All we can say about most of the 4,000+ known exoplanets is that they exist. Their physical characteristics are unknowable with current technology, but a few have given up some secrets. Astronomers using the Hubble Space Telescope have identified a magnetic field around the exoplanet HAT-P-11b. Earth’s magnetic field is essential for our continued existence, and this is the first time we’ve confirmed one around an exoplanet.
Earth and several other objects in our solar system have magnetic fields, a consequence of the way planets and moons interact with the solar wind. On Earth, the magnetosphere deflects damaging radiation, which would otherwise render the surface inhospitable. Fields surrounding exoplanets could serve a similar purpose. There was every reason to think exoplanets could have magnetic fields like the ones we see locally, but this is the first time we’ve been able to confirm that.
Astronomers from the University of Arizona observed the exoplanet HAT-P-11 b across six transits — that’s when the exoplanet passes in front of its host star from our perspective. This is how the HATNet Project discovered HAT-P-11 b in 2009. It was confirmed and further characterized later using radial velocity measurements from the Keck Observatory, which is the other standard method for detecting distant planets. Although, HAT-P-11 b is relatively close in the grand scheme at just 123 light years away.
Size comparison of Neptune with HAT-P-11b (gray).
Hubble detected a cloud of ionized carbon atoms, a sure sign HAT-P-11 b, which is about the size of Neptune, has an active magnetic field. The particles were not just around the planet, though. The carbon ions extend away from HAT-P-11 b like a tail, reaching about 1 AU (the distance between Earth and the sun). The team speculates the tail is a consequence of the planet’s proximity to its sun. At just five percent of an AU from the star, the upper atmosphere is superheated and ejected into space.
Hubble wasn’t designed with this kind of investigation in mind, but it has still managed to pick up some tantalizing hints of the exoplanet’s composition. HAT-P-11 b is just eight percent of Jupiter’s mass, but has similarly low metal content and a powerful magnetic field for its size. Meanwhile, smaller gas giants in our solar system (like Neptune) have higher metal and weaker magnetospheres. This suggests we’re missing part of the puzzle when it comes to planetary formation. We might get there with The James Webb Space Telescope, which has successfully launched after decades of work. Deployment will be the next major test for the new observatory.
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