Author: Jiahao Wu
Editor: Kira Tian
Artist: Cici Zhang
Earth is the planet that we call home, but while it might seem massive, it pales in comparison to the size of the sun, a star. So how exactly do these massive structures form and exist? According to NASA, a planet must fulfill three requirements: orbit a star, be big enough for its gravitational pull to form a spherical shape, and clear the surrounding space in its orbit. A star, on the other hand, is formed within clouds of dust. As this cloud collapses under its gravitational attraction, it starts to heat up and condense. Then it starts gathering dust and gas, some of which will form planets.
When a star is still gathering gas, it is called a protostar. Eventually, nuclear fusion starts to occur, and it becomes stable. This is called the main-sequence star. Our sun, like many other stars, will stay in this stage for billions of years before gaining enough mass to become a red giant, a star that loses its hydrogen supply (which is converted into helium through nuclear fusion) and becomes unstable. In this process, the core contracts, and the outer shell—mostly still made of hydrogen—expands, cools, and glows a red color. Subsequently, helium starts to fuse into carbon. Then, in smaller mass stars, the core collapses again. While the core collapses, it expels the outer layers and the core stays as a white dwarf (very dense and small mass) while the outer layers form a planetary nebula. The white dwarf eventually turns into a black dwarf when it doesn’t emit any light anymore. Larger mass stars, in contrast, undergo a supernova explosion instead of collapsing. A neutron star would form if the remnants of the explosion are about 1.4 to 3 times the mass of our sun. If it is greater than 3 times the mass of our sun, something different will happen: gravity overcomes the forces that keep protons and neutrons from combining. The core is then swallowed up by its gravity, creating a black hole.
Planets form differently than stars. Instead of forming from gathering gas, planets start as tiny pieces of dust sticking together. Dust clumps up into pebbles, then the pebbles clump up into rocks, and those rocks clump up to make even bigger rocks. This process repeats itself until we have a planet. However, this process is different for planets that are forming further away from a star. With longer distances, fragments cling onto dust and roll up into planetary cores, similar to a snowball. Gases are slowed down enough to be drawn into the planet, creating gas planets such as Jupiter, Saturn, Uranus, and Neptune.
The mystery of how our universe operates is still largely unknown, but we humans have gone back far enough to see how planets such as ours formed. This gives us insight into what the future holds. Research on exoplanets helps us find suitable habitats for organisms on Earth and potentially alien life. Nevertheless, though we know about stars and planets, the universe still contains lots of unknowns waiting for us to explore.
Citations:
NASA. “Background: Life Cycles of Stars.” NASA, NASA, 2021,
imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html.
NASA. “How Do Planets Form? – Exoplanet Exploration: Planets Beyond Our Solar
System.” NASA, NASA, 2021, exoplanets.nasa.gov/faq/43/how-do-planets-form/.
NASA. “Stars.” NASA, NASA, 2021,
science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve.
NASA. “What Is a Planet?” NASA, NASA, 27 June 2019,
spaceplace.nasa.gov/planet-what-is/en/.
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