TON 618’s event horizon stretches roughly 0.04 light-years across, or about 2,600 astronomical units in diameter. That makes it far larger than our entire solar system as measured by the orbits of the planets. But that number rests on the black hole’s estimated mass of around 66 billion times that of the Sun, and recent research suggests that figure could be significantly off. The story of TON 618’s size is really a story about how we measure the most extreme objects in the universe, and how much we trust those measurements.
From Mass to Size
A black hole’s event horizon scales directly with its mass. Double the mass, double the radius. For a non-spinning black hole, the boundary is called the Schwarzschild radius. For our Sun, that radius would be about 3 kilometers. For TON 618, with a commonly cited mass of 66 billion solar masses, the math gives a Schwarzschild radius of roughly 1,300 astronomical units, meaning the full diameter of the event horizon comes to about 2,600 AU. One light-year is about 63,241 AU, so that diameter translates to approximately 0.04 light-years, or around 15 light-days.
If TON 618 is spinning, as most supermassive black holes are thought to be, the event horizon would shrink somewhat. A maximally spinning black hole has an event horizon radius half that of a non-spinning one of the same mass. So even in the most extreme case, the event horizon would still span well over 1,000 AU across. Whether the black hole spins fast or slow, its event horizon dwarfs anything in our planetary neighborhood.
Putting 2,600 AU in Perspective
Numbers like 0.04 light-years or 2,600 AU do not mean much on their own. To get a sense of scale, consider that Neptune orbits the Sun at about 30 AU, so TON 618’s event horizon stretches roughly 40 times farther than Neptune’s orbit in every direction from the center. If you dropped this black hole where the Sun sits, everything out past Pluto, past the Kuiper Belt, and deep into the scattered disc would be inside the event horizon.
The Voyager 1 spacecraft, the most distant human-made object, is currently about 160 AU from the Sun after nearly half a century of travel. It would have to travel more than eight times farther to reach the edge of TON 618’s event horizon if it were centered on our solar system. The nearest star to the Sun, Proxima Centauri, is about 4.24 light-years away. TON 618’s event horizon is roughly one percent of that distance, which sounds small until you remember we are talking about a single object’s boundary, not the gap between two stars.
How We Got the Mass Estimate
TON 618 was first cataloged in the 1950s as an unremarkable-looking star in the Canes Venatici constellation. It took decades of follow-up work to reveal it as a quasar, one of the most luminous objects in the observable universe, powered by matter falling into a supermassive black hole. Its enormous distance of about 10.4 billion light-years means we see it as it was when the universe was only a few billion years old.
The mass of the black hole powering TON 618 has been estimated using what astronomers call single-epoch spectroscopy. The idea is straightforward in principle: you look at the broad emission lines in the quasar’s spectrum, measure how fast the gas is moving near the black hole based on how wide those lines are, estimate how far the gas is from the center using the quasar’s brightness, and combine those to get a mass. The most commonly used emission lines for very distant quasars like TON 618 are in the ultraviolet, particularly a line from triply ionized carbon known as C IV.
This method has produced the headline figure of 66 billion solar masses. But the method comes with serious caveats. The gas producing those emission lines is not on simple circular orbits. It could be flowing inward, flowing outward in winds, or moving in complicated patterns influenced by radiation pressure. All of those introduce uncertainty into the mass calculation.
Why the Mass Could Be Wrong
A growing body of work has shown that C IV-based mass estimates are prone to systematic bias. An analysis of 85 quasars with good ultraviolet spectra demonstrated that the average properties of quasar samples used to calibrate the C IV method carry built-in biases, and those biases propagate into the mass estimates. The result is that black hole masses based on C IV can be nearly 50 percent too high for a typical quasar, with the error spanning close to an order of magnitude depending on the specific spectral characteristics of the object being measured.1arXiv. Bias in C IV-based quasar black hole mass scaling relationships from reverberation mapped samples
A 50 percent overestimate applied to TON 618 would drop its mass from 66 billion to around 44 billion solar masses. The event horizon would shrink proportionally, falling to about 0.027 light-years, or roughly 1,700 AU across. That is still staggeringly large, but it is a meaningful difference when you are trying to figure out where TON 618 sits among the most massive black holes ever found. And the uncertainty could go in the other direction too: some individual quasars with particular spectral properties may have their masses underestimated by the same method.
