Some things in space are not stars. They are too small to glow. They can be like big planets. These objects can be very cold. They might even have water in the air. It is fun to look at them! Do you like looking at the stars?
Some things in space are not stars.
They are too small to glow. These objects lack the mass to make energy. Because they lack energy, they slowly cool down. They also slowly shrink.
Some of these objects are like big planets. They can even have water in their air. Some of them orbit a star. Others float all alone in space.
It is fun to find them!
Some things in space are not quite stars. We call them substellar objects. These objects do not have enough mass to make power through hydrogen fusion. Fusion is the way stars make light and heat.
Because they cannot fuse hydrogen, they behave differently than stars. They slowly cool down and shrink. This happens as they give off their own stored power. Some may fuse deuterium, which is a type of hydrogen. This lasts for about 1,100 million years.
These objects can be many things. Some are brown dwarfs. Others are large planets. They can even have water vapor in their air.
Size is also interesting. A substellar object might be as big as Jupiter. Even if its mass changes, its size stays about the same. This is because the density changes to balance the mass. Some of these objects orbit a star. Others float all alone in the dark. Some people call very large planets "Super-Jupiters."
Some objects in space are not quite stars. We call these substellar objects or substars. They are smaller than the smallest star. A star needs a certain mass to keep hydrogen fusion going. This is the process that makes stars shine. A substellar object has less than 0.08 solar masses. This means it cannot sustain that heat and light.
How do these objects work? They do not undergo the usual life stages of stars. Some might start hydrogen fusion for just a short time. This does not provide enough energy to stop them from shrinking. Other objects might fuse deuterium for a while. This source of energy lasts about 1,100 million years. Most of their light comes from gravitational potential energy. This energy is released as they slowly cool and shrink.
Scientists have thought about these objects for a long time. In 1918, William Duncan MacMillan made a special classification. He grouped objects by their density and state. He named solid objects like Earth and our Moon. He called Uranus and Neptune transitional objects. He called gas giants like Jupiter and Saturn gaseous. This helped people understand different types of space bodies.
There are many interesting facts about their size and heat. Many substellar objects have a size similar to Jupiter. This is true even if their mass changes. Their density changes to keep their radius about the same. These objects are often cool enough to have water vapor. We can use infrared spectroscopy to see this water. This tool detects the special color of water in gas giants.
You can think of these objects as the middle ground of space. They sit between the huge stars and the small planets. Some orbit a star like an exoplanet. Others might be a brown dwarf orbiting a star. Some large planets are even called Super-Jupiters. An object as small as 8 Jupiter masses can be a brown dwarf. This shows how many different things can be substellar.
In the vastness of space, not every glowing object is a star. Astronomers use a specific term for objects that fall below a certain size threshold. These are called substellar objects, or sometimes substars. A substellar object is defined by its mass. It has less mass than the smallest possible star. Specifically, it has less than approximately 0.08 solar masses. This mass is the limit required to sustain hydrogen fusion. Without enough mass, an object cannot maintain the nuclear reactions that power a true star.
To understand how these objects work, we must look at the process of fusion. In a star, hydrogen fusion provides constant energy. A substellar object lacks the mass to keep this process going. Some objects near the top of the mass range might ignite hydrogen fusion temporarily. However, this energy is not enough to stop gravitational contraction. This means the object continues to shrink under its own weight. Other objects with at least 0.013 solar masses can fuse deuterium. This is a type of hydrogen, but this energy source lasts only about 1,100 million years. Most of their light comes from the release of gravitational potential energy. As they release this energy, they gradually cool and shrink.
Despite their different masses, many substellar objects share a similar size. If an object has a composition like the Sun and at least the mass of Jupiter, its radius remains steady. A Jupiter-sized mass is about 0.001 solar masses. Even as mass changes, the radius stays comparable to Jupiter, which is about 0.1 solar radii. This happens because of a balance in density. At the high end of the mass range, the center is quite degenerate. This means it has a very high density of approximately 10^3 g/cm3. As the mass decreases toward the mass of Jupiter, this degeneracy lessens. The central density drops to less than 10 g/cm3. This decrease in density balances the decrease in mass, keeping the radius nearly constant.
Classification of these objects has evolved over time. In 1918, William Duncan MacMillan proposed a system based on density and phase state. He identified three distinct categories. Solid objects include Earth, moons, and smaller terrestrial planets. He classified Uranus and Neptune as transitional objects between solid and gaseous states. Large gas giants like Saturn and Jupiter are considered fully gaseous. Today, we also look at how these objects exist in relation to stars. A substellar object might be a companion orbiting a primary star. These can be exoplanets or brown dwarfs.
There is often debate regarding the exact boundaries between these categories. Astronomers use different mass limits to name these companions. Objects orbiting a star are often called planets if they are below 13 Jupiter masses. If they are above that limit, they are called brown dwarfs. However, some researchers have called objects as small as 8 Jupiter masses brown dwarfs. There is also a middle ground known as Super-Jupiters. These are found in systems like the star Kappa Andromedae. Some studies have even identified substellar companions as large as 823 Jupiter masses.
We can learn much about these objects through specialized tools. Because they are cooler than stars, they often have water vapor in their atmospheres. Scientists use infrared spectroscopy to study them. This technique allows them to detect the distinctive color of water in gas-giant-sized objects. This works even if the object is not orbiting a star. By looking at these light signatures, we can understand the chemical makeup of distant worlds. This helps us distinguish between different types of substellar bodies in the dark.
Substellar objects represent a vital link in astronomical study. They bridge the gap between the massive stars and the small planets. They show us the transition from nuclear-powered stars to gravity-powered worlds. By studying brown dwarfs and massive exoplanets, we learn about the limits of formation. We see how mass, density, and energy dictate the life of a celestial body. Whether they are isolated or orbiting a star, they provide a window into the physics of the universe.
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