What If We Replaced the Moon With a Black Hole? The Full Math Behind Every Answer

Look up on a clear night and there it is, the same pale face that every human who ever lived has seen. The Moon pulls the oceans into tides twice a day. It helps hold Earth’s tilt steady, and that tilt gives us our seasons. Over billions of years it has even slowed Earth’s spin, which is why your day is 24 hours long and not something much shorter.

Now imagine you could reach up and swap it for a black hole.

It sounds like the plot of a disaster movie, and in one version of this experiment it absolutely is. But in another version, almost nothing happens. Your tides stay. Your seasons stay. Your month stays the same length. And yet the night sky changes forever, a coral reef in Australia loses its calendar, and the black hole turns out to be one of the coldest objects in the entire universe.

Which version you get depends on one tiny word that most people never notice. And by the end of this article, you will have done the actual math yourself, with every number traced back to the scientists who worked it out first. Let’s go.

Where did our Moon come from?

Before we throw the Moon away, it is worth knowing what we are throwing away. The best explanation for the Moon’s birth is called the giant impact. Early in Earth’s history, a body roughly the size of Mars slammed into the young Earth at an angle. The collision blasted molten and vaporized rock into orbit, and that debris pulled itself together into the Moon. Computer simulations by planetary scientists Robin Canup and Erik Asphaug showed that a single oblique hit like this could produce a Moon with the right mass and an Earth with the right spin (Canup and Asphaug, Nature, 2001).

When did that happen? By dating tiny zircon crystals brought back by the Apollo 14 astronauts, a team led by Melanie Barboni at UCLA found that the Moon had formed by about 4.51 billion years ago, only around 60 million years after the solar system itself was born (Barboni et al., Science Advances, 2017, via Princeton University). So “about four and a half billion years” is right on the money.

The Moon today has a mass of about 7.35 × 10²² kilograms, a radius of about 1,737 kilometers, and sits an average of 384,400 kilometers from Earth, moving along its orbit at about 1.02 kilometers per second (NASA Moon Fact Sheet). Keep those four numbers in your head. We are going to use every one of them.

The sneaky question hiding in plain sight

When someone says “replace the Moon with a black hole,” most of us picture it simply. Moon out, black hole in, done. But there is a hidden choice in that sentence.

Do you want a black hole that is the same size as the Moon? Or one that has the same mass as the Moon?

For ordinary objects, size and mass go roughly together. A bigger rock is usually a heavier rock. But black holes break that rule completely. To understand why, we need one equation, and it is surprisingly friendly.

The only equation you need

In 1916, just months after Albert Einstein published his theory of gravity, the German astronomer Karl Schwarzschild found the first exact solution to Einstein’s equations while serving on the Russian front in World War One (Schwarzschild, 1916, English translation on arXiv). Hidden inside his solution was a special distance. If you squeeze any mass inside this distance, not even light can escape. We now call it the Schwarzschild radius, and it is the “size” of a simple black hole:

R = 2GM ÷ c²

Here G is the gravitational constant (6.674 × 10^-11), M is the mass, and c is the speed of light (about 3 × 10⁸ meters per second). Notice what this formula says. The size of a black hole depends on just one thing you can choose, which is its mass. Double the mass and you double the size. You cannot pick a size and a mass separately. Pick one, and nature picks the other for you.

So our two experiments are genuinely different universes. Let’s try the scary one first.

Part 1: A black hole as wide as the Moon

We want a black hole whose Schwarzschild radius equals the Moon’s radius, 1,737 kilometers. So we just flip the formula around and solve for mass.

The math: M = R × c² ÷ 2G = (1.737 × 10⁶ m) × (8.99 × 10¹⁶) ÷ (1.335 × 10^-10) ≈ 1.17 × 10³³ kg. The Sun’s mass is 1.989 × 10³⁰ kg (NASA Sun Fact Sheet), so this black hole weighs about 588 Suns.

Not 588 Moons. 588 Suns. And the Sun is no small thing. Its volume is about 1.3 million times the volume of Earth, according to the same NASA fact sheet. Now take 588 of those and crush them into a ball about 3,474 kilometers across. That is a distance you could drive across in about two days at a steady 72 kilometers per hour without stopping. (Astronomers would call this an intermediate mass black hole, a class that sits between the small ones left by dead stars and the giants at the centers of galaxies.)

