HD 189733 b

Lore & Background

HD 189733 b is a hot Jupiter exoplanet located in the constellation Vulpecula, orbiting the star HD 189733. It was discovered by astronomers in France on October 5, 2005, via the transit method, and confirmed shortly after through Doppler spectroscopy, which also detected the Rossiter–McLaughlin effect. The planet has a mass about 11.2% greater than Jupiter’s and a radius about 11.4% larger, completing an orbit every 2.2 days at a high orbital speed. It is the closest transiting hot Jupiter to Earth, making it a prime target for atmospheric study. Its deep blue color, first determined in 2013, likely results from Rayleigh scattering, and its weather includes rain of molten glass. HD 189733 b was the first exoplanet to have its thermal map constructed, its overall color determined, its transit observed in X-rays, and carbon dioxide confirmed in its atmosphere. The planet is thought to be tidally locked, with a permanent day and night side, and its atmosphere is evaporating at a rate of 1–100 gigagrams per second. It has no detectable satellites larger than 0.8 Earth radii and no ring system.

Reader's Guide

HD 189733 b holds significance as the first exoplanet to have its thermal map constructed, its overall color determined (deep blue), and its transit viewed in the X-ray spectrum. It was also the first exoplanet confirmed to have carbon dioxide in its atmosphere. The planet's weather includes raining molten glass, and it exhibits winds up to 8,700 km/h from day to night side. In July 2014, NASA announced its very dry atmosphere. The blue color, likely due to Rayleigh scattering, was confirmed in 2013, though polarimetric measurements have been disputed. The planet is tidally locked and has no satellites or ring system.

Did You Know?

• HD 189733 b was the first exoplanet to have its thermal map constructed.
• The planet's weather includes raining molten glass.
• It was among the first exoplanets with carbon dioxide detected in its atmosphere, though not the first confirmed.

Discovery Through Transit and Spectroscopy

In October 2005, a group of French astronomers made a landmark announcement: a new world was circling the star HD 189733 in the constellation Vulpecula, roughly 64.5 light-years from our own Sun. The detection relied on the transit method, in which the planet periodically slides across the stellar disk and dims its light. Before photometric confirmation arrived, real-time radial velocity measurements had already picked up the Rossiter–McLaughlin effect, a subtle wavelength shift produced as the planet blocks portions of the rotating star. The following year, Drake Deming and colleagues used secondary-eclipse photometry to capture strong infrared thermal emission from the planet's dayside. By early 2007, Carl Grillmair and his team at NASA's Spitzer Science Center had extracted detailed infrared spectra, a result published in the Astrophysical Journal Letters alongside parallel work on HD 209458 b. Together, these two worlds became the first exoplanets ever to be directly spectroscopically characterized, a distinction that has kept them at the center of atmospheric research ever since.

A World of Glass Rain and Dry Air

Despite its resemblance to Jupiter in bulk size, HD 189733 b harbors an atmosphere unlike anything in our own solar system. Carbon dioxide has been confirmed as a constituent of the atmosphere, while high-resolution observations with the UVES spectrograph on the Very Large Telescope identified atomic sodium and probed further thermal properties. A condensate of magnesium silicate, with particle sizes on the order of ten to the minus second through minus first micrometers, appears to float in the haze. In July 2014, NASA reported that the planet's atmosphere is remarkably dry, a finding shared with HD 209458 b and WASP-12b. Early models classified the atmosphere as pL type, lacking the temperature-inversion stratosphere seen in cooler L-dwarf objects, and the apparent absence of a cooler shaded layer beneath the outer envelope supports that picture.

The First Blue Planet

One of the most celebrated achievements in exoplanet science came when researchers determined the overall visible color of HD 189733 b. The polarization signal indicated that the scattering atmosphere extends more than 30 percent beyond the opaque disk seen during transit. Follow-up work in 2011 showed the albedo is markedly stronger in blue wavelengths than in red, a signature consistent with Rayleigh scattering and molecular absorption at longer wavelengths. By 2013 the deep cobalt-blue hue had been confirmed through a second independent technique, making this the first planet whose overall color was established by two different methods. However, two later teams employing more sensitive polarimeters cast doubt on the original polarimetric measurements, placing upper limits on the signal and leaving the exact mechanism under debate.

X-ray Shadows and a Leaking Atmosphere

HD 189733 b has also pushed the boundaries of what astronomers can observe in extreme wavelengths. In July 2013, NASA reported the first-ever detection of a planet's transit in the X-ray spectrum. The result was striking: the planet's extended atmosphere absorbs three times more X-ray flux than visible light during the crossing, revealing a vast, tenuous envelope that stretches far beyond the opaque core. It was the second world, after HD 209458 b, for which such atmospheric escape had been measured. The planet's blistering 2.2-day orbit at speeds near 152 kilometers per second, combined with tidal locking that fixes one hemisphere in perpetual daylight, creates the conditions that drive this continuous hemorrhage of gas into space.