What would Earth look like to aliens?

To an alien civilization, Earth’s appearance would depend a lot on how far away they are and what kinds of instruments they use.

Up close, from a nearby spacecraft, Earth would likely look like a vibrant pale blue dot: blue oceans, swirling white clouds, green and brown landmasses, and bright polar ice.

From several light-years away, Earth would just be a tiny unresolved point of light. But with advanced spectroscopy and careful monitoring, aliens could still infer a lot:

• Atmospheric biosignatures: they might detect gases such as oxygen, water vapor, carbon dioxide, and methane. The coexistence of oxygen and methane is especially interesting because those gases tend to react away unless something is continually replenishing them.
• The red edge: Earth’s vegetation reflects strongly in the near-infrared, so they could potentially pick up the vegetation “red edge” in our spectrum.
• Ocean glint and rotation: changes in brightness as Earth rotates, plus glints from sunlight off oceans, could reveal continents, clouds, and water.

If they were listening for artificial signals, Earth might also stand out as technologically active. Human radio emissions have been leaking into space since the mid-20th century, creating a radio environment around Earth that extends outward by many light-years, but the exact range and detectability depend on the signal type, directionality, and the receiver’s sensitivity.

So the short version is: up close, Earth would look like a blue, cloud-covered water world; from far away, it would mainly be inferred through its atmosphere, reflectance, and possible technosignatures rather than seen in detail.