Imagine finding a piece of ice inside a giant furnace. It sounds impossible, but something surprisingly similar may be happening in the space around our Solar System.
The Sun is located inside a huge region of space called the Local Bubble. This bubble stretches roughly 100–200 parsecs around the Sun, covering hundreds of light-years of space. Much of its interior contains extremely hot plasma, with temperatures reaching around 1 million degrees Kelvin.
Yet, inside this incredibly hot region, astronomers have found clouds of gas that are almost unbelievably cold.
One of them is the Local Leo Cold Cloud (LLCC). It is located only about 11–24 parsecs from the Sun, or roughly 36–78 light-years away. While the surrounding Local Bubble can contain million-degree plasma, the LLCC has a temperature of only about 20 Kelvin, which is around −253°C.
How can such a cold cloud exist inside such a hot environment?
A new study by researchers Rathjen and Linsky provides an important possible answer. Their computer simulations suggest that these cold clouds do not necessarily have to come from somewhere else. Instead, they may be able to form right inside the Local Bubble itself.
The Mystery of the Local Bubble
The Local Bubble is not an ordinary empty region of space. It is thought to have been shaped by powerful events involving stars, including stellar winds and supernova explosions.
These events released enormous amounts of energy into the surrounding interstellar medium. Over time, they created a large cavity filled with very hot, low-density gas.
The Solar System happens to be located inside this enormous structure.
But scientists have long been puzzled by the presence of extremely cold clouds within it.
The Local Leo Cold Cloud is particularly interesting because it is not located at the edge of the Local Bubble. It appears to be well inside the hot region.
If the surrounding environment is so energetic, why hasn't the cold cloud been heated and destroyed?
One possible explanation is that the cloud formed somewhere else and later travelled into the Local Bubble.
Another possibility is that warm clouds collided and produced colder material.
However, these explanations are difficult to test because the interstellar medium is constantly moving and changing.
Rathjen and Linsky wanted to investigate another possibility: Could cold clouds actually form locally inside this violent environment?
Scientists Created a Virtual Galaxy
To answer the question, the researchers used advanced computer simulations from the SILCC Project.
These simulations are designed to reproduce many of the complicated processes that occur in interstellar space.
The researchers included processes such as:
Star formation
Stellar winds
Supernova explosions
Ultraviolet radiation
Magnetic fields
Cosmic rays
Chemical reactions
Heating and cooling of gas
This is important because space is not controlled by a single physical process. Gas can be heated by radiation, pushed by stellar winds, compressed by explosions and influenced by magnetic fields and cosmic rays—all at the same time.
The researchers then searched the simulations for cold diffuse gas with temperatures below 100 Kelvin.
They also used special virtual tracer particles to follow pieces of gas through time.
These particles helped the researchers understand where the gas came from, how it became cold, how long it stayed cold and what eventually happened to it.
Cold Gas Can Form Locally
The results were surprising.
The simulations showed that cold diffuse gas can form inside warm and hot surroundings created by stellar feedback.
This means an LLCC-like cloud does not necessarily need to enter the Local Bubble as a fully formed cold cloud.
Instead, gas already present in the region can come together and gradually become cold.
Some of the material can begin as relatively cold gas, while some can come from thermally unstable warm gas. As these materials interact, they can collect into larger structures.
Over time, these structures can become extremely cold.
The simulations also showed that the resulting cold material does not always look like a traditional round cloud.
It can form long filaments and thin sheets, creating complicated shapes throughout the interstellar medium.
This is important because the real interstellar medium is highly turbulent and messy. Gas is constantly being compressed, stretched, heated and cooled.
The Gas Does Not Simply Freeze
One of the most interesting parts of the study is how the gas reaches temperatures close to 20 Kelvin.
You might expect the gas to become cold simply because it expands into a larger volume.
But the simulations show that this is not the main reason.
Instead, the gas undergoes non-adiabatic cooling.
In simple terms, this means the gas loses energy through processes other than expansion.
As the gas loses energy, its temperature can fall dramatically—even while it remains surrounded by much hotter material.
This provides a possible explanation for how gas can eventually reach temperatures similar to those observed in the Local Leo Cold Cloud.
Magnetic Fields and Cosmic Rays Help
But forming cold gas is only part of the mystery.
The bigger question is: How does it survive?
If million-degree plasma surrounds a 20-Kelvin cloud, you might expect heat to quickly destroy the cold material.
The simulations suggest that the situation is more complicated.
The pressure inside and around the cold structures is not controlled only by temperature.
Other forms of pressure also matter.
In particular, cosmic-ray pressure remains relatively continuous across the boundaries between cold and warmer gas. This creates a kind of pressure floor that helps support the structures.
Magnetic pressure also contributes.
Magnetic fields can partially balance the much greater thermal pressure from the surrounding hot gas.
Together, these non-thermal effects help the cold material survive in an environment that initially seems completely hostile to it.
The Clouds Can Survive for Millions of Years
The researchers found another important clue.
The cold structures survived in the cold diffuse phase for a median of about 2.5 million years.
That is an enormous amount of time compared with human timescales.
Even when a cold structure eventually loses its original shape, much of its material can remain cold.
This means that a cloud does not necessarily have to remain a clearly defined object to preserve its cold gas.
It may change shape, break apart and lose its identity while much of the material continues to exist in a cold state.
This makes the survival of cold material inside the Local Bubble much more realistic.
A New Explanation for the Local Leo Cold Cloud
The findings provide a possible solution to the mystery surrounding the LLCC.
Scientists do not necessarily need to assume that the cloud was created somewhere far away and transported into the Local Bubble.
Instead, it could have assembled locally from nearby gas.
The surrounding stellar activity may have helped create the conditions needed for this process.
Supernova explosions, stellar winds, radiation, magnetic fields and cosmic rays may all contribute to a constantly changing environment where warm and cold gas interact.
Under the right conditions, some of that material can lose energy, become extremely cold and gather into structures resembling the Local Leo Cold Cloud.
Space Is More Complicated Than It Looks
This study gives scientists a new way to think about our cosmic neighborhood.
From Earth, space may appear to be an almost completely empty vacuum. But the region around the Sun is filled with different forms of matter and energy interacting over enormous distances.
Hot plasma can exist alongside warm gas.
Magnetic fields can influence the movement of matter.
Cosmic rays can provide pressure.
And, surprisingly, extremely cold gas can survive in the middle of it all.
The Local Leo Cold Cloud may therefore not be an ancient cold object that simply wandered into a million-degree environment.
Instead, it could be a cloud that formed locally, cooled through energy loss and survived because of the complex balance of pressures around it.
The discovery also shows how important computer simulations are for understanding regions of space that are difficult to observe directly.
Most importantly, it reveals something fascinating about the universe: even inside an environment filled with million-degree plasma, nature can create and preserve pockets of gas that are only a few degrees above absolute zero.
Reference: Tim-Eric Rathjen, Jeffrey L. Linsky, "Why is there cold gas inside the Local Bubble?", Arxiv, 2026. https://arxiv.org/abs/2608.27603
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