How Do Astronomers Measure Wind Speeds on Other Planets?

Jupiter's Great Red Spot has been spinning for at least 350 years. Neptune's winds scream at more than 1,200 miles per hour, faster than the speed of sound on Earth. Saturn's polar hexagon, a six-sided storm wider than our entire planet, has been churning at least since we first spotted it in the 1980s.

We've never put a weather station on any of these worlds. So how do astronomers know how fast the winds are blowing on other planets?

The answer is a mix of physics, cutting-edge technology, and some surprisingly old-school detective work.

Tracking Cloud Features Over Time

The most intuitive method is one we’ve been doing since long before we could see other planets. Astronomers watch clouds to see where they go.

Planets like Jupiter are covered in distinct cloud bands and storm systems. By taking images of the same region at precise intervals, scientists can track how far a feature has moved. Divide the distance by the time, and you have a wind speed.

This technique, called cloud tracking, has been used since the Voyager flybys of the 1970s and 1980s and is still a workhorse method today. The Hubble Space Telescope, the Cassini spacecraft at Saturn, and the Juno mission at Jupiter have all produced stunning time-lapse datasets that reveal wind patterns in extraordinary detail.

The challenge is that scientists need a cloud feature that's stable enough to track. On a world like Neptune, where storms appear and disappear rapidly, this requires careful image selection.

Doppler Spectroscopy

If you've heard of the Doppler effect, you know that waves shift in frequency depending on whether the source is moving toward you or away from you. That’s what you observe when an ambulance siren drops in pitch as it passes. The same principle applies to light.

When astronomers point a spectrograph at a planet's atmosphere, they can measure the Doppler shift of specific spectral lines, signatures of particular molecules like methane, ammonia, or hydrogen. If one side of the planet's atmosphere is moving toward Earth (winds blowing in our direction), those lines shift toward shorter wavelengths (blueshift). If it's moving away, they shift red.

By comparing the shifts across different regions of a planet's disk, scientists can build a detailed map of wind velocities. This method is especially powerful for measuring winds in layers of the atmosphere that aren't visible in ordinary images, because different molecules absorb and emit light at different altitudes.

Radio Waves and Thermal Emission

Gas giants also emit radiation we can't see with our eyes. Jupiter, for instance, is a powerful source of radio waves. Some are generated by its intense magnetic field, others by thermal processes in its deep atmosphere.

By analyzing the subtle variations in microwave and radio emission, scientists can infer temperature gradients. Temperature differences drive wind. Juno's microwave radiometer has been especially revolutionary, allowing researchers to peer beneath Jupiter's visible cloud tops and study wind patterns extending hundreds of miles into the planet's interior. Some of those deep winds, it turns out, run even faster and deeper than anyone expected.

Atmospheric Probes

In 1995, NASA took a more direct approach. The Galileo spacecraft dropped a probe into Jupiter's atmosphere. It was the only time we've ever directly sampled the interior of a gas giant's weather system. The probe survived for about an hour before being crushed by pressure, but in that time it transmitted wind speed measurements directly back to Galileo in orbit.

The results showed winds of around 400 mph, far stronger than models had predicted, and no clear evidence of a weather layer powered by the Sun. Jupiter's storms are likely driven from within, according to Cornell astronomers.

Future missions, including proposals for ice giant probes at Uranus or Neptune, aim to repeat this feat on worlds we understand even less.

Stellar Occultations

If you have the right stargazing equipment, wait for a planet to pass in front of a distant star, and watch what happens to the starlight as the planet's atmosphere crosses it.

As the star's light filters through different layers of the atmosphere, it bends and dims in ways that reveal density and temperature profiles at various altitudes. Temperature gradients can then be used to infer wind speeds through a technique based on the thermal wind equation, a relationship from fluid dynamics that connects temperature differences to atmospheric flow.

This method has been used successfully on Neptune, Uranus, and even Pluto.

Understanding how gas giants circulate heat, energy, and material has direct implications for our models of how solar systems form and evolve, which is also useful in understanding the thousands of gas giant exoplanets we've discovered orbiting other stars.

When the James Webb Space Telescope turns its instruments toward a distant hot Jupiter, the same physics that governs Jupiter's cloud bands applies to those alien skies. Every wind speed measurement we make in our own solar system sharpens the tools we use to decode worlds we'll never visit.

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Written by- Matt Herr