How to SUPPPPRESS Light From a Star That Is Ten Billion Times Brighter Than Its Habitable Exoplanet

Searching for Earth 2.0 has been an obsession of almost all exoplanet hunters since the discipline’s dawn a few decades ago. Since then, they’ve had plenty of technological breakthroughs help them in their quest, but so far, none of them have been capable of providing the clear-cut image needed to prove the existence of an exo-Earth. However, some of those technologies are undoubtedly getting closer, and one of the most interesting is utilizing a system called a multi-grated vector vortex coronagraph (mgVVC). Researchers funded by ESA think it may hold the optical properties to enable space-based telescopes like the Habitable Worlds Observatory (HWO) to finally capture the holy grail of exoplanet hunting – and it may be ready for prime time as early as next year.

That’s the timeline provided by the project team for the Substantiating Unique Patterned Polarization-sensitive Polymer Photonics for Research of Exoplanets with Space-based Systems (SUPPPPRESS) project, based out of Leiden University. ESA funded the project in October 2023 and plans to run for two years. For those two years, its primary focus will be building and testing a mgVVC designed to eliminate one of the biggest challenges related to its implementation—polarization leakage.

To understand why that’s a problem, it’s best first to understand what a vector vortex coronagraph is. A standard coronagraph uses some optical mask or physical disk to block out a star’s light. This allows the light from that star’s exoplanets to shine directly onto its optical system, allowing even relatively standard optics to make out details of the planet, like whether it has water in its atmosphere.

A vector vortex coronagraph uses a type of liquid crystal mask that shifts the phase of the starlight, essentially eliminating it. However, light from objects slightly off the mask’s axis, such as an exoplanet, isn’t affected by the phase shift, allowing it to pass through directly to the accompanying telescope’s detector. 

Polarization leakage happens because of manufacturing defects in the liquid crystal mask used by VVCs. These could result from alignment errors, deformities in the liquid crystals, or stress or strain on the mask. Ironically, the way to fix this might be to make more masks.

The concept of a multi-grated vector vortex coronagraph is to layer multiple masks on each other. Since many of the defects are created in the manufacturing process, they should be unique to each individual mask, and as such, they shouldn’t stack but cancel each other out when placed in series with one another. And the more grates there are, the more effective this solution is. According to the paper, a single-grated VVC could capture light from an exoplanet that is about 10,000 times dimmer than its host star. In contrast, a triple-grated VVC would be capable of capturing light from exoplanets that are 10 billion times dimmer than their stars.

That level of contrast is what would be needed to find a true exo-Earth. But the research team isn’t quite there yet. As part of a recent paper, the SUPPPPRESS project team did some modeling, built some preliminary prototypes, and performed some testing using JPL’s In-Air Coronagraphic Testbed and the University of Arizona’s Space Coronagraph Optical Bench. Results were promising, though they “highlighted the need for further refinements,” according to the paper. Each prototype test showed some errors in the mgVVC design, including the ominous-sounding “dark-hole regions” and the slightly less dire-sounding “uniform speckle field.” 

These hurdles aren’t impossible to overcome, and further testing is ongoing at the THD2 test bed in Paris – as long as the researchers aren’t distracted by all the sports going on around them. They did pass a preliminary design review in April of this year and plan to wrap up the next round of testing by December.

If continued testing and development go as planned, the researchers at the University of Leiden could provide one of the critical components for HWO by the time it is ready to move to the manufacturing phase. But even if it isn’t used on that mission, given this system’s impressive optical characteristics, it will undoubtedly be used somewhere.

Learn More:
Laginja et al – Prototyping liquid-crystal coronagraphs for exo-Earth imaging
UT – The Search for the Perfect Coronagraph to Find Earth 2.0
UT – Webb Directly Images a Jupiter-Like Planet
UT – Suppressing Starlight: How to Find Other Earths

Lead Image:
This artist’s concept features one of multiple initial possible design options for NASA’s Habitable Worlds Observatory.
Credits: NASA’s Goddard Space Flight Center Conceptual Image Lab