A targeted, parallax-based search for Planet Nine

TL;DR

This study conducts a targeted parallax-based search for Planet Nine in an ≈$98$ deg$^2$ field near the predicted high-probability region, leveraging consecutive-night parallax displacements of roughly $4''$–$7''$ to isolate distant Solar System objects. Using two observing campaigns with the JAST/T80 telescope and the T80Cam in Sloan $r$, the authors derive 85% exclusion limits for $r$-band magnitudes brighter than $21.0$–$21.4$, with an average sensitivity of $r\approx21.3$; no credible Planet Nine candidate is found. A large set of false positives from bright-star artefacts and cosmic rays is carefully curated and ruled out, yielding robust constraints but highlighting the challenges of shallow, wide-field searches near the Galactic plane. The work provides complementary constraints to archival surveys and other detection strategies, and demonstrates a simple, rapid method that can be extended with deeper imaging to probe fainter planetary candidates.

Abstract

The hypothesized Planet Nine is thought to reside in the distant outer solar system, potentially explaining various anomalies in the orbits of extreme trans-Neptunian objects (ETNOs). In this work, we present a targeted observational search for Planet Nine in a field of approximately 98 square-degrees. This field is close to the highest probability region of finding Planet Nine, according to simulations, but poorly constrained by previous searches. Our observations and search methodology, based on the detection of parallax position shifts between consecutive nights, work well in these conditions. We provide 85% confidence exclusion limits for objects with Sloan r-band magnitudes brighter than between 21.0 and 21.4, with an average sensitivity limit of 21.3. No credible Planet Nine candidates were identified within this field and magnitude limits. A caveat to our approach is that it would miss a candidate if its position were affected by scattered light from bright stars in at least one of the nights. However, we estimate that the probability for this is very low, around 0.4%. We discuss several possible reasons for our Planet Nine non-detection. Our study complements prior searches, particularly those using archival survey data that are limited in the Galactic plane or at fainter brightness limits. While our consecutive-night observation approach offers high sensitivity to minimal motion, extending the search for Planet Nine to fainter magnitudes (which may be crucial, according to recent predictions), will require higher sensitivity instrumentation

Figures (3)

• Figure 1: Four examples of false positives (see text for explanation). Left column: First night. Right column: Second night. The red (yellow) circle marks the source position in the first (second) night, with the red line indicating the apparent motion expected for Planet Nine due to parallax. The $x$ and $y$ units are pixels.
• Figure 2: Histograms of the number of sources detected per magnitude interval. Green line: Linear fit. Red line: Range of data fitted. Vertical dashed lines: Magnitudes at which 85% (red) and 50% (yellow) of the sources are detected (assuming that the true number of sources is well approximated by the fit, see text). Upper panel: Combining all frames with seeing better than 0.95$^{\prime\prime}$ . Lower panels: Histograms for a sample of randomly selected subfields.
• Figure 3: Estimated probability of missing sources ($m_1(r)$ and $m_2(r)$) and combined probability $m_{12}(r)$ of not detecting Planet Nine in the two best images of subfield A01. Symbols show original histogram values, with curves interpolated to a finer grid using cubic splines. Negative values of $m$ on the left side of the curve are spurious, caused by random fluctuations around zero as the number of observed sources $N_{Obs}$ fluctuates above and below the fit that approximates the true number of sources $N_{True}$. This regime is irrelevant for our analysis.