We may or may not have imaged a planet around Alpha Centauri A, possibly a ‘warm Neptune’ at an orbital distance of roughly 1 AU, the distance between Earth and the Sun. Let’s quickly move to the caveat: This finding is not a verified planet, and may in fact be an exozodiacal disk detection or even a glitch within the equipment used to see it.
But as the paper notes, the finding called C1 is “is not a known systematic artifact, and is consistent with being either a Neptune-to-Saturn-sized planet or an exozodiacal dust disk.“ So this is interesting.
As it may be some time before we can make the call on C1, I want to emphasize the importance not so much of the possible planet but the method used to investigate it. For what the team behind a new paper in Nature Communications has revealed is a system for imaging in the mid-infrared, coupled with long observing times that can extend the capabilities of ground-based telescopes to capture planets in the habitable zone of other nearby stars.
Lead author Kevin Wagner (University of Arizona Steward Observatory) and colleagues describe a method showing a tenfold improvement over existing direct imaging solutions. Wavelength is important here, for exoplanet imaging usually works at infrared wavelengths below the optimum. Wagner points to the nature of observations from a warm planetary surface to explain why the wavelengths where planets are brightest can be problematic:
“There is a good reason for that because the Earth itself is shining at you at those wavelengths. Infrared emissions from the sky, the camera and the telescope itself are essentially drowning out your signal. But the good reason to focus on these wavelengths is that’s where an Earthlike planet in the habitable zone around a sun-like star is going to shine brightest.”
With exoplanet imaging up to now operating below 5 microns, where background noise is low, the planets we’ve been successful at imaging have been young, hot worlds of Jupiter class in wide orbits. Let me quote from the paper on this as well:
Their high temperatures are a remnant of formation and reflect their youth (~1–100 Myr, compared to the Gyr ages of typical stars). Imaging potentially habitable planets will require imaging colder exoplanets on shorter orbits around mature stars. This leads to an opportunity in the mid-infrared (~10 µm), in which temperate planets are brightest. However, mid-infrared imaging introduces significant challenges. These are primarily related to the much higher thermal background—that saturates even sub-second exposures—and also the ~2–5× coarser spatial resolution due to the diffraction limit scaling with wavelength. With current state-of-the-art telescopes, mid-infrared imaging can resolve the habitable zones of roughly a dozen nearby stars, but it remains to be shown whether sensitivity to detect low-mass planets can be achieved.
Getting around these challenges is part of what Breakthrough Watch is trying to do via its NEAR (New Earths in the Alpha Centauri Region) experiment, which focuses on the technologies needed to directly image low-mass habitable-zone exoplanets. The telescope in question is the European Southern Observatory’s Very Large Telescope in Chile, where Wagner and company are working with an adaptive secondary telescope mirror designed to minimize atmospheric distortion. That effort works in combination with a light-blocking mask optimized for the mid-infrared to block the light of Centauri A and then Centauri B in sequence.
Remember that stable habitable zone orbits have been calculated for both of these stars. Switching between Centauri A and B rapidly — as fast as every 50 milliseconds, in a method called ‘chopping’ — allows both habitable zones to be scrutinized simultaneously. Background light is further reduced by image stacking and specialized software.
“We’re moving one star on and one star off the coronagraph every tenth of a second,” adds Wagner. “That allows us to observe each star for half of the time, and, importantly, it also allows us to subtract one frame from the subsequent frame, which removes everything that is essentially just noise from the camera and the telescope.”
Among possible systematic artifacts, the paper notes the presence of ‘negative arcs’ due to reflections that are introduced within the system and must be eliminated. The image below shows the view before the artifacts have been removed and a second after that process is complete.
Image: This is Figure 2 from the paper. Caption: a high-pass filtered image without PSF subtraction or artifact removal. The α Centauri B on-coronagraph images have been subtracted from the α Centauri A on-coronagraph images, resulting in a central residual and two off-axis PSFs to the SE and NW of α Centauri A and B, respectively. Systematic artifacts labeled 1–3 correspond to detector persistence from α Centauri A, α Centauri B, and an optical ghost of α Centauri A. b Zoom-in on the inner regions following artifact removal and PSF subtraction. Regions impacted by detector persistence are masked for clarity. The approximate inner edge of the habitable zone of α Centauri A13 is indicated by the dashed circle. A candidate detection is labeled as ‘C1’. Credit: Wagner et al.
Over the years, we’ve seen the size of possible planetary companions of Centauri A and B gradually constrained, and as the paper notes, radial velocity work has excluded planets more massive than 53 Earth masses in the habitable zone of Centauri A (by comparison, Jupiter is 318 Earth masses). The constraint at Centauri B is 8.4 Earth masses, meaning that in both cases, lower-mass planets could still be present and in stable orbits. We already know of two worlds orbiting the M-dwarf Proxima Centauri.
You can find the results of the team’s nearly 100 hours of observations (enough to collect more than 5 million images) in the 7 terabytes of data now made available at http://archive.eso.org. Wagner is forthcoming about the likelihood of the Centauri A finding being a planet:
“There is one point source that looks like what we would expect a planet to look like, that we can’t explain with any of the systematic error corrections. We are not at the level of confidence to say we discovered a planet around Alpha Centauri, but there is a signal there that could be that with some subsequent verification.”
A second imaging campaign is planned in several years, which could reveal the same possible exoplanet at a different part of its modeled orbit, with potential confirmation via radial velocity methods. From the paper:
The habitable zones of α Centauri and other nearby stars could host multiple rocky planets–some of which may host suitable conditions for life. With a factor of two improvement in radius sensitivity (or a factor of four in brightness), habitable-zone super-Earths could be directly imaged within α Centauri. An independent experiment (e.g., a second mid-infrared imaging campaign, as well as RV, astrometry, or reflected light observations) could also clarify the nature of C1 as an exoplanet, exozodiacal disk, or instrumental artifact. If confirmed as a planet or disk, C1 would have implications for the presence of other habitable zone planets. Mid-infrared imaging of the habitable zones of other nearby stars, such as ε Eridani, ε Indi, and τ Ceti is also possible.
It’s worth keeping in mind that the coming extremely large telescopes will bring significant new capabilities to ground-based imaging of planets around nearby stars. Whether or not we have a new planet in this nearest of all stellar systems to Earth, we do have significant progress at pushing the limits of ground-based observation, with positive implications for the ELTs.
The paper is Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12: 922 (2021). Abstract / full text.