Stare up at the sky on a clear evening and you will see about 2,000 twinkling stars with the naked eye. Along with this field of flickering lights, several of the planets in our solar system can easily be seen without magnification. At a glance, they look like ordinary stars, but unlike stars, they do not appear to twinkle.

Stars are glowing balls of hot gas that are typically millions of kilometres in diameter. Since they are extremely far away, they only appear as tiny points of light on Earth. Even when viewed with our best telescopes, virtually all stars look like pinpoints of light without a measurable size.

On the other hand, the planets in our solar system are much smaller than most stars, but they are much closer. This allows them to be viewed as disks of light with visible details under just a small amount of magnification by using binoculars or amateur telescopes.

When we look at a star, we must also stare through Earth’s atmosphere. According to Professor Norm Murray of the Canadian Institute for Theoretical Astrophysics (CITA), “[Light] gets bent around a bit by the air in the atmosphere, and twinkling comes about because the amount of bending depends on the conditions in the air between us and the star.”

The greatest influence on the bending of starlight in our atmosphere is temperature. Our atmosphere is a violent dance of warm and cool air patches. This effect continually produces changes in the index of refraction in the air. The index of refraction is the amount that light bends as it changes media—like the way light bends when it passes through water. The index of refraction of air depends on its temperature.

“What happens is that the image gets shifted around on the sky, and it gets focused and defocused,” said Murray. “When it gets focused, the star looks brighter, and when it gets defocused, the star looks dimmer.”

This is the same effect that causes us to see shimmering images above asphalt roadways on a hot summer’s day.

But what about planets?

“The patches of sky that cause twinkling are fairly small and a planet’s [image] actually covers a big enough angle in the sky that you would see different parts of the planet through different patches. You get an average value, which doesn’t vary as much as if you had a point source,” explained Murray.

Each ray of light we receive from a nearby planet twinkles as much as any star does. However, since planets are much larger in the sky than a star, we receive many more rays of lights from the planet. The total effect from all of the individual rays is a steady image.

Murray concludes, “Planets do twinkle, but just not noticeably.”