Astronomers from Wesleyan University have detected the shock waves produced by a high-speed “hot Jupiter” exoplanet caught in a tight orbit around its host star. It’s a potential indication of an incredibly powerful magnetic field around the planet.

Also known as “roaster planets,” hot Jupiters are so named because they have many characteristics in common with the largest gas giant in our solar system, most notably mass. But they have much hotter surface temperatures because they orbit much closer to their parent stars.


HD 189733b is located about 63 light-years from Earth, making it it the closest known transiting hot Jupiter to Earth. It features a mass 13% higher than that of Jupiter, and it orbits its host star once every 2.2 days at a breakneck orbital speed of 152.5 km/s, or 341,000 mph. Given this extreme velocity, astronomers have wondered if this hot Jupiter, like other celestial bodies, produces a bow shock—a disturbed boundary separating a magnetosphere from its surrounding gas. In this case, the hot Jupiter’s magnetosphere is pushing up against the stellar wind and corona produced by its host star.

Given its relative close proximity to Earth, a team of astronomers led by Wilson Cauley of Wesleyan University speculated that the bow shock should emit a “pre-transit signature” before the exoplanet itself moves in front of the host star. In theory, a bow shock should disrupt the optical spectra of the system as seen in high resolution from Earth—a dip that would be noticeable in the hydrogen spectrum, providing a telltale signature.

As Cauley explains at his website:

If the planet is moving supersonically through the stellar wind or coronal plasma, a bow shock will form between the planet and the star at an angle that is determined by the relative velocity of the planet and the plasma. If the planet has a magnetosphere, the bow shock will form where the pressure between the plasma and the magnetosphere balance. For planets with strong magnetic fields, the bow shock can form many planetary radii ahead of the planet in it’s orbit. If the compression of the stellar wind material in the bow shock is high enough, the line-of-sight column density of material in the bow shock between us and the star can be high enough to produce a visible absorption signature in the stellar spectrum. This absorption signature occurs before the planet normally transits the star.

Cauley’s new study, which appears at the Astrophysical Journal, shows that a recently detected signature is consistent with the geometry ahead of HD 189733b.

Here’s a graphical representation of the new study, showing the transit of the bow shock material as it crosses the stellar disk, and the absorption values the model predicts (solid colored lines in the bottom panel). “The parameters in our model that best fit the observed absorption allow us to estimate the magnetic field of the planet,” writes Cauley. “These measurements demonstrate an exciting path forward in attempting to measure exoplanet magnetic fields.” The upper panel shows the plane of the sky perpendicular to our line of sight, while the right side panel shows a top-down view of the planet’s orbit. All of the sizes and distances are to scale. (Video and caption credit: Wilson Cauley)


Their model suggests that a shock is leading the planet at a distance of 12.75 times the planet’s radius, or 125 minutes ahead of the predicted optical transit of the hot Jupiter.

As Susanna Kohler reports in AAS Nova, this suggests the presence of a particularly powerful magnetic field around the exoplanet:

This stand-off distance is surprisingly large. Assuming that the location of the bow shock is set by the point where the planet’s magnetospheric pressure balances the pressure of the stellar wind or corona that it passes through, the planetary magnetic field would have to be at least 28 Gauss. This is seven times the strength of Jupiter’s magnetic field!

As Kohler points out, observations like this will help astronomers model the interiors of planets, and help them to better understand their mass loss rates and how they interact with their host stars.

More at AAS Nova, and be sure to read the entire study at the Astrophysical Journal: “Optical hydrogen absorption consistent with a thin bow shock leading the hot Jupiter HD 189733b”. The pre-print version can be found here.

Email the author at and follow him at @dvorsky. Top image by NASA/ESA/A. Schaller