Polarimetry seems like a good method for finding exoplanets. The light from quiescent stars is unpolarized (the net signal, we should say, is unpolarized: it cancels itself out) so if we look for polarized photons only, we can reasonably assume the signal is from the scattering and reflection from a planet’s surface or atmosphere.
This suggests that hot Jupiter exoplanets — large gaseous planets orbiting close to their star — would be outstanding targets because being so near their light source and with such a large scattering surface, a large fraction of the star’s available light would be scattered, and therefore polarized, by them. In fact this was a hot topic around the turn of the millennium when the first optimistic predictions for the polarized light signals from these planets were made (e.g. Seager, Whitney & Sasselov 2000). Those predictions suggested signals as high as about 100 parts-per-million but typically in the 10s of ppm.
As we learned more about hot Jupiters, however, we learned that they are often dark, so while a lot of light might reach them, little of it would be polarized and scattered on to our telescopes. Furthermore the activity of some host stars may add problematic noise.
Campaigns to detect polarization from hot Jupiter systems seemed to succumb to these hinderances. Observations from three groups (POLISH, HIPPI and PlanetPol) produced only non-detections providing upper limits to the albedo and some insight into the nature of the planets, but no direct detections. One planet, HD 189733b (Permadeath) did have a claimed detection in the late 00s, but it was subsequently contradicted by follow up by two independent groups. They did not see the strong polarization signal and suggested that Saharan dust, a known source of atmospheric polarization, might have created a false signal in the earlier work. You can read more about this saga here.
This year the HIPPI polarimetry team have published their exoplanet system observations taken during a campaign over several years for four systems. We published inconclusive non-detections for two of the four systems in the hopes that the datasets may be useful to others.
For one system, however, we seem to detect stellar activity.
And for another, possibly… finally… the planet itself.
For HD 189733b (Permadeath) the expected polarization is only on the order of about 20 ppm, yet the polarization varies much more than this and the star is known to be active. Five observing runs spanning five years did not improve the errors. The variations also are not in sync with the orbit of the planet. Finally, the polarization from this system is akin to the broadband polarization (from Zeeman splitting) seen in other stars from previous stellar characterization campaigns (see Cotton et al 2019). This, along with the Saharan dust, could help explain the previous wildly changing levels of polarization seen from the system.
The data taken over several years overlaid for the four systems considered in the paper. On the left is the data with the data reduction technique used in our previous HIPPI team papers and on the right with an improved approach. See the paper for details.
The possible detection of polarization from a planet itself, is from 51 Peg b… coincidently the first exoplanet discovered around a sun-like star with any method in 1995. In this case the signal is fit to “Rayleigh Lambert” models, which simplify light scattering for planets where we expect Rayleigh scattering is the primary source of polarization.
Our best fit for 51 Peg b is for a polarization signal at 11.2 ± 4.0 ppm. This is in good agreement with predictions from more complex models where either the radius of the planet is on the smaller side (it’s not currently pinned down because it doesn’t transit) and the planet is a proficient Rayleigh scatterer, or a case with a larger radius where additional opacity sources are present as we’ve seen previously for WASP-18b.
Still, this is only a 2.8 sigma detection with a 1.9% false alarm probability (based on a bootstrap analysis) so this is not a claimed detection. (A signal is usually only considered a detection over 3 sigma for direct methods). By eye (below) you can see that while the data is more convincing than, say, HD 179949’s, it still has room for improvement. We hope that with polarimeters being designed by the HIPPI team and others, this possible detection can soon be either proven or disproven.
In the meantime, we’ve done our best with this paper to provide a thorough analysis, considering things like the interstellar medium and host star contributions, which are often overlooked.
The data (black) for four hot Jupiter systems observed with HIPPI/-2 presented in this paper compared to their best fit Rayleigh-Lambert phase curves. HD 179949 and Tau Boo in the top panels do not have any reported trends. However, HD 189733 (bottom left) seems to show polarization due to stellar activity, and 51 Peg (bottom right) possibly shows polarization from the planet itself.
For more details on this I turn you to our paper accepted to MNRAS. (And if you’re really into this sort of thing, you should know that banding on a brown dwarf was detected with polarimetry last year as well!)