Look towards the north-west after
sunset and there is currently one bright point of light that easily stands out
relative to everything around it. That is the planet Jupiter, shining with an
intense and steady glow.
After a journey of five years, and
decades in the planning, NASA's Juno spacecraft has achieved orbit around Jupiter. Soon begins the
next stage in humanity's quest to explore the largest of the gas giants in our solar system.
But what do we know of Jupiter to date?
Brightest, biggest, first
Jupiter's brightness in the night
sky is due to its enormous size. It is by far the biggest planet of the solar
system, containing more than double the mass of all the other planets, moons,
comets and asteroids combined.
Its great size suggests that Jupiter
was also the first planet to form around the sun. The planets emerged
out of the debris left over when an interstellar
cloud of gas and dust coalesced to form our star. Early in its life,
the young sun generated a wind that blew away most of the remaining
interstellar cloud, but Jupiter was able to hold on to that history.
Locked up in Jupiter, therefore, is
the recipe for how a solar system is made – the ingredients from which the
planets and other smaller bodies came to be, and the processes and conditions
that enabled this material to come together to form such amazing and diverse
worlds.
King of the planets
Jupiter, along with Mercury, Venus,
Mars and Saturn, have all been observed since ancient times as they are easily
visible in the night sky. Different cultures who studied the
stars also realised that these objects were unique; they did not stay fixed in
their relative patterns or constellations as the stars did but moved according
to different rules.
They were referred to as the
wandering stars by the ancient Greeks and derived from this term came the name
planet.
It is remarkable just how aptly
Jupiter is named. We are now aware that Jupiter is the largest and most massive
of the planets so it is fitting to be named for the Roman king of the gods, who
was also the god of the sky. While in Greek mythology, Jupiter's counterpart is
Zeus, the supreme god of ancient Greece.
But Jupiter
is not the brightest of the planets, that record is held by Venus. However,
Jupiter and Venus are very different in how they wander across the sky.
Nowadays we can explain that difference by where they are positioned in the
solar system.
Venus, being
an inner planet, remains close to the sun, appearing as the evening star after
sunset or the morning star before sunrise. Whereas Jupiter, being an outer
planet, can wander across the entire sky.
Jupiter
holds the secrets to the early solar system. Credit: NASA/ FUