If the sun turned black, meaning it stopped emitting light and heat while keeping its mass, Earth would remain in its current orbit but rapidly become a frozen, dark world. The planet’s surface would drop below freezing within a week, plants would die within weeks, and the oceans would eventually freeze solid. But the timeline isn’t as instant as you might expect, and a few surprising factors would keep pockets of the planet habitable longer than you’d think.
Earth’s Orbit Wouldn’t Change
One common assumption is that a “black” sun would somehow pull Earth in or fling it away. It wouldn’t. Gravity depends on mass, not on light or color. As long as the sun retains the same mass, Earth continues orbiting at the same distance, at the same speed, on the same path. This is the same reason physicists point out that if you replaced the sun with a black hole of equal mass, the planets would keep orbiting normally. The only thing that changes is what reaches us: no visible light, no ultraviolet radiation, no warmth.
How Fast the Cold Sets In
Earth’s surface temperature averages about 15°C (59°F), and nearly all of that warmth comes from sunlight. Without it, the cooling would be dramatic. Within days to a week, according to calculations from Caltech, the surface would drop below the freezing point of saltwater, and the oceans would begin forming a complete ice cap.
That ice layer would actually act as insulation. Water is excellent at holding heat, and a thick shell of ice on top of the ocean would slow further freezing considerably. The deepest parts of the ocean wouldn’t freeze for roughly 1,000 years. In the short term, the atmosphere itself would start to change too. Within a few decades, average surface temperatures would fall to around negative 70°C (negative 94°F). Eventually, the air itself would begin to liquefy, with nitrogen condensing at negative 196°C and oxygen at negative 183°C, though that process would take centuries.
Plants Die Fast, but Oxygen Lasts
Without sunlight, photosynthesis stops immediately. Plants can survive for a short time on stored energy reserves, growing slightly in darkness the way a potato sprouts in a cellar. But that window is narrow. Most crops and grasses would die within a few weeks. Large trees might linger somewhat longer thanks to greater energy stores in their trunks and root systems, but they’d follow the same path.
The collapse of plant life would ripple through every food chain on the surface. Herbivores lose their food source first, then predators follow. The ocean food web, which starts with photosynthetic plankton, would collapse in parallel.
You might worry about running out of breathable air, but that’s actually one of the least pressing problems. Earth’s atmosphere contains an enormous reservoir of oxygen, roughly 1.2 billion billion tons. Even if all photosynthesis stopped and every bit of organic matter on the planet slowly decomposed (consuming oxygen in the process), it would only reduce atmospheric oxygen by about 1.2%. That’s a negligible change. Oxygen depletion from the loss of photosynthesis operates on timescales of millions of years. You’d freeze long before you’d suffocate.
Geothermal Heat as a Lifeline
Earth generates its own heat, independent of the sun. About half of this internal heat is left over from the planet’s formation 4.5 billion years ago, and the other half comes from the ongoing decay of radioactive elements like uranium, thorium, and potassium in the crust and mantle. According to Stanford University’s energy research, just 0.1% of Earth’s total internal heat could supply all of humanity’s energy needs for 2 million years.
In a sunless scenario, geothermal vents on the ocean floor would become the most important ecosystems on the planet. Deep-sea communities already thrive around these vents today, running on chemical energy rather than sunlight. Bacteria convert minerals from superheated water into energy, and entire food chains of worms, shrimp, and fish build on top of them. These ecosystems wouldn’t notice the sun was gone.
For humans, geothermal energy could theoretically power underground or heavily insulated settlements. Iceland already heats most of its buildings with geothermal energy. The challenge wouldn’t be energy production so much as food. Growing crops under artificial lights powered by geothermal electricity is technically possible, but scaling it to feed billions of people is another matter entirely. A small, well-organized population near volcanic hotspots could potentially survive for a very long time.
Permanent Darkness and the Human Body
Even if you solved the food and warmth problems, permanent darkness would take a serious toll on human health. Your body’s internal clock relies on the cycle of light and dark to regulate sleep, hormone production, and mood. Without any natural light cycle, circadian rhythms would drift and fragment, leading to chronic sleep disruption.
The psychological effects would be severe. Seasonal Affective Disorder, a form of depression triggered by reduced daylight exposure, gives a preview of what permanent darkness might look like on a larger scale. Researchers have documented that the absence of natural light increases the risk of circadian rhythm sleep disorders, depressive episodes, and worsening symptoms in people with conditions like ADHD and schizophrenia. Bright light therapy is an effective treatment for these conditions today precisely because light is so fundamental to brain chemistry. In a world without sunlight, artificial light on a strict schedule could partially compensate, but the psychological weight of knowing the sky will never brighten again is harder to engineer around.
What Would Actually Survive
The organisms best equipped for a black sun scenario are the ones that already live without sunlight. Deep-sea vent communities, underground bacteria, and organisms in subglacial lakes all run on chemical energy. Some bacteria live miles below the surface in rock, feeding on hydrogen produced by geological reactions. These life forms would be largely unaffected.
On the surface, the picture is bleaker. Insects, small mammals, and cold-resistant microbes might persist for months or even years in insulated environments, but the collapse of photosynthesis-based food webs would eventually catch up with almost everything. The planet wouldn’t become lifeless, but the rich, diverse biosphere we recognize would be gone within a few years. What would remain is a frozen world with scattered pockets of warmth near geothermal sources, inhabited primarily by microbes and, if we’re resourceful enough, a small number of humans huddled around the planet’s own internal furnace.