Astronomers have gained insights into the life cycle of stars and their interactions with planetary systems by studying numerous stars at different stages of evolution. They have confirmed that sun-like stars expand up to 1,000 times their original size as they near the end of their life, engulfing their inner planets in the process. This phenomenon is rare, occurring only a few times a year in the entire Milky Way. While the aftermath of planetary engulfments has been observed before, scientists have never seen one occur until now.
Using the powerful Gemini South Adaptive Optics Imager (GSAOI), astronomers have directly observed a dying star expanding to swallow one of its planets. The evidence was found in a “long and low-energy” outburst from a star in the Milky Way, located about 13,000 light-years away from Earth. This event is likely a preview of what will happen to Mercury, Venus, and Earth when our sun reaches the end of its life in approximately five billion years.
According to Ryan Lau, an astronomer from NOIRLab and co-author of the study published in the journal Nature, “These observations offer a new perspective on the discovery and study of the billions of stars in our Milky Way that have already devoured their planets.”
During the majority of its lifespan, a sun-like star generates energy by fusing hydrogen into helium within its dense core. This process counteracts the gravitational pressure of the star’s outer layers. However, once the hydrogen fuel in the core runs out, the star expands as helium fuses into carbon. The outer layers of the star expand, transforming it into a red giant.
This transformation is disastrous for any planets located within the star’s inner system. As the star’s surface engulfs one of its planets, a burst of energy and material is triggered by their interaction. Additionally, the planet’s orbital velocity slows down, causing it to ultimately crash into the star.
The Zwicky Transient Facility first discovered evidence of this event through optical images. Subsequently, the engulfment event, known as ZTF SLRN-2020, was confirmed by examining archived infrared data from NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE). According to NOIRLab astronomer Aaron Meisner, who co-authored the paper, “Our team’s customized analysis of all-sky infrared maps from NEOWISE showcases the significant potential for discovering new events through archival survey datasets.”
Identifying a planetary-engulfment outburst amidst other types of outbursts, such as solar-flare-type events and coronal-mass ejections, is a challenging task that demands high-resolution observations to accurately locate the outburst’s position and long-term measurements of its brightness, free from contamination by neighboring stars.
Thanks to its adaptive-optics capabilities, Gemini South provided the crucial data required to conduct this study. “Gemini South’s observations advance our understanding of the cosmos, and these latest findings validate predictions about the future of our planet,” remarked Martin Still, NSF Gemini Observatory program director. “This discovery highlights the remarkable results that can be achieved when world-class telescope operations and state-of-the-art scientific collaboration come together.”
As Kishalay De, an astronomer at the Massachusetts Institute of Technology and lead author on the paper, stated, “With these innovative new optical and infrared surveys, we are witnessing these events in real time in our very own Milky Way galaxy, a clear indication of what awaits our planet in the future.”
The outburst from the engulfment lasted for about 100 days, and its lightcurve characteristics, as well as the expelled material, provided astronomers with valuable information on the star’s mass and the planet it engulfed. The expelled material comprised roughly 33 Earth masses of hydrogen and 0.33 Earth masses of dust. “This burp of matter contains both planetary and stellar material that is being recycled or ejected into the interstellar medium as a result of the star devouring its planet,” explained Lau. Based on these observations, the team estimated that the parent star’s mass ranges between 0.8 and 1.5 times that of our Sun, while the engulfed planet’s mass was between 1 and 10 times that of Jupiter.
The recent discovery of the first-ever observed planetary engulfment event has given astronomers a better understanding of how to detect similar phenomena occurring in other parts of the universe. This new knowledge will be especially useful when the Vera C. Rubin Observatory becomes operational in 2025. By studying the chemical signatures of remnants from engulfed planets, scientists will be able to identify more instances of planetary engulfment. The findings from this event also fill a gap in our knowledge about the ultimate fate of planetary systems, including our own.
The event’s significance is not lost on the researchers involved in the study. “This discovery speaks to the ephemeral nature of our existence,” says Lau. “After billions of years of existence, our solar system’s end will likely be a fleeting moment that lasts just a few months.”