In 2021, astronomers identified a gargantuan comet — an ancient mass of ices, dust, and rocks — hurtling through our cosmic neighborhood. Fortunately, it won’t come within a billion miles of Earth. Named Comet Bernardinelli-Bernstein, it was perhaps the largest comet ever detected, likely some 10 times larger than the 6-mile-wide object that pummeled Earth and triggered the dinosaurs’ extinction.

Now, new research more accurately gauges the comet’s size. It’s even bigger than some astronomers supposed. In the new study, to be published in the science journal Astronomy & Astrophysics, scientists estimate it’s some 85 miles wide.

If stood next to Mount Everest, it would be around 15 times taller.

“It’s huge,” marveled Samantha Lawler, an astronomer at the University of Regina who researches deep objects in our solar system. “It’s by the far the biggest comet that’s ever been discovered.” (Lawler had no role in the new research.)

“It’s huge.”

There are almost certainly other profoundly giant comets out there. We just have to keep looking. After all, Comet Bernardinelli-Bernstein, Lawler noted, was only barely discovered. It was unknowingly picked up during a survey of galaxies in the deep cosmos in 2014. Then, it took years and the help of intensive computing for scientists to sift through loads of observations and ultimately identify this distant behemoth (as of June 2021, it was 1.8 billion miles from the sun).

“These big things are out there,” she said.

Like many other comets, Bernardinelli-Bernstein came from the Oort cloud, a sphere of ancient, icy objects surrounding the solar system. Out there, perturbations, like another massive object passing by, can send a great ball of ice hurtling through our solar system. The comet Hale-Bopp, another Oort cloud visitor, enthralled skywatchers in 1996 and 1997.

Crucially, Hale-Bopp passed 122 million miles from Earth, which is relatively close in cosmic terms. Bernardinelli-Bernstein, over twice the size of Hale-Bopp, won’t come closer than the orbit of Saturn, about a billion miles away, in 2031.

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Comet ISON photographed in 2013, at some 80 million miles from Earth.
Credit: NASA / MSFC / Aaron Kingery

How can astronomers measure the size of such a distant object?

Just looking at its brightness (meaning how much sunlight is reflecting off) won’t cut it, explained Emmanuel Lellouch, an astronomer at the Observatoire de Paris and one of the study’s authors. From Earth, a large and dark object could have the same brightness as a small but shiny comet.

So the astronomers measured the comet’s “thermal flux,” meaning how much heat the object is emanating. They do this by looking at a type of light called “infrared.” It’s not visible to the human eye, but we feel this light when the sun shines on our skin. A larger object will absorb more sunlight and then radiate this energy out. This information, combined with the object’s distance, gave Lellouch and his team a quality estimate of the comet’s size.

“It was in deep freeze storage for billions of years.”

“This is one way we can find out how big something is in the outer solar system without sending a probe there,” Lawler said.

In the coming years, the giant Bernardinelli-Bernstein will reveal bounties about our solar system. Scientists don’t think the comet has ever traveled near the sun, meaning the sun’s heat hasn’t evaporated its surface and formed an iconic tail of dust and gas (called a coma). Instead, the comet’s existed for eons on the periphery of our solar system. It’s a scientifically prized, frozen artifact from the beginnings of our cosmic home. It’s a glimpse into what happened here, some 4 billion years ago, just as Earth started to form.

“It was in deep-freeze storage for billions of years,” said Lawler.

As the comet approaches the sun over the coming decade, Lellouch noted that astronomers will observe the dust and gases on this giant, ancient, preserved chunk of ice and rock.

“It hasn’t ever come this close to the sun,” Lawler said.