Universe Today
We’re still in the definition phase of the Habitable Worlds Observatory (HWO), but it seems like every week a new research group comes out with a paper helping to contribute to what is shaping up to be one of the most important space telescopes of the 2040s. A new paper from a team of researchers led by Daniel Jaffe of the University of Texas at Austin contributes to this ongoing definition work by arguing that it’s time HWO adopted a high-resolution near-IR spectroscopy capability, - which sounds great in practice, but so far hasn’t been attempted due to technological limitations. But, according to the paper, two recent inventions finally make a working version of an extremely high resolution exoplanet hunter viable.
The current record holder for the highest resolution infrared sensor in space is, unsurprisingly, the James Webb Space Telescope (JWST). However, its resolving power of around 3600 is considered low–to-moderate resolution compared to ground-based sensors. At that level of resolution, the clear spectral lines needed to differentiate critical components of an exoplanet’s atmosphere, such as CO2, become blurred. In addition, it makes it even more difficult to filter out the light from the exoplanet’s host star, contributing to a signal to noise issue that could wipe out critical data.
Dr. Jaffe and his team believe it’s time for an upgrade. They think the HWO team should equip the spacecraft with a high-resolution spectrograph, operating at a resolution of 45,000, more than twelve times the resolving power of the JWST. This offers three huge advantages for astronomers. First, and most obviously, it makes it possible to detect molecules with “weak” spectral signatures, like CO2, dramatically increasing the signal-to-noise ratio (SNR).
Fraser discusses the limits of HWO.In addition to molecules, it can also help scientists track the weather on these exoplanets. By measuring precise Doppler shifts in these spectral lines, researchers can determine orbital velocities - in other words how weather is moving on a planet light years away. Doing so will require a coronagraph to block out the light coming directly from the exoplanet’s star, but no coronagraph is perfect and will always let some starlight through. Higher resolution spectrographs will make it much easier to separate that “noise” from the signal of the light from an actual planet.
This all sounds great in theory, so why haven’t we done it already? Simply put, the technology was too big, too heavy, and too overcome with noise to be useful. Weight is a critical factor in any telescope, as it directly ties to the cost of the mission. And sensitivity to “dark current” (i.e. electric current caused even when there is no light hitting a sensor) made much of the data older generations of higher-resolution sensors collected useless anyway.
According to Dr. Jaffe and his team, though, those problems have largely been solved - at least on the ground. The first is by a new technology called silicon immersion gratings and grisms. These force light to diffract from inside a high-refractive material like silicon, as compared to traditional gratings that bounce light off a mirrored surface. This allows engineers to drastically reduce the size (and therefore weight) of the spectrograph, and has the added bonus of not requiring any moving parts to adjust any mirrors.
Fraser interviews Lee Feinberg, the lead architect of the HWO.The second technological breakthrough is in the area of avalanche photodiode arrays (APAs). These new detectors have near-zero “dark current” and the noise introduced by the sensor itself is less than the signal introduced by a single photon. These baselines make it much more feasible to capture the right kind of light from an exoplanet and ensure it can be differentiated from the starlight of its host star.
That being said, while these technologies have been thoroughly tested on the ground, such as in the IGRINS instrument in the Gemini South telescope, they still need to be tested in space before they can be adopted by such a high profile mission as the HWO. To that end, Dr. Jaffe and his team suggest flying a technology demonstration mission with the express intent of testing both silicon immersion gratings and APAs in space before their adoption onto the Flagship class mission.
Keep in mind that we’re still only in the definition phase of the HWO’s development cycle. It could literally be twenty years before this telescope launches. So there’d be plenty of time for such a mission, if it is funded. As of now, there’s no clear path to that funding though, so perhaps this will be another informative paper helping to define what is sure to be an iconic telescope. But without such a demonstration mission, and the eventual adoption of a high-resolution spectrograph, it’s hard to see how HWO can live up to its full potential.
Learn More:
D. Jaffe et al - The Case for High-Resolution Infrared Spectroscopy with the Habitable Worlds Observatory
UT - The Habitable Worlds Observatory Will Need Astrometry To Find Life
UT - Is the Habitable Worlds Observatory a Good Idea?
UT - The Habitable Worlds Observatory Could Find More Very Massive Stars
