Silicate snow on HD 189733b.

HD 189733b was discovered in 2005 by astronomers working at the Haute-Provence Observatory; its discovery was announced in a paper in the journal Astronomy & Astrophysics by a team of researchers lead by François Bouchy of the Laboratoire d'Astrophysique de Marseille and the Haute-Provence Observatory. It is a Hot Jupiter type planet 63 light years from Earth in the constellation of Vulpecula, orbiting a K-type Orange Dwarf star (HD 189733A), with a mass 80% that of the suns, at a distance of 0.03 AU, i.e. 3% of the distance between Earth and the Sun, or one tenth of the distance at which Mercury orbits the Sun.

Illustration of HD 189733b by Martin Kornmesser of the Hubble European Space Agency Information Centre.

HD 189733b is one of the best studied Hot Jupiter type-planets. It has a mass of 1.13 × that of Jupiter, 6.17 × Jupiter's volume. It orbits HD 189733A once every 53.25 hours. The planet has an atmosphere comprised primarily of hydrogen (H₂) and helium (He), but significant amounts of water (H₂O), methane (CH₄) and carbon monoxide (CO) have been also detected.

In a paper published on the online arXiv database at Cornell University Library on 21 February 2012, and accepted for publication in the Monthly Notices of the Royal Astronomical Society, a team of scientists lead by Catherine Huiston of the Astrophysics Group at the School of Physics at Exeter University describe an attempt to produce a vertical temperature profile for the atmosphere of HD 189733b using the Hubble Space Telescope, and the conclusions derived from their results.

Huitson et al. resolved the temperature of HD 189733b to 1280 K in the lower atmosphere, bellow heights of 500 km, rising to 3600 K in the outer layers of the atmosphere. They also found a distinct scattering effect in the atmosphere that they attributed to the presence of magnesium silicate (MgSiO₃).

MgSiO₃ is of interest to geologists on Earth as it forms different minerals at different pressures, thereby serving as a proxy for how deep within the Earth's mantle rocks originated at. In the lower atmosphere of HD 189733b it could have far stranger properties, since it sublimates (turns directly from a solid to gas) at about 1300 K (depending on pressure). Thus it could evaporate within the lower atmosphere, rise till it met a temperature inversion, where the temperature fell below the sublimation point, then fall as snow till the temperature rose high enough to sublimate it again.

See also Wasp-12b; slowly boiling away... KOI-13b, a big, hot planet not a Brown Dwarf, Water in the stratosphere of Jupiter, A new study of XO-2b, a planet in a wide binary star system and Exoplanets on Sciency Thoughts YouTube.

Sunset on Osiris.

'Osiris' is an informal name given to the planet HD 209458b, the first transiting exoplanet (planet in another solar system, detected as its orbit regularly brings it in front of its star when seen from Earth), discovered by the European Space Agency's Hippacaros Satellite in 1999, when press interest in exoplanets still warranted their being given names rather than just numbers. It is a Hot Jupiter type planet, with a mass 69% of that of Jupiter and a volume 250% of Jupiter's (the lower mass of the planet makes for less gravity to pull it into a tighter ball, and the higher temperature also makes the planet expand, leading to a planet with a smaller mass but a larger volume than Jupiter) orbiting a sun-like star 150 light years from Earth in the constellation of Pegasus. The planet orbits the star at a distance of 0.044 AU; 4.4% of the distance between the Earth and the Sun, or 12.5% of the distance at which Mercury orbits the sun. It has a year of only 84 hours.

Later studies by the Spitzer Space Telescope, and the Very Large Telescope revealed the planet to have a circular orbit, a surface temperature of at least 750 °C, and an atmosphere rich in Carbon Monoxide (CO) with clouds of silica dust (SiO₂), and superstorms with winds reaching 7000 km/h.

In 2008 a team lead by David Sing of the Institute d'Astrophysique de Paris at Université Pierre et Marie Curie was able to create a complete optical spectrum for the planet, using data gathered by the Hubble Space Telescope, their results being published in a paper in the Astrophysical Journal. Sing et al. found sodium and hydrogen ions (Na⁺ and H⁺) as well as Vanadium and Titanium Oxides. They also found that the planets atmosphere has a temperature inversion and distinct troposphere and stratosphere, as on Earth. They calculated that the atmospheric temperature at the bottom of the atmosphere would be around 1700 °C, falling to about 500 °C at the top of the troposphere, then rising again slowly in the stratosphere. They calculated that this could led to clouds of liquid sodium at the top of the troposphere, leading to sodium 'rain' that would evaporate in the hotter lower troposphere to rise up and recondense in the cloudy layer.

On 6 January 2012, Frédéric Pont of the University of Exeter published an image of a sunset on Osiris on the ExoClimes Website based upon the Hubble spectrographic data. This has a strange, greenish tinge to it, due to the absorption of light in the orange part of the spectrum by the gaseous sodium. Pont acknowledges that this image was influenced by an earlier video of a sunset on Osiris that appeared on the Instítut national des sciences de l'univers website in 2008, and may also have been influenced by the fact that David Sing now works at Exeter.

Pont's image of a sunset on Osiris.

Pont also produced an image of a sunset on HD 189733b, another Hot Jupiter type planet with a well studied atmosphere. HD 189733b lies in a binary system 63 light years from Earth in the constellation of Vulpecula. The planet orbits close to the system's primary star (HD 189733A), an orange dwarf with 82% of the mass of the sun, a radius 75% of the suns and 26% of its luminosity. A second star, a dim red dwarf (HD 189733B) orbits every 2300 years at a distance of 216 AU (i.e. 216 times the distance at which the Earth orbits the Sun, or seven times the distance at which Neptune orbits the Sun). HD 189733b is only 0.03 AU from HD 189733A (3% of the distance between the Earth and the Sun), and is slowly boiling away into space as a consequence. It orbits the star once every 53 hours. Sodium, Potasium and Carbon Dioxide have been found in its atmosphere, which is thought to lack an inversion layer and distinct stratosphere and troposphere, due to the extreme heating caused by the close-by star. None of this appears to be visible in the sunset picture,

Pont's image of a sunset on HD 189733b.

See also A fresh look at Wasp-43b, A planet disintegrating in the heat of its own sun, Wasp-19b: a highly irradiated Hot Jupiter, The atmosphere of GJ1214b and Exoplanets on Sciency Thoughts YouTube.