Astronomers discover an iron planet stripped of its skin around an exhausted star | Science

In a glimpse of what might be reserved for our own solar system, astronomers have discovered what appears to be the broken remains of a planet orbiting a white dwarf, extinguished ember. a star like our sun. If the team's calculations are correct, the object in orbit may be the iron core of a small planet whose outer layers have been torn off by the intense gravity of the white dwarf.

Although astronomers know the existence of thousands of exoplanets in the Milky Way, they find it hard to see anything much smaller than the Earth. The new object is by far the smallest, more an asteroid than a planet. His discovery also provides a clue to the fate of the planets with the aging of their stars. When sun-like stars lack hydrogen and begin to burn elements such as helium and carbon, they swell into red giants and consume the planets that gravitate too closely. Those who have survived witness what can happen next when the fuel of the red giant is exhausted: it collapses into a small white and dense dwarf, which cools for billions of years. Its intense gravity can tear any surviving planet that strays too close, consuming a material and leaving the rest in a swirling disk of dust.

Finding planetesimal, 400 light-years from Earth, was not easy. A team of astronomers, led by Christopher Manser of the University of Warwick in Coventry, UK, was watching this white dwarf especially for 15 years. They saved time on the world's largest optical telescope, the 10.4-meter Gran Telescopio Canarias located in La Palma, Canary Islands, Spain, in 2017 and 2018. The white dwarf, known as SDSS J122859.93 + 104032.9, or SDSS J1228 + 1040 for his friends, this is one of the few white dwarfs surrounded by a disk containing both gas and debris, and the team wanted to study the variations minute by minute of gas.

Most exoplanets are not directly visible, but are found when they cast a shadow on the face of their star or they shoot the star with the force of their gravity. The Manser team used a similar indirect method. They separated light from the disc to see its frequency spectrum and zoomed in on three bright spectral lines produced by the calcium ions, which serve as a flag for gas flowing through the disc.

When the gas – including calcium ions – slips around the white dwarf, its light shifts in Doppler towards slightly higher frequencies as it moves towards the Earth and lower frequencies as it moves away. The effect also extends the normally narrow calcium emission lines into broad bands with peaks at each end, shaped like a hammock suspended between two poles.

Manser said his team expected to see such extended lines with perhaps some random fluctuations in the peaks, caused by the collision of debris and the production of flares. Instead, they found that the two peaks of each calcium line increased and decreased against each other every 2 hours, as on wheels. "It was a really exciting discovery," Manser said.

The researchers gave several possible explanations for meteorological peaks, including a large planet in orbit and vortices in the dust disk. But write in Science today, they reject everything but one: that it is the signature of a planetesimal orbiting the star. They argue that the calcium lines do not come from the planetesimal itself, but from a cloud of gas that surrounds it, either because the disk debris destroys it, or because the radiation emitted by the star makes it emit gas. When this cloud of gas follows the planetesimal in its orbit, it amplifies a peak emission while moving towards Earth and, an hour later, the other peak while moving away.

"It's unbelievable for me to be able to infer the existence of such a small object," said astronomer Ben Zuckerman of the University of California at Los Angeles, who did not participate in this work. But he and astronomer Mukremin Kilic of the University of Oklahoma at Norman agree that the team's explanation is most likely. "Is this a planetesimal?" Asks Kilic. "Given the information available, it is probably the best conclusion."

The result is also surprising because the object is so close to its star the size of the Earth. If it was in our solar system, it would be in orbit on the surface of the sun. Anything close to a white dwarf would normally be torn. The researchers calculated that if the planetesimal was simply held together by its own gravity, the whole thing should be the density of iron, which would bring it closer to the metal asteroids that we find in our solar system. If it had differentiated layers to give it strength, it could be less dense and have a width of up to 720 kilometers, tied with the dwarf planet Ceres. The researchers say that, whatever the nature of the object, its outer rock layers had to be torn off by the white dwarf, leaving only its metal core.

The fact that this object was found around one of the very few white dwarfs that have both dust and gas in its disc suggests that the gases could be "a firearm for planetesimals", said Manser. The team therefore hopes to be able to examine other white dwarves with gassy records to find other survivors in orbit.

Meanwhile, the fate of the SDSS J1228 + 1040 and his companion gives us a sobering picture of the future of our solar system. It is thought that when the sun swells into a red giant, it will consume Mercury, Venus and the Earth. Other planets can move outward and survive, but these movements can cause a gravitational shock that completely ejects the planets or sends them spiraling toward their destiny. It's not a good idea, but we have about 6 billion years to consider our destiny.