So now picture yourself in your backyard at night. Where the Moon used to hang, there is a black disc blotting out the stars. Actually, it would look even bigger than the Moon did. Light passing close to a black hole gets bent, so the dark “shadow” we see is about 2.6 times wider than the black hole itself, the same effect that let the Event Horizon Telescope photograph the black hole in galaxy M87. The stars around its edge would appear smeared and warped, as if seen through the bottom of a glass.

But you would not have long to admire it. So what happens to you?

The elevator with the snapped cable

Here is the strange part. The black hole is pulling on you incredibly hard, but it is also pulling on your house, the trees, the ocean, and the whole planet underneath you. Everything starts falling toward it together. When everything falls together, you do not feel the pull as weight. It is like being in an elevator when the cable snaps. You, the floor and the walls all drop at the same rate, so you would feel weightless.

This is exactly why astronauts float on the International Space Station. They are not beyond Earth’s gravity. At the station’s height, gravity is still about 90 percent as strong as on the ground. They float because they and the station are falling around Earth together (NASA, What Is Microgravity?).

So does that mean you would feel nothing at all? Not even close. And this is where the original video script needs a small correction.

Tides on a planet-ripping scale

The side of Earth facing the black hole is about 6,371 kilometers closer to it than Earth’s center. Gravity weakens with distance, so the near side gets pulled harder than the middle, and the middle gets pulled harder than the far side. That difference in pull is called a tidal force. It is the very same effect that the real Moon uses to raise ocean tides, just turned up to an unimaginable level.

The math: For a distance d that is much bigger than Earth’s radius R, the tidal acceleration is about 2GMR ÷ d³. Plugging in M = 1.17 × 10³³ kg, R = 6.371 × 10⁶ m and d = 3.844 × 10⁸ m gives about 17,500 meters per second squared. Divide by Earth’s surface gravity (9.81) and you get roughly 1,785 g.

So the script’s “about 1,800 times stronger than the gravity holding you on the ground” is correct. But here is the correction. That stretching force would not wait for the “floaty” moment to end. It would switch on the instant the black hole appeared. If the black hole were overhead, you would be pulled upward, relative to the ground, about 1,785 times harder than Earth pulls you down. You would be flung off the surface immediately. People on the sides of the planet, at right angles to the black hole, would be squeezed inward at roughly 890 g. Earth’s own gravity is only 1 g, so there is nothing holding the planet together against this. It would be stretched like taffy from the very first second.

In fact, astronomers have a name for the distance inside which an object gets torn apart this way: the tidal disruption radius. For Earth and a 588 Sun black hole, it works out to roughly 3.7 million kilometers. Earth would be sitting almost ten times deeper than that. It does not stand a chance.

Thirty seconds

The Moon is far away. You could line up about 30 Earths in the gap (384,400 km ÷ 12,742 km ≈ 30.2). So surely falling that far would take days?

No. For an object dropped from rest at distance d toward a mass M, the time to fall all the way in is given by a classic formula from orbital mechanics:

The math: t = (π ÷ 2) × √(d³ ÷ 2GM). With d = 3.844 × 10⁸ m and M = 1.17 × 10³³ kg, that is (1.571) × √(5.68 × 10²⁵ ÷ 1.56 × 10²³) ≈ 1.571 × 19.07 ≈ 30 seconds.

About the length of one TV advertisement. (Near the very end, the falling material would be moving at a large fraction of light speed, where Einstein’s corrections matter, but the 30 second ballpark holds.)

Why doesn’t Earth just orbit it?

Earth orbits the Sun without falling in, so why can’t it orbit the black hole? Because to stay in orbit, you need to move sideways fast enough that you keep “missing” the thing you are falling toward.

The math: The speed for a circular orbit is v = √(GM ÷ d) = √(7.81 × 10²² ÷ 3.844 × 10⁸) ≈ 14,250 km per second. That is about 4.75 percent of the speed of light.

How fast is Earth moving sideways relative to where the Moon was? The Moon orbits us at about 1.02 km per second, so if the black hole took over the Moon’s motion, Earth’s sideways speed relative to it would be about 1 km per second. That really is roughly the speed of a high-powered rifle bullet. (You might be thinking, wait, isn’t Earth flying around the Sun at 30 km per second? It is, but the black hole would be traveling along with us, so that motion cancels out.) Compared with the 14,250 km per second needed, 1 km per second is basically standing still. So Earth does not orbit. It falls.