Additional complications arise from how bolometric luminosity is calculated. Building a complete picture of a quasar’s energy output across all wavelengths requires combining X-ray, optical, and ultraviolet observations. Researchers have shown that the correction factors used to estimate total luminosity from a single wavelength band depend on how rapidly the black hole is consuming matter relative to its maximum possible rate.2Oxford Academic. Simultaneous X-ray/optical/UV snapshots of active galactic nuclei from XMM–Newton: spectral energy distributions for the reverberation mapped sample Getting the luminosity wrong feeds directly into the mass estimate, compounding the uncertainty from the spectral line method.
None of this means TON 618 is not an ultramassive black hole. Even at the low end of plausible estimates, it ranks among the heaviest ever observed. But the precise number matters when you are converting mass into a physical size, because the event horizon scales linearly with mass. An uncertainty of a factor of two in mass is an uncertainty of a factor of two in diameter.
Is There a Limit to How Large a Black Hole Can Grow?
TON 618 sits in uncomfortable territory for theoretical models. Work on the physics of accretion disks has shown that there is a natural ceiling for how massive a black hole can become while still actively feeding. The argument is that as the black hole gets more massive, the innermost stable orbit around it eventually moves so far out that a conventional accretion disk can no longer form. Without the disk, there is no efficient way to funnel matter inward. That ceiling comes in at roughly 50 billion solar masses, though the exact number depends on assumptions about the disk’s viscosity and the rate of accretion.3Monthly Notices of the Royal Astronomical Society: Letters. How big can a black hole grow?
TON 618’s commonly cited mass of 66 billion solar masses would exceed that theoretical maximum. There are a few possible explanations. The mass estimate might be too high, which the spectral-line biases discussed above make plausible. The theoretical limit might be more flexible than the simple calculation suggests, especially for black holes that grew through mergers rather than steady accretion. Or the physics of super-Eddington accretion, where matter falls in faster than the standard maximum rate, could push the ceiling higher. Simulations of triple quasar mergers have explored scenarios in which accretion is allowed to exceed the Eddington rate by up to a factor of two, producing ultramassive black holes through pathways that do not rely solely on smooth disk feeding.4The Astrophysical Journal Letters. Ultramassive Black Holes Formed by Triple Quasar Mergers at z ∼ 2
The tension between TON 618’s apparent mass and the theoretical cap is one reason this object attracts so much attention. It sits at the edge of what our models say should be possible, which either means the models need updating or the mass needs revising.
What Lies Beyond the Event Horizon
The event horizon is the defining boundary of a black hole, but it is not the full extent of the object’s influence. TON 618 is a quasar, meaning it has an active accretion disk surrounding the black hole and pouring out radiation. The accretion disk likely extends thousands of AU beyond the event horizon. The broad-line region, where fast-moving gas clouds produce the emission lines used to measure the black hole’s mass, can extend tens of light-days or more from the center. And the narrow-line region, where slower-moving gas produces sharper spectral lines, can reach thousands of light-years from the black hole.
So when you ask how big TON 618 is, the answer depends on what you count. The event horizon is about 0.04 light-years across. The accretion disk probably extends a few times farther. The region of space dominated by the quasar’s radiation stretches to galactic scales. And the gravitational sphere of influence, where the black hole’s gravity outweighs the pull of the surrounding galaxy’s stars, likely extends tens of thousands of light-years, depending on the galaxy’s properties.
The quasar’s luminosity is staggering. TON 618 shines at about 140 trillion times the luminosity of the Sun, placing it among the most luminous quasars known. That brightness is generated in a region barely larger than our solar system, which gives you a sense of how extreme the energy density is near the event horizon. Much of that light comes from the inner part of the accretion disk, where temperatures reach millions of degrees.
Why We Cannot Simply Photograph It
The Event Horizon Telescope famously imaged the shadow of the supermassive black hole in M87, which sits about 55 million light-years away and has a mass of roughly 6.5 billion solar masses. TON 618’s black hole is about ten times more massive, so its event horizon is about ten times larger. But it is also about 190 times farther away. The apparent angular size on the sky is therefore much smaller than M87’s shadow, far too small for any current or planned telescope to resolve directly.
