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3.2. The transiting planet HD 209458

The low-mass companion to the star HD 209458 was the first extrasolar planet found to transit the optical disk of its parent star (Charbonneau et al. 2000, Henry et al. 2000, Mazeh et al. 2000). As I explained in the previous section, transits offer a unique opportunity to determine the properties of the orbiting planet. The Hubble Space Telescope followed four transits (Brown et al. 2001) and obtained an exquisitely detailed light curve (Fig. 8; note that the depth of the eclipse is only 1.7%!). The orbital period was determined to be 3.5247 days, the lower limit on the mass of the planet Mp sin i = 0.69 ± 0.05MJ (with the inclination angle i = 86°.6 ± 9°.14), and the radius of the planet Rp = 1.347±0.060 RJ. Most impressively, the precision of the HST light curve allowed even for searches for rings around the planet, for constraints on planetary satellites, and for probing the planet's atmosphere.

Figure 8

Figure 8. Light curve of the slight dimming of the star HD 209458 due to a planet passing directly in front of it, HST/STIS, April-May 2000. Credit: NASA, T. M. Brown, D. Charbonneau, R. L. Gilliland, R. W. Noyes, & A. Burrows.

A ring system with significant opacity around the planet would cause distortions of the light curve relative to that of a spherical body. In particular, one would expect small dips in the light curve before the first and after the fourth contact. The observations were consistent with no rings (maximum ring radius consistent with observations was 1.8 Rp).

A satellite orbiting the planet might be detectable either from its photometric signature (the satellite would block light in addition to that obstructed by the planet), or from its influence on the orbital motion of the planet. Brown:2001 showed that satellites larger than 1.2 Roplus (where Roplus is the Earth radius), or with masses larger than 3 Modot are excluded by the data.

Finally, precision spectrophotometric observations in the region of the sodium resonance doublet at 589.3 nm revealed that the photometric dimming during transit was deeper by (2.32 ± 0.57) × 10-4 relative to simultaneous observations in adjacent bands (Charbonneau et al. 2002). This additional dimming has been interpreted as absorption by sodium in the planet's atmosphere. In fact, the existence of a detectable sodium feature in the spectrum has been predicted by theoretical models (e.g., Seager and Sasselov 2000). Furthermore, observations of brown dwarfs (stellar objects below the hydrogen burning limit) with similar surface temperatures show strong absorption in alkali metal lines (e.g., Burrows et al. 2000). HD 209458 therefore represents the first detection of an atmosphere of an extrasolar planet. While not a shocking result in itself, since giant planets are clearly expected to have atmospheres, the actual detection marks the beginning of a new era in extrasolar planets research.

Even more interestingly perhaps, HST observations of HD 209458 during three transits revealed atomic hydrogen absorption of ~15% over the stellar Lyman alpha line (Vidal-Madjar et al. 2003). A comparison of the observations with models showed that the observations could be explained in terms of hydrogen atoms evaporating and escaping the planet in an asymmetric cometary-like tail. To account for the observed absorption depth, the simulations implied a minimum escape flow rate of ~ 1010 g s-1.

In the future, other spectral features (e.g., water vapor; with implications for the searches for extraterrestrial life) can be searched for. Furthermore, in principle, HST can detect even the secondary eclipse (when the planet is eclipsed by the star), expected to be characterized by a ~ 10-5 decrease in the luminosity. This will allow for a determination of the planet's albedo, and for a comparison with models of planetary atmospheres. All of these results, whether already obtained, or expected, are somewhat of a surprise, since HST has been heralded primarily as a "cosmology machine," not as an exquisite tool in exo-planetary research.

Moving now from planets to stars, the Hubble Space Telescope has been particularly instrumental in revealing new details of stellar births and deaths.

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