And then it eats the solar system

Here is a twist people rarely consider. With 588 solar masses sitting where the Moon was, the black hole is now by far the heaviest thing around. The Sun is no longer the center of our solar system. The black hole is. And everything else is moving far too slowly to orbit it.

Using the same free-fall formula for each body’s distance (the Moon and Earth are only 0.0026 AU apart, so we can treat the black hole as sitting at Earth’s position):

The math: An easy shortcut is that free-fall time scales with distance to the power 1.5 and with 1 ÷ √M. From 1 AU (150 million km), falling onto one Sun would take about 64.6 days. Divide by √589 ≈ 24.3 and you get about 2.7 days for the Sun. Jupiter at 5.2 AU: 2.7 days × 5.2^1.5 ≈ 32 days. Saturn at 9.6 AU: about 79 days. Neptune at 30 AU: about 440 days, or 1.2 years.

So the script is right. The Sun gets there in under three days, Jupiter in about a month, and Neptune in a little over a year.

Not quite falling in

One refinement worth adding. The planets and the Sun are not perfectly still. They all carry some sideways motion, so instead of dropping straight down the hole, each one swings inward on an extremely stretched out orbit and whips around the black hole at its closest point. The timings above are basically the time to reach that first close pass.

For the Sun, that closest point would be only about 130,000 kilometers from the black hole, smaller than the Sun’s own radius of 695,700 km. Long before then, the Sun would cross the tidal disruption radius, which for the Sun works out to about 5.8 million kilometers (the Sun’s radius × 588^1/3). There, the black hole pulls on the Sun’s near side so much harder than its far side that the star is torn into a long, glowing stream of gas.

This really happens out there

This is not science fiction. The idea that stars could be shredded by black holes was worked out in the 1970s and 1980s, most famously by the British astronomer Martin Rees, who predicted that roughly half the debris of a shredded star would be flung away and the rest would fall back and feed the black hole, lighting it up (Rees, Nature, 1988). Today telescopes catch these tidal disruption events in distant galaxies as bright flares in visible, ultraviolet and X-ray light that rise over weeks and fade over months. A 2021 review by astronomer Suvi Gezari covered 56 such events (Gezari, Annual Review of Astronomy and Astrophysics, 2021).

The same fate awaits Jupiter, whose tidal disruption radius is about 6 million km, and the other planets as they swing in. Within a year or two, the solar system as we know it is gone.

Who survives?

The script says a few icy chunks way out in the dark would get away. That is nearly right, with a small tweak. The comets of the distant Oort cloud, tens of thousands of times farther out than Earth, would take something like 80,000 years just to make their first trip inward. Because they are tiny and their closest passes would stay well outside the danger zone, many would survive, looping around the black hole on huge orbits rather than escaping. The material that truly gets away is the debris flung out during all that shredding, just as Rees predicted.

So version one is a total catastrophe. But wait. Did we really replace the Moon? We secretly invited 588 Suns into our backyard and then acted surprised when it went badly. Let’s do it fairly this time.

Part 2: A black hole that weighs the same as the Moon

This time we keep the Moon’s mass exactly the same, all 7.35 × 10²² kilograms. We just squeeze it, and squeeze it, and keep squeezing, until it collapses into a black hole. How big is it now?

Before you read on, take a guess.

The math: R = 2GM ÷ c² = 2 × 6.674 × 10^-11 × 7.35 × 10²² ÷ 8.99 × 10¹⁶ ≈ 1.09 × 10^-4 meters. That is a radius of 0.11 millimeters, so a diameter of about 0.22 millimeters.

About a fifth of a millimeter. That is the size of a grain of fine sand. The entire Moon, every crater, every mountain, every bootprint the Apollo astronauts left, packed into something you could lose on a beach.

So go back to your backyard. You look up. There is nothing there. Just stars. What happens to you this time?

Almost nothing.

Gravity doesn’t care how big you are

That sounds wrong, but here is why. Outside an object, gravity depends only on how much mass it has and how far away its center is. Isaac Newton proved this more than three centuries ago with what is now called the shell theorem: a round body pulls on things outside it exactly as if all its mass were squeezed into a single point at its center. Einstein’s theory says the same thing, and Schwarzschild’s 1916 solution is exactly that point-mass field.

