You Can't Know the True Size of an Exoplanet Without Knowing its Star's Magnetic Field

Artist's impression of a "hot Jupiter" orbiting close to a Sun-like star. Credit: NASA

In 2011, astronomers with the Wide Angle Search for Planets (WASP) consortium detected a gas giant orbiting very close to a Sun-like (G-type) star about 700 light-years away. This planet is known as WASP-39b (aka. “Bocaprins”), one of many “hot Jupiters” discovered in recent decades that orbits its star at a distance of less than 5% the distance between the Earth and the Sun (0.05 AU). In 2022, shortly after the James Webb Space Telescope (JWST) it became the first exoplanet to have carbon dioxide and sulfur dioxide detected in its atmosphere.

Alas, researchers have not constrained all of WASP-39b’s crucial details (particularly its size) based on the planet’s light curves, as observed by Webb. which is holding up more precise data analyses. In a new study led by the Max Planck Institute for Solar System Research (MPS), an international team has shown a way to overcome this obstacle. They argue that considering a parent star’s magnetic field, the true size of an exoplanet in orbit can be determined. These findings are likely to significantly impact the rapidly expanding field of exoplanet study and characterization.

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A Hot Jupiter With a Comet-Like Tail

The hot jupiter exoplanet WASP-69b orbits its star so closely that its atmosphere is being blown into space. Researchers made detailed observations of the planet, located about 160 light-years from Earth. They found that it has a comet-like tail extending about 560,000 km into space, about seven times the planet's diameter. Image Credit: Adam Makarenko/W. M. Keck Observatory

About 164 light-years away, a Hot Jupiter orbits its star so closely that it takes fewer than four days to complete an orbit. The planet is named WASP-69b, and it’s losing mass into space, stripped away by the star’s powerful energy. The planet’s lost atmosphere forms a trail that extends about 560,000 km (350,000 miles) into space.

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