Stars Versus Planets: What’s the Difference
Both stars and planets are massive, large, and round, and look almost the same to the naked eye from the Earth. Yet they're entirely different objects. What makes them different? Let's find out!
- What is a star?
- What is a planet?
- What is the primary difference between stars and planets?
- Can a planet become a star?
- Difference between stars and planets
- How to tell planets from stars in the sky?
- Bottom line
What is a star?
A star is a luminous, hot, and massive ball of gas, mostly hydrogen and helium, held together by its own enormous gravity (the result of a star's huge mass). Gravity also causes a process of nuclear fusion in the core of a star, during which stars produce light and heat. Fusion occurs because a star's massive gravity compresses hydrogen to the point where high pressures and temperatures squeeze two hydrogen atoms into a helium atom. This process creates an enormous amount of energy that makes a star bright. The closest star to us is the Sun.
What is a planet?
A planet is a natural body that orbits around a star and dominates its orbit, displacing all similarly sized objects nearby. Planets are massive enough to have a spherical shape but not massive enough to cause nuclear fusion inside. They can consist of rock, like the Earth and Mars, or of gas, like Jupiter and Saturn. Planets outside of the Solar System are called exoplanets.
What is the primary difference between stars and planets?
Their key difference is:
Stars generate their own light and heat through nuclear fusion in their cores. They emit energy in the form of light and electromagnetic radiation, which makes them visible from great distances. On the contrary, planets do not produce light. Instead, they reflect light from their parent stars. That's why we can't see exoplanets like we can see other stars — a star like the Sun is about a billion times brighter than the reflected light from an orbiting extrasolar planet.
Can a planet become a star?
If a planet is just one thing short of becoming a star, does it have a chance of becoming one? Theoretically, yes. A planet could become a star by adding enough mass that it would compress and heat up, causing a nuclear fusion reaction. In order for this hypothetical planet to transform into a star, it has to be mostly hydrogen. This is necessary to enable the nuclear fusion process that turns hydrogen into helium.
Let's take Jupiter, which is mostly hydrogen, as an example. Its mass is 1.898 × 10²⁷ kg, while the Sun's mass is 1.989 × 10³⁰ kg. So Jupiter is about 1,000 times less massive than the Sun. In other words, to make Jupiter a Sun-like star, we'd have to crash 1,000 Jupiters together. Struggling to find one thousand Jupiters? No problem! There are stars less massive than the Sun: if we take about 7.5% of the mass of the Sun's hydrogen and put it together, we'd have a red dwarf. For this, we only need to crash 80 Jupiters into each other.
So hypothetically, it’s possible to turn a planet into a star, but it would require a series of massive collisions. And, who knows, maybe it’s happening right now, somewhere in the vastness of space.
Difference between stars and planets
Besides the ability to create light and heat, there are other differences between stars and planets.
Stars are formed from huge clouds of gas and dust that collapse under the force of gravity and heat up, igniting nuclear fusion in their cores. Planets form from leftover materials that didn't contribute to a star's formation.
Most of the found stars consist primarily of hydrogen and helium. Speaking of planets, there are two types based on their composition — gas planets (that also consist of hydrogen and helium) and terrestrial (rocky) ones.
Stars don't orbit planets, but planets usually orbit stars. However, there are exceptions, such as rogue (or free-floating) planets. They're not gravitationally bound to any star or brown dwarf and casually wander through space on their own. Yes, that's possible! Even our Sun used to have more planets. Rogue planets appear when several large planets fight for a place around a single star and eventually kick their rivals out of that planetary system.
While planets have a stable, long-term existence as long as nothing destroys or captures them, stars have a well-defined life cycle from birth to death. This cycle depends on the star’s size — the bigger the star, the shorter its lifetime. For instance, the most massive stars can die after only a few million years, while a Sun-like star can live for about 10 billion years.
Usually, stars have a bigger diameter than planets. However, there are exceptions, such as white dwarf stars. They are remnants of stars that were once like the Sun but died, shedding their outer layers and leaving only the core behind. That core is only about the size of the Earth. If the star had planets bigger than the Earth orbiting it before it died, it's possible that some of them will survive, and you’ll get a planet bigger than its star.
Stars always have more mass than planets. As mentioned above, if a gaseous planet gains as much mass as a star, it most likely becomes a star. As for rocky planets, there is no known rocky planet with a mass anywhere near that of a star.
Stars’ atmosphere is mostly composed of hot gasses and plasma. Planets, on the other hand, have atmospheres that vary in composition and density. For example, the Earth’s atmosphere is 99% nitrogen and oxygen, while Venus’ and Mars’ atmospheres have more than 98% of carbon dioxide and nitrogen.
Stars can't be habitable because of the lack of surface area and the intense heat and radiation emitted from their cores. And there are also unhabitable planets with extreme temperatures, lack of breathable air, or toxic environments. But we're a living example of how some planets, like the Earth, can support life.
Stars are incredibly hot and have high temperatures; planets have relatively low temperatures. But there are curious cases. For example, in 2017, scientists found KELT-9b — a planet with a "surface" temperature of over 4,000°C, almost as hot as our Sun. The reason KELT-9b is so hot is that its star is hot itself, and the planet is very close to it.
10. Number in the Universe
Planets are more common than stars in our universe. Most likely, the total number of planets exceeds the number of stars by a factor of 100 to 100,000. What's even more surprising is that there may be even more rogue planets (not orbiting any star) than stars in the Milky Way.
How to tell planets from stars in the sky?
Now that you know the difference between a star and a planet in space, it's time for some down-to-earth problems. From the Earth's surface, stars and planets look very similar; here are some clues to help you tell them apart.
See if the object twinkles. If it does, it’s a star; if it shines with a constant brightness, it’s a planet.
Compare the apparent brightness. The brightest dot you see in the sky is most likely Venus. Its magnitude varies from -3 to -4.9, while the brightest star, Sirius, has a magnitude of -1.46. Jupiter and sometimes even Mars are also more luminous than stars.
Find planets near the ecliptic. The ecliptic represents the visible path of the Sun in the sky. Since the orbits of all the planets are more or less in the same plane, they all move across pretty much the same constellations as the Sun in our sky — the ecliptic constellations. So do not expect to see a planet in Ursa Major or in the Monoceros constellation. Look for planets in the ecliptic constellations!
Observe the color. Each planet has its own color, and this will help you determine which specific planet you're looking at. Although stars also have a tinted appearance, planets’ color is more prominent. Venus is white, Jupiter and Saturn are yellow, and Mars is reddish. The color of Mercury is difficult to determine because this planet is dim. The color of Uranus is indeterminable without a telescope. Neptune is not visible at all without optics.
Use a stargazing app like Star Walk 2 or Sky Tonight. If you see a shiny dot and you're not sure if it's a star or a planet, take out your phone, point it at the sky, and the app will resolve your doubts.
The main difference between planets and stars is that the latter can generate their own light and heat. This is why we can see them in the vastness of space, even with the naked eye, while planets outside of the Solar System are not visible. From the Earth, you can tell a planet from a star by whether it twinkles or not, by its color and its location.