Transmission spectrum of Venus as a transiting exoplanet


An outline of the paper: Transmission spectrum of Venus as a transiting Exoplanet by Ehrenreich. et al.

On the 5-6 June 2012 the planet Venus will transit our Sun as seen from Earth. Go get your calendar and pen out right now and make note of this date, because the next time this event occurs the year will be 2117.

The transit of Venus presents a unique opportunity to assess the feasibility of studying Earth-like planets in the habitable zone using the transmission spectroscopy technique.

The 2004 transit of Venus

Ehrenreich et al. present in their paper a theoretical transmission spectrum of Venus ranging from the ultraviolet to the infrared. This spectrum can later be compared to this summer’s Venus observations  as well as previous observations of Earth’s transmission spectrum, which were obtained by studing reflected moonlight during a Lunar eclipse (Vidal-Madjar et al. 2010).

Figure from the paper showing the predictions for the transmission spectrum of Venus without hazes (black), and with two different haze models (red and green). In blue a model for Earth's transmission spectrum.

Ehrenreich et al. note that using transmission spectroscopy to study the atmosphere of an Earth-like planet is very challenging. Dense rocky planets are known for their small scale heights and their very compact atmospheres. Detecting these atmosphers would require a high photometric precision, at the 0.1 ppm level (more than a hundred times better than the present best space spectrophotometry). Having a detailed and well understood transmission spectrum of Venus will greatly aid in the identification of Venus-like planets (planets with a similar mass and waistline).

Feature Image: Fred Espenak, NASA/GSFC, taken from

Further Reading:

The Earth as an extrasolar transiting planet. Earth’s atmospheric composition and thickness revealed by Lunar eclipse observations by Vidal-Madjar et al. 2010

Local Transit Times (NASA)


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