Could a Comet Really Become Bigger Than the Sun?

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In October 2007, astronomers and stargazers worldwide watched in absolute amazement as a normally invisible comet suddenly burst into brilliance, becoming visible to the naked eye almost overnight. More incredibly, this tiny cosmic wanderer, named Comet Holmes, briefly developed a gaseous atmosphere larger than our Sun itself. How is that even possible? What mysterious force could make a small, frozen rock transform into the Solar System's largest object?

This article is written by FreeAstroScience only for you, to unravel one of the most spectacular and puzzling astronomical events of our century. We invite you to read every word, for by the end, you'll understand not just what happened to Comet Holmes, but why this event matters to anyone who's ever wondered about the cosmos. Remember: the sleep of reason breeds monsters—so let's keep our minds awake and curious together.

What Is Comet Holmes and Why Did It Capture the World's Attention?

Comet 17P/Holmes isn't your typical celestial showstopper. It's what astronomers call a periodic comet—a cosmic "dirty snowball" that loops around the Sun approximately every seven years. Discovered by British amateur astronomer Edwin Holmes in 1892, this comet normally remains so faint that even powerful telescopes struggle to detect it. With a nucleus measuring just 3.4 kilometers across, Holmes spends most of its time quietly orbiting between Mars and Jupiter, far from the Sun's intense heat.

Under ordinary circumstances, Comet Holmes shines at a dismal magnitude of 16 to 17—roughly 25,000 times too dim for human eyes to perceive. For decades after its discovery, astronomers actually lost track of it entirely. The comet went missing from 1906 until 1964, when astronomer Elizabeth Roemer finally recovered it using sophisticated equipment.

Think of Holmes as the ultimate cosmic introvert—quiet, unassuming, and preferring to stay in the background. Yet this humble ice ball had a dramatic secret hidden beneath its frozen surface, one that would soon shock the astronomical community.

When Did Comet Holmes Explode and What Did People See?

On October 23-24, 2007, something extraordinary happened. Comet Holmes, positioned about 240 million kilometers from Earth, suddenly erupted in what became the largest cometary outburst ever recorded in history.

The transformation was mind-blowing.

In less than 48 hours, the comet's brightness skyrocketed from magnitude 17 to approximately 2.5—a million-fold increase in luminosity. To put that in perspective, imagine a single candle flame suddenly becoming as bright as a million candles. Astronomers described it as unprecedented, with one researcher noting the "amplitude of Comet Holmes' outburst is truly exceptional".

Observers stepping outside that October night found what appeared to be a brand-new "star" shining in the constellation Perseus. The comet initially looked star-like because we were viewing it nearly head-on, with its tail pointing directly toward us. But within days, that stellar point expanded into a fuzzy, circular ball of light.

Through binoculars and telescopes, Holmes resembled a ghostly sphere—a pale, round nebula suspended against the starry background. Night after night, the cosmic bubble continued to inflate like some celestial balloon. By late October, its apparent size reached roughly half the diameter of the full Moon in our sky.

What made this event even more remarkable was its duration. Most cometary outbursts fade quickly, but Holmes remained visible to the naked eye for months. Even as its coma expanded and thinned, the comet held onto magnitude 3-4 brightness through late November 2007. For amateur astronomers worldwide, it became the astronomical event of the year—no telescope required.

What Triggered This Massive Cometary Explosion?

Here's where the mystery deepens. Despite extensive study, scientists still don't know exactly what caused Comet Holmes to explode.

Several hypotheses have been proposed, each intriguing in its own way.

The Meteoroid Collision Theory

Perhaps a meteoroid or small asteroid crashed into Holmes's nucleus. The impact could have shattered part of the surface, ejecting an enormous cloud of dust and gas into space. While this explains a sudden burst, there's no direct evidence of such a collision.

The Pressure Cooker Scenario

A more likely explanation involves internal pressure buildup. Imagine Holmes with a thick, airtight layer of dust sealing its surface like a cork in a bottle. Beneath that seal, volatile ices were warming and vaporizing as the comet moved through space, with nowhere for the gas to escape.

Eventually, the mounting pressure cracked through the surface "cork" and exploded outward in one massive release. This process resembles what happens when you shake a carbonated beverage—pressure builds until the cap finally pops, releasing everything in a sudden burst.

Studies published in Astronomy and Astrophysics suggest that this thick dust cover could have been storing solar energy for months before the outburst, with the comet's porous structure providing extra surface area for sublimation—potentially one order of magnitude greater than normal.

