Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of $α$ Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits (2508.03814v1)

Abstract: We report on coronagraphic observations of the nearest solar-type star, $\alpha$ Cen A, using the MIRI instrument on the James Webb Space Telescope. With three epochs of observation (August 2024, February 2025, and April 2025), we achieve a sensitivity sufficient to detect $T_{\rm eff}\approx$ 225-250 K (1-1.2 $R_{\rm Jup}$) planets between 1"-2" and exozodiacal dust emission at the level of $>$5-8$\times$ the brightness of our own zodiacal cloud. The lack of exozodiacal dust emission sets an unprecedented limit of a few times the brightness of our own zodiacal cloud$-$a factor of $\gtrsim$10 more sensitive than measured toward any other stellar system to date. In August 2024, we detected a F$\nu$(15.5 $\mu$m) = 3.5 mJy point source, called $S1$, at a separation of 1.5" from $\alpha$ Cen A. Because the August 2024 epoch had only one successful observation at a single roll angle, it is not possible to unambiguously confirm $S1$ as a bona fide planet. Our analysis confirms that $S1$ is neither a background nor a foreground object. $S1$ is not recovered in the February and April 2025 epochs. However, if $S1$ is the counterpart of the object, $C1$, seen by the VLT/NEAR program in 2019, we find that there is a 52% chance that the $S1+C1$ candidate was missed in both follow-up JWST/MIRI observations due to orbital motion. Incorporating constraints from the non-detections, we obtain families of dynamically stable orbits for $S1+C1$ with periods between 2-3 years. These suggest that the planet candidate is on an eccentric ($e \approx 0.4$) orbit significantly inclined with respect to $\alpha$ Cen AB orbital plane ($i{\rm mutual} \approx 50^\circ$, or $\approx 130^\circ$). Based on the photometry and orbital properties, the planet candidate could have a temperature of 225 K, a radius of $\approx$1-1.1 $R_{\rm Jup}$ and a mass between 90-150 $M_{\rm Earth}$, consistent with RV limits.

• The paper demonstrates direct imaging detection of a candidate giant planet in α Cen A’s habitable zone using JWST/MIRI coronagraphy and advanced PSF subtraction.
• It establishes robust detection metrics (S/N=4–6) and constrains the planet’s orbit, mass (<150 M⊕), and effective temperature (~225 K) through multi-epoch observations.
• The study sets stringent exozodiacal dust upper limits via injection-recovery tests, emphasizing improved sensitivity over previous surveys for future exoplanet imaging.

Direct Imaging of a Candidate Giant Planet in the Habitable Zone of α Cen A

Introduction and Observational Strategy

This paper presents a comprehensive analysis of JWST/MIRI coronagraphic imaging of the α Centauri A system, targeting the direct detection of exoplanets and exozodiacal dust within the habitable zone (HZ) of the nearest solar-type star. The proximity of α Cen A (1.33 pc) enables spatial resolution of its HZ at mid-infrared wavelengths, providing a unique opportunity to probe for both planetary and dust signatures at unprecedented sensitivity. The observational campaign comprised three epochs (August 2024, February 2025, April 2025), utilizing the MIRI F1550C 4QPM coronagraph to optimize contrast for 200–350 K planets at 1–3 au separations.

The observing strategy addressed several technical challenges, including the presence of the bright companion α Cen B, high proper motion and parallax, and the need for precise target acquisition. Reference star differential imaging (RDI) and principal component analysis-based PSF subtraction were employed to mitigate stellar speckles and instrumental artifacts. The achieved sensitivity allowed detection of planets with $T_{\rm eff} \approx 225$–250 K and radii $\sim$1–1.2 $R_{\rm Jup}$ at 1–2 arcsec separations, and exozodiacal dust at levels $>$5–8$\times$ the solar zodiacal cloud.

Figure 2: JWST/MIRI F1550C image of the α Cen system from August 2024, showing the region around α Cen A and the location of the candidate S1.

Detection and Validation of a Planet Candidate

A point source (S1) was detected at 1.5 arcsec east of α Cen A in the August 2024 epoch, with a flux density of 3.5 mJy and a contrast of $5.5 \times 10^{-5}$ relative to the host. The detection significance was S/N = 4–6. S1 was not recovered in the February and April 2025 epochs. Multiple lines of evidence rule out S1 as a background or foreground object:

• No source is detected at the expected background position in subsequent epochs, eliminating the stationary background hypothesis.

  Figure 1: PSF-subtracted image for April 2025, showing no source at the expected background position of S1.

• Archival Spitzer, 2MASS, and VLT/NACO images show no source at the S1 position, even accounting for high extinction scenarios.

  Figure 3: Limits from archival imaging at S1's position, demonstrating that a background star or galaxy would have been detected at other wavelengths.

• The probability of a chance alignment with a bright extragalactic source is $<4 \times 10^{-4}$ within the field of view.
• The possibility of a Solar System object is excluded by the lack of motion during the observation and the absence of known asteroids at the position.

The analysis in the companion paper (Sanghi & Beichman et al. 2025, in press) further supports the astrophysical nature of S1, ruling out detector and PSF subtraction artifacts.

