Stars Twinkle but Planets Do Not

Super fact 102: Stars twinkle but planets do not. Planets reflect sunlight, like the moon, whereas stars emit intense light like the sun. However, stars are thousands of times further away.

Bright white star with a planet and a moon. | Stars Twinkle but Planets Do Not
The light from stars is intense allowing stars to have much smaller discs in the sky for the same amount of light as a planet. Shutter Stock Illustration ID: 566774353 by Nostalgia for Infinity.

The Sun and the Moon appear to have roughly the same sized disk in the sky, about 0.5 degrees of arc, because the Sun is roughly 400 times wider than the Moon but also about 400 times farther away. However, the moon, which reflects sunlight but does that emit any, is much fainter than the sun in our sky. The Sun provides approximately 400,000 times more light to Earth than the full Moon. If you were to move the sun farther away until it provided the same amount of light as the moon you would have to move the sun 632 times farther away until it became a tiny point in comparison to the moon.

The planets in our solar system, Venus, Jupiter, Mars, Mercury, Saturn, etc., appear in the sky as small discs reflecting light whereas the stars appear as intense points of light with a disc diameter that is thousands of times smaller than that of the planets. However, we can’t see the difference with the naked eye. Both planets and stars appear like points of lights to us, but the difference in disc size in the sky is very big.

The picture shows a big bright star (but much smaller than the sun) shining down on planet Earth lighting up the planet a bit. The light from the star is intense. | Stars Twinkle but Planets Do Not
Planets reflect light whereas the stars appear as intense points. Shutterstock asset id: 2709145593 by buradaki

Because stars are tiny intense points of light Earth’s atmosphere can easily refract (bend) their light. This is known as scintillation. I can add that the light from stars that are low in sky go through more atmosphere and therefore twinkle more. Since planets appear as tiny discs rather than tiny points, planets create a steadier beam of light, which averages out atmospheric turbulence. In addition, dust particles in space and in the atmosphere can more easily block the light from stars compared to that of planets, because the planets have much larger discs in the sky (thousands of times larger).

In the picture there is a disc representing a planet and a little yellow star and a small piece of space dust. The text says, “In a telescope the planet looks like a little disc whilst the star is still a bright point. However, to the naked eye both look like a star. Guess which one the space dust can make to twinkle by passing in front of it.”
To the naked eye a planet and a star point look light points of light. In a telescope the planet will turn into a disc, but the star will remain a bright point of light. For example, a piece of space dust can easily make the star twinkle by moving in front of it but won’t do that to the planet. Note: planets are much smaller than stars, but they are much closer and can look bigger.

Since this fact is not scientifically under question, a lot of people don’t know it, and it is kind of important to anyone interested in the night sky, I consider it a super fact. I should say that I borrowed this super fact from Jacqui Murray’s blog worddreams.

Planets Reflect Light like the Moon

Planets reflect light. They don’t shine and emit light like the sun or the stars.

A colorful photo of Jupiter (orange and white) including the Great Red Spot.
Jupiter in true color, taken by the Hubble Space Telescope in January 2024
A photo of Saturn including the impressive rings.
Saturn and its prominent rings, as captured by the Cassini orbiter. This natural color view of the planet Saturn was created from images collected shortly after Cassini began its extended Equinox Mission in July 2008.
This picture features the photo of Pluto taken by NASA’s New Horizons spacecraft in 2015 plus some text. | Stars Twinkle but Planets Do Not
Pluto from NASA/JHUAPL/SwRI. NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute, Public domain, via Wikimedia Commons. NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto in 2015.
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Author: thomasstigwikman

My name is Thomas Wikman. I am a software/robotics engineer with a background in physics. I am currently retired. I took early retirement. I am a dog lover, and especially a Leonberger lover, a home brewer, craft beer enthusiast, I’m learning French, and I am an avid reader. I live in Dallas, Texas, but I am originally from Sweden. I am married to Claudia, and we have three children. I have two blogs. The first feature the crazy adventures of our Leonberger Le Bronco von der Löwenhöhle as well as information on Leonbergers. The second blog, superfactful, feature information and facts I think are very interesting. With this blog I would like to create a list of facts that are accepted as true among the experts of the field and yet disputed amongst the public or highly surprising. These facts are special and in lieu of a better word I call them super-facts.

