The fireball from the edge of the solar system is not what astronomers expected | Science

Just before dawn on February 22, 2021, a ball of fire lit up the skies of the Canadian province of Alberta when a 2-kilogram space rock vaporized as it plunged into Earth’s atmosphere. Although the object came from the Oort Cloud, a conglomerate of comets on the outskirts of the solar system, it was not a comet, the researchers now claim. Data collected during its fall suggests the object was made of rock rather than ice and behaved more like an asteroid.

Independent observers of the new work say the discovery sheds light on the processes that formed our solar system and challenges the conventional wisdom that the Oort Cloud only contains icy comets. “This tells us that there was scattering and deposition of material from across the solar system into the Oort Cloud,” says Karen Meech, a planetary scientist at the University of Hawaii’s Institute of Astronomy.

The discovery could provide support for models suggesting that objects from the asteroid belt were dispersed into the Oort cloud shortly after the birth of the solar system 4.6 billion years ago, says Bill Bottke, a solar system dynamics specialist at the Southwest Research Institute. “It’s very exciting,” he said. “Now we have to see what we can do to explain it.”

First proposed by Dutch astronomer Jan Oort in 1950, the Oort Cloud is a spherical halo of comets that extends halfway from Proxima Centauri, the Sun’s closest neighbor, far beyond from the view of larger telescopes. “Anything we know about it is indirect,” says Denis Vida, a meteorological astronomer at Western University who led the new study.

Scientists speculate that the Oort Cloud became populated with comets when the gravitational muscle of Jupiter and the other giant planets dispersed the icy objects left over from the formation of the outer solar system into the distance. Occasionally, a passing star will gravitationally push an object from the Oort cloud and send it plummeting into the inner solar system. These objects are known as long-period comets, defined by their eccentric trajectories that take hundreds or even thousands of years to orbit the Sun.

In 2016, Meech and his colleagues reported the discovery of an unusual long-period comet that was dark and lacking a bright tail of vaporized ice. In fact, the object looked much more like an asteroid, a hint that the composition of the Oort cloud might not be so homogeneous. Meech called it a Manx Comet, after a breed of cat without a tail. Although astronomers have since detected dozens more of these comets, they have yet to definitively demonstrate that the objects are asteroids because they are so faint and fast moving.

Now, with the Canadian Fireball, researchers believe they have caught one of these rare objects crashing into Earth’s atmosphere. “It was very bright, very fast, and it left a light trail for several seconds,” says Vida, who presented the work today at a meeting of the Planetary Science Division of the American Astronomical Society.

In addition to hundreds of reports of the witnesses who caught the fireball on the dashboard and security cameras, Vida and her colleagues also worked with footage from the Global Fireball Observatory, a network of high-precision celestial cameras. Even a lightning monitor on an orbiting satellite picked up the flash of the fireball. Combining these observations, the team calculated the object’s trajectory and discovered that it had an orbit of around 1000 years, proof that it came from the Oort cloud.

Despite its provenance, the object was not comet-like at all. Most cometary fireballs are fragile; they fragment and burn high in the Earth’s atmosphere. But this object, plunging at 62 kilometers per second, penetrated much deeper, Vida says, suggesting it was hard and rocky rather than icy. It also broke into two phases at two discrete pressures, mirroring the breakup of a common type of asteroid that drops meteorites to Earth.

Vida and her colleagues turned to historical data to see if other objects like this had been overlooked. They found that in 1979 a network of fireball cameras in Canada tracked the disappearance of a 20-gram object which, like the Alberta object, was in a long-period orbit characteristic of an Oort cloud object. He too fell through the atmosphere like rock, not ice. After comparing the events of 2021 and 1979 with the total number of long-period comets captured by the two fireball arrays, they calculated that between 1% and 20% of the Oort cloud must be rocky.

Bottke, however, is skeptical of extrapolating from such a small number of events. He also thinks there could be a “survival bias” towards detecting rocky objects because they are tougher than comets, skewing the true proportion of rocky objects in the Oort Cloud downwards. of the estimate.

But even though the Oort Cloud is only 1% rocky, it will challenge theorists to explain how these objects arrived from the asteroid belt, says Alan Jackson, planetary astronomer at Arizona State. University, Tempe. He says the discovery could support a hypothesis called the Great Tack, which suggests that just 3 million years after the solar system was born, Jupiter plunged inward toward the Sun, almost into Earth’s orbit, before return close to its contemporary position. . “As you can imagine, by doing this it stirs things up” — including throwing lots of rocky objects on its way to the Oort Cloud, Jackson says.

Like Bottke, Meech also worries about doing too many two fireball events. But she can’t wait for Vida and her colleagues to capture more of these unusual streaks in the sky. “It’s very interesting,” she said. “Hopefully they get more.”

Comments are closed.