Stars are points, planets are discs
The nearest star after the Sun, Proxima Centauri, sits roughly 40 trillion kilometres away, about 4.2 light-years. From Earth, even the largest telescope can’t show it as anything other than a point: a single spark with no measurable size.
The planets of our solar system are vastly closer, minutes of light away rather than years. Jupiter, Mars and Venus are near enough to appear as tiny discs, hard to resolve with the naked eye but easy to measure through a telescope. That difference in scale is what changes everything.
The air gives the light away
The atmosphere above your head is never still. Layers of warm and cool air drift, mix, and form pockets with slightly different densities. Each pocket bends the incoming light by a hair, like a faint, shifting lens. Astronomers call this atmospheric seeing.
When a star’s light (that single point) crosses some hundred kilometres of turbulent air, it’s nudged constantly. Tenth of a second by tenth of a second, the point is pushed a little to the right, a little to the left, a little brighter, a little dimmer. That trembling is what we see as twinkling. The star itself hasn’t moved — it’s the air between you and it that’s dancing.
Planets hold steady
For a planet, the same atmosphere does the same work, with a different result. Because a planet reaches us as a small disc, each point on that disc gets its own deviation. Some are pushed right, some left, some brightened, some dimmed. Averaged across the whole disc, the variations cancel out. The disc stays steady, the light arrives smooth.
That’s the trick for telling a planet from a star at night: if the point shines with a steady glow, it’s probably a planet. If it trembles, it’s a star.
Modern observatories fight this in several ways: by sitting at altitude where the atmosphere is thinner (the observatories of Chile, of Hawaii), by correcting the distortion in real time with deformable mirrors (adaptive optics), or by leaving the atmosphere altogether: which is why Hubble, in orbit since 1990, and the James Webb, parked further out still, deliver images sharper than anything Earth’s surface will ever allow.