42 thoughts on “Stars Twinkle but Planets Do Not”

  2. I had no idea the planets in our solar system appear as small discs that reflect light. I heard somewhere (in the media, I think) that Venus shines like a star, which just shows how misonformed people are! Thanks for sharing this interesting post.

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    1. Yes the planets look like stars to the naked eye but if you use a telescope, even a small one, or binoculars, you see that they look little moons in the sky. However, the stars will still look like points, only brighter points, even in a strong telescope. Venus is often called the evening star when you see it in the west in the evening, and it is often called the morning star when you see it in the east in the evening. However, it is of course not a star, but a planet. Those are nick names. Since the Venus is closer to the sun than Earth you tend to see it closer to the sun and never right above in the sky, except for during a total solar eclipse (I’ve seen that).

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            1. Ha ha. IAU voted in 2006 to classify Pluto as a dwarf planet rather than just a “planet”, but it was far from unanimous and the planetary scientists (different from astronomers) were upset. It is a lot more controversial than one might think. Basically, at first a planet was considered an object that orbited the sun but was big enough to squeeze itself into a sphere (or closely a sphere). What happened was that a third condition was added, that it had to clear its neighborhood/orbit of debris. That third condition is true for Mercury, Venus, Earth,, Mars, Jupiter, Saturn, Uranus, and Neptune, but not for Pluto. So it was reclassified as a dwarf planet instead of a planet.

              There are advantages and disadvantages to the decision. Pluto is part of the Kuiper belt and there are probably a lot of Kuiper belt objects that are like Pluto but haven’t been discovered yet so it would complicate things if they all would be considered planets. That’s an advantage of the new definition. A disadvantage of the new definition is that there are many very large exo-planets (planets orbiting other stars), larger than Jupiter that have not cleared their neighborhood of debris, so applying that third condition to exo-planets would make these “giant planets”, not planets anymore. It is an interesting drama.

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  5. This is just as well, it would be a little bit tougher to do the poem otherwise. Hmm … “Twinkle twinkle little planet, you are probably made of granite. Aliens up in the skies, looking down with envious eyes.” Wow that went right to War of the Worlds, didn’t it?

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    1. Thank you so much Debbie. An interesting coincidence is that the NASA space probe that took the Pluto photo, New Horizons, was the creation of Alan Stern, who went to the same school as my two boys, St.Marks School of Texas. Alan Stern came back to the school and spoke to the kids about Pluto and the mission.

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  9. After all the science fiction I’ve watched, you’d think I’d know this. But in all honesty, I thought it was the tree branches making the stars appear to twinkle. Now that we have more open space in our backyard, I’m going to be watching for those twinkles

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  10. One of the earliest Superfacts I learned on my path to being an astronomer! Related to this is that if the atmosphere is very calm and still, stars twinkle much less than they do when the atmosphere is very agitated. So on ideal nights for ground-based astronomy, the stars don’t twinkle as much as they do on windier more turbulent nights.

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      1. Yes, in fact “delivered image quality,” which is a measurement of how much the turbulent atmosphere is affecting the starlight, is something we pay close attention to and report on each night of our observations. In short, how much the stars twinkle affects the quality of our data, so it’s an important thing for us to measure.

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  15. Long ago a friend of mine bought a telescope and invited me over to check it out. He pointed it at a star. Turns out that “star” was actually a galaxy! It was just so far away it looked like a single point of light to the naked eye. That blew my mind!

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  16. Pingback: Stars Twinkle but Planets Do Not – Côté lumière, light side

  17. Just a question. Compared to yellow stars, do blue stars twinkle more and red stars twinkle less?
    Also, planets closer to their stars reflect more brightly, while planets farther from their stars reflect more faintly, do they not? Therefore, Saturn ought to reflect more faintly than Jupiter, and Mars more faintly than Venus or Mercury, right? Neptune ought to reflect more faintly than Uranus.

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    1. Great questions! The wider apparent angular diameter that red giants have in the night sky will make them twinkle less. I admit I had to look up that one. But the second question I knew. Yes planets that are closer to the sun reflect more per angular diameter in the night sky, assuming the material is the same. Venus has a large apparent angular diameter in the night sky (close to us), it is relatively close to the sun and therefore get more light, and the thick white atmosphere is also very reflectant. That is why it is so bright in the night sky.

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