The star in question is in our galaxy, about 12,000 light-years away, near the constellation Eagle. Astronomers have observed an intense burst of light as it swallowed a planet — presumably the size of Jupiter — orbiting closest to it. This is the first time that scientists have witnessed the phenomenon firsthand. This is also the fate that the Earth will know in five billion years when the Sun reaches the end of its life.
When a star has exhausted all the hydrogen in its core at the end of its life, it expands to a million times its original size, engulfing the nearest planets in the process — which probably results in luminous mass ejections from the star. However, this phase of stellar evolution had never been observed directly. Scientists at MIT, Harvard University, Caltech, and elsewhere report first observing a planet being devoured.
"Like many scientific discoveries, this is an accidental discovery that really opened our eyes to a new kind of phenomenon," Kishalay De, a postdoctoral researcher at MIT, told the Guardian. Astronomers spotted this dying star for the first time in 2020, thanks to observations made by the Zwicky Transient Facility (ZTF) at Palomar Observatory in California. In just 10 days, the star became more than 100 times brighter, then faded rapidly. For scientists, this sudden flash of light could only mean one thing: a planet had just plunged into the burning depths of the star.
An atypical chemical composition
The ZTF is an astronomical sky survey designed to detect objects whose brightness changes rapidly, such as supernovas, gamma-ray bursts, or collisions between two neutron stars. The discovery was fortuitous: while searching ZTF data for signs of flares in binary star systems, Kishalay De noticed a star whose brightness had increased exponentially. "I had never seen such a stellar explosion in my life," he says.
De and his team initially assumed it was a stellar merger, but observations by the Keck Observatory in Hawaii cast doubt on this hypothesis. The telescopes at this observatory take spectroscopic measurements of starlight, from which scientists can deduce their chemical composition. Most binary systems produce hydrogen and helium when one star engulfs the other, but no trace of these two elements was noticeable in the new light source.
Instead, the team spotted signs of "particular molecules," which are usually only seen in very cool stars. "But when a star lights up, it usually gets hotter. So low temperatures and bright stars don't go hand in hand," De says. The light flash was therefore not linked to a stellar merger, but its origin remained unknown.
About a year after its discovery, the researchers examined data this time from an infrared camera at the Palomar Observatory. They then found that the source was "incredibly bright" in the near-infrared, suggesting that after the initial flash, the star released cooler material into space, likely resulting from the condensation of gas from the star. Again, the team assumed from this new data that the star was merging with another. But observations from NASA's NEOWISE infrared space telescope have provided other information to solve the question once and for all.
The energy released is too low for stellar fusion
From this data, the researchers were able to estimate the total amount of energy released by the star since the initial bright flash: they found that it was surprisingly low — about 1,000 times less than that typically measured during a stellar merger. Therefore, the object engulfed by the star could not be another star. In other words, this object must necessarily have been about 1000 times smaller than all the stars seen before.
However, by a "happy coincidence", the mass of Jupiter is precisely about equal to one-thousandth of the mass of the Sun. The nature of the object that had just been disintegrated by the star was now clear: "The exceptionally low optical luminosity (about 1035 erg/s) and the radiated energy (6.5 × 1041 erg) indicate the engulfment of a planet of less than ten Jupiter masses," notes the team in Nature. The researchers estimate that this planet orbited its host star extremely quickly, completing an orbit in less than a day.
It began to brush the surface of the star about nine months before being permanently dragged into its atmosphere, causing an explosion of light. Then, when the planet fell into the core of the star, the outer layers exploded and then deposited as cold dust over the next year. "The star today looks like it did before the eruption, except it's surrounded by this dust shell that was ejected during the eruption itself," De told the Guardian.
This image traces the life of a Sun-like star. After spending most of its life in the main sequence, the star's core begins to gradually warm up, the star expands — engulfing the surrounding planets in the process — and becomes redder until it turns into a red giant. The stars 18 Scorpii (or 18 Sco) and HIP 102152 are considered binoculars of our sun. © ESO/M. Kornmesser
For decades, astronomers have observed the before and after of this type of event — planets orbiting closest to a magnifying star or giant stars that have already engulfed surrounding planets. This unprecedented and real-time observation, therefore, makes it possible to complete current knowledge on the death of stars. "We estimate that the galactic rate of such subluminous red novae is between 0.1 and several per year. Future surveys of the galactic plane should make it possible to identify them systematically, thus showing the demographic characteristics of the engulfment of the planets and the final fate of the planets in the inner solar system.
When the Sun reaches this stage in about five billion years, it will swell enough to engulf Mercury, Venus, and then Earth. But our planet will become uninhabitable long before it is swallowed up: the Sun will become so powerful that the temperature on the Earth's surface will rise by several hundred degrees, causing all the water it contains to evaporate.