So from 384,400 kilometers away, a Moon spread across 3,474 kilometers and a Moon crushed into a sand grain pull on Earth in precisely the same way. The orbit stays the same. The month stays the same, about 27.3 days for one orbit.

Tides, tilt and the long goodbye

Tides were never about the Moon’s rocks. They are about the Moon’s tug on the oceans, and that tug, which depends only on the Moon’s mass and distance, has not changed one bit. So the tides keep rolling in and out roughly twice a day, just as before.

The seasons stay too. In 1993, the French astronomer Jacques Laskar and his colleagues showed that the Moon’s pull keeps Earth’s tilt locked close to its current 23.4 degrees. Without the Moon, they calculated, Earth’s tilt could wander chaotically over enormous ranges (Laskar, Joutel and Robutel, Nature, 1993). Later work suggests a moonless Earth might wobble less wildly than first feared, at least for billions of years (Li and Batygin, Astrophysical Journal, 2014), but that debate does not even matter here. Our sand grain has the full mass of the Moon, so it steadies Earth’s tilt exactly as well as the Moon does.

Even the Moon’s slow goodbye carries on. Apollo astronauts left mirrors on the lunar surface, and scientists have been bouncing lasers off them for more than 50 years. Those measurements show the Moon drifting away from Earth by about 3.8 centimeters every year (NASA JPL, Apollo 11 experiment continues to return valuable data; detailed in Dickey et al., Science, 1994). That is close to how fast your fingernails grow, a few centimeters a year.

Why does it drift? Because the Moon raises a bulge of water and rock on Earth. Earth spins faster than the Moon orbits, so that bulge is carried slightly ahead of the Moon. The bulge’s gravity tugs the Moon forward, nudging it into a slightly bigger orbit. And by the same exchange, the Moon tugs back on the bulge, gently braking Earth’s spin. Studies of ancient eclipse records show the tidal braking alone adds about 2.3 milliseconds to the length of the day every century, although other effects, such as Earth’s crust rebounding after the ice ages, trim the net rate to about 1.8 milliseconds (Stephenson, Morrison and Hohenkerk, Proceedings of the Royal Society A, 2016). Over deep time this adds up. Layered tidal rock deposits show that around 620 million years ago, a day lasted only about 21.9 hours (Williams, Reviews of Geophysics, 2000).

Same mass, same bulge, same tug. So our tiny black hole keeps drifting away, and your days keep getting a tiny bit longer, just as they always have.

So if almost nothing changes, would you notice anything at all? Yes. And it is the thing we usually take most for granted.

The night the lights went out

The Moon does not make its own light. It is a reflector, bouncing sunlight back toward us. Here is a small correction to the popular “giant mirror” picture, though. The Moon is actually quite dark. Its surface reflects only about 11 to 12 percent of the sunlight that hits it (NASA Moon Fact Sheet), roughly like worn asphalt. It only looks brilliant because it is lit by the full blaze of the Sun against a black sky.

A black hole the size of a sand grain reflects nothing. There would be no full moon, no crescent, no half moon, no moonlit walks. Every night, all over the world, would be as dark as the night of a new moon. Forever.

To us, that is mostly a loss of beauty. But for some living things, moonlight is a clock.

The reef that reads the Moon

On the Great Barrier Reef, something extraordinary happens each year. Over a few nights following the full moons of October, November and December, colonies of many coral species release clouds of eggs and sperm into the water at nearly the same time. According to the Great Barrier Reef Marine Park Authority, it usually happens one to six nights after the full moon, with water temperature and the lunar cycle acting as triggers (GBRMPA, Coral spawning 101). The first big description of this mass spawning, recording 105 coral species spawning in sync, was published in 1986 (Babcock et al., Marine Biology, 1986).

How do animals with no eyes and no brain know which night to choose? Part of the answer is light. In 2007, researchers found that corals carry light-sensing proteins called cryptochromes that respond to the faint blue light of the Moon (Levy et al., Science, 2007). Then in 2021, a team in Taiwan ran a clever experiment. They found that what triggers spawning in one coral species is a stretch of darkness between sunset and moonrise, a gap that only appears in the nights after a full moon, because the Moon rises later each night. When they shaded corals from moonlight, spawning shifted. When they kept dim light on in the evening, spawning was delayed by three or four nights (Lin et al., PNAS, 2021).

