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# Extrasolar Planet HD 209458 b (Osiris)

The exoplanet HD 209458 b is one of only a handful that has a nickname (Osiris). It was one of the first transiting exoplanets to be discovered, and is one of the best-studied hot Jupiters. There are many results in the literature concerning the physical properties of HD 209458 b and the system in which it resides.

HD 209458 b was one of the first exoplanets that lent itself to the studies of atmospheres of planets outside our solar system. Sodium was discovered in its atmosphere, as well as Oxygen, Carbon, and water vapour. The planet is also known to have a comet-like tail that stretches to a size of about 4 times the size of the planet. The issue of whether the atmosphere of HD 209458 b is escaping has been hotly debated. The planet is also one of the gas giants that show what is known as the “radius anomaly,” which refers to the fact that the average density of the planet is so low that it cannot be explained by standard theory concerning the composition and physics of its structure. Also, direct infrared emission has been detected from HD 209458 b (Deming et al. (2006)).

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The star HD 209458 is about located at about 153 light years from Earth, and has a mass ($M=1.148\pm0.022 \ M_{\odot}$) and size ($R=1.146\pm0.059 \ R_{\odot}$) that are similar to our Sun. The mass of the planet HD 209458 b is about three-quarters that of Jupiter ($M=0.714\pm0.017 \ M_{J}$), and a size that is about 40/ larger than Jupiter ($R=1.38\pm0.018 \ R_{J}$). However, remember that the planet has a large comet-like tail. The orbital period is, as for other hot Jupiters, very short, at about 3.52 Earth days, corresponding to a maximum star-planet distance (semimajor axis) of less than 5% of the Earth-Sun distance ($0.04747\pm0.00055$ astronomical units (AU)). The orbit is nearly circular.

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File under: HD 189733 b; Well-studied transiting system and hot Jupiter; Hot Jupiter atmospheres.