Magnetic moment and plasma environment of HD 209458b as determined from Lyα observations

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Science  21 Nov 2014:
Vol. 346, Issue 6212, pp. 981-984
DOI: 10.1126/science.1257829

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Transit observations of HD 209458b in the stellar Lyman-α(Lyα) line revealed strong absorption in both blue and red wings of the line interpreted as hydrogen atoms escaping from the planet’s exosphere at high velocities. The following sources for the absorption were suggested: acceleration by the stellar radiation pressure, natural spectral line broadening, or charge exchange with the stellar wind. We reproduced the observation by means of modeling that includes all aforementioned processes. Our results support a stellar wind with a velocity of ≈400 kilometers per second at the time of the observation and a planetary magnetic moment of ≈1.6 × 1026 amperes per square meter.

Transit marked by magnetosphere effects

Life on Earth exists under the protective sheath of our magnetosphere that deflects charged particles blown out by the Sun. Kislyakova et al. calculated the strength of the magnetic field of a well-studied hot-Jupiter–type exoplanet that produces similar effects. During the planet's transit in front of its host star, HD 209458, hydrogen atoms leave a peculiar asymmetric signature in the transmitted spectrum.

Science, this issue p. 981

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