And human light is already scrambling that clock. A 2021 study linked light pollution to delayed egg development and out of sync spawning in two reef building corals (Ayalon et al., Current Biology, 2021). In 2023, scientists analyzing 2,135 spawning records from around the world found that corals on artificially lit reefs spawned one to three days closer to the full moon than corals on dark reefs (Davies et al., Nature Communications, 2023).

So what happens if the Moon’s light vanishes altogether? Nobody has run that experiment, so this part is an educated guess. But if the corals partly rely on the rhythm of moonlit and dark evenings, a sky with no moonlight at all would remove that rhythm entirely. One of the biggest synchronized events in nature could lose its calendar.

No more eclipses

A total solar eclipse happens when the Moon slides in front of the Sun and its disc covers the Sun almost perfectly. A grain of sand 384,400 kilometers away covers nothing. So no more total solar eclipses, and no more partial ones either.

A lunar eclipse, the famous “blood moon,” happens when the Moon passes into Earth’s shadow and glows red from sunlight filtered through our atmosphere. With nothing up there to catch the shadow, those are gone too.

Here is a fun bonus the original video didn’t mention, though. The black hole would still bend light. When it passed directly in front of a distant star, it would act like a tiny magnifying lens, an effect called gravitational microlensing.

The math: The angular “Einstein radius” for a lens at distance d is about √(4GM ÷ c²d) = √(2.18 × 10^-4 ÷ 3.844 × 10⁸) ≈ 7.5 × 10^-7 radians, or about 0.16 arcseconds. The Moon moves across the sky at about 0.55 arcseconds per second relative to the stars, so a star lined up behind our black hole would briefly brighten for roughly half a second.

So a careful astronomer with a fast camera could still catch the invisible Moon, by watching for stars that flicker as it passes. Astronomers actually use this exact trick to hunt for dark, compact objects in our galaxy.

Colder than the universe

In 1974, Stephen Hawking published a short paper with a startling title: “Black hole explosions?” He showed that, once quantum physics is taken into account, black holes are not perfectly black. They should glow faintly, as if they had a temperature, slowly leaking energy we now call Hawking radiation (Hawking, Nature, 1974). The smaller the black hole, the hotter it glows.

The math: The Hawking temperature is T = ħc³ ÷ (8πGMk), where ħ is the reduced Planck constant and k is Boltzmann’s constant. A handy version is T ≈ 6.17 × 10^-8 K × (mass of the Sun ÷ M). The Sun is about 27 million times heavier than the Moon, so T ≈ 6.17 × 10^-8 × 2.71 × 10⁷ ≈ 1.67 kelvin.

That is 1.67 degrees above absolute zero, the coldest temperature possible. But here is the delicious part. The whole universe is bathed in faint leftover heat from the Big Bang, called the cosmic microwave background, and it has a temperature of 2.7255 kelvin (Fixsen, Astrophysical Journal, 2009). So our black hole is colder than empty space itself. Like an ice cube in a warm room, it absorbs more energy than it gives out. Instead of shrinking, it very slowly gains.

How slowly? Let’s check. A black hole swallows light over an area of 27π R², where R is its Schwarzschild radius. That works out to about one square millimeter here. The cosmic background delivers only about 3 millionths of a watt per square meter, so the black hole would gain about 10^-21 kilograms a year from it. Even soaking in direct sunlight, it would gain only about half a nanogram per year.

And here is a twist for the very far future. As the universe expands, the cosmic background keeps cooling. Using standard cosmology, it should fall below 1.67 kelvin in roughly 8 billion years. After that, our black hole would finally become the warmer of the two and start to evaporate. But the evaporation time for a Moon mass black hole is about 10⁴⁵ years, a number so large it makes the age of the universe look like a blink.

So the black hole can grow, but only by eating. Which raises the real question. Could this tiny sand grain one day grow into the monster from Part 1?

Part 3: Does it ever grow up?

