Key Differences Between a Star and a Planet: Star VS Planet
Both stars and planets are massive, large, and round, and look almost the same to the naked eye from Earth. Yet they're entirely different objects. What makes them different? Let's find out!
What is the primary difference between stars and planets?
The key difference is that stars generate their own light and heat, mostly² through nuclear fusion in their cores. This process releases enormous amounts of energy, including visible light and other forms of electromagnetic radiation, allowing us to see stars across vast distances.
Planets, by contrast, do not generate their own visible light and do not sustain nuclear fusion (although they do emit thermal infrared radiation from their internal heat). Planets are mainly visible because they reflect light from their host stars. This is why we can see stars with the naked eye but not exoplanets¹: a star like the Sun can be about a billion times brighter than the light reflected by a planet orbiting it.
¹In simple terms, an exoplanet is a planet outside the Solar System.
²There are exceptions, such as white dwarfs ("dead" stars), which radiate light from residual heat.
Contents
- What is the primary difference between stars and planets?
- Can a planet become a star?
- How to tell planets from stars in the sky
- Difference between a star and a planet: bottom line
Difference between stars and planets
A star is a massive sphere of hot gas (mostly hydrogen and helium) and plasma that generates energy through nuclear fusion. The Sun is the closest star to Earth.
A planet is a large, nearly spherical natural body that orbits a star³ and dominates its orbit, displacing all similarly sized objects nearby. Planets can’t generate energy through hydrogen fusion.
³An exception is a rogue planet.
Besides the ability to create light and heat, there are other differences between stars and planets. Prefer a quick visual summary? Check out our infographic: Stars vs Planets.
1. Origin
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. During this process, hydrogen nuclei fuse through a series of reactions that ultimately convert four protons into one helium-4 nucleus, releasing an enormous amount of energy that makes the star shine.
Planets form from leftover materials around a star that didn't contribute to a star's formation — particles of dust and gas crash into one another, clumping into larger objects.
2. Composition
Most known stars consist primarily of hydrogen and helium. Planets include rocky worlds, gas giants, and ice giants, as well as many intermediate types found around other stars.
3. Orbit
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! There are theories suggesting that even our Sun once had more planets. Some planets became “rogue” when several large planets fought for a place around a single star and eventually kicked their rivals out of that system. Others may have formed independently through the collapse of small gas clouds.
Do you think the Solar System once had more planets?
4. Mass
Stars always have more mass than planets. 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.
Between planets and stars are brown dwarfs — objects commonly called "failed stars". They form like stars and can be massive enough to briefly fuse deuterium (same as stars), but they cannot sustain the hydrogen fusion that powers true stars. Brown dwarfs usually have 13 to 80 times the mass of Jupiter. So, a gas giant would have had to pass through the brown-dwarf range before becoming a low-mass star.
5. Diameter
Usually, stars have a larger 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 Earth. If the star had planets bigger than 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.
6. Temperature
Stars are incredibly hot, while 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. KELT-9b is so hot because it orbits extremely close to an unusually hot A-type star.
7. Atmosphere
A star’s atmosphere is mostly composed of hot gases and plasma. Planets, on the other hand, have atmospheres that vary in composition and density. For example, the Earth’s dry atmosphere is 78% nitrogen and 21% oxygen. Venus has an atmosphere dominated by carbon dioxide with a few percent nitrogen, while Mars has a thin atmosphere composed mostly of carbon dioxide, followed by nitrogen and argon.
8. Habitability
Life as we know it cannot exist on stars because of the intense heat and radiation from their cores and the lack of a solid surface. Many planets are also uninhabitable — with extreme temperatures, a lack of breathable air, or toxic environments. But we're a living example of how some planets, like Earth, can support life.
Have you ever wondered if there is life beyond Earth? Our infographic makes it easy to explore this possibility. Learn more about life in the Universe in pictures.
9. Lifespan
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 mass — the bigger it is, 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.
10. Number in the Universe
Planets outnumber stars in our Universe. It’s estimated that, in the Milky Way alone, the total number of planets (including the rogue planets) exceeds the number of stars by a factor of 100 to 100,000.
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 Sun’s total mass 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.
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.
1. Check whether it twinkles
Stars usually twinkle more noticeably, while planets tend to shine more steadily.
A distant star is effectively a point source, so turbulence in Earth’s atmosphere can rapidly change its apparent brightness and position. Planets are close enough to show tiny disks, even if those disks are too small for the naked eye to resolve. Light arrives from multiple points across the disk, so atmospheric fluctuations tend to average out.
This is a clue, not an absolute rule. A planet close to the horizon can twinkle strongly because its light passes through more turbulent air. A bright star high in the sky may appear relatively steady. Learn more in our guide to why stars twinkle.
2. Consider the brightness
If Venus is visible, it’s the brightest starlike object in the sky. Its magnitude varies from −3 to −4.9, while the brightest star, Sirius, has a magnitude of −1.46. However, as it always remains relatively close to the Sun, Venus appears in the evening or morning sky rather than in the middle of the night.
Jupiter is also exceptionally bright and is often the most prominent object when Venus is absent. Mars can outshine most stars near opposition, but becomes much fainter at other times. Mercury, meanwhile, can be bright but always appears in twilight.
3. Look 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 within or near the ecliptic constellations.
4. Observe the color
A planet’s color can help you identify which one you’re looking at.
Venus appears white, while Jupiter and Saturn have a yellowish tint. Mars is easy to recognize by its reddish color. Mercury usually looks white or pale yellow, although its color can be difficult to distinguish because the planet is often seen low above the horizon in bright twilight.
The two most distant planets are too faint to identify by color with the naked eye. Through binoculars or a telescope, Uranus may show a faint blue-green tint. Neptune requires optical aid and appears pale blue through a telescope.
5. Use magnification
If you have binoculars or a telescope, take a closer look at the object. Even under magnification, stars remain points of light. Some planets, however, appear as tiny disks and may reveal additional details, such as phases, rings, or moons.
Find out what you can observe with binoculars in our guide: Best Night-Sky Objects to See With Binoculars.
6. Use a stargazing app
Use the free Star Walk 2 app to quickly identify whether you’re looking at a star or a planet. Just take out your phone, point it at the sky, and the app will resolve your doubts.
Difference between a star and a planet: bottom line
The key difference between stars and planets is that stars generate their own light, while planets reflect the light of the stars they orbit. This is why distant stars are visible to the naked eye, whereas planets outside the Solar System are not.
From Earth, you can try to distinguish a planet from a star by its color, position in the sky, and how much it twinkles. However, none of these clues is completely reliable on its own. The easiest way to identify an unfamiliar object is to use Star Walk 2, which shows what it is and where to find it in the sky from your location.
