20 May 2022

Astronomers from Spain and Romania have scrutinised two distant trans-Neptunian object (TNO) candidates, identified by automated software analyses of data from NASA's TESS mission. They used PF-QHY, a CMOS camera mounted at the renewed prime focus of the William Herschel Telescope (WHT), and could not recover these objects.

Observing Solar System objects located beyond 100 AU from the Sun is intrinsically difficult. Firstly, they are faint (apparent magnitude > 24.5 unless they are larger than Pluto or Eris), and secondly, their movement across the sky is so slow that they can be easily confused with stars. Hence, it is not surprising that so far only two objects have been discovered at distances beyond 100 AU. 2018 VG18 was the first such object, detected in 2018 when it was located at 123 AU, while the discovery of 2018 AG37 located at a distance of 132 AU was announced in 2021. Both TNOs were detected via the classic blink technique by ground-based telescopes.

NASA's Transiting Exoplanet Survey Satellite (TESS) is performing a homogeneous survey of the sky from space, searching for transiting exoplanets. As a by-product, TESS can also detect passing Solar System small bodies.

In 2020 Malena Rice and Greg Laughlin from Yale University used a technique called "shift-stacking" to explore the trans-Neptunian space using a limited amount of TESS data. They found eight candidate objects located farther than 100 AU with predicted apparent magnitudes brighter than 23. Several of their candidates move towards the circumpolar region which implies that these candidates must have high orbital inclinations (above 60 degrees).

In short, if real, some of the distant TNO candidates suggested by the automated software analyses of data from the TESS mission might be dwarf planets moving in highly inclined orbits, and their possible existence could be inconsistent with our current understanding of the orbital architecture of the outer Solar System.

Within the framework of the EUROpean Near Earth Asteroids Research (EURONEAR) collaboration, a group of astronomers from Spain and Romania attempted to recover two distant TESS candidates. The observations, covering two areas of over one square degree each in the Cepheus and Camelopardalis constellations, were carried out using the PF-QHY camera, mounted temporarily from May to July 2021. Data were collected in service mode over 5 nights under similar seeing conditions.

The astronomers were unable to recover either of the candidates at Sloan r < 23 magnitude and, after careful analysis of both visibility and detectability issues, this null result is interpreted as evidence that either both candidates are actually much fainter than predicted, or that they are false positives.

Null results can be important, and finding nothing at all when something was expected or predicted can lead to improved algorithms and software. This is particularly important when the current technology is being pushed to its limits.

The data from the mini-survey performed using PF-QHY were processed and analysed using the automatic software pipeline UMBRELLA, and the results visualised using the web-based WEBRELLA application developed for EURONEAR by Malin Stănescu, with both software applications running on the EURONEAR server at the University of Craiova, Romania. The computer-generated results were validated by independent, manual analyses. The negative detections were checked using the Astrometrica software tool, and were also confirmed by eye. Both machine- and human-powered data processing and analyses produced fully consistent results.

A null result was not completely unexpected as Rice and Laughlin had warned that some of their candidate detections could originate in unmodelled systematic errors. Successes and failures in recovering candidates using ground-based telescopes can therefore be used to improve the "shift-stacking" algorithm and produce better automated software that may lead to robust detections when the entire TESS dataset is processed.

[Image]

Fields and pointings observed in the mini-survey. In both panels, North is up, East to the left. The left panel shows the observed field in the Cepheus constellation while the right panel displays the one in the Camelopardalis constellation.