Let’s look at the menu. The space around Earth is astonishingly empty. The main thing drifting past is the solar wind, a thin stream of charged particles, mostly protons, blowing out from the Sun at a few hundred kilometers per second. Near Earth, it typically carries only about five to ten particles per cubic centimeter, roughly a sugar cube’s worth of space (Schwenn, Living Reviews in Solar Physics, 2006). For comparison, the air you are breathing holds about 25 billion billion molecules per cubic centimeter.

The paperclip calculation

How fast would a black hole swallow such a thin breeze? Astrophysicists use a formula developed by Fred Hoyle and Raymond Lyttleton in 1939 and refined by Hermann Bondi in 1952, known as Bondi-Hoyle accretion (Bondi, Monthly Notices of the Royal Astronomical Society, 1952). For an object moving through gas at speed v, the rate it gathers mass is roughly:

The math: Ṁ ≈ 4πG²M²ρ ÷ v³. Take 5 protons per cubic centimeter, so a density ρ ≈ 8.4 × 10^-21 kg per cubic meter, and a wind speed of 400 km per second. Then Ṁ ≈ 4π × (4.9 × 10¹²)² × 8.4 × 10^-21 ÷ (4 × 10⁵)³ ≈ 4 × 10^-11 kg per second. Multiply by the 31.6 million seconds in a year and you get about 1.2 grams per year.

About one paperclip. Per year. The script nails this number.

And honestly, that is a generous estimate. The Bondi-Hoyle formula treats the gas like a fluid that collides and funnels inward. The solar wind is so thin that its particles almost never bump into each other. If each particle has to actually aim itself at the black hole to be captured, the math gives only a few micrograms per year. On top of that, for several days around each full moon, the Moon (and our black hole) sits inside Earth’s magnetic tail, where Earth’s magnetic field shields it from much of the solar wind (NASA Scientific Visualization Studio, The Moon passes through Earth’s magnetotail).

What about space dust? Earth sweeps up about 40,000 tonnes of cosmic dust each year (Love and Brownlee, Science, 1993), but Earth is a huge target. A sand grain black hole can only capture dust grains that pass within about 3 meters of it, a target trillions of times smaller than Earth’s. That adds only about a hundredth of a gram per year.

How long to double?

Our black hole already weighs as much as the Moon. So how long until it doubles?

The math: 7.35 × 10²² kg ÷ 1.2 × 10^-3 kg per year ≈ 6 × 10²⁵ years. Because the Bondi-Hoyle rate grows with the square of the mass, the black hole would eat slightly faster as it grows, which brings the doubling time down to about 3 × 10²⁵ years.

So the honest answer is somewhere between 10²⁵ and 10²⁶ years, a 1 followed by 25 or 26 zeros. The universe is 13.8 billion years old. You would need to replay the entire history of the universe a few thousand trillion times just to double the black hole’s mass once. And doubling it would still leave it about 8 billion times too light to become the 588 Sun monster from Part 1.

What about a big asteroid?

Surely one big hit would speed things up? Here is the problem. Today, the Moon is a target 3,474 kilometers wide. Our black hole is a target a fifth of a millimeter wide. Even counting gravity bending paths toward it, an asteroid moving at a typical 20 km per second has to pass within roughly 3 meters to be swallowed directly. That makes the black hole hundreds of billions of times harder to hit than the Moon. Almost every asteroid would sail right past, just as most of them sail past the real Moon.

And even a perfect bullseye would not deliver the whole meal. Long before arriving, a loosely bound rocky asteroid would be pulled apart by tidal forces, within a few thousand kilometers of the black hole. The pieces would whip around and fly off, and only the thin sliver of material aimed within a few meters would actually fall in. For something as heavy as the Moon, that is a whale swallowing a single crumb.

So no. That black hole is never coming for us. Not on any timescale that means anything.

But something up there is.

The real danger was always the Sun

Our Sun is slowly getting brighter. As it burns hydrogen into helium in its core, the core becomes denser and hotter, and the Sun’s output creeps upward by roughly 1 percent every hundred million years (Gough, Solar Physics, 1981). You would never notice in a lifetime, but over geological time it adds up.

Climate scientists have modeled what this does to Earth. One leading study found that Earth should not tip into a runaway greenhouse, where the oceans boil away into the sky, for at least another billion years (Leconte et al., Nature, 2013). After that, Earth stops being a home for anything like us.

