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TESS Researchers Discover Six-Planet System Using New Algorithms

Animation showing six “sub-Neptune” exoplanets in rhythmic orbits around their star—with a musical tone as each planet passes a line drawn through the system. The line is where the planets cross in front of (transit) their star from Earth’s perspective. In these rhythms, known as “resonance,” the innermost planet makes three orbits for every two of the next planet out. Among the outermost planets, a pattern of four orbits for every three of the next planet out is repeated twice. Animation credit: Hugh Osborn, University of Bern

An international group of scientists have recently uncovered a remarkable system of six exoplanets from data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS). Their years of planetary detective work—which began with light curve processing on the Pleiades supercomputer at the NASA Advanced Supercomputing (NAS) facility—was detailed in a paper published in Nature.

The six “sub-Neptune” planets—smaller than our Neptune—orbit their host star in a tight gravitational formation known as resonance, where the innermost planet makes three orbits for every two of the next planet out, and the outermost planets make four orbits for every three the next planet makes.

While orbital resonance is fairly rare, this particularly delicate configuration makes the star system a unique “fossil” for astronomers, since it suggests a lack of violent events in its billion-year history that impacted the development of its planets’ orbits. Future study of this system might shed light on the process of planetary migration and the properties of protoplanetary disks in a “pristine” environment.

Tracking Down a Planetary Dance

The bright orange star HD 110067 at the center of this system was first observed by TESS in 2020. From imagery collected during this period, scientists from the TESS Science Processing Operations Center (SPOC) at NASA’s Ames Research Center were able to identify two potential transiting planets, in addition to some curious features that they couldn’t initially explain.

The SPOC pipeline processes imagery from TESS by calibrating pixels, performing simple aperture photometry, and running transiting planet searches for exoplanets passing in front of their host star each orbital period, blocking a small fraction of light. One of the two planetary candidates was identified close to the star, while the other couldn’t be pinpointed based on its single observed transit. Other dips in the stellar brightness data suggested there might be more happening than initially observed.

HD 110067 was observed by TESS for a second time in 2022, deepening this astronomical puzzle. The SPOC scientists identified 14 individual transits in the combined data sets and proposed a large number of possibilities for which transits belonged to which planet. With the help of additional photometric data from the European Space Agency’s Characterizing ExOPlanets Satellite (CHEOPS)—which was designed to follow up on transiting planet systems identified by other observatories and survey satellites—and several ground-based observatories, NASA researchers were able to identify a third planet and recognize the rare 3/2 orbit resonance of the three planets. Amazingly, when compared to other examples of this phenomenon, the data suggested that there might be a fourth planet involved in the chain, in addition to two more transit-like dips that would suggest more planets with longer orbits. Unfortunately, these couldn’t be confirmed, since their transits were only clearly recorded once.

Manually validating these additional light curves proved challenging for several reasons, including the loss of a large portion of the data due to high levels of scattered light and background “noise” from the Earth and Moon that coincided with the predicted transits of the star’s two outer planets. Consequently, the team reached out to the SPOC scientists who were developing new a computational code to recover lost data due to scattered light effects. When this code was applied to the TESS SPOC data, the team was able to significantly mitigate the scattered light and confidently identify the “missing” transits for the outer two planets—right where the math had suggested they would be.

The Role of NASA Supercomputers

These six new planet candidates are just a few of the thousands identified by TESS in the five years since its launch in 2018. As of December 2023, 6,996 planet candidates have been identified through tens of thousands of potential transits, with 410 planets confirmed by scientists. All of that data starts its journey with the SPOC, powered by NASA’s petaflop-scale Pleiades supercomputer, which uses thousands of CPUs to process terabytes of raw image data from TESS each month. The NAS facility also provides and maintains the SPOC servers and filesystems, which allow multi-sector and multi-year transit searches to store hundreds of terabytes of data for comparison and analysis.

This science data pipeline is a continuation of a collaboration that started with the Kepler and K2 missions over a decade ago, which used an older configuration of Pleiades to process light curves from raw images from TESS’s predecessor. Since then, Pleiades has been upgraded to faster hardware several times, helping to keep up with the fifty-fold increase in the volume of data collected by TESS—just one of the many NASA projects that depend on reliability and availability of NAS supercomputing resources to support their science and engineering research.

—Katharine Pitta, NASA Ames Research Center