The Another

    Star caught in the act of swallowing a planet



    The Samuel Oshin telescope at the Palomar Observatory in California.

    For the second time in less than six months, the ZTF has been featured in the prestigious journal Nature. Behind the acronym is a camera at the Zwicky Transient Facility, installed on one of the telescopes on Mount Palomar (California) that scans the sky with a wide field of view. It’s searching for what astronomers call transient events (comet or asteroid transits, but also star explosions). On November 30, 2022, Nature announced that the ZTF had detected a star torn apart by a black hole. This time, in its May 4 issue, Nature published a US study showing another cosmic event as brutal as it is unprecedented: the swallowing of a giant planet by its star.

    It all started in May 2020. The paper’s main author, Kishalay De, a postdoctoral fellow at the Massachusetts Institute of Technology, was examining ZTF data looking for bright flares in double star systems. “One night,” he said, “I noticed that a star brightened by a factor of 100 over the course of a week, out of nowhere. It was unlike any stellar outburst I had seen in my life.” What was going on up there?

    Read more Article reserved for our subscribers Astronomers detected a star that was ripped apart by a supermassive black hole

    To find out, De and several astrophysical colleagues collected additional data from other instruments to see what had happened before and after the sudden brightness increase. And they proceeded by elimination. An astrophysicist’s first reaction when faced with such a phenomenon is to think of a classic nova. Imagine a dying double star, with a red giant at the end of its life, and a white dwarf, a kind of still-smoldering, extremely dense stellar corpse. The latter pulls matter from its companion, which “piles up” on its surface until all the accumulated gas produces a thermonuclear explosion. And a beautiful flash.

    A red nova

    The authors of the study had to discard this scenario because the data they collected did not fit. The phenomenon of May 2020 did not give rise to a fantastic and brief expulsion of very hot gas. On the contrary, for months after the bright outburst, the star was instead regularly releasing cold material into space, clearly visible in infrared. The researchers then turned to a lesser known – and also calmer – type of nova, the red nova, which signals the fusion of two stars.

    But here again, there was a snag in the hypothesis. When De and his colleagues took measurements of the original event, they realized that the amount of energy emitted was ridiculously small compared to what would have been released if one star had ingested another. About 1,000 times weaker. What was “eaten” must have been 1,000 times smaller than a star. Now, we know that the largest planet in our Solar System, the gas giant Jupiter, has a mass almost exactly one-thousandth of the Sun. Then the inevitable conclusion became apparent: what the ZTF had observed, 12,000 light-years from the Earth, was a giant planet being swallowed by its star.

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