There's been a controversy in exoplanet polarimetry. I know that sounds like something CSI Miami worthy, but I'm going to try to summarise it here in less sexy terms.
The first detection of polarised light from an exoplanet system was done with aperture (not imaging) polarimetry by Berdyugina et al. in 2008. It was for what is now a very well-studied exoplanet with blue skies, glass rain, temperatures over 1100 degrees K, and winds whipping around at 8,700 km/h: HD 189733b (aka Permadeath).
The level of polarisation Berdyugina and her group found was way higher than we would expect from the planet. This is estimated from models that take into account things like multiple scattering, which reduces the overall polarisation of the light as it bounces off many molecules and gets polarised in random directions, and the source, in this planet's case Rayleigh scattering is believed to be the primary source of polarisation.
Net starlight is usually unpolarised, and a planet can polarise the light as it scatters off an ocean or through the atmosphere via Rayleigh scattering or rainbow effects.
When you look for the polarised light from a planet and you observe over that planet's period, you expect there to be particular fluctuations in the polarised light. If Rayleigh scattering, like that observed in our own atmosphere, is really the cause, you expect two peaks (or troughs) in the polarised light signal depending on things like the inclination and eccentricity (actually if an orbit is face on you wouldn't see the variation over time because the angle never changes between us, the planet, and the star).
Examples of the polarised light signal from exoplanets with various orbital parameters (right) compared to the variation in albedo (left) from Seager et al. 2000. Rayleigh scattering maxes out when the star-planet-Earth are at 90 degrees, the polarised light curve (shown) has the shape of the sum of the Rayleigh curve and the albedo; the extrema of a transiting planet thus are at +/- 70 degrees from the secondary eclipse (here 0 is SE).
And this is what Berdyugina et al. observed. But it was a very high signal.
When Wiktorowicz followed the observations up in 2009 with the POLISH polarimeter, the signal wasn't there. The polarised light did not vary over time as dramatically as Berdygina et al. observed, and certainly not with the period of the planet producing those two peaks we expect from Rayleigh scattering.
Not long after this though, in 2011, it seemed Berdyugina et al. solved the discrepancy. If one looks for polarised light in blue light, you see a huge signal, but if you look in red light, as Wiktorowicz had, you see nothing. This would make sense if Rayleigh scattering is the process polarising the light, since it scatters shorter wavelengths much more efficiently than longer wavelengths. As most of us know, that is why our sky on Earth looks blue. However the level of polarisation in blue light, while lower than their initial finding was still strangely high, so high that it would require the giant inflated planet atmosphere to not have multiple scattering.
Berdyugina et al. 2011 saw more variation in polarised light through the orbit of the planet in blue light, corresponding to a much higher albedo in blue. This was confirmed by Evans et al. 2013 but for values corresponding to much lower polarisation.
Photometrically, we do know that HD 189733b has a deep blue colour. Evans et al 2013 found this using Hubble data of the planet's reflected visible light in different bands. It reflects or scatters blue much more efficiently than red. This paper also gave us albedos for the planet in blue light though, and using Buenzli et al 2009's model grid for polarised signals from realistic planet atmospheres we can see that in the best case HD 189733b would only have a polarised light signal of about 38 ppm (parts per million). Estimates using Lucas et al. 2009 suggest it would be no more than about 30 ppm in an idealised case.
New aperture polarimeters like HIPPI in Australia and POLISH2 in the United States are built to be extremely sensitive to polarised blue light, accurately detecting polarisations down to a few ppm. They are the most sensitive polarimeters in the world. And in 2015, separate observations done by different teams on each polarimeter found that even in blue light they could not reproduce the Berdyugina et al. 2011 findings.
The (binned) variation we observed, with a least squares fit Rayleigh scattering curve in red, compared the the curve fit to Berdyugina et al.'s data in blue. The offset from zero would be predominantly due to the interstellar polarisation from the ISM. Q/I and U/I are the normalised Stokes parameters, so their variation in relation to each other tells us about the planet's orbital orientation.
In our paper on the planet, we find variations of only about 30 ppm and offsets more akin to those seen by the POLISH2 team. The data do not perfectly line up to that two-peak shape we expect, but fitting a Rayleigh curve by least squares 30 ppm is about the level of polarisation the data would suggest if the variation was due to the planet. This is also around the same level of variation as that found by Wiktorowicz et al. in their 2015 paper.
But this is only a 2-sigma detection: it is still possible that this is just noise or created by other polarising effects such as star spots. Importantly, it fits the albedo in blue light detected by Evans et al. 2013 as well. So while the terrifying, strange and hellish hot Jupiter, Permadeath, still has its ironic Rayleigh blue sky, it is probably not the super polariser it once seemed to be. As for the unusually high measurements from the Berdyugina et al. observations, this could be due to Saharan dust events known to affect polarimetry from the Canary Islands (see our paper linked below or Bailey et al. 2008).
If you'd like to read more about our findings, and in particular the other effects that can cause polarised light signals for this and other systems, please do take a look at our paper, available on arXiv here, and out soon in MNRAS.