Why is it important to look for exoplanets in triple star systems and how many can we find there? This is what a study recently accepted by Astrophysics & Space Science seeks to clarify by two researchers from the University of Texas at Arlington examining the statistical likelihood of triple star systems hosting exoplanets. This study has the potential to help researchers better understand the formation and evolution of triple star systems and whether they are suitable for finding life as we know it.
Here, Universe Today discusses this incredible research with Dr. Manfred Cuntz, a physics professor at the University of Texas at Arlington and lead author of the study, discussed the motivation behind the study, the key findings and importance of studying triple star systems, and the likelihood of finding exolife in triple star systems. So what was the motivation behind the study?
Dr. Cuntz explains to Universe Today: “Age and metallicity (i.e. the amount of heavy elements = elements other than hydrogen and helium) are fundamental properties of stars – a statement that applies to all stars.” Considering that most stars ( (which is not the case for the Sun) are members of higher order systems, the study of stars in triple star systems is a natural extension of research focused on individual stars.”
For the study, researchers conducted a statistical analysis of both the age and metallicity of triple star systems with a total of 27 confirmed exoplanets based on previous research, with the number of exoplanets in each system ranging from 1 to 5. The age of the triple star system was between 20 million and 7.2 billion years, with margins of error. For comparison, our sun is estimated to be just over 4.6 billion years old.
The metallicities of the star systems ranged with margins of error between -0.59 and +0.56, which is often calculated based on the ratio of iron to hydrogen (Fe/H) and is also calculated using the equation X + Y + Z =1 , where X is the proportion of hydrogen, Y is the proportion of helium, and Z is everything else (e.g. carbon, oxygen, silicon, iron, etc.). These values range from -4.5 to +1.0, with stars having 0, -1, greater than 0, and less than 0, meaning that a star has the same iron abundance as our Sun, one-tenth the iron abundance of ours Sun, i.e. a higher metal content than our sun or a lower metal content than our sun. So what were the key findings of this study?
“Two extremely significant results were identified,” says Dr. Cuntz told Universe Today. “First, stars in triple star systems are, on average, significantly younger than stars in the solar environment. The most plausible explanation is a possible dual selection effect due to the relatively high mass of these systems' planet-hosting stars (which spend less time on the main sequence than low-mass stars) and the fact that planets in triple star systems can be long-term orbitally unstable. The stellar metallicity of these stars, on average, is similar to that of the Sun; However, due to the limited amount of data, this result is not inconsistent with the previous finding that stars with planets tend to be metal-rich because the derived metallicity distribution is relatively broad.”
The distances to the respective triple star systems are between 4.3 and 1,870 light-years from Earth, but only 6 of the 27 triple star systems are less than 100 light-years away. These six triple star systems include Alpha Centauri (4.3 light years), Epsilon Indi (11.9 light years), LTT 1445 (22.4 light years), Gliese 667 (23.6 light years) and 94 Ceti (73.6 light years). and Psi1 Draconis (74.5 light years), with the total number of exoplanets (with exoplanet candidates) in each system being 3 (2), 1, 1, 2 (1), 1 and 1, respectively. For comparison, as of September 2024, the total number of confirmed exoplanet systems in our cosmos is more than 4,300, which includes almost 5,800 exoplanets. But what is the importance of studying triple star systems despite the small number of triple star systems that host exoplanets?
Dr. Cuntz explains to Universe Today: “Most stars (although this does not apply to the Sun) are members of higher order systems, particularly binary star systems – and in rarer cases of triple star systems and even higher order systems.” Hence the study of planets in triple star systems a natural extension of the standard approach that focuses on planets around individual stars. The current study focuses on some properties of stars in triple star systems that are also known to host one or more planets – a relatively rare situation. The significance of the current study is to expand our general understanding of star-planet systems.”
For Alpha Centauri, the exoplanet Proxima Centauri b was confirmed to be terrestrial (rocky), approximately the size of Earth in radius and mass, and orbiting within the habitable zone (HZ) of Proxima Centauri, one of the stars that form the Alpha triple star system Centauri. The only other terrestrial exoplanet orbiting within the HZ of its star is Gliese 667 Cc, which has a mass and radius greater than Earth's, designating it a super-Earth. Given the small number of triple star systems with exoplanets, and even fewer that host terrestrial exoplanets orbiting in their HZ, what is the probability of finding exolife in triple star systems?
“The only planet where we know for certain that life exists is Earth,” says Dr. Cuntz told Universe Today. “However, through observations and theoretical studies over many decades of dedicated work, scientists are convinced that exolife is almost certainly real. This statement should also apply to planets in triple star systems. However, these planets typically experience relatively different environmental influences (e.g. different amounts of radiation absorbed by stellar components), which is expected to reduce the likelihood of advanced life forms, but should still allow microbial life, particularly extremophiles.”
As the number of confirmed exoplanets continues to grow, the confirmed number of triple star systems hosting exoplanets should also grow. When science fiction fans read about multiple star systems, they almost immediately think of the iconic scene in Star Wars: A New Hope in which Luke Skywalker sees two stars setting on the horizon. While Tatooine was habitable for humans and other interesting life forms, this may not be the case in the real world, as demonstrated by Proxima Centauri b, which is currently the only Earth-like exoplanet orbiting within 100 light-years of Earth in its HZ. So what limitations should scientists apply when searching for life in triple star systems? Since the movie Avatar features the semi-inhabitable moon Pandora orbiting a much larger exoplanet in the Alpha Centauri system, should we study their moons instead? Are triple star systems with exoplanets as rare as statistics show today?
“The search for life outside planet Earth continues to be a fascinating topic,” says Dr. Cuntz told Universe Today. “Political and social support for ongoing and future space missions is greatly appreciated. We as scientists are grateful for the continued support of taxpayers around the world, but especially here in the United States.”
What new discoveries about triple star systems will researchers make in the coming years and decades? Only time will tell, and that's why we do science!
As always, keep up the science and keep looking up!
Reference: Cuntz, Manfred & Patel, Shaan D. “On the age and metallicity of planet-hosting triple star systems.” Astrophysics and Space Sciences (2024) (accepted)
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