloud tops
The sky from WASP-17b
WASP-17 would look 29× as wide as the Sun does from Earth, and the planet gets 1 400× as much light as Earth gets.
Its colour comes from its surface temperature of 6 550 K (6 280 °C); the Sun's is about 5 772 K (5 500 °C).
How fast WASP-17b spins is unknown, so the length of its day is too.
Apparent size = star radius ÷ orbit; light = size² × (temperature ÷ 5 772 K)⁴. Both from the values on this page. The landscape is imagined.
WASP-17b is a giant planet about 1 320 light-years away, found in 2009 by the SuperWASP survey as it crossed the face of its star every 3.7 days. Measured against Jupiter it is a strange object: nearly twice as wide, but with only about half the mass. Its average density is about 0.08 grams per cubic centimetre, less than a tenth of Jupiter’s and far less than water. It is one of the most swollen planets known.
Inflated by heat
WASP-17b orbits only 0.0515 AU from its star, a hot F-type star of about 6 550 K (6 280 °C), and its temperature is around 1 750 K (1 480 °C). Heat alone does not explain its size. The team that found it suggested that the planet is also being heated from inside, by tides from the star as its orbit is pulled into a circle. Why some hot Jupiters are inflated this much is still one of the open questions about them.
Going the wrong way
As a planet crosses its spinning star, it blocks first the side turning towards us and then the side turning away, and the starlight’s colour shifts in a pattern that reveals the planet’s direction. For WASP-17b the pattern came out backwards. Its orbit runs against the star’s spin, at an angle of about 150° as seen from Earth, which a second team confirmed in 2010. It was among the first planets found on such a retrograde orbit. Planets are born in a disc that turns the same way as their star, so something must have tipped its orbit over later; what did it is not known.
Clouds of quartz
On 12 and 13 March 2023 the James Webb Space Telescope watched WASP-17b cross its star for about ten hours, measuring the starlight filtered through its atmosphere at mid-infrared wavelengths. At 8.6 microns the spectrum showed the fingerprint of silica, the material of quartz. The particles are tiny, about 10 nanometres across, and float high in the atmosphere, where the pressure is about a thousandth of Earth’s at sea level. It was the first time silica had been found in the atmosphere of a planet beyond the Solar System.
The detection depends on the model used: it is strong compared with a cloud-free atmosphere, weaker against a generic haze. On Earth quartz forms in rock; on WASP-17b it seems to condense straight out of hot gas.
Water and a warm night side
Hubble and Spitzer had already found water vapour and a hint of carbon dioxide in its atmosphere, and later JWST spectra measured more water than a gas with the Sun’s composition would hold. In 2025 another JWST study measured the night side at about 1 000 K (727 °C), cooler than the day side but still hot enough to glow.
Its orbit, to scale
WASP-17b goes around once every 3.7 days, at 0.0515 AU from its star. The whole view is tiny on Solar System scale: Mercury orbits the Sun about 8 times farther out than the outermost orbit shown here.
Earth days passing: 0
How hot is it?
Earth (average)288 K · 15 °C
Water boils373 K · 100 °C
Venus surface737 K · 464 °C
Iron melts1 811 K · 1 540 °C
Sun's surface5 772 K · 5 500 °C
WASP-17b1 755 K · 1 480 °C
WASP-17b: equilibrium temperature. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
How long to get there?
WASP-17b is about 1 320 light-years away.
Light299 792 km/s1 320 years
Breakthrough Starshotplanned light sail, 20% of light speed6 600 years
Parker Solar Probefastest spacecraft so far, 192 km/s2.1 million years
Voyager 1leaving the Solar System, 17 km/s23.3 million years
At Voyager 1's speed the trip would take about 290 975 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
A brilliant white star on the left and a huge, pale, hazy planet beside it, an impression of the swollen gas giant WASP-17b close to its hot star. AI-generated painting.A sea of pale clouds stretching to a curved horizon under a misty, glittering sky, with a white-hot star glaring through the haze, an impression of the quartz-crystal clouds of WASP-17b. AI-generated painting.WASP-17b, 3D artist's impression, rendered from the interactive model.WASP-17b, 3D artist's impression, from another angle, rendered from the interactive model.WASP-17b, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2009SuperWASP discovers WASP-17b, on a probable backward orbit.
2010A second team confirms the retrograde orbit.
2013Hubble finds hints of water in its atmosphere.
2022Hubble and Spitzer data confirm water and show signs of carbon dioxide.
2023The James Webb Space Telescope finds quartz crystals in its clouds.
Questions people ask
What are WASP-17b's clouds made of?
Partly quartz. In 2023 the Webb telescope found silica crystals about 10 nanometres across high in its atmosphere.
Why is WASP-17b so puffy?
Nobody is sure. It is very hot, and its discoverers suggested extra heating from tides as its orbit was pulled into a circle.
Does WASP-17b orbit backwards?
Yes. Its orbit runs against its star's spin, at about 150° as seen from Earth.