Astronomy News 2007/03/20 HEAG meeting Astronomers Puzzled by Spectra of Transiting Planet Orbiting Nearby Star

First spectra of exoplanets Two teams reported on Feb 21 on their independent findings about two transiting exoplanets. These are the first spectra from planets outside our own solar system.  C. J. Grillmair et al., ApJ 658, L115-L118, 2007 April 1 “A Spitzer spectrum of the exoplanet HD b”  L. J. Richardson et al., Nature 445, , 2007 Feb 22 “A spectrum of an extrasolar planet”

HD b HD ➢ K0 V star ➢ 19.3 pc HD b ➢ orbital period = 2.2 days ➢ orbital radius= AU ➢ duration of secondary eclipse ~ 1.9 hr

Observations Infrared Spectrograph on the Spitzer Space Telescope 2006 October 21 and November hr of telescope time μm flux ranging from 0.6 to 1.8 mJy 0.49%±0.02% of the flux of the parent star

Flat flux ratio Theoretical calculations by different teams unanimously predicted that water vapor should be the most obvious spectral feature. However, the variation in the measured fractional flux is very nearly flat over the entire wavelength range and shows no indication of significant absorption by water or methane.

Why lack of water First of all, there could really be no water, which is not very likely. Second, there could be some other chemicals in the planet's atmosphere that emit radiation just where water absorbs it, thereby effectively camouflaging the signature of the water. This too seems unlikely. Third, the water could be hidden underneath an opaque cloud layer the Spitzer telescope can't see through. Fourth, a theoretical model suggests that, if the planet is in tidal lock (in other words, is so close to its sun that the same side always faces the same way), the atmospheric temperature profile on the day-side of the planet could be such that spectral features are suppressed.

Is something hiding the molecules from sight? One clue comes from the spectrum of a second planet, HD b. That spectrum shows hints of silicates. Such molecules form rocks on the Earth, but on the scorching-hot worlds studied with Spitzer, silicates exist as tiny dust grains that can form clouds. They think that both planets may be cloaked in dark silicate clouds.

Spectrum of HD b

New era of extrasolar planets The best way to clear up the mystery is to study additional "hot Jupiters" to determine if they show similar signs in their atmospheres. Astronomers also will continue to study HD b and HD b in more detail. Thank you ~

 In secondary eclipse, a so-called transiting planet is monitored as it circles behind its star, temporarily disappearing from our Earthly point of view.  By measuring the dip in infrared light that occurs when the planet disappears, Spitzer can learn how much light is coming solely from the planet.  The technique will work only in infrared wavelengths, where the planet is brighter than in visible wavelengths and stands out better next to the overwhelming glare of its star. L. Jeremy Richardson (NRC, NASA Goddard)

Analysis (HD b)

Flux (HD b)