HOW COME? Is it a planet -- or star?

How come planets look like stars in the night sky? asks Emma Pinezich, a student in Brookville?Planet, or star? It's easy to figure out when it's Venus, huge and dazzling in the twilit western sky on a winter evening. But what about the other four planets visible to the naked eye, lurking against the background of stars? How do we tell, say, Saturn from Sirius?

Planets shine like stars because they reflect their own nearby star's light, making it appear as if they're producing their own. And while planets are much smaller than the average star (a million Earths would fit into our Sun), the stars are so distant that planets in our own solar system look similarly sized.

Of course, stars are intrinsically much brighter than planets. But their distance (the next-nearest star to the Sun is 24 trillion miles away) means their light has dimmed significantly by the time it reaches us. Take our big, round Sun and move it trillions of miles away, and it too will be just a point of light in the dark.

So it's not surprising that the five planets visible from Earth with the naked eye -- Mercury, Venus, Mars, Jupiter and Saturn -- can pass for bright stars. However, there are clues to their real identities. Observe over the course of many nights, and the planets will seem to wander among the constellations of stars. Which is why the word "planet" comes from an ancient Greek word, meaning "wanderer."

But there's an easier way to tell you're looking at a planet instead of a star: Stars twinkle, while planets usually shine with a steady light.

How come? Stars twinkle because we are looking at them through our planet's turbulent atmosphere. All day long the ground heats in the sun, and all night long, the Earth radiates stored heat into space. As air just above the ground warms and rises, it mixes with cooler air, creating roiling motion.

Starlight, on its way to the ground, passes through cooler, denser air into warmer, thinner air. As the light makes it way through Earth's unsteady air, it's bent this way and that by its interactions with gas molecules.

So as you look at a star from your vantage point on the ground, its light seems to spark and jump, and appears to dim and brighten. That's because light rays get alternately bunched up (causing brightening) and spread out again (causing dimming) by the swirling air.

Stars are so far away that they appear as points of light. But planets are close enough that, through a telescope, they look like discs. While light from different parts of the disc is shifting like starlight on its journey through Earth's air, the twinkling effects from across the disc average out. Leaving a nearly steady light.