The strange behaviour of 3I/ATLAS, the third interstellar object seen in our solar system, has made scientists and sky-watchers curious. Its tail appears and disappears, making it look unusual. Even though it has been labelled a comet, its tail isn’t clearly visible.

But comets aren’t the only things with tails. Earth has one too. And so does Mercury. NASA recently shared that Mercury shows a long tail at certain times, something that has been known for years. Now, experts say that Earth also has a huge tail stretching behind it, on the dark side of our planet.

Scientists estimate that Earth’s tail reaches at least two million kilometres into space.

Mercury’s tail forms because the planet’s surface contains sodium. When Mercury moves closest to the Sun, sunlight and solar radiation push the sodium atoms outward.

NASA explains that scattered sunlight gives the sodium "a bright orange glow." The atoms are pushed away by "radiation pressure", which strips the planet’s thin atmosphere and creates a shining, comet-like tail.

Earth’s tail forms for a very different reason. It is linked to our planet’s magnetic field, also known as the magnetosphere.

NASA says the magnetosphere "dominates the behaviour of electrically charged particles in space near Earth and shields the planet from the solar wind."

This magnetic field also traps plasma, which is a type of charged gas. Some of this plasma flows outward into space and creates Earth’s long tail.

NASA believes this happens because of the solar wind, a constant stream of plasma from the Sun.
When the solar wind hits Earth’s magnetosphere:

  • The front side (day side) is pushed in, like the head of a raindrop

  • The back side (night side) is stretched out, forming a long tail

This return flow of plasma creates what scientists call the “magnetotail.”

Scientists are unsure of its exact size, but they know it is enormous.
The European Space Agency says Earth’s tail extends at least two million kilometres into space on the night side. Some estimates say it could be 1,000 times Earth’s radius.