You can tell whether a train or a police car is moving toward or away from you just by listening. Its horn or siren changes pitch – higher if it’s moving toward you, lower if it’s moving away.

The same principle applies to the stars. Their light is shifted to longer or shorter wavelengths. Measuring that shift reveals the star’s motion – its “radial velocity.”

An example is Pollux, the brightest star of Gemini. It’s close to the Moon at first light tomorrow. Its radial velocity tells us that the star is moving toward us at more than 7,000 miles per hour. That sounds fast. But at the star’s distance of almost 34 light-years, it would take millions of years to reach us. Pollux’s orbit is also carrying it sideways, so the star will never even get close.

By measuring its radial velocity, astronomers at McDonald Observatory discovered a planet orbiting Pollux. As it orbits, the planet tugs at the star, changing its motion toward us by a few miles per hour.

Precise measurements of that change revealed some details. The planet is a bit more than twice as massive as Jupiter, the giant of our own solar system. It’s a little farther from Pollux than Mars is from the Sun. And it orbits the star once every 19 months.

Astronomers named the planet Thestias – a version of Leda, the mother of Pollux – a planet discovered by measuring a tiny shift in the star’s light.

Script by Damond Benningfield