The Science Behind Space’s Cold Temperatures: Kinetic Energy Explained
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.”
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.
The Role of Particle Motion in Space Temperature
In the vacuum of space, the lack of particles leads to extremely low temperatures. The average kinetic energy of a system is directly related to the motion of its particles. In space, where particles are sparse, there is little jiggling or motion to generate heat. This is why most of space is cold, as the absence of particles means there is nothing to possess kinetic energy.
Without the presence of a dense collection of particles, such as in stars or planets, the transfer of heat through conduction and convection is limited. Instead, radiation becomes the primary method of heat transfer in the vast emptiness of space. This inefficiency in heat transfer contributes to the overall cold temperatures experienced in space.
The Influence of Cosmic Microwave Background Radiation
The cosmic microwave background radiation, a remnant of the Big Bang, pervades the universe and contributes to its overall cold temperature. This faint radiation has been cooling off since the birth of the universe and serves as a constant reminder of the extreme conditions present in space. The sparse nature of this radiation, coupled with the vast distances between particles, results in a lack of significant heat generation in most regions of space.
While there are exceptions, such as the extreme temperatures experienced on certain planets and within stars, these instances are rare compared to the predominantly cold conditions found in the universe. The presence of the cosmic microwave background radiation serves as a constant reminder of the frigid temperatures that dominate the vast expanse of space.
Exploring the Limits of Temperature in Space
When considering the entirety of the universe as a single system, the average kinetic energy translates to a bone-chilling negative 454 degrees Fahrenheit. This temperature is continually decreasing over time, with certain regions experiencing even colder conditions. The Boomerang Nebula, for example, set a record with a temperature just 1 degree Kelvin above absolute zero, nearly reaching negative 460 degrees Fahrenheit.
Absolute zero, the theoretical limit of temperature, represents a state where all particles in a system come to a complete rest. However, achieving this state is impossible due to the principles of quantum mechanics, which dictate that the position and velocity of a particle cannot be simultaneously determined with perfect certainty.
Despite the extreme cold temperatures found in space, scientists have managed to reach even colder temperatures on Earth. In a groundbreaking achievement, a team of researchers successfully cooled rubidium atoms to a remarkable 38 trillionths of a degree above 0 Kelvin in 2021. This feat, while impressive, highlights the stark difference between the coldness of space and the innovative techniques used on Earth to achieve ultra-low temperatures.
Conclusion
In conclusion, the cold temperatures experienced in space are a result of the sparse distribution of particles and the limited heat transfer mechanisms present in the vacuum of the cosmos. While there are exceptions, such as the extreme temperatures found on certain planets and within stars, the overall temperature of the universe remains predominantly cold. Through advancements in scientific research and technology, we continue to push the boundaries of temperature extremes, both on Earth and in the vast expanse of space.