In an effort to keep these informative but approachable links to more information and definitions for general public are embedded throughout.
(Most of) the HIPPI team observing at the AAT in 2014. Credit: J.Bailey
If you wanted to get information about an exoplanet you couldn't see directly, you might try either looking at how it affects its star's light or by making the planet more visible by making the star's light less overwhelming. Most techniques used to characterise exoplanets so far have relied more on the first approach, but one area that is more akin to the second and perhaps hasn't gotten the attention it deserves, is polarimetry.
Polarimetry is a great tool for getting information on exoplanets because it
- improves the contrast - stars light isn't very polarised but a planet can have lots of polarised light that has been scattered by gases, aerosols or a surface
- gives you orbital information - polarised light is directional so it can be combined with other measurements to give a very detailed picture of the planetary system
- provides information about the atmosphere including the presence of clouds; plus, by the effect of Rayleigh scattering or rainbows, it can tell you what sort of molecules form the condensates (clouds and aerosols) in the atmosphere
The angle at which a rainbow forms changes depending on the type of molecule forming a droplet because the index of refraction will change. Credit: Wiki
This March our group had our first paper on the HIPPI polarimeter accepted by MNRAS. The paper outlines the design of the polarimeter itself. It is the most sensitive astronomical polarimeter in the world (in use) currently. Its predecessor, PlanetPol was more sensitive but better suited for red light and is no longer available for observations.
Small but effective-- testing HIPPI in the lab. HIPPI uses carefully selected commercially available and 3-D printer components which helps drive the cost down while maintaining quality. Credit: J. Bailey
HIPPI (High Precision Polarimetric Instrument) is geared more for blue light sensitivity since we wanted to look for polarised light from exoplanets. Many hot Jupiter exoplanets are expected to have a high blue albedo, primarily from Rayleigh scattering (Burrows 2008, Berdyugina 2011). This polarimeter gets very high precision, detecting fractional polarisation (from a system) down to a few (3--4) parts per million. Hot Jupiter exoplanets are theorised to only produce fractional polarisation at ten parts per million at best (Seager et al 2000), so this type of precision is vital.
Along with an outline of the instrument itself, preliminary data about how HIPPI is being used to help us map the interstellar medium is available at the first author's website.
We have also obtained data on a few exoplanet systems and have been awarded more time on the AAT to observe in May, so a paper on our findings for exoplanets should be out soon.
Prior to the photometric albedo measurements by Evans et al 2013, Berdyugina et al 2011 had polarimetric evidence that the exoplanet HD 189733b would appear blue in colour. Credit: Berdyugina et al 2011.
Interested in past attempts to detect polarised light from exoplanets?
1. First Detection http://arxiv.org/abs/0712.0193
2. Refutation http://arxiv.org/abs/0902.0624
3. Confirmation of surprisingly high signal http://arxiv.org/abs/1101.0059
4. Attempts at other systems http://arxiv.org/abs/0807.2568
The HIPPI Instrument Paper: http://arxiv.org/abs/1503.02236