Imaging low-mass planets within the habitable zone of α Centauri

Abstract

Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, α Centauri. Based on 75-80% of the best quality images from 100 h of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of α Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around α Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.

Fig. 1. Simulated planet brightness and basic properties of the α Centauri system.

a N-band (10–12.5 μm) contrast vs. angular separation of planets around α Centauri A (blue) and B (green), assuming face-on circular orbits, a Bond albedo of 0.3 and internal heating that provides an additional 10% of the planets’ equilibrium temperatures. The curves correspond from bottom to top to planetary radii equivalent to that of Earth, a Super-Earth (1.7 × Earth’s radius, R_⊕), Neptune, and Jupiter. The blue and green shaded regions show the location of the classical habitable zones around α Centauri A and B, respectively. b Diagram of the orbital properties and approximate habitable zones of the α Centauri AB system. Note that this diagram does not show the 79° inclination of the orbit as seen from Earth, or the tertiary dwarf star, Proxima Centauri, at ~10⁴ au.

Fig. 2. Mid-infrared images of α Centauri.

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 A is indicated by the dashed circle. A candidate detection is labeled as ‘C1’.

Fig. 3. Simulated planet injections.

NEAR campaign image (a) and those with simulated planets (b–f). Each image has been PSF-subtracted following removal of known artifacts. The location of C1 has been masked in (b–f) so that the simulated planets (indicated in these panels by white circles) can be clearly identified. These examples demonstrate the lower brightness limit at which simulated planets are identifiable. The bottom right panel represents the limiting case at which the source is marginally identifiable among speckles of similar brightness.

Fig. 4. Sensitivity of the NEAR data.

Results for α Centauri A & B are shown in (a) and (b), respectively. The bold curve shows the sensitivity to point sources computed from simulated injection and recovery tests and the dashed curve shows the background noise contribution from the variation of pixel-to-pixel intensities. The red and blue curves represent simulated planets of equivalent radii to those in the solar system, with the addition of an R = 1.7 R_⊕ Super-Earth (SE). Each model planet’s temperature is set by the assumption of thermal equilibrium at a given separation with an A_B = 0.3 Bond albedo and internal heating included as 10% or 50% of the equilibrium temperature; similar to conditions of the solar system planets^,.

Fig. 5. Completeness to planets of various radii and orbital semi-major axes.

(a) and (b) assume A_B = 0.3 and internal heating contributing 10% of the equilibrium temperature, whereas (c) and (d) assume A_B = 0.3 and internal heating contributing 50% of the equilibrium temperature. Radius and semi-major axis were uniformly sampled along with an inclination prior of P(i) ∝ sin i (see supplementary methods, completeness analysis). The dashed lines correspond to the radii of Neptune, Saturn, and Jupiter (N, S, and J, respectively). Contour units are normalized.