A search for Planet Nine using the Zwicky Transient Facility public archive

Abstract: Recent estimates of the characteristics of Planet Nine have suggested that it could be closer than originally assumed. Such a Planet Nine would also be brighter than originally assumed, suggesting the possibility that it has already been observed in wide-field moderate-depth surveys. We search for Planet Nine in the Zwicky Transient Facility public archive and find no candidates. Using known asteroids to calculate the magnitude limit of the survey, we find that we should have detected Planet Nine throughout most of the northern portion of its predicted orbit -- including within the galactic plane -- to a 95% detection efficiency of approximately $V=20.5$. To aid in understanding detection limits for this and future analyses, we present a full-sky synthetic Planet Nine population drawn from a statistical sampling of predicted Planet Nine orbits. We use this reference population to estimate that this survey rules out 56% of predicted Planet Nine phase space, and we demonstrate how future analyses can use the same synthetic population to continue to constrain the amount of parameter space effectively searched for Planet Nine.

• The paper presents a systematic search for Planet Nine using a novel orbit-linking algorithm on ZTF data.
• It calibrates survey depth with known asteroids and shows a 56% detection potential in predicted orbital phase space.
• The study refines theoretical models by establishing detection thresholds, guiding future enhanced astronomical surveys.

An Analytical Examination of the Search for Planet Nine

The recent investigation conducted by Brown and Batygin has scrutinized the existence of the hypothesized Planet Nine utilizing the public archives from the Zwicky Transient Facility (ZTF). The study aims to exploit the possibility that Planet Nine, if it exists, might be closer and thus more luminous than previously estimated, which supports the possibility of its detection via extant wide-field surveys.

Methodology and Findings

The methodology leveraged involves a comprehensive scan within the ZTF's database, deploying a novel orbit-linking algorithm designed to efficiently detect celestial bodies across multiple observational points. A critical aspect of the paper is the development of a method to determine the detection limits of the ZTF survey using known asteroids as a reference for calibration. These asteroids provided a benchmark to self-calibrate the survey with respect to various observational parameters such as depth and cadence.

Despite an extensive search across the full-sky visible zone provided by the Oschin Schmidt telescope at the Palomar Observatory, the authors found no candidates fitting the criteria for Planet Nine. The survey data suggested that Planet Nine would have been detected in 56% of its predicted orbital phase space, were it within the survey's observational limits of approximately $V=20.5$ magnitude at 95% efficiency.

Important Outcomes and Implications

Critical among these outcomes is the contribution to ongoing debates regarding the existence of Planet Nine. The reference population developed by the authors, drawing from statistically modeled orbits, allows for an unprecedented examination of the unsampled regions of parameter space for Planet Nine. Subsequent surveys can readily use this reference population to constrain further the viable parameter space for Planet Nine hypotheses.

Remarkably, the detection efficiency exceeded 92% for brighter magnitude bodies ($V < 19.75$), but no orbital linkages matching hypothetical models of Planet Nine manifested within the ZTF data. This leads to significant implications for the theoretical models predicting the existence and location of Planet Nine. The absence of detections consolidates hypotheses suggesting a more distant celestial body or alternatively refines the perceived magnitude and location of search efforts.

Forward-Looking Perspectives

Looking forward, this study is a pivotal framework for astronomers pursuing identification and observational correlations of hypothesized planetary bodies. The synthesized statistical models combined with an innovative detection algorithm hold promise for adaptation in future surveys, perhaps with more profound celestial capabilities. Furthermore, the notions of observational biases and self-calibration within transient surveys set a methodological precedent for analogous extraterrestrial investigations.

In sum, this paper presents a robust methodological approach towards the empirical search for Planet Nine, setting concrete procedural foundations and establishing a comprehensive synthetic model framework that future astronomical studies can build upon. Although evidence for Planet Nine remains elusive, the precedence set by Brown and Batygin is likely to drive successive collaborative efforts in this domain, bringing new light to the extreme regions of our Solar System.