About the image. Drawn glowing yellow-white on its dayside, as a body near 4 600 K (4 330 °C) would. The temperature is measured; the cloud bands are imagined.
72×as wide as our Sun looks from Earth
34 000×the light Earth gets
Tidally locked: the star stays putNo ground: the view from the cloud tops
The sky from KELT-9b
KELT-9 would look 72× as wide as the Sun does from Earth, and the planet gets 34 000× as much light as Earth gets.
Its colour comes from its surface temperature of 9 270 K (9 000 °C); the Sun's is about 5 772 K (5 500 °C).
KELT-9b is tidally locked: one side always faces the star.
There are no sunrises. Where you stand decides where the star hangs, overhead on the day side, low at the edge, gone on the night side.
Apparent size = star radius ÷ orbit; light = size² × (temperature ÷ 5 772 K)⁴. Both from the values on this page. The landscape is imagined.
When B. Scott Gaudi and colleagues published KELT-9b in Nature in June 2017, it was the hottest planet ever found, and it still holds the record. Its dayside reaches about 4 600 K (4 330 °C). That is warmer than the surfaces of some stars.
A planet around a hot star
Most known planets orbit stars like the Sun or cooler. KELT-9 is a hot, blue-white star of type B9.5 to A0, more than twice the Sun’s size and mass. KELT-9b circles it in a day and a half, with one face always turned to it.
Too hot for molecules
At these temperatures, molecules on the dayside are torn apart into single atoms. In 2018 Jens Hoeijmakers and colleagues detected iron and titanium in the atmosphere as atomic vapour: metals that, on Earth, we only know as solids. On the cooler night side, atoms can recombine.
Evaporating
The planet’s outer atmosphere reaches almost to the edge of its gravitational hold, and radiation from the star is driving it away. In 2020 the loss was measured at between 18 and 68 Earth masses per billion years. KELT-9b is nearly three times as massive as Jupiter, so it will survive for a long time, but it is visibly being cooked.
Its orbit, to scale
KELT-9b goes around once every 1.5 days, at 0.03368 AU from its star. The whole view is tiny on Solar System scale: Mercury orbits the Sun about 11 times farther out than the outermost orbit shown here.
KELT-9b: dayside temperature. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
How long to get there?
KELT-9b is about 667 light-years away.
Light299 792 km/s667 years
Breakthrough Starshotplanned light sail, 20% of light speed3 335 years
Parker Solar Probefastest spacecraft so far, 192 km/s1.0 million years
Voyager 1leaving the Solar System, 17 km/s11.8 million years
At Voyager 1's speed the trip would take about 147 030 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Your weight on KELT-9b
On KELT-9b
Gravity there is about 1.94× Earth's, at the cloud tops; there is no surface. Calculated from the measured mass and radius.
Gallery
Artist's impressions rendered from the same model as the view at the top of this page.
Timeline
2016Announced from KELT-North observations.
2017Published in Nature as the hottest known planet.
2018Iron and titanium found as atomic vapour in its atmosphere.
2020Its rate of atmospheric loss is measured.
Questions people ask
What is the hottest known planet?
KELT-9b, with a dayside of about 4 600 K (4 330 °C), hotter than the surfaces of some stars.
Is KELT-9b evaporating?
Yes. Radiation from its star drives gas off the planet, measured in 2020 at 18 to 68 Earth masses per billion years.
Why is KELT-9b so hot?
It orbits very close to a hot blue-white star, more than twice the Sun's size, completing a year in 1.5 days.