Sensitivity to Planets and Exozodiacal Dust

The combined sensitivity maps across all epochs demonstrate that the observations are sensitive to planets with $T_{\rm eff} \approx 250$ K (for 1 $R_{\rm Jup}$) at 1–2 arcsec, and to even colder planets at wider separations.

Figure 4: Two-dimensional 5$\sigma$ planet effective blackbody temperature sensitivity map, showing the regions of highest sensitivity to cold giant planets.

For exozodiacal dust, injection-recovery tests with physically motivated asteroid belt analog (ABA) models show that the observations are sensitive to exozodi levels as low as 5–8 times the solar value, a factor of $\gtrsim$5–10 more sensitive than previous photometric or interferometric surveys.

Figure 5: Recovery of an injected ABA-3 exozodi model in the April 2025 dataset, demonstrating the sensitivity to faint, edge-on dust disks.

Figure 6: Surface brightness profiles of injected zodi models, with the fiducial 1-zodi model for comparison.

No significant exozodiacal emission is detected, setting the most stringent upper limits to date for any stellar system.

Orbital and Physical Characterization of the Candidate

The astrometric position of S1 is consistent with the candidate C1 detected by VLT/NEAR in 2019. By combining these detections and accounting for non-detections in 2025, the authors identify four families of dynamically stable orbits (prograde/retrograde, $a < 2$ au/$a > 2$ au) with periods of 2–3 years, eccentricities $e \approx 0.4$, and mutual inclinations of $\sim$50° (prograde) or $\sim$130° (retrograde) relative to the binary plane.

Figure 7: 1$\sigma$ and 3$\sigma$ contours for the sky-projected positions of S1+C1 consistent with non-detections in 2025.

Figure 8: Posterior distributions for key orbital parameters of stable S1+C1 orbits consistent with all constraints.

Figure 9: Example stable orbits for each orbital family, showing the diversity of possible configurations.

Radial velocity constraints ($K_{RV} < 3$ m/s) limit the planet mass to $<100$–150 $M_\oplus$ for most orbits.

Figure 10: RV semi-amplitude distribution for a 100 $M_\oplus$ planet in stable orbits, compared to the RV noise floor.

The equilibrium temperature for the candidate, set by stellar irradiation, is bimodal: $\sim$225 K for $a < 2$ au, $\sim$195 K for $a > 2$ au.

Figure 11: Distribution of flux-averaged mean planet temperatures for the allowed orbits.

Atmospheric model fits (ATMO2020++, Sonora, PICASO) to the photometry yield plausible solutions for $T_{\rm eff} = 225$ K, $R_p \approx 1.1$–1.15 $R_\oplus$, and $M_p \approx 90$–150 $M_\oplus$, consistent with RV limits. Models with optically thick circumplanetary rings can also explain the observed flux for a smaller planet.

Figure 12: Range of star-planet separations and instantaneous ring temperatures at the S1 and C1 epochs for the favored orbital family.

Figure 15: Blackbody emission from a circumplanetary ring plus a planet atmospheric model, matching the observed photometry.

The mass-radius locus of the candidate is consistent with the coldest transiting giant planets known.

Figure 13: Mass-radius diagram for planets cooler than 750 K, with the S1+C1 candidate properties overplotted.

Dynamical and Formation Implications

The best-fit orbits for S1+C1 are highly inclined and eccentric, placing the candidate in the regime of von Zeipel-Kozai-Lidov (vZKL) oscillations, which can drive periodic exchanges between eccentricity and inclination in hierarchical triples.

Figure 14: Probability density of minimum mutual inclination for prograde and retrograde orbits, highlighting the vZKL regime.

N-body simulations show that the presence of S1+C1 precludes the existence of other stable planets in the HZ beyond $\sim$0.4 au, due to dynamical clearing.

Figure 16: Fraction of stable test particles as a function of semi-major axis, demonstrating the lack of stable orbits exterior to $\sim$0.4 au in the presence of S1+C1.

The architecture of the candidate system is analogous to other S-type planets in close binaries (e.g., HD 196885 Ab, γ Cep Ab), supporting the plausibility of such configurations.

Prospects for Confirmation and Future Observations

The analysis predicts that S1 will be optimally positioned for recovery in August 2026, with a separation $>$1 arcsec and clear of coronagraphic mask boundaries.

Figure 17: Predicted position and separation histogram for S1 in August 2026, favoring recovery for $a < 2$ au orbits.

Follow-up with JWST, Roman, ELT/METIS, and high-precision RV and astrometric monitoring will be critical for confirmation and characterization. The stringent exozodi limits demonstrate the power of resolved mid-IR imaging for future direct imaging missions targeting terrestrial planets.

Conclusion

This work reports the direct imaging detection of a candidate giant planet in the HZ of α Cen A, with robust evidence against background or instrumental origins. The candidate is consistent with a cold ($\sim$225 K), moderately massive ($\sim$100 $M_\oplus$), and dynamically excited planet on a 2–3 year orbit, with no detectable exozodiacal dust at levels $<$5–8$\times$ the solar value. The results have significant implications for planet formation and dynamical evolution in close binaries, and for the design of future direct imaging surveys. Confirmation of the candidate will require additional epochs, but the system represents a benchmark for exoplanetary science in the solar neighborhood.