Supersonic Winds Blowing on an Extreme Exoplanet

The exoplanet WASP-127b is an unusual world. It is about 30% larger than Jupiter but has just a fifth of Jupiter’s mass. It is an example of a super-puff planet because of its extremely low density. These puffy worlds are so unusual that we don’t know if they would resemble the gas giants of our solar system, or something more exotic, such as a large super-Earth. But a recent study of WASP-127b shows that super-puff worlds can have tremendous winds.

The team used the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES+) on ESO’s Very Large Telescope to watch the world as it passed in front of its star. Since some of the starlight passes through the planet’s atmosphere with each transit, the team could capture the absorption spectra of the atmosphere. From this, they identified some molecules in the atmosphere that we would expect, such as carbon monoxide and water vapor, which are present in the atmospheres of our gas planets.

But the team also found a surprise. As the planet began a transit, the spectra of the atmosphere were red shifted, and as it ended the transit, the spectra were blue shifted. By itself, this isn’t unusual, as it simply indicates that the atmosphere is moving through some kind of wind. But when the team calculated the speed of the wind, they found it was nearly 9 km/s, or about 33,000 km/hr (more than 20,000 mph for those in the states). That’s ten times faster than the peak winds of Neptune, which are the fastest winds in the solar system.

This rapid motion isn’t due to a fast rotation of the planet itself. WASP-127b orbits its star very closely, so it is almost certainly tidally locked. This means it rotates on its axis once for every orbital period, which is a bit more than four hours. The winds are six times faster than that. They are, in fact, the fastest wind speeds ever observed in the Universe.

The planet’s locked rotation and low density may explain the world’s intense winds. Since one side of the planet is forever in light and the other is always in darkness, there would likely be a large temperature difference between the two sides. Just as cold and warm regions of Earth’s atmosphere can generate strong winds, so would winds arise on WASP-127b. But with a constant temperature differential and a diffuse atmosphere, the winds on this puffy planet are like nothing we’ve ever experienced.

Reference: Nortmann, L., et al. “CRIRES+ transmission spectroscopy of WASP-127b. Detection of the resolved signatures of a supersonic equatorial jet and cool poles in a hot planet.” arXiv preprint arXiv:2404.12363 (2024).

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