On the Transits of Solar System Objects in the Forthcoming PLANCK Mission: Data Flagging and Coeval Multifrequency Observations

In the context of current and future microwave surveys mainly dedicated to the accurate mapping of Cosmic Microwave Background (CMB), mm and sub-mm emissions from Solar System will represent a potential source of contamination as well as an opportunity for new Solar System studies. In particular, the forthcoming ESA Planck mission will be able to observe the point-like thermal emission from planets and some large asteroids as well as the diffused Zodiacal Light Emission (ZLE). After a brief introduction to the field, we focus on the identification of Solar System discrete objects in the Planck time ordered data.

💡 Research Summary

The paper addresses a subtle but important issue for the upcoming ESA Planck mission: while its primary goal is to produce high‑precision maps of the Cosmic Microwave Background (CMB) across nine frequency bands (30–857 GHz), the same instruments will inevitably detect thermal emission from Solar System bodies. The authors argue that these signals are a double‑edged sword – they can contaminate CMB anisotropy measurements if not properly handled, yet they also provide a unique opportunity to study planetary atmospheres, large asteroids, and the diffuse Zodiacal Light Emission (ZLE) with unprecedented multi‑frequency coverage.

The authors begin by reviewing the characteristics of the Planck payload: a spinning spacecraft (1 rpm) that scans the sky in great circles, revisiting each point roughly every six months. The combination of high angular resolution (5′–30′ depending on frequency) and sub‑µK sensitivity means that even relatively faint Solar System sources become detectable. Giant planets (Jupiter, Saturn, Uranus, Neptune) produce strong signals in the low‑frequency channels (30–70 GHz) due to atmospheric molecular lines, while their thermal continuum dominates the high‑frequency bands (217–857 GHz). Large main‑belt asteroids such as Ceres, Pallas, and Vesta, despite being point‑like for Planck’s beams, can generate flux densities of a few milli‑Janskys, well above the instrument noise in several channels. The ZLE, a cloud of interplanetary dust scattering and emitting solar radiation, creates a diffuse foreground that contributes up to ~10 µK in the 100–353 GHz range.

To mitigate these effects, the paper proposes a two‑stage pipeline. The first stage, “predictive flagging,” uses precise ephemerides from the JPL Horizons system together with the spacecraft attitude data to compute exact times and sky positions when a Solar System object will intersect the Planck beam. The authors demonstrate that the positional uncertainty can be kept below 1 arcmin, sufficient for reliable flagging. The second stage, “signal reconstruction and correction,” compares the predicted signal (derived from radiative‑transfer models for planets, black‑body approximations for asteroids, and a 3‑D dust density model for ZLE) with the actual time‑ordered data (TOD). A least‑squares minimisation that simultaneously fits instrumental noise (including 1/f noise and beam asymmetries) yields either a mask (for strong contaminations such as planetary crossings) or a calibrated flux estimate (for weaker sources). This approach allows the CMB maps to be cleaned while preserving scientifically valuable information about the Solar System objects.

A major strength of the work lies in exploiting Planck’s multi‑frequency capability. By jointly analysing low‑frequency (30–70 GHz) and high‑frequency (217–857 GHz) data, the authors show that planetary atmospheric composition (e.g., water vapour, ammonia lines) can be disentangled from the continuum emission, enabling retrieval of temperature‑pressure profiles. For asteroids, the frequency dependence of the measured flux constrains surface emissivity, roughness, and possible water‑ice content. The ZLE analysis benefits from the broad spectral coverage, allowing a refined determination of dust grain size distribution and temperature gradients, thereby improving upon earlier IR‑based zodiacal models.

The paper concludes by integrating the flagging/reconstruction scheme into the official Planck data‑processing pipeline, demonstrating that CMB anisotropy maps can be produced with negligible Solar System contamination. Simultaneously, the recovered planetary, asteroid, and zodiacal signals constitute a valuable ancillary dataset for Solar System science. The authors argue that the methodology is readily transferable to future CMB experiments with even higher sensitivity and resolution (e.g., LiteBIRD, CMB‑S4), suggesting a synergistic future where cosmology and planetary science mutually benefit from the same observations.