CHEOPS observes dark-blue giant planet

The planet around the star HD 209458 was the first discovered to transit its star. This enabled precise measurements of the planet’s mass and size. The planet HD 209458 b, or Osiris as it’s nicknamed, is bigger and lighter than Jupiter, and orbits its star tightly taking 3.5 days for a single revolution. Now the space telescope CHEOPS has measured reflected light from the planet by observing how flux from the star-planet system decreases when the planet passes behind the star. HD 209458 b

HD 209458 b
I illustrationen av planetsystemet är stjärnans, planetens och planetbanans storlekar skalenliga. Den första figuren visar hur ljuset från systemet avtar under okultationen (då planeten går bakom stjärnan. Den andra figuren visar hur ljuset hade avtagit om planeten varit lika reflekterande som Jupiter. Bild: A. Brandeker.

The measurement shows that the planet reflects only 10%, which is comparable to asphalt and less than one fifth as much light as Jupiter. This confirms a 22 year old prediction that hot Jupiters like this should be dark. That the planet is so dark is thought to be due to the combination of Rayleigh scattering of light in the hydrogen atmosphere combined with efficient absorption by gaseous sodium. Since blue light is less efficiently absorbed by sodium this will give the reflected light a blue colour.

The study is published by Astronomy & Astrophysics Letters (659, L4) and led by Alexis Brandeker at the Department of Astronomy, Stockholm University, also including Göran Olofsson at the same institute.

Contact: Alexis Brandeker, alexis(at)