Malena Rice - Research Highlight #4

Research Highlight #4

Exploring Trans-Neptunian Space with TESS

(Rice & Laughlin 2020, PSJ 1, 81)

The Transiting Exoplanet Survey Satellite (TESS) was designed to search for undiscovered extrasolar planets around bright, nearby stars. But, it turns out that its observations are also ideally suited for another purpose: searching for a proposed but as-yet undiscovered ninth planet in the solar system, dubbed "Planet Nine".

Planet Nine was hypothesized to explain the puzzling clustering of orbits observed among some of the most distant known trans-Neptunian objects. If it exists, Planet Nine would be a super-Earth-sized planet located between 12-23 times as far from the Sun as Pluto. Despite its large size, this enormous distance would render its signal extremely faint, explaining why the planet would not yet have been found.

Fortunately, faint signals can be dramatically strengthened by summing over a large number of frames each containing the object of interest. The video below demonstrates how Sedna (V=20.64), a distant trans-Neptunian object (TNO) below the TESS single-frame detection limit, can be recovered by shifting and stacking reduced TESS frames along Sedna's orbit. TESS observes the vast majority of the high-inclination solar system, collecting 27 days of data at each pointing. This rich dataset can be leveraged to search for undiscovered solar system objects, including Planet Nine.

To search for undiscovered objects, we extend the idea of shift-stacking to produce "best-ever" aggregate frames in which we search for objects along all orbits of interest.

The schematic below details the process of creating both shift-stacked frames to recover known objects (Steps 1-3) and best-ever frames to blindly search for undiscovered objects (Steps 4-6). We have developed a new TESS pipeline designed for both of these purposes: we use two separate methods to subtract the baseline flux of each pixel and shift-stack the remaining frames to search for significant deviations from the baseline flux along orbital paths of interest.

Figure 1: Schematic demonstrating shift-stacking (Steps 1-3) and best-ever frames (Steps 4-6).

We applied both of these methods to recover three known distant TNOs: Sedna, 2015 BP519, and 2007 TG422, with each recovery shown below. Because we did not recover 2007 TG422 in our blind search, we find that V~22 is roughly our magnitude limit in this parameter space.

Figure 2: Recoveries of each object along their known paths in the sky (Column 1) and with blind best-ever frame recoveries that incorporate no input orbital information (Columns 2 and 3). The location of each object in its orbit is provided in Column 4.

Afterwards, we applied our blind search method to look for undiscovered objects in Sectors 18 and 19, both located along the galactic plane. While our existing framework is not sensitive to Planet Nine itself, we found 17 candidate objects recovered with both of our independent baseline subtraction methods. We expect that most of these candidates are likely false positives due to the low expected number density of large, distant, and highly-inclined TNOs; however, if even one of these objects is real, it could provide important new insights to narrow down the remaining Planet Nine parameter space.

This proof-of-concept will pave the way for a much larger-scale survey of the outer solar system, which we plan to accomplish by incorporating convolutional neural networks into our pipeline framework for a more automated candidate identification process. Furthermore, we have identified several promising paths forward to push our detection limits toward fainter and slower-moving objects such as Planet Nine, particularly as new data arrives from the ongoing TESS extended mission. Keep your eyes peeled — the search has only just begun!

Figure 3: Footprints of TESS sectors 18 and 19, with 150 random Planet Nine orbits overplotted in light blue. Candidate objects identified in this work are shown in green for reference.