The Crystallizing Ice Mechanism

Another fascinating theory centers on the nature of water ice inside the comet. Holmes might contain water ice in an unusual amorphous (non-crystalline) form. Over time, this ice can suddenly reorganize into a crystalline structure—a phase change that releases heat and trapped gases.

If a subsurface layer underwent this transformation, it could trigger a rapid gas buildup that blew off the overlying material. Researchers examining various trigger mechanisms—including amorphous-to-crystalline water ice phase transition, CO₂-driven pressure pockets, and cliff collapse—found that subsurface cavities sealed by refreezing volatiles offer a compelling explanation for such dramatic outbursts.

What's truly remarkable is that Holmes survived intact. One researcher noted how extraordinary it was that the comet didn't tear itself completely apart during such a violent eruption. This resilience suggests Holmes might be "built for fireworks", possibly due to its unique internal composition or structure.

Intriguingly, Comet Holmes had experienced a similar massive outburst back in 1892 when Edwin Holmes first discovered it. This pattern suggests the comet has an inherent volatility, leading scientists to describe its outbursts as "pretty much in a class by themselves".

How Could Comet Holmes Become Larger Than the Sun?

This is where the story becomes truly cosmic in scale.

As Comet Holmes hurled dust and gas into space, that ejected material expanded outward in an ever-growing sphere around its tiny 3.4-kilometer nucleus. By late October 2007, the comet's coma stretched about 13 arcminutes across in our sky—roughly half the apparent diameter of the Moon.

Given Holmes's distance from Earth (about 2 astronomical units), astronomers calculated the coma's true diameter had already swollen to over 1 million kilometers—approximately 70% of the Sun's diameter.

And it kept growing.

By mid-November 2007, observations showed the coma exceeded 1.4 million kilometers across. For comparison, the Sun's diameter measures about 1.39 million kilometers. Comet Holmes's atmosphere had officially become larger than the Sun itself.

Let that sink in for a moment: a fragile ball of ice measuring only 3.4 kilometers wide created a gossamer atmosphere so immense it could swallow our Sun whole.

Object	Diameter
Comet Holmes nucleus	3.4 km
Moon's orbit around Earth	~760,000 km
Comet Holmes coma (late October)	~1 million km
The Sun	~1.39 million km
Comet Holmes coma (mid-November)	~1.4 million km

Astronomers at the University of Hawaii captured stunning comparison images showing Holmes's coma superimposed on the Sun and even Saturn. In terms of sheer size (though certainly not mass), Comet Holmes briefly held the title of the largest object in the Solar System.

Of course, this gigantic coma was incredibly tenuous—thinner than the best laboratory vacuum on Earth. The gas and dust were spread into an extraordinarily thin veil. As the coma expanded, it also grew more diffuse and dimmer, much like a balloon's surface gets thinner as you inflate it.

Still, for several weeks in November 2007, Holmes wore a "cosmic halo" wider than the orbit of our Moon around Earth. It's staggering to think that a comet typically too faint for backyard telescopes suddenly sported an atmosphere dwarfing our Sun.

Why Was This Outburst So Historically Significant?

The brightness surge of Comet Holmes was unprecedented. In modern observational history, no other comet has ever been observed to brighten so much so quickly.

An increase of 14 to 15 magnitudes represents approximately a million-fold change in brightness—a scale almost incomprehensible in everyday terms. Even famous comets that dazzled stargazers, like Halley (1986) or Hale-Bopp (1997), didn't undergo explosive outbursts. They were bright due to their size and proximity to the Sun, not because they suddenly erupted.

Another spectacular comet of 2007, Comet McNaught, was extremely bright—even visible in daylight—but that was normal behavior for a comet passing very close to the Sun. Holmes, by contrast, exploded while relatively far from the Sun and Earth, making its visibility all the more remarkable.

Other comets experience minor outbursts occasionally. For example, Comet 29P/Schwassmann-Wachmann is known for frequent mini-flares, and Holmes itself had a much smaller outburst in 2015 (about a 3-4 magnitude rise). But those events required telescopes to detect. Nothing in modern astronomical records matches the Holmes event for sheer luminous drama.

To find comparable brightness changes, you'd almost have to look beyond comets to stellar explosions like novae and supernovae, where stars suddenly increase in brightness by factors of thousands to billions. Within our Solar System, Comet Holmes's performance was like a "miniature supernova"—as if this modest comet briefly became an honorary star.

The 2007 outburst prompted numerous scientific studies. Researchers modeled the dust trail ejected during the blast, finding that it formed a long-lived trail along the comet's orbit—a lingering footprint of the explosion observable with telescopes for years afterward. Interferometric observations at 90 GHz revealed two distinct dust components: a fast-expanding shell with typical velocities of 50-100 meters per second, and a slowly-moving core component.

