Thursday 23 February 2012

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 Fran├žois 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.