(Image: The Wolf-Rayet star WR 124 in the constellation Sagittarius, imaged by the Hubble Space Telescope.)

The prevailing theory of the formation of the solar system is that a large, nearby supernova ejected material into the dense cloud of interstellar dust which collapsed to form our sun and the leftover material that orbits as planets, moons, asteroids and the rest. Astronomers developed this theory because of the high abundance the isotope aluminum-26, which is liberally ejected in a supernova explosion.

However, more recent measurements of meteorites from the early solar system suggest there is not as much iron-60 as in the rest of the galaxy, which is also created in high abundances in supernovae. “It begs the question of why one was injected into the solar system and the other was not,” said coauthor Vikram Dwarkadas, a research associate professor in Astronomy and Astrophysics at the University of Chicago, in a press release.

Dwarkadas and his team have developed a new solar system formation theory in a study published today in the Astrophysical Journal, one that could explain the apparent imbalance of aluminum-26 and iron-60 compared to what you would expect to see from supernova material.

The new theory suggests that the sun formed within the walls of a dense bubble of material surrounding one of the hottest-burning stars in the galaxy, a Wolf-Rayet star. Wolf-Rayet stars are rare and diverse stars full of heavy elements that tower about 50 times larger than our sun. Their surfaces burn at temperatures ranging from 30,000 K to more than 200,000 K (50,000 to 360,000°F), making them hotter and brighter than almost all other stars.

Slices of a simulation showing how bubbles around a massive star evolve over the course of millions of years (moving clockwise from top left). Courtesy ofV. Dwarkadas & D. Rosenberg

The immense heat and volatile conditions cause Wolf-Rayet stars to hemorrhage heavy elements out into space. These stars with hundreds of times the mass of our sun are short-lived, only lasting for millions of years compared to the 4.6-billion-year-old sun, which still has about 5 billion years to go. They quickly burn through any hydrogen as Red Supergiants, and then begin fusing helium and other heavier elements as Wolf-Rayets, all the way up to iron by the end. These heavy elements bubble up to the surface in large amounts, where vicious stellar winds rip them up and eject them into space.

A bubble with a thin dense shell of heavy material forms around the raging star, inflated by stellar winds. These bubbles vary in size based on the initial mass of the star, but they can stretch as big as tens of parsecs in diameter with the volatile star in the middle. The new theory suggests that the sun could have formed within the walls of this immense cosmic bubble, where the high densities cause rapid star formation.

"These are really large, and the sun's radius is very small compared to the bubble size," Dwarkadas told Popular Mechanics in an email. "We postulate that a part of the shell will collapse to form the early solar system."



Wolf-Rayet star WR 124 imaged by the Hubble Space Telescope, with a still-forming nebula bubble of about 2 parsecs, or 6 light-years. WR 124 lies 15,000 light-years away towards the constellation of Sagittarius. NASA/ESA/Hubble

Wolf-Rayet stars rapidly form abundant aluminum-26, fusing it in their cores while some rises to the surface and is ejected out into space, where it collides with the surrounding dense shell of the bubble and enriches it with the isotope. Unlike a supernova, a Wolf-Rayet stars does not eject much iron-60.

“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed—trapping the aluminum inside the shell,” said Dwarkadas.

Once the Wolf-Rayet star dies, the shell cools and parts of it collapse to form stars, including our sun. Based on the number of Wolf-Rayet stars and the distribution of material in their bubbles, Dwarkadas and his team believe that from 1 to 16 percent of all sun-like stars could form in these conditions.



When the star does die however, it likely erupts in a supernova. If this happens to the Wolf-Rayet star, the researchers suggest the iron-60 that is generated in the explosion could be distributed unequally, missing the part of the shell where our solar system formed, or perhaps it could not puncture the shell wall. Alternatively, the Wolf-Rayet star could collapse directly into a black hole, which would produce little iron-60.

The mystery of the solar system's formation will likely endure for years and years, as scientists scrutinize the Wolf-Rayet star theory to see if it holds, and come up with new ones altogether. But the clues we have left written in ancient space rocks that formed at the beginning of the solar systems life can tell us a great deal about the past. Perhaps when OSIRIS-REx—a NASA spacecraft on its way out to an old asteroid that orbits nearby, named Bennu—returns a sample of that ancient material to scientists on Earth in September 2023, astronomers will answer the question of the solar system's formation more completely.

Until that time, take comfort in the fact that there is a descent probability that everything you own, everything you can see from the sun out to beyond Pluto, and even the atoms of your body were once superheated plasmas of heavy elements in a dense shell surrounding an inferno of rapid fusion and eruptions that is a Wolf-Rayet star.

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