Most exoplanets are found with the transit method: when a planet passes in front of its star, the light dips very slightly. To a well-placed alien observer, Earth would dim the Sun by only about 0.008%, once a year, for some thirteen hours. Even so, the Kepler telescope hunted for signals like that across 150,000 stars at once.
A large share of Earth's water molecules formed in the interstellar cloud that existed before the Sun was even born, more than 4.6 billion years ago. You are literally drinking a cosmic fossil.
By day, the Martian sky looks butterscotch because of fine dust hanging in the air. But at dusk, those same grains scatter blue light forward, creating a bluish glow around the Sun. Rovers have photographed these blue sunsets: the exact reverse of what we see on Earth.