Have you ever gazed up at the stars on a clear night? There are a million things that you
wonder about, and sometimes you get a strange question, do stars have the capability of
becoming planets on their own? Absolutely! A star can become a planet, but only in the
case of a particular type of star called a brown dwarf.
A star's core contains a limited quantity of hydrogen. Once all the hydrogen in its
core has been used up, the nuclear fusion reaction in the core stops.
As the core collapses inward, its temperature increases. The outermost layers of the star
start disappearing as the core becomes hotter. The result is that the outer layers of the
star become cool and expand, giving it a reddish glow. It is now known as a red giant.
The red giant stage lasts about 1 billion years. The star attempts to produce more energy
during this phase by converting the helium it contains into energy through nuclear reactions.
These reactions can only sustain the star for a short time.
The star starts losing even more of its outer layers as these reactions become unstable.
Stars go through this process until all the layers are shed and the core is revealed.
At this point, it is now referred to as a white dwarf, and will slowly cool and disappear.
For a star with a mass greater than 1.4 times that of our sun, its first core will collapse
inwards and then explode. These explosions are called supernovae.
A supernova releases so much energy that it can shine brighter than an entire galaxy for
a few weeks. An explosion of this kind leaves behind either a neutron star or a black hole.
During the formation of a star, a disc of gas, dust, and debris surrounds it. The dust
particles in this disc are the building blocks of rocky planets.
Gravity and other forces cause these particles to collide with one another. Mild collisions
cause these particles to stick together. The process continues until rocks of slightly
larger masses are formed. Gravity allows these rocks to pull even more particles towards
them. As a result of these processes, small planetary
bodies called planetesimals are created. Like the smaller particles, these planetesimals
collide and fuse to form planets. As long as the distance between a planetesimal
and its star is large enough, its core will be icy and it will develop into a gaseous
planet. These regions have an abundance of hydrogen and helium compounds.
The gravitational pull of a planetesimal increases with its size. Consequently, it attracts all
nearby materials. The surrounding materials of a planetesimal are gases.
Stars are usually considered stars because they are massive enough to ignite nuclear
fusion, resulting in light emissions and the classic appearance of a star.
Many scientists will not accept brown dwarfs as stars simply because they do not have the
same properties as regular stars. Even though a brown dwarf does not undergo nuclear fusion,
it is still classified as a star. This is because they tend to sit in the center
of a planetary system, just as a star does. As a result, accepting a brown dwarf as a
planet is more difficult than accepting it as a star.
The light emitted by a brown dwarf will be similar to that of a star. However, while
most stars have enough gravitational force to ignite the fusion of normal hydrogen, brown
dwarfs will only be able to ignite the fusion of heavy hydrogen. Heavy hydrogen is different
from regular hydrogen. As an example, let's look at our Sun. There
is a category of stars called main-sequence stars that includes the Sun. Inside their
cores, these stars produce energy through the nuclear fusion of hydrogen to form helium.
Light and heat are released from this process. The energy released also keeps the pressure
required, so they don't collapse inwards. In general, stars remain in this phase for
about ten billion years. Brown dwarfs are often referred to as failed
stars. They are objects that are too compact to be stars, but too large to be planets.
They have characteristics of both stars and planets. Their sizes range from twice Jupiter's
mass to 90 times Jupiter's mass. In general, they are found at the center of
their solar system and are orbited by planets. They do not have enough gravitational force
to support the nuclear fusion of hydrogen. A brown dwarf may not be able to support hydrogen
fusion, but it may be able to support deuterium fusion (heavy hydrogen). So, early in its
life, it gets energy from this reaction and emits heat and light, just like any other
star. Because brown dwarfs consume heavy hydrogen
faster than they consume regular hydrogen, they will cease emitting light very early
in their lives. There is, however, a limited supply of deuterium
in the universe. A brown dwarf's deuterium fuel exhausts very quickly as a result. At
this point, all reactions cease, and the brown dwarf stops emitting light and heat.
Dimming and cooling down, it begins to resemble a planet. The result is a group of planets
orbiting a giant, central planet. Wait... But in that case, are these planets orbiting
the big planet considered moons instead? Aghh... Nevermind.
Only about 3,000 brown dwarfs have been discovered so far. The reason for the small number is
that brown dwarfs stop emitting light very early in their life cycle.
With conventional telescopes, they are difficult to spot because they live in a cool, dark
environment for almost their entire lives. Brown dwarfs may also be able to contribute
to the mystery of dark matter in a small but crucial way.
Some scientists believe that the number of brown dwarfs in our galaxy is similar to the
number of regular stars. These stars will eventually resemble planets.