The End of a World as We Know It - Planet Eaten up by a Dying Star

In May 2023, astronomers shared the news that they had witnessed a star swallowing a planet and this first-of-its kind observation offered the first direct glimpse of a gnarly process called ‘planetary engulfment’ - a fate that most likely awaits our Earth in the far future. 

Infographic on a star engulfing a planet. Credit: International Gemini Observatory / NOIRLab / NSF / AURA / P. Marenfeld

Accidental Discovery

The scientists first noticed the event in 2020 about 12,000 light years away in the constellation Aquila while searching for the fireworks associated with stellar mergers (red novas). According to the scientists, the star grew bigger and more than 100 times brighter in just 10 days. This could have been the result of two stars merging. But a second look by NASA’s NEOWISE space telescope suggested this wasn’t the case.

The merger of two stars typically produces hot hydrogen and helium, which leaves bright emission lines in the spectrum. However, these were missing and instead, the researchers saw signs of molecular gasses that can only exist at much colder temperatures. 

Using additional infrared data from NASA’s Neowise space telescope, the team estimated the total amount of energy released by the star to be very small - about 1,000 times smaller than past observations of stars merging with one another. This led the scientists to conclude that whatever the star merged with must have been comparatively small. Given that - as a happy coincidence - the mass of Jupiter is about 1 /1,000 the mass of the Sun, the team realized that the star was engulfing a planet.

According to the researchers’ analysis, the planet must have had a maximum mass of about 10 times the mass of Jupiter, being swallowed by and falling towards the core of an expanding red giant. As the star swallowed the planet, its expanding outer envelope continued to cool, forming a glowing dust cloud around the star that explained the long-lived infrared brightness. 

Life Cycles of Stars

The life cycles of stars are linked to their masses. Small stars, like red dwarfs, may shine for trillions of years, whereas the most massive stars explode just a few million years after their births. Astrophysicists have long known that when a Sun-like star runs out of fuel, it becomes a red giant, expanding hundreds of times in size and consuming anything within their advancing borders. The growth occurs when a star exhausts the hydrogen fuel in its core and the fusion reactions that make the star shine spread outward in search for more fuel, swelling the star’s outer layers. 

When a sun-like star nears the end of its life, it expands into a red giant, 100 to 1,000 times its original size, eventually overtaking nearby planets. Credit: ESA / Hubble Space Telescope

Signs of engulfment events are littered across the Milky Way. The light of some stars is polluted with the chemical signatures of planets and this suggests that whole worlds are being digested before our eyes.

However, while stars clearly consume planets occasionally, capturing this moment is challenging because the light sparked by these events is faint and brief. Previous observations caught the stages just before and after the planetary engulfment, but the recent discovery was the first time the act was actually seen which is why the observation was groundbreaking. 

Our Earth’s Last Supper

Now that scientists have seen a real example of planetary engulfment, they can search the skies for similar patterns. They suspect thousands of planets around other stars will suffer the same fate as this one did and, eventually, so will our solar system. Many astronomers believe that Earth will ultimately be consumed when the Sun reaches its red-giant phase. However, there is still about five billion of years before the Sun is expected to grow large enough to engulf Earth - after swallowing Mercury and Venus first. Also, according to estimates, it would still take tens of thousands of years for the aged Sun to expand from Mercury to Earth (and humans likely will not be around for this event because Earth will have become uninhabitable by then).  

The sobering future of our home planet aside, the recent observation will make it easier to detect planet engulfments - which probably occur in our galaxy once every few years - given that scientists now know what to look for in regards to chemical composition, temperature and infrared signatures. According to Dr. De, who made the discovery together with his team, it opens up an entire new field of research. 

Sources

1. https://www.science.org/content/article/dying-star-consumes-planet-foreshadowing-earth-s-fate

2. https://www.washingtonpost.com/climate-environment/2023/05/03/dying-star-eats-planet-study/

3. https://www.nytimes.com/2023/05/03/science/star-eating-planet.html

4. https://www.sciencealert.com/a-star-was-caught-swallowing-a-planet-in-an-astronomical-first

5. https://www.spektrum.de/news/erstmals-beobachtet-sterbender-stern-verschluckt-planeten/2135463

6. https://www.snexplores.org/article/first-time-star-eating-planet-seen-astrophysics