HD 189733 b is the exoplanet famous for its deep blue color caused by atmospheric haze.
This hot Jupiter orbits the star HD 189733, about 64 light-years away in the constellation Vulpecula. It is a gas giant roughly comparable in size to Jupiter, but it circles its star extremely closely, completing an orbit in only about 2.2 days. That proximity heats its atmosphere to well over 1,000 kelvins and makes the planet a valuable target for transit and eclipse observations.
The blue appearance is not an ocean-like reflection. Astronomers associate it with light scattering in the planet's atmosphere, including fine particles or haze. The effect is broadly analogous to why Earth's daytime sky looks blue, although the chemistry, pressure, temperature, and particle environment are very different. Observations have also suggested that the planet's atmosphere is dynamic and may contain high-altitude clouds or hazes.
A frequent mistake is to call HD 189733 b an Earth-like blue world. Its color does not indicate liquid water or a habitable surface: the planet is a swollen, intensely irradiated gas giant. Its host star is also much cooler and smaller than the Sun, making the system very unlike our own.