About 550 light years way in the direction of the Pisces constellation is a pre-main sequence star called RZ Piscium. The star has long been known to undergo unusual dimming events, which can last for as long as two days. The star becomes as much as ten times fainter during these dimming events. New evidence suggests that there is a sinister reason for these dimming events – the star is consuming the planets that it produced.
Another feature about the star is that an unusually large portion of the total radiation observed emanating from it is in the infrared spectrum. The levels of infrared radiation is seen only from a few of thousands of nearby stars studied over the previous years. The excessive infrared radiation indicates plenty of debris in orbit around the star. The debris is also really hot, around 230 degrees Celsius, which indicates just how close the disc is to the star. The ring of debris is within 50 million kilometres from the surface of the star. To put that number in perspective, the debris is closer to the star than Mercury is to the Sun, and is falling into the star.
These kinds of discs around stars are known to occur during two phases of the stellar evolution. Towards the beginning of the formation of a system, the young planets are in erratic orbits and often collide into each other. In the early period of our own solar system, Neptune was actually closer to the Sun than Uranus. However, the orbits of the planets eventually stabilise. Towards the end of the star’s life, before it explodes in a supernova, the star balloons up into a red giant, swallowing up at least the planets closest to it. In 2012, astronomers found evidence that BD+48 740, a red giant about 2,000 light years away in the direction of Perseus, had swallowed up one of the planets in orbit around it. Earth may meet with the same fate, about 5 billion years from now.
As a star ages, the lithium within it is destroyed because of the nuclear reactions. The amount of lithium measured in RZ Piscium indicates that the star is between 30 million to 50 million years old. For a sun like star, which can live up to 10 billion years, this is positively young. There are other indications as well, that the star is young. It has a surface temperature of 5,330 degrees Celsius, just a bit cooler than the Sun, and it produces 1,000 times the amount of X-rays as the Sun. And although the star is young, it is still too old to have a protoplanetary disc.
Ben Zuckerman, a professor at the University of California, and a member of the team that made the finding says, “Most Sun-like stars have lost their planet-forming disks within a few million years of their birth. The fact that RZ Piscium hosts so much gas and dust after tens of millions of years means it’s probably destroying, rather than building, planets.”
Observations of the debris disk and the gas within it indicates that the astronomers have been observing the aftermath of the destruction of a planet. Material from a substellar gas giant such as Jupiter or a really massive comet may be getting stripped away, or even entirely destroyed. Another possibility is that two gas giants have been ripped apart after colliding into each other. The observations indicate that RZ Piscium is a relatively young star surrounded by a debris disk containing unstable planets. Celestial objects that have been ripped apart by their host stars are known as “disrupted planets”.
The finding provides insights into the early periods of solar system formation, including what might have happened in our own. Observing such a system is rare because the debris discs are not very long lasting in comparison to the lifespan of a star. It also helps explain why some systems survive, while others do not.
Catherine Pilachowski, an astronomer at Indiana University and co-author of the study says, “This discovery really gives us a rare and beautiful glimpse into what happens to many newly formed planets that don’t survive the early dynamical chaos of young solar systems. It helps us understand why some young solar systems survive – and some don’t.”
The research paper containing the findings is called “Is the Young Star RZ Piscium Consuming Its Own (Planetary) Offspring?” and has been published in The Astronomical Journal.
This is not the first time that a star has been suspected of devouring its own children, before long before it goes nova. In October 2017, researchers from the University of Princeton found evidence that HD 240430, about 250 light years from the Earth had swallowed up to 15 rocky planets. The star has been given the nickname Kronos, after the Titan from Greek mythology known for child cannibalism. An alien megastructure was a proposed explanation for dimming events observed on another star, KIC 8362852. The star has several nicknames, including the WTF star, Boyajian’s star, or Tabby’s star. One of the explanations for the sudden dimming events is that the star is gulping the planets it has spawned.