Using Hubble Space Telescope ultraviolet data, researchers have identified the first signature of carbon ions surrounding exoplanet HAT-P-11b, a Neptune-sized planet 123 light-years from Earth. The charged carbon ions trace the extent of the exoplanet’s magnetosphere, which is sculpted by the host star’s stellar wind.
Charged Carbon Ions
The finding, described in a paper in the journal Nature Astronomy, marks the first time such a feature has been seen on an exoplanet. The team obtained ultraviolet spectra with Hubble to characterize the atmosphere and periphery of HAT-P-11b as the exoplanet transited the face of its host star six times.
A magnetic field best explains the observations of an extended region of exoplanet HAT-P-11bs that surround the planet and stream away from it in a long tail, reports the University of Arizona. Magnetic fields play a crucial role in protecting planetary atmospheres, so the ability to detect the magnetic fields of exoplanets is a significant step toward better understanding what these alien worlds may look like.
In the illustration below, the planet is depicted as the small circle near the center. Carbon ions fill an immense region around it. In the magnetotail, not shown to its full extent, ions escape at the observed average speeds of about 100,000 mph. 1 AU equals the distance between the Earth and the sun. Credit: Lotfi Ben-Jaffel/Institute of Astrophysics, Paris.
Protection from Deadly Solar Winds?
“This is the first time the signature of an exoplanet’s magnetic field has been directly detected on a planet outside our solar system,” said Gilda Ballester, principal investigator and adjunct research professor at the University of Arizona Lunar and Planetary Laboratory and one of the paper’s co-authors. “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.”
The discovery of HAT-P-11b’s magnetosphere is a significant step toward an improved understanding of the habitability of an exoplanet. Not all planets and moons in our solar system have their own magnetic fields, and the connection between magnetic fields and a planet’s habitability still needs more study, according to the researchers.
“HAT-P-11 b has proven to be a very exciting target, because Hubble’s UV transit observations have revealed a magnetosphere, seen as both an extended ion component around the planet and long tail of escaping ions,” Ballester said, adding that this general method could be used to detect magnetospheres on a variety of exoplanets and to assess their role in potential habitability.
A key discovery was the observation of carbon ions not only in a region surrounding the planet, but also extending in a long tail that streamed away from the planet at average speeds of 100,000 mph. The tail reached into space for at least 1 astronomical unit, the distance between Earth and sun.
Researchers led by the paper’s first author, Lotfi Ben-Jaffel at the Institute of Astrophysics in Paris, then used 3D computer simulations to model interactions between the planet’s uppermost atmospheric regions and magnetic field with the incoming solar wind.
Physics in the Magnetospheres of Earth and HAT-P-11b
“Just like Earth’s magnetic field and its immediate space environment interact with the impinging solar wind, which consists of charged particles traveling at about 900,000 mph, there are interactions between HAT-P-11b’s magnetic field and its immediate space environment with the solar wind from its host star, and those are very complex,” Ballester explained.
The physics in the magnetospheres of Earth and HAT-P-11b are the same; however, the exoplanet’s close proximity to its star—just one-twentieth of the distance from the Earth to the sun—causes its upper atmosphere to warm and essentially “boil off” into space, resulting in the formation of the magnetotail.
Researchers also found that the metallicity of HAT-P-11b’s atmosphere—the number of chemical elements in an object that are heavier than hydrogen and helium—is lower than expected. In our solar system, the icy gas planets, Neptune and Uranus, are rich in metals but have weak magnetic fields, while the much larger gas planets, Jupiter and Saturn, have low metallicity and strong magnetic fields. HAT-P-11b’s low atmospheric metallicity challenges current models of exoplanet formation, the authors say.
Mystery of Jupiter’s Enormous Magnetic Field
“Although HAT-P-11b’s mass is only 8% of that of Jupiter, we think the exoplanet more resembles a mini-Jupiter than a Neptune,” Ballester said. “The atmospheric composition we see on HAT-P-11b suggests that further work needs to be done to refine current theories of how certain exoplanets form in general.”
How Jupiter generates its powerful magnetic field, the strongest in the solar system, is a mystery. One theory is that about halfway to Jupiter’s core, the pressures and temperatures become so intense that the hydrogen that makes up 90 percent of the planet loses hold of its electrons and begins behaving like a liquid metal. Oceans of liquid metallic hydrogen surrounding Jupiter’s core would explain its powerful magnetic field. At high temperatures and pressures – the conditions that exist within planets and perhaps exoplanets like Jupiter – hydrogen takes on the properties of a liquid metal and behaves like an electrical conductor.
The Last Word
“This exoplanet is a poor candidate for habitability,” wrote Gilda Ballester in an email to The Daily Galaxy. “It is a gas planet so it does not have a surface. It is at about 0.05 AU from a mid-K star so it is a warm exoplanet, with a temperature is about 800 K in the lower atmosphere.
“The planet is like a mini-Jupiter in two aspects: its atmospheric metallicity is low (that of Jupiter and Saturn is ~5 and that of Neptune and Uranus is ~ 100), and has a strong versus weak magnetic field (surface field strength of a few Gauss like Jupiter vs at least x10 weaker in Uranus and Neptune). Keep in mind that mass is small, 8% of Jupiter’s mass (only about x1.5 larger than Neptune) so interiors should be different.”
Source: Lotfi Ben-Jaffel et al, Signatures of strong magnetization and a metal-poor atmosphere for a Neptune-sized exoplanet, Nature Astronomy (2021). DOI: 10.1038/s41550-021-01505-x
Image credit top of page: Artist’s impression of the exoplanet HAT-P-11b and its host star.
Harvard Center for Astrophysics/D. Aguilar