This past Saturday, a speck of light shimmering in the shadows behind Jupiter erupted. And then it kept erupting, with two more violent jets of material firing into space on Sunday, followed by a fourth paroxysm on Monday. As it raged and flared, it became 250 times brighter than usual, like a lit match becoming a bonfire.
This is not a distant tempestuous star, nor is it an effervescent world covered in erupting volcanoes. This is Comet 29P. And it put on the performance of a lifetime that anyone with a powerful enough backyard telescope could see.
As far as astronomers are aware, this is the first time that this comet has exhibited four closely spaced eruptions.
“Some are calling this a super outburst,” said Maria Womack, an astrophysicist at the National Science Foundation. “This requires an immense amount of energy.”
What’s causing this comet’s riotous cascade?
“We don’t know,” Dr. Womack said. “And that’s what makes it so interesting.”
Comets are icy leftovers from the anarchic birth of the solar system that occasionally are yanked by the forces of gravity toward the sun. Each comet is novel in its own way. But Comet 29P/Schwassmann-Wachmann — or 29P — is “odd on many levels,” said Kacper Wierzchoś, an astronomer at the University of Arizona.
Unlike most comets, it does not dramatically plunge toward the sun before zipping back out to the outermost frontier of the solar system. Instead, this 37-mile-wide ball of ice revolves around the sun in the space between Jupiter and Saturn — making it a type of object known as a centaur — on a freakish, near-circular orbit, completing one circumnavigation every 14.6 years.
Despite being 560 million miles from the sun, 29P is in a state of near-constant fury, frequently blasting gas and dust into the enveloping dark. It is “always active and never turns off,” Dr. Womack said.
This hyperactivity is perhaps a result of its preponderance of carbon monoxide — a volatile gas — which needs only a soupçon of sunlight to dramatically heat up and outgas into space in large volumes. These eruptions briefly brighten the comet’s atmosphere, or coma, by topping it up with sunlight-reflecting dust.
There are at least seven brightening outbursts per year. “No other known comet in the solar system undergoes outbursts with such frequency and intensity,” Dr. Wierzchoś said.
But this exhibitionist comet outdid itself last week with its four consecutive outbursts. Amateur and professional astronomers around the world quickly noticed “its brightness increased in jumps,” said Richard Miles, director of the asteroids and remote planets section of the British Astronomical Association.
The cause of this cometary pageantry, however, remains unknown.
The problem is that “we don’t know what drives 29P’s eruptions” on a regular basis, Dr. Womack said. Perhaps they are triggered by the explosive vaporization of some of the comet’s chemistry. But work by Dr. Wierzchoś found that you can get a very dust-rich eruption without an accompanying outburst of carbon monoxide gas.
These eruptions could be the consequence of cryovolcanism, Dr. Miles said. Sunlight warms and softens parts of the surface, under which a subterranean slush of exotic ices — a cryomagma — resides. When the comet’s weakened crust ruptures, its cryomagma could get exposed to the vacuum of space. Its dissolved carbon monoxide would then vigorously bubble out, propelling the cryomagma into the cosmic expanse like champagne rushing out of a shaken-up bottle that’s been speedily uncorked.
Historically, more outbursts happen after 29P makes its closest approach to the sun, which was in 2019. But why this super-outburst now?
Perhaps there was a gargantuan landslide, or a vast chunk of 29P has broken off. “Who knows,” Dr. Wierzchoś said.
Whatever the cause of this sky show, 29P remains “a rare, always-open natural laboratory to study the composition and behavior of primitive icy objects in the solar system,” Dr. Womack said. Although it has been nearly a century since it was discovered, the comet’s latest record-breaking quartet of eruptions highlights why many astronomers love to study these ostentatious objects.
“They will surprise you,” Dr. Womack said.
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