Physicists publish assessment of planet-hosting multiple-star systems

A planetary physicist at The University of Texas at Arlington is the lead author of a study that catalogs all known triple (and higher) stellar systems—those with planets having three or more suns.

Manfred Cuntz, professor of physics, led the project, titled “An Early Catalog of Planet-hosting Multiple-star Systems of Order Three and Higher”. This study provides a thorough bibliographic assessment of planet-hosting triple stellar systems.

It was recently published in The Astrophysical Journal Supplements, a journal of the American Astronomical Society (AAS) which has one of the highest impact factors in the field. Co-authors include UTA alumni G.E. Luke, Matthew Millard, and Lindsey Boyle, as well as Shaan D. Patel, a Ph.D.-bound graduate student.

The paper offers a template of system classification, which particularly considers the various types of planetary orbits. Additionally, the authors evaluate cases of controversies and planet retractions based on the criteria for what constitutes a planet-hosting triple star system. They also provide selected statistical analyses and interpretation, encompassing both the planets and the host stars.

In 1995, Swiss astronomers Michel Mayor and Didier Queloz made the first detection of a planet beyond the solar system in orbit about a Sun-like star. Since then, more than 5,000 exoplanets have been confirmed. Examples of planet-hosting triple star systems include 16 Cygni, where a Jupiter-mass planet was discovered by William Cochran and collaborators in 1997 based on data taken at McDonald Observatory in West Texas and at the Lick Observatory in California.

Most planets, such as all in Earth’s solar system, are hosted by single stars. However, there are a notable number—about 100, depending in part on the cutoff regarding the stellar separation distance—that are members of stellar binaries, the authors wrote in their study. 

“The number of planets found to be hosted by higher-order systems is relatively small—about 40 for triple and quadruple systems combined, with the exact number depending on whether some controversial or unconfirmed cases are included,” Cuntz said. “The number of confirmed planets in triple stellar systems currently stands at about 30, which is approximately 0.5 percent of the total number of planets identified. This aspect makes those planets very special.”

Cuntz said that the science of discovering planet-hosting triple stellar systems benefited considerably from the NASA Kepler Space Telescope, which was operational from 2009-18. He also noted that scientists expect the number to increase, particularly with the abilities of the James Webb Space Telescope, which was launched in 2021 and among other goals is devoted to studying star and planet formation.

The authors note that the overwhelming majority of triple stellar system planets are Jupiter-type, meaning they are gas giants, and the host stars are relatively massive, compared to typical main-sequence stars. However, some Earth-mass planets have been found as well.

All triple stellar systems with planets are identified as highly hierarchical, Cuntz said. This means that a system’s stars can be divided into two subgroups, each of which travels in a relatively large orbit around the system’s center of mass. In a triple stellar system, two of the stars usually form a close binary pair (two stars that are gravitationally bound to and in orbit around each other), and the third orbits that pair at a distance much larger than that of the binary orbit. Hierarchical systems with more than three stars are expected to produce even more complicated orbiting arrangements.

The work of Cuntz’s team recently received heightened attention during his visit to the Aryabhatta Research Institute of Observational Sciences (ARIES) in Nainital, India. He also participated in the inauguration of the International Liquid Mirror Telescope (ILMT), which has a 4-meter diameter mercury mirror and is the first liquid mirror telescope for astronomy in the world. This facility is devoted to fundamental studies of the universe as well as the assessment of transits and multiple stellar systems.

“The existence of planets in triple star systems is extremely challenging theoretically, both regarding their formation and orbital stability,” Cuntz said. “These topics are a stark motivation of future UTA research, also involving students.”

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