ALMA Discovers Companion Orbiting Giant Red Star π 1 Gruis

In a groundbreaking discovery, an international team of scientists has identified a close companion orbiting the giant red star π 1 Gruis, located approximately 530 light-years from Earth. Utilizing the advanced capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA), researchers have directly observed the companion’s orbit, shedding light on the complexities of stellar evolution.

Unveiling the Mysteries of a Red Giant

Known to astronomers as pi-one-Gru, π 1 Gruis is classified as an asymptotic giant branch (AGB) star. Once similar to our sun, this aging star has expanded to over 400 times the size of the sun and now radiates thousands of times more light than its younger counterpart. As AGB stars like π 1 Gruis evolve, they undergo significant changes, producing new elements and losing mass at an astonishing rate. Over a four-year period, π 1 Gruis can eject an amount of material equivalent to that of Earth.

Detecting companions around such luminous stars has posed a considerable challenge for astronomers, as these companions can be outshone and vary greatly in brightness. The recent findings, published in Nature Astronomy, mark a significant advancement in understanding the dynamics of these celestial bodies.

Insights from ALMA’s Observations

Yoshiya Mori, a Ph.D. candidate in astrophysics at Monash University, played a pivotal role in this research. He conducted detailed comparisons between the observed properties of π 1 Gruis and cutting-edge stellar evolution models. “A key part of understanding the orbit of the companion is knowing the mass of the AGB star,” Mori stated. “Our team helped better constrain this mass by using its observed luminosity and pulsation characteristics to find the best-suited stellar model.”

The study reveals that contrary to earlier predictions of an elliptical orbit for the companion, it exhibits an almost perfectly circular orbit. This surprising finding implies that orbital evolution occurs at a faster rate than previously anticipated, prompting the need to revise existing models of giant star evolution.

Project lead Mats Esseldeurs from KU Leuven emphasized the broader implications of this research for understanding stellar evolution. “Our sun will one day go through a similar stage as π 1 Gruis. Understanding how close companions behave under these conditions helps us better predict what will happen to the planets around the sun and how the companion influences the evolution of the giant star itself,” he explained.

The analysis also indicates that current models may have underestimated the rates of circularization for orbits involving companions. This opens new avenues for research into tidal interaction physics and the evolution of binary star systems.

This discovery not only advances our understanding of AGB stars but also provides crucial insights into the life cycles of stars and their potential impact on surrounding planetary systems.

For further details, refer to the study by Mats Esseldeurs et al., titled “Evidence for the Keplerian orbit of a close companion around a giant star,” published in Nature Astronomy on November 14, 2025.