Astronomers use several techniques to discover planets in other star systems. By far the most prolific is the transit method. It looks for a planet to pass in front of a star, causing the star’s light to fade by a tiny bit. Planet-hunting space telescopes have used this technique to discover thousands of exoplanets.

A transit takes place in our own solar system early today. Mercury, the innermost planet, will pass across the face of the Sun. All or most of the transit will be visible across all of the United States except parts of Alaska.

You might wonder if astronomers in other star systems could detect Mercury’s transit. It all depends on the sensitivity of their instruments.

Our transit-hunting telescopes can detect a tiny drop in a star’s light — in many cases, less than a hundredth of a percent. Mercury’s transit will produce a smaller drop than that — about three-thousandths of a percent. So it would take really sensitive instruments to detect such a tiny change.

If they do see the transit, alien astronomers could use it to determine Mercury’s size, its distance from the Sun, and how long it takes to orbit the Sun. And when combined with other techniques, they could learn Mercury’s mass, density, and much more.

We recommend that you watch Mercury’s transit online or at a public venue that’s displaying it. Don’t look at the Sun directly, though — you need eye protection to gaze upon this rare astronomical alignment.

Tomorrow: a “double” foot.

Script by Damond Benningfield