The Habitable Worlds Observatory (HWO) is slated to be the next Great Observatory for the world. Its main focus has been searching for biosignatures in the atmospheres of at least 25 Earth-like exoplanets. However, to do that, it will require a significant amount of effort with only a coronagraph, the currently planned primary instrument, no matter how powerful that coronagraph is. As new paper from Fabien Malbet of the University of Grenoble Alpes and his co-authors suggest an improvement - add a second instrument to HWO’s payload that will be able to astrometrically track planets down to a precision of .5 micro-arcseconds (µas). That would allow HWO to detect Earth-size planets around hundreds of nearby stars - dramatically increasing the number of potential candidates for atmospheric analysis.
Currently, only 12% of sun-like stars within 65 light years are known to have planets - and all of those have been gas giants. We haven’t found any rocky exoplanets anywhere near our own solar system, but that doesn’t mean they don’t exist. Its more likely that the signal from those planets get lost in the noise of missions like Gaia. While Gaia is the most advanced planet hunter we currently have, its highest precision is on the order of 20-30 µas. That is still orders of magnitude more than what would be needed to detect an Earth-like planet even around a nearby star.
HWO, with the suggested additional astrometry instrument, would be 400-600 more precise than Gaia, allowing it to find exo-Earths that could then be the focal point of further exploration by Gaia’s coronagraph. To reach that level of precision will require two things: a really advanced calibrator and lots and lots of pictures.
NASA's video describing the HWO. Credit - NASA Goddard YouTube ChannelAstrometric sensors are nothing new - we have been finding exoplanets using them for decades. They measure a star’s “wobble” as it is pushed around by an orbiting planet. The more precise an instrument is, the smaller “wobble” it is capable of seeing, corresponding to a lower planet mass, like that of Earth compared to Jupiter. It also have major advantages over other exoplanet hunting techniques by allowing astronomers to calculate an exoplanet’s complete orbit as well as its absolute mass.
However, astrometers have a weakness - they are prone to error. This can be introduced via a wide variety of causes, such as detector imperfections or slight offsets in a mirror, but overall they can compound into significantly limiting the precision of an instrument. To combat this, the authors suggest a technology known as a “Detector Calibration Unit” (DCU). This tool would produce a series of light and dark bands across the CMOS sensor that makes up the astrometer, allowing it to isolate and correct small errors before they become big ones. It allows researchers to calibrate the exact position of every pixel in the CMOS’ field of view to ensure stability of multiple images.
Those multiple images are the second key ingredient to the astrometric pie. Dr. Malbet and his team estimate that HWO would have to take over 100 individual measurements of an individual over the course of its 3-4 year operational life in order to get down to the precision level necessary to fully confirm a planet. That high number is necessary to ensure that any random errors that aren’t corrected by the DCU don’t sway the final result too much. By combining hundreds of images, whatever other random errors there might be residually would be offset by one another.
Dr. Becky discusses how HWO will work.An added benefit of this new technology would be in a completely different realm of astrophysics - it could test one of the prevailing theories of Cold Dark Matter (CDM). CDM theory predicts that there are “cusps” of dark matter in the center of galaxies, but collected data shows that, while there is dark matter in those centers, they are shaped more like “cores”. In a “cusp” their density would spike near the center of the galaxy by the gravity of the dark matter pulling more of itself to itself. In a “core” on the other hand, there isn’t necessarily a spike in dark matter density anywhere in the galaxy as the dark matter is distributed semi-evenly.
HWO’s new astrometer could pick up the fine wobbles created by the “spikes” of dark matter as they lensed light passing by them. It could add valuable insight into what might be causing these flat discs of dark matter - whether that’s supernovae explosions or some unique property of dark matter itself.
This paper isn’t the first time the idea was discussed though - Dr. Malbet was also a critical player in the Theia mission proposal, which uses the same fundamental idea but would have been launched as its own separate mission. The Theia team has been working on the concept for years, so strapping it to the HWO seems like a logical platform for it, especially since it can enhance the observatory’s main mission.
HWO is the Habitable Worlds Observatory after all, and its main mission is to find habitable worlds. Any tool in its toolkit that could help it do that, or even lessen the time that it takes to do so, would be welcome. Given al the work that has already gone into developing the precision astrometer, it seems a waste not to use it on some platform eventually. Development on the HWO isn’t planned to start in earnest until the 2030s as it is, and it likely won’t launch until the 2040s, so there’s still plenty of time to implement this improvement. It just remains to be seen if the project’s manager will allow its ambitions to grow to fit the need.
Learn More:
F. Malbet et al - Very High Precision Astrometry for Exoplanets and Dark Matter with the Habitable Worlds Observatory
UT - Simulating HWO’s Ability to Characterize Earth-Sized Exoplanets
