Without its magnetic field, life on Earth would be impossible, or at least a shadow of what we know. The disappearance of the Martian magnetic field is a big part of the reason it lost its atmosphere, making hunting for planetary magnetic fields a big part of the search for life beyond the Solar System. Now, for the first time, astronomers are confident they've found one.
Hubble space telescope observations of the planet HAT-P-11b reveal a stream of charged carbon particles surrounding it and forming a comet-like tail pointing away from its star. In Nature Astronomy, the scientists who made the discovery argue this represents compelling evidence for the presence of a strong magnetic field.
"This is the first time the signature of an exoplanet's magnetic field has been directly detected on a planet outside our solar system," Professor Gilda Ballester of the University of Arizona said in a statement. "A strong magnetic field on a planet like Earth can protect its atmosphere and surface from direct bombardment of the energetic particles that make up the solar wind. These processes heavily affect the evolution of life on a planet like Earth because the magnetic field shelters organisms from these energetic particles."
With a sample of only eight planets to work with (sorry Pluto fans) no one knows why some have strong magnetic fields and others don't. Every extra example takes us closer to learning how common such fields are, and where they are likely to be found. Yet the paper describes efforts to detect fields around so-called “hot Jupiters” as “inconclusive”, enhancing the importance of this discovery.
The observations were possible because HAT-P-11b passes across the face of its star, as seen from Earth, every five days, giving astronomers a regular chance to observe the effects of the particles on starlight. In this case, Hubble used its capacity to see into the ultraviolet to detect the presence of those particles in the magnetosphere around the planet and a long tail behind. The tail represents escaping ions pushed away from the star by its stellar wind at speeds of 160,000 kilometers per hour (100,000 miles per hour). The tail could be detected for a distance of an astronomical unit (AU) – the space between the Earth and Sun.
HAT-P-11b is slightly larger than Neptune and hotter than Venus so it is not a likely location for life. At 123 light-years away, it's much harder to study than our nearest neighboring planets. Nevertheless, it's attracted more attention from astronomers than most among the thousands of known planets.
At one point it was the smallest exoplanet to have had specific molecules detected in the atmosphere – and better still one of those was water. Five years ago radio signals coming from HAT-P-11b were attributed to frequent powerful lightning strikes creating hydrogen cyanide in its atmosphere.
We don't observe a similar phenomenon in Earth's magnetosphere because we are much more distant from our parent star. HAT-P-11b is just a twentieth of an AU from HAT-P-11, causing its upper atmosphere to boil into space.
HAT-P-11b's atmosphere has relatively low concentrations of elements heavier than helium, making it more like Jupiter or Saturn in composition than Neptune. "Although HAT-P-11b's mass is only 8 percent of that of Jupiter, we think the exoplanet more resembles a mini-Jupiter than a Neptune," Ballester said. That will force a rethink of standard models of large planet formation.