Can You Still Observe Comet Holmes Today?

After its starring role in 2007, Comet Holmes gradually faded as its dust dispersed. By spring 2008, it was no longer visible without a telescope. Today, you cannot see Comet Holmes with your eyes—it has returned to its quiescent, faint state.

Holmes continued along its seven-year orbit and returned to the inner Solar System in 2014 and 2021, reaching perihelion without any comparable fireworks. These subsequent passages showed only modest activity boosts of a few magnitudes at most, keeping the comet at magnitude 12-17—accessible only to sensitive instruments.

Right now in 2025, Comet Holmes is likely a dim speck, observable only with decent telescopes and detailed star charts. Its orbit between Mars and Jupiter means it never comes extremely close to Earth or the Sun, which is why it usually stays so faint.

The comet's next perihelion is expected around January 2028. However, there's no particular reason to anticipate another major event then. Outbursts like the 2007 spectacle are rare and possibly once-in-a-lifetime for this particular comet.

That said, comets are notoriously unpredictable. Holmes taught us that even a "boring" comet can transform into a celestial wonder overnight. Astronomers will surely keep monitoring it during future orbits. And who knows? If Comet Holmes ever decides to erupt again, you can bet skywatchers worldwide will be ready to gaze upward in wonder, remembering the cosmic surprise of 2007.

What Can Comet Holmes Teach Us About Our Universe?

The story of Comet Holmes is an uplifting lesson in curiosity and humility.

Back in 2007, this comet's outburst transformed many ordinary people into impromptu astronomers. Folks who might not have cared about comets suddenly found themselves in their backyards, looking up, and asking questions about the universe. Holmes reminded us that the cosmos is full of surprises, some happening right above our heads on any given night.

We still don't have all the answers about what caused the explosion—and that's perfectly okay. The mystery only deepens our sense of wonder. It's like a poem written in stardust, prompting us to ponder and investigate rather than giving up its secrets easily.

For anyone new to astronomy, Comet Holmes offers a perfect example: you don't always need fancy equipment or decades of experience to witness profound cosmic events. Sometimes the universe offers a spectacle to anyone willing to simply look up. Holmes appeared suddenly, like a cosmic beacon, igniting the curiosity of millions worldwide.

In many ways, we can think of Comet Holmes as a messenger. Its ephemeral glow has long faded, but it left behind a spark in our collective imagination. The giant expanding sphere of dust was like a sign in the sky saying, "There is so much we still don't know". Instead of fear, that unknown invites excitement.

Just as Comet Holmes came back from the dark to surprise us, who knows what other mysterious space phenomena await discovery? Each outburst, each comet, each twinkling star has a story to tell. Holmes's story encourages us to keep asking questions and embrace the awe that comes with not fully knowing all the answers. That spirit of curiosity is what drives discovery and makes the cosmos such a beautiful, poetic place for us all.

Conclusion

Comet Holmes's 2007 outburst remains one of the most spectacular and scientifically significant cometary events in recorded history. This humble periodic comet, normally invisible to all but the most powerful telescopes, suddenly exploded with a million-fold increase in brightness and grew an atmosphere larger than the Sun itself—briefly becoming the largest object in our Solar System.

The exact trigger for this dramatic explosion remains uncertain, highlighting how much we still have to learn about these icy wanderers from the outer Solar System. Whether caused by internal pressure buildup, crystallizing ice, or some other mechanism, the outburst demonstrated the violent and unpredictable nature hidden within even small comets.

For those of us who witnessed it, Comet Holmes offered an unforgettable reminder that the universe is dynamic, mysterious, and capable of surprising us at any moment. It showed that you don't need to be a professional astronomer with expensive equipment to experience cosmic wonder—sometimes you just need to look up at the right time.

As we continue to explore and study our Solar System, comets like Holmes will keep teaching us about the conditions that existed when our planetary system formed billions of years ago. Each observation, each unexpected outburst, brings us closer to understanding these frozen time capsules from the dawn of the Solar System.

So keep looking up at the stars, dear readers. Keep your minds engaged and your curiosity alive. The next cosmic wonder might be just around the corner, waiting to transform our sky and deepen our love for the universe. Remember what we've learned together today, and return to FreeAstroScience.com whenever you hunger for knowledge about the cosmos. The sleep of reason breeds monsters—but awakened reason breeds wonder.

Could a Comet Really Become Bigger Than the Sun?

What Is Comet Holmes and Why Did It Capture the World's Attention?

When Did Comet Holmes Explode and What Did People See?