'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.
