Using ultraviolet data from the NASA/ESA Hubble Space Telescope, astronomers have detected carbon ions — charged particles that interact with magnetic fields — in the magnetosphere of the Neptune-sized exoplanet HAT-P-11b and found that the metallicity of its atmosphere is lower than expected.
An artist’s impression of the exoplanet HAT-P-11b with its extended helium atmosphere blown away by the star, an orange dwarf star smaller, but more active, than the Sun. Image credit: Denis Bajram.
HAT-P-11b is located approximately 122 light-years away in the constellation of Cygnus.
Discovered in 2009, the planet is four times the radius of Earth and about 26 times Earth’s mass.
Also known as Kepler 3b, it orbits very close to its host star, the 10th magnitude K-type star HAT-P-11, and therefore is incredibly hot, with temperatures around 605 degrees Celsius (1,100 degrees Fahrenheit).
HAT-P-11b probably has a rocky core, wrapped in a thick, gaseous envelope of about 90% hydrogen.
“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 Dr. Gilda Ballester, an astronomer at the University of Arizona Lunar and Planetary Laboratory.
“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.”
Hubble’s observations of an extended region of charged carbon particles that surround the exoplanet HAT-P-11b and streaming away in a long tail are best explained by magnetic field, the first such discovery on a planet outside of our Solar System. The planet is depicted as the small circle near the center. Carbon ions fill an immense region it. In the magnetotail, not shown to its full extent, ions escape at the observed average speeds of about 161,000 kmh. Image credit: Lotfi Ben-Jaffel / Institute of Astrophysics, Paris.
Using Hubble, Dr. Ballester and colleagues observed carbon ions not only in a region surrounding HAT-P-11b, but also extending in a long tail that streamed away from the planet at average speeds of 161,000 kmh (100,000 mph).
The tail reached into space for at least 1 AU (astronomical unit), the distance between Earth and the Sun.
The astronomers then used 3D computer simulations to model interactions between the planet’s uppermost atmospheric regions and magnetic field with the incoming solar wind.
“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 1.45 million kmh (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,” Dr. Ballester said.
“The physics in the magnetospheres of Earth and HAT-P-11b are the same; however, the exoplanet’s close proximity to its star causes its upper atmosphere to warm and essentially ‘boil off’ into space, resulting in the formation of the magnetotail.”
The team 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.
“HAT-P-11b’s low atmospheric metallicity challenges current models of exoplanet formation,” the researchers said.
“We think the exoplanet more resembles a mini-Jupiter than a Neptune.”
“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.”
The new results appear in the journal Nature Astronomy.
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L. Ben-Jaffel et al. Signatures of strong magnetization and a metal-poor atmosphere for a Neptune-sized exoplanet. Nat Astron, published online December 16, 2021; doi: 10.1038/s41550-021-01505-x
