How to tell a star from a planet?
If somebody asked you what was the difference between a star and a planet, would you be able to answer? Maybe you would think of the Earth and the Sun, and you would say something like: planets are rocky and stars are balls of gas. But what about Jupiter or Saturn? They’re planets, but they’re balls of gas too. Why aren’t they stars?
What exactly is a star?
A star is a celestial body made mostly of hydrogen and helium, and held together by its own gravity. The life of a star consists of one task: fusing hydrogen into helium, through a process called nuclear fusion. This is how we get heat and light down here on Earth — the nuclear fusion in Sun’s core produces energy (heat) and photons (light).
But how does matter accumulate into a giant ball to form a star?
Space might seem empty, but it’s actually full of dust and gas. Most is hydrogen and helium, but there are also some heavier elements left behind after death of giant stars. When gravity starts to pull these gases and particles together, they form enormous clouds, called nebulae.
Nebulae form in interstellar space — the space between the stars — and are mindbogglingly huge. They can measure from several light years to more than one million light years across. Some of the most famous ones are Orion, Eagle and Crab nebula.
Why are these gigantic clouds of gas important? Because they are also called stellar nurseries, meaning they give birth to stars.
Eventually, the gravity in a nebula becomes so strong that particles begin to collapse in on themselves and start to spin. This causes the cloud to flatten and become a pancake like disk. In the center, the material starts to accumulate into what we call a protostar.
It takes tens of millions of years for this young star to start fusing hydrogen into helium. And that’s when a star is born.
More than 4,5 billion years ago, our Sun went through the same process. Some of the cloud from which it was born was left behind. This would provide material for planets to form. But what is a planet?
Birth of planets
If you ask Wikipedia, a planet is “an astronomical body orbiting a star that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals”. That’s a lot to take in in one sentence, so let’s break it down a bit.
A planet is a celestial body that is orbiting a star. In our solar system, there are 8 bodies that we qualify as planets and that orbit our star, called the Sun. These planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are made of rock and metal, and the outer ones are what we call gas giants.
For the past 30 years, we’ve also known there are planets outside our solar system, orbiting stars in other solar systems. These are known as exoplanets. But how did all these planets form? Do planets out there resemble our planets?
Planets are born from a cloud of dust and gas swirling around a new star. This material, held in orbit by the star’s gravity, is called an accretion disk.
At first, each planet is just tiny grains of dust in the accretion disk. Then the atoms and molecules begin to stick together, or accrete, into larger particles. They collide and build up to form small objects a mile in diameter, called planetesimals.
These objects are big enough to attract each other due to their gravity. Through gentle collision, they start to combine and grow.
These protoplanets can orbit the star for some 10 or 100 million years. Further collisions cause protoplanets to grow into planets.
A planet orbits its star and it’s massive enough to become spherical due to its own gravity. By the end of its formation, a planet has also cleared its surroundings of smaller objects.
This means that it is gravitationally dominant. Other objects in its neighborhood need to either collide with the planet and become part of it, or else become gravitationally bound to it by becoming its satellites. This way, a planet only shares its orbital region with bodies that are under its gravitational influence.
This is one of the reasons Pluto is no longer considered a planet since 2006. It does not meet this last condition.
So why are some planets rocky and others gaseous? Our Sun sends out a constant stream of energy and particles, called stellar wind. During the formation of our solar system, these winds were so strong they blew out most of the gases of the four most inner planets. Thus, these planets became smaller and were left only with rocks and metal. That’s why they’re called rocky planets.
The outer four planets were too far away, so the solar wind left their gases intact. Today we know them as gas giants. They are made mostly of hydrogen and helium, and have a small rocky or metal core.
If gas planets are mostly made of hydrogen and helium, just like stars, why is there no thermonuclear fusion going on in their core? Well, the answer to that is really simple: they’re not massive enough.
Jupiter, the biggest planet in our solar system, is more than 300 times more massive than Earth, but that’s still not enough. It takes immense gravitational pressure for nuclear fusion to ignite. Jupiter would actually have to be at least 75 times more massive than it is to become a star.
What about planets outside our solar system, the so-called exoplanets? Do they follow the same rules as our planets or are they different?
Well, to this date, we have discovered more than 4000 exoplanets. This might seem like a lot, but it’s a tiny fraction of what’s really out there. But those that we’ve discovered so far only come in two flavors: small and rocky or big and gaseous, just like the planets in our solar system.
Of course, in reality things are surely more complicated. Currently puzzling the scientist is a recently discovered rocky giant TOI-849b. About 730 light years from us, this weird planet is 40 times more massive than Earth, meaning it should have evolved into a gas giant. But it’s actually rocky with a very thin atmosphere.
In the future, we will undoubtedly discover more new worlds that will astound us and make us question existing theories about planet formation.
Bottom line
So, next time somebody asks you if you know the difference between a star and a planet, or you just want to start an interesting topic, you can say this: A star is a huge ball of gas that fuses hydrogen into helium in its core, through a process called nuclear fusion. This is how the Sun provides us with heat and light.
A planet, on the other hand, is smaller and orbits the star. It’s made of rocks and metal, like Earth, or of light gases and a rocky/metal core, like Jupiter. Either way, the main difference is that a planet is not massive enough for nuclear fusion to ignite in its core.
There you have it. Simple enough!
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