It has long been theorised that planets located relatively close to a parent star may have their ultimate fate sealed by their proximity – that of being engulfed by their star as it approaches the end of its main sequence and becomes a red giant. The star itself has to be at least as massive as a low mass star – that is, not a red dwarf, so that it would expand to a red giant towards the end of its life. This had never been observed – until now, when a recent observation by astronomers seems to confirm this theory.

Through observation of seve­ral sun-like stars over decades and countless simulations, astronomers have pieced together what they expect to occur in the final throes of stars with at least 40 per cent the mass of the sun. As they run out of hydrogen at the core, which they use up in fusion, their core contracts and heats up, causing the outer layers of the star to expand and cool, forming the formidable red giant. In the process, planets close to their parent star would be engulfed by the expanding outer layers of the dying star. For the first time, a star has been observed in the exact moment that this process was occurring.

For the first time, a star has been observed in the exact moment that this process was occurring

A burst of radiation from a star, located some 12,000 light years away, was detected at the Zwicky Transient Facility (ZTF) in 2020. They noted that at the same time that this observation was made, a star in the constellation Aquila brightened about 100 times its normal brightness, but they also noted that the burst of radiation they observed, dubbed ZTF SLRN-2020, had a spectrum atypical of novae, with cold gas rather than hot gas surrounding the source. Infrared observations of the same event also showed that it had genera­ted extraordinary amounts of dust, again not typical of known nova events.

Finally, through observations with the NEOWISE telescope, any possibility of a stellar merger was discounted since the amount of energy released was, in fact, significantly lower – by a factor of 1,000 – when compared with normal stellar merger events. This led to the conclusion that the collision event was indeed not between two stars, but between a star and a much smaller body – probably a Jupiter-sized planet!

And what about the Earth? While astronomers are yet unsure about the ultimate fate of our planet, we do know that our sun will indeed swell to become a red giant in about five billion years’ time. When that happens, we do know that both Mercury and Venus will very likely also be engulfed by the sun.

The Earth, on the other hand, might actually escape this fate, even though at its current distance from the sun, the Earth would also likely be engulfed. However, the sun might lose enough mass over the next five billion years to nudge the Earth slightly further out in its orbit, increasing its distance from the sun enough to avoid being engulfed by the swelling red giant.

Dr Josef Borg completed a PhD in Astronomy at the Institute of Space Sciences and Astronomy, University of Malta, and is currently a post-doctoral researcher in space biology at the Faculty of Health Sciences at the University of Malta. He is also Malta’s representative on the European Astrobiology Network Association (EANA) council.

Sound Bites

•        China to attempt first-ever sample return mission from far side of the moon: With its Chang’e 6 mission, China aims to return samples from the Apollo basin on the far side of the moon. The mission is expected to launch in May 2024 from Wenchang, and will be followed with subsequent missions (Chang’e 7 and Chang’e 8) in 2026 and 2028 respectively. The aim of the mission is to collect material from the Apollo basin site, which lies in an ancient impact crater, with material strewn inside the basin hopefully shedding light on the history of the moon.

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DID YOU KNOW?

•        The less massive the star, the longer its lifetime: While stars like our sun tend to have an expected lifetime of approximately 10 billion years, not all stars have a similar lifetime. Indeed, the most massive of stars – termed giant stars – have an expected lifetime of only a few million years, reaching such high temperatures and pressures at their cores that they fuse their core hydrogen at a significantly higher rate. On the other hand, the least massive of stars – named red dwarfs – have an estimated lifetime of one to 10 trillion years, with slower fusion rates and also remaining fully convective, meaning that all their stellar hydrogen will be available for fusion.

•        The sun will not explode when it dies: After reaching the red giant phase, there will be no supernova explosion – the sun will simply lose its outer layers over time, exposing its core and forming a white dwarf – the final remnant of our star. It takes a much more massive star than the sun to continue fusing heavier and heavier elements, all the way up to iron, and finally go supernova.