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Dr. Loïc Rossi
Office 2409
11 Boulevard d'Alembert
78280 Guyancourt


Venus is the sister planet of Earth. Nearly the same size and mass, but while Earth is a pretty nice place to live (mostly), Venus is quite like hell: surface temperature is about 400°C and pressure is 92 times that of the Earth. And the atmosphere is nearly pure CO2, so not nice either. The planet is also completely surrounded by thick clouds of sulphuric acid. So much like hell that amongst the few probes that reached the surface, the longest they could hold was about 2 hours… Some history of Venus polarimetry

Polarimetry has provided significant contribution in our knowledge of Venus clouds. The first observations date from the 1920s by Bernard Lyot during his PhD. Several ground-based observations followed in the following decades. In a famous paper from 1974, Hansen and Hovenier took those measurements and compared them with a radiative transfer model. They managed to derive microphysical properties of the cloud droplets and found that they were composed of concentrated sulphuric acid (75%).

Later the US probe Pioneer Venus carried the Orbiter Cloud Photopolarimeter (OCPP) which made polarimetric maps of Venus during the 1980s until 1992. These measurements were used to study the overlying hazes.

My work on Venus is in the continuation of these observations, as I study the clouds of Venus using polarimetric observations from SPICAV-IR onboard Venus Express.


Since 1995 and the discovery of the first exoplanet (Dimidium/51 Peg b) the field of exoplanetary science is growing fast. I don't even dare to put a number of known exoplanets, because it is probably going to be outdated as soon as you'll have finished reading this sentence…

A lot of new techniques have appeared to detect and study these new worlds. But my favourite is polarimetry, which has a huge potential.

Polarimetry for exoplanets

The light coming from a star like the Sun is not polarized. But the scattering of light by the atmosphere, by the clouds or by the surface of a planet can generate significant amounts of polarization.

Hence, even if we can't distinguish the planet and the star (like what is done by direct imaging), by measuring the degree of polarization, one could detect periodical variations of the polarization of the star-planet couple and hence detect an exoplanet! While in intensity, one can typically expect a contrast between the planet and the star of the order of 10-9, in polarimetry it could go down to 10-6, so several orders of magnitude difference.

But not only can we detect planets with polarimetry, we can also characterize them! Scattering by a clear atmosphere occurs in the Rayleigh scattering regime. It produces a lot of polarization with a peak around 90°. Polarization by clouds can produced very different features like glories, rainbows, etc. These are also polarized and detecting them is a good indication that the planets has clouds. The surface can also produce polarization and can also potentially be characterized with polarimetry.

I'm currently working on models of exoplanetary atmospheres and running simulations of diverse cloud coverages to determine whether we could tell how much cloudy is a given planet, and if we can tell what kind of cloud pattern we see!