Further out, in roughly five billion years, the Sun will run out of hydrogen in its core and swell into a red giant. Modeling by Klaus-Peter Schröder and Robert Connon Smith found that the Sun reaches its largest size about 7.6 billion years from now, swallowing Mercury and Venus, and that “planet Earth will not be able to escape engulfment” either (Schröder and Smith, Monthly Notices of the Royal Astronomical Society, 2008).

So the dangerous thing in our sky was never the black hole. It is the warm, friendly light you wake up to every morning.

Could there be a real black hole like this out there?

Here is the part that turns this from a thought experiment into a live question. Black holes made by dying stars are always several times heavier than the Sun. But some physicists think the violent early universe may have produced primordial black holes of almost any mass, including ones as light as a moon or a planet.

In 2020, physicists Jakub Scholtz and James Unwin suggested that the hypothetical Planet Nine, an unseen object that some astronomers think tugs on distant icy bodies beyond Neptune, might not be a planet at all. It could be a primordial black hole of five to ten Earth masses, about the size of a grapefruit (Scholtz and Unwin, Physical Review Letters, 2020). Nobody has found it, and it remains a speculative idea. But it shows that a small, quiet, invisible black hole sitting in our own solar system is not something physics rules out.

And everything we just worked out tells us what such a thing would be like: invisible, harmless at a distance, keeping its orbit perfectly, and starving in the emptiness of space.

Checking the script: what we confirmed and what we refined

Since this article is built on the Live Qurious video script, here is an honest scorecard. Almost every number holds up under the math.

Confirmed: the Moon formed about 4.5 billion years ago in a giant impact; a Moon wide black hole needs about 588 Suns; it would be about 3,474 km across; the tidal stretch is about 1,785 g; the fall takes about 30 seconds; orbiting would need about 4.75 percent of light speed; Earth’s sideways speed relative to the Moon is about 1 km per second; the Sun, Jupiter and Neptune take about 2.7 days, 32 days and 1.2 years to fall; a Moon mass black hole is about 0.22 mm across; tides, month and tilt stay the same; the Moon recedes about 3.8 cm per year; the Hawking temperature is about 1.7 K against a 2.7 K universe; and the solar wind feeding rate is about a gram a year.

Refined: the tidal stretch would hit instantly, not after a weightless moment; the planets would swing in on stretched orbits and be shredded at their closest pass rather than drop straight in; far out comets would mostly survive by looping around the black hole rather than escaping; the Moon is more like dark asphalt than a mirror; the Sun holds about 1.3 million Earths; the gram a year figure is an upper estimate; and the doubling time is roughly 10²⁵ to 10²⁶ years once the faster eating at higher mass is included.

So, would you trade?

Let’s put it all together. Swap the Moon for a black hole of the same size, and you have secretly invited 588 Suns into the neighborhood. Earth is shredded in seconds, the Sun in days, and the rest of the solar system within a year or two.

Swap it for a black hole of the same mass, and you get a sand grain that keeps your tides, steadies your seasons, drifts away at the same fingernail pace, and quietly steals two things: your moonlight and your eclipses. It is colder than the universe, it eats about a paperclip a year at most, and it will never, ever grow up.

That black hole would never hurt us. The Sun eventually will.

Which leaves one question that physics cannot answer for you. If you had to choose, would you keep the Moon, with its silver light, its blood moons and its coral fireworks? Or would you trade it for a black hole that keeps everything working perfectly, but leaves every night dark forever?

Tell us in the comments. And if you watched the video first, you now know the math behind every single claim in it.

Sources

• Canup, R. M. and Asphaug, E. (2001). Origin of the Moon in a giant impact near the end of the Earth’s formation. Nature 412, 708 to 712.
• Barboni, M. et al. (2017). Early formation of the Moon 4.51 billion years ago. Science Advances (Princeton University release).
• NASA Goddard Space Flight Center, Moon Fact Sheet.
• NASA Goddard Space Flight Center, Sun Fact Sheet.
• Schwarzschild, K. (1916). On the gravitational field of a mass point according to Einstein’s theory (English translation).
• NASA, What Is Microgravity?
• Rees, M. J. (1988). Tidal disruption of stars by black holes of 10⁶ to 10⁸ solar masses in nearby galaxies. Nature 333, 523 to 528.
• Gezari, S. (2021). Tidal Disruption Events. Annual Review of Astronomy and Astrophysics 59.
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