What we observe instead is the quasar’s total light output, blended together into a single brilliant point. Separating the quasar’s glare from its host galaxy is a challenge even with the most powerful telescopes. The James Webb Space Telescope has opened new possibilities in this area. JWST observations have successfully detected the stellar light from quasar host galaxies at very high redshifts, using careful image decomposition to separate the blinding central point source from the faint surrounding glow of stars.5The Astrophysical Journal. SHELLQs-JWST Unveils the Host Galaxies of 12 Quasars at z > 6 These techniques have revealed that some early quasar host galaxies are less massive than expected given the enormous black holes at their centers, suggesting that ultramassive black holes may have grown disproportionately fast compared to their host galaxies.6The Astrophysical Journal. Undermassive Host Galaxies of Five z ∼ 6 Luminous Quasars Detected with JWST
JWST’s integral field unit spectroscopy has also proven capable of disentangling quasar light from host galaxy properties for extremely red and obscured quasars, providing a new window into the environments of actively feeding supermassive black holes.7The Astrophysical Journal. JWST Integral Field Unit Observations Uncover Host Galaxy Continua in Extremely Red and Obscured Quasars While these observations do not resolve the event horizon itself, they bring us closer to understanding the full physical environment around objects like TON 618.
Comparisons with Other Giant Black Holes
TON 618 is often called the largest known black hole, but that distinction shifts as new measurements come in and old ones get revised. Phoenix A, the ultramassive black hole at the center of a galaxy cluster about 5.8 billion light-years away, has been estimated at roughly 100 billion solar masses, which would give it an event horizon close to 0.06 light-years across. Holm 15A*, another contender, has been estimated at about 40 billion solar masses. All of these numbers carry the same kinds of uncertainty that plague TON 618’s estimate.
The black hole at the center of our own Milky Way, Sagittarius A*, has a mass of about 4 million solar masses. Its event horizon is roughly 25 million kilometers across, or about a sixth of the Earth-Sun distance. TON 618’s event horizon is roughly 16,000 times wider. M87’s black hole, the first ever imaged, is about 6.5 billion solar masses, making its event horizon roughly a tenth the size of TON 618’s. These comparisons underscore just how far TON 618 sits beyond typical supermassive black holes, even the ones we already consider enormous.
Why “How Big” Is a Harder Question Than It Sounds
A black hole is not a solid object with a surface you can measure. The event horizon is a mathematical boundary, the point beyond which light cannot escape. It is not a physical barrier you could touch. If you were falling into TON 618 feet-first, you would cross the event horizon without any particular sensation at that moment, at least according to general relativity. The tidal forces that would rip apart a human body would not become dangerous until well inside the event horizon, because the gravitational gradient across a human-sized object is gentle when the black hole is this massive.
There is also the question of what coordinate system you are using. A distant observer watching something fall into the black hole would see it slow down and dim as it approached the event horizon, never quite appearing to cross it. The object itself would experience crossing in finite time by its own clock. These are well-established predictions of general relativity, but they mean that even the concept of “the edge of the black hole” depends on who is doing the measuring.
For practical purposes, the Schwarzschild radius or its spinning equivalent is the standard answer astronomers give when asked about a black hole’s size. For TON 618, that gives a diameter of roughly 0.04 light-years, assuming the commonly cited mass holds up. But that number should be understood as an estimate built on other estimates, not a measurement anyone has made directly. The event horizon of TON 618 has never been observed. Its size is inferred from a mass that is itself inferred from the width of spectral lines in light that has traveled for over ten billion years. Each step in that chain introduces uncertainty, and the honest answer is that we know the size to within perhaps a factor of two.
That range, from maybe 0.02 to 0.06 light-years, still describes something breathtaking. Even at the low end, it is a single object’s boundary that would swallow our solar system many times over. The precision will improve as better calibrations for quasar mass estimates emerge and as next-generation telescopes refine our understanding of ultramassive black hole environments. For now, TON 618 remains one of the most dramatic examples of how far gravity can go when given enough mass and enough time.