The Earth sits in a stream of accelerated particles coming in from the Sun, interstellar material, and galactic sources.
Instead, they could be interstellar rovers captured by the Sun's gravity.
The greatest challenge of interstellar travel is the enormous distances between stars.
The solar system formed 4.6 billion years ago from an interstellar cloud of gas and dust.
Most will crash into the Sun, strike another planet, or be flung by Jupiter's gravity into interstellar space.
Some scientists tend to scoff at the idea of interstellar travel because of the enormous distances that separate the stars.
More dark clouds of interstellar dust with intriguing shapes are seen silhouetted against the glowing interstellar gas at the top right.
They may not exit the comet clouds and into proper interstellar space for up to another 40,000 years after that.
Very few species had achieved interstellar travel at this time in the Galaxy.
Technologies based on resources drawn from all over the Solar System will make interstellar trips affordable.