Judging only by the blistering summer sun, you’d be forgiven for assuming that space is a hothouse. But despite the tremendous energy pouring out of trillions and trillions of stars, our universe is surprisingly arctic.

To understand why, we first need to wrap our heads around temperature because its true nature isn’t obvious when you burn your hand on the stove or dive into an icy lake. When scientists talk about hot and cold, they’re referring to the average kinetic energy of a system (whether it be a snowball or an entire galaxy), which is based on the motion of its particles: the more they jiggle, the hotter they are.

But far from stars, planets, and other cohesive objects, in empty tracts of the cosmos, the concentration of particles drops precipitously — and so does the thermometer.

“Most of space is cold,” says Emily Hardegree-Ullman, an astronomy professor at Colorado State University, “because there is just nothing to possess kinetic energy.”

And because they’re so far apart, the few particles in this near-vacuum can’t transfer much heat via conduction and convection, relying instead on the less efficient method of radiation.

The Temperature in Space

Obviously, it isn’t cold everywhere. Earth’s atmosphere maintains a mild, life-giving climate; other planets in our solar system undergo extreme temperature swings (Mercury, for example, pendulums from 800 degrees Fahrenheit during the day to negative 280 degrees at night); our sun’s corona blazes at 2 million degrees, and quasar 3C273 — a spiraling disk of plasma around a black hole in the constellation Virgo — has been estimated to reach the mind-melting figure of 18 trillion degrees.

Nevertheless, these space heaters (pun very much intended) are few and far between, vastly outweighed by, well, a whole lotta nothing. Most of the universe is filled only by the cosmic microwave background, a sparse but pervasive radiation that’s been cooling off ever since the Big Bang.

To picture this, Hardegree-Ullman likes to imagine the observable universe as a giant bubble, its inner surface like a faint star that fully surrounds us.

“The light from that surface has been stretched out over billions of years as it has traveled toward us,” she says, “so by the time it reaches Earth, it looks like it comes from the surface of a star that is only 3 [degrees] Kelvin.”

Read More: 5 Planets with Extreme (and Weird) Weather Patterns in Our Solar System

Why Is it so Cold in Space?

In other words, when we consider the entire universe as one giant system, the average kinetic energy equates to a spine-chilling negative 454 degrees Fahrenheit. It’s only getting colder over time, and even now, temperatures plunge farther in certain regions. The record goes to the Boomerang Nebula, which registered just 1 degree Kelvin above absolute zero — nearly negative 460 degrees Fahrenheit — when scientists measured it in 1995.

In fact, a smidge above 0 Kelvin would seem to be the theoretical limit on temperature.

“In order for a system to achieve absolute zero,” Hardegree-Ullman explains, “all its particles would have to come to a rest.”

Yet we know that isn’t possible because, as quantum mechanics tells us, we can never pin down both the position and the velocity of a particle with perfect certainty.

Ironically (given its generally cozy conditions), our own planet is actually the source of the coldest temperatures ever documented. Scientists routinely get closer to absolute zero than anything you’d find in space. The best success so far came in 2021, when a team of German researchers reported they had cooled rubidium atoms to an astonishing 38 trillionths of a degree above 0 Kelvin.

Still, as if in homage to extraterrestrial frigidness, they simulated microgravity by dropping the system from a tower, allowing them to squeeze out a bit more energy. When it comes to cold, we couldn’t ask for a better teacher than space.

Read More: There’s No Wind or Rain On The Moon, But There Are Extreme Temperatures

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor’s degree in journalism and media production from Washington State University.