How Earth will meet its demise: Astronomers witness a star devour a planet

In a huge astronomical discovery, astronomers have pieced together an understanding of the life cycle of stars and their interactions with surrounding planetary systems as they age. 

By studying countless stars at various stages of their evolution, researchers have confirmed that when a Sun-like star nears the end of its life, it can expand anywhere from 100 to 1000 times its original size. This dramatic expansion often results in the engulfment of the system’s inner planets, a rare event estimated to occur only a few times each year across the entire Milky Way.

Although past observations have confirmed the aftermath of such planetary engulfments, astronomers have never actually witnessed one in progress. That changed recently, thanks to the power of the Gemini South Adaptive Optics Imager (GSAOI) on Gemini South, one half of the International Gemini Observatory, operated by NSF’s NOIRLab. 

For the first time, astronomers have observed direct evidence of a dying star expanding to engulf one of its planets (see image here).

What astronomers saw

The evidence came in the form of a telltale “long and low-energy” outburst from a star located approximately 13,000 light-years from Earth in the Milky Way. This event, the devouring of a planet by an engorged star, is believed to foreshadow the ultimate fate of Mercury, Venus, and Earth when our Sun begins its death throes in about five billion years.

“These observations provide a new perspective on finding and studying the billions of stars in our Milky Way that have already consumed their planets,” said Ryan Lau, NOIRLab astronomer and co-author of the study published in the journal Nature.

Throughout most of its life, a Sun-like star fuses hydrogen into helium within its hot, dense core. This fusion process allows the star to push back against the crushing weight of its outer layers.

However, when the core’s hydrogen supply runs out, the star begins fusing helium into carbon, while hydrogen fusion migrates to the star’s outer layers. This shift causes the outer layers to expand, transforming the Sun-like star into a red giant.

Unfortunately, this transformation often spells doom for any planets located within the inner reaches of the star’s planetary system. As the star’s surface expands to engulf one of its planets, their interaction triggers a spectacular outburst of energy and material. 

Simultaneously, this process puts the brakes on the planet’s orbital velocity, causing it to plunge into the star. 

How the study was done

The initial hints of this rare event were discovered through optical images from the Zwicky Transient Facility. NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which specializes in peering into dusty environments in search of outbursts and transient events, then confirmed the engulfment event, named ZTF SLRN-2020.

“Our team’s custom reanalysis of all-sky infrared maps from NEOWISE exemplifies the vast discovery potential of archival survey data sets,” said NOIRLab astronomer Aaron Meisner.

Distinguishing a planetary-engulfment outburst from other types of outbursts, such as solar-flare-type events and coronal-mass ejections, is challenging. 

High-resolution observations are needed to pinpoint the location of an outburst and long-term measurements of its brightness without contamination from nearby stars. Gemini South, with its adaptive-optics capabilities, provided the essential data for this observation.

“Gemini South continues to expand our understanding of the Universe and these new observations support predictions for the future of our own planet,” said NSF Gemini Observatory program director Martin Still. “This discovery is a wonderful example of the feats we can accomplish when we combine world-class telescope operations and cutting-edge scientific collaboration.”

“With these revolutionary new optical and infrared surveys, we are now witnessing such events happen in real time in our own Milky Way – a testament to our almost certain future as a planet,” said Kishalay De, an astronomer at the Massachusetts Institute of Technology and lead author on the paper.

More details about what the astronomers found

The outburst resulting from the planetary engulfment lasted approximately 100 days, and the characteristics of its light curve, as well as the ejected material, provided astronomers with information about the mass of the star and its engulfed planet. The ejected material consisted of about 33 Earth masses of hydrogen and about 0.33 Earth masses of dust.

“That’s more star- and planet-forming material being recycled, or burped out, into the interstellar medium thanks to the star eating the planet,” said Lau. The team estimated that the progenitor star is about 0.8−1.5 times the mass of our Sun, while the engulfed planet was 1−10 times the mass of Jupiter.

Now that the signatures of a planetary engulfment have been identified for the first time, astronomers have improved metrics they can use to search for similar events happening elsewhere in the cosmos. This information will be particularly important when the Vera C. Rubin Observatory comes online in 2025.

For instance, the observed effects of chemical pollution on the remnant star, when seen elsewhere, can hint that an engulfment has taken place. The interpretation of this event also provides evidence for a missing link in our understanding of the evolution and final fates of planetary systems, including our own.

“I think there’s something pretty remarkable about these results that speaks to the transience of our existence,” said Lau. “After the billions of years that span the lifetime of our Solar System, our own end stages will likely conclude in a final flash that lasts only a few months.”

The recent observations of this phenomenon have given astronomers a unique opportunity to study the interactions between dying stars and their planetary systems, shedding light on the life cycle of stars and the fates of their planets.

More about the life cycle of stars 

The life cycle of a star depends on its mass, which determines its longevity and the stages it goes through during its life. Here is an overview of the typical life cycle of a star:

Stellar birth

Stars form in large, cold molecular clouds primarily composed of hydrogen gas and dust. Gravity causes the denser regions of the cloud to collapse, forming a rotating protostar. As the protostar continues to contract, its temperature and pressure increase, eventually reaching the point where nuclear fusion can begin.

Main sequence

This is the longest stage in a star’s life, where it spends the majority of its time. Nuclear fusion occurs in the core of the star, where hydrogen atoms combine to form helium, releasing a tremendous amount of energy in the form of light and heat. This energy counteracts the inward pull of gravity, maintaining the star’s size and stability. The more massive a star is, the shorter its main sequence stage, as it burns through its fuel more rapidly.

Red giant or supergiant phase 

As the hydrogen in the core is depleted, the core contracts, and hydrogen fusion moves to the outer layers. The core heats up, allowing helium to fuse into carbon (and oxygen in more massive stars). The outer layers of the star expand and cool, causing it to become a red giant or, in the case of more massive stars, a red supergiant. This phase marks the beginning of the end for a star’s life.

Stellar death: The fate of a star depends on its mass:

Low-mass stars (like our Sun)

After the red giant phase, the outer layers of the star are expelled into space, forming a planetary nebula. The remaining core becomes a white dwarf, which eventually cools and fades over billions of years, turning into a black dwarf (although none exist yet in the universe, as the process takes longer than the current age of the universe).

Medium-mass stars 

These stars may undergo a similar process as low-mass stars, but with more intense planetary nebulae and a more massive white dwarf at the end.

High-mass stars

These stars end their lives in a violent explosion known as a supernova. The core collapses under the force of gravity, causing the outer layers to be ejected into space. The remaining core can form a neutron star, an incredibly dense object made almost entirely of neutrons. If the core is massive enough, it may collapse further and form a black hole, an object with such strong gravity that not even light can escape it.

Stellar remnants

The remnants of a star, whether a white dwarf, neutron star, or black hole, continue to evolve over time. White dwarfs eventually cool and become black dwarfs, while neutron stars and black holes can interact with their surroundings, such as accreting matter from nearby objects or merging with other compact remnants.

Throughout their life cycles, stars play a crucial role in the universe. They create and disperse heavy elements through supernovae and planetary nebulae, which contribute to the formation of new stars, planets, and even life.

Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani

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