Radiation belts—doughnut-shaped areas bounded by magnetic fields where
particles are trapped and accelerated and glow in radio light—are present on
every planet in our solar system with global magnetic fields. All of this
implies that radiation belts should exist everywhere there is a constant,
global magnetic field.
But it is difficult to resolve the diffuse light of a radiation belt,
making it difficult to detect the modest emission from an extrasolar
radiation belt. However, difficult does not equate to impossibly difficult:
for the first time, scientists have captured a picture of a radiation belt
encircling an extrasolar object.
That object is LSR J1835+3259, an extremely low-mass red dwarf star that is
little over Jupiter's diameter, roughly 77 times Jupiter's mass, and is
located around 20 light-years distant.
According to astronomer Melodie Kao
of the University of California, Santa Cruz, "We are actually imaging the
magnetosphere of our target by observing the radio-emitting plasma - its
radiation belt - in the magnetosphere." "For something the size of a gas
giant planet outside of our Solar System, that has never been done
before."
The Van Allen belts on Earth are chock-full of solar wind debris. Radiation
belts are present on Saturn, Mercury, Neptune, and Uranus.
Io, Jupiter's volcanic moon, provides the bulk of the massive radiation
belts with large gouts of volcanic material. Even Ganymede, the only moon in
the Solar System with its own magnetic field,
has a radiation belt.
Ganymede is a moon orbiting Jupiter.
And although if extrasolar objects had not been discovered by the radiation
belts and magnetic fields that contained them, we had observed signs of
their existence.
Low-mass stars and brown dwarfs have displayed behavior like the solar
system's auroras. When accelerated charged particles are directed down
magnetic field lines and fall into a planet's atmosphere where they interact
with other particles, auroras—which are visible on many planets—are
created.
LSR J1835+3259 was the ideal target to check attentively for radiation
belts since it had showed signals of this auroral activity, which indicated
the existence of a global magnetic field.
Kao and her colleagues made observations of the star, paying close
attention to the area surrounding it where a radiation belt, when viewed
from the side, would appear as two radio-emitting lobes. They did this by
utilizing a network of 39 radio telescopes located all over the world to
essentially build an Earth-sized radio telescope.
Images confirmed the presence of a double-lobed object surrounding the star
that was generating radio waves of a fainter frequency than Jupiter's
radiation belt. The star's radio lobes are approximately 10 million times
more intrinsically brilliant than Jupiter's because of the star's greater
distance from Earth.
Additionally, the radiation detected is of a sort that has previously been
detected in brown dwarfs and low-mass stars, but it was previously
attributed to outbursts in the stellar corona.
These discoveries not only show that stars and other similar objects can
have radiation belts, but they also suggest that humans may have already
noticed radiation belts in other similar objects without realizing it.
"Now that we've established that this particular steady-state, low-level
radio emission traces radiation belts in the large-scale magnetic fields of
these objects, we can more confidently say they probably have a big magnetic
field, even if our telescope isn't big enough to see the shape of it,"
Kao says. "When we see that kind of emission from brown dwarfs- and eventually from
gas giant exoplanets."
Astronomers anticipate that this finding will aid their hunt for possibly
habitable planets as methods and equipment advance. That's because it's
believed that for life to thrive, the Earth's magnetic field is necessary.
By preventing dangerous solar radiation from reaching the surface, it
shields the atmosphere and the weak creatures that live there from
damage.
We will be able to identify planets that are similarly shielded if we have
the means to detect magnetic fields around other worlds.
Even if that's still a ways off, this discovery puts us on the correct
track.
According to astronomer Evgenya Shkolnik of Arizona State University, "this is a crucial first step in finding many
more such objects and honing our skills to search for smaller and smaller
magnetospheres, eventually enabling us to study those of potentially
habitable, Earth-size planets."
The research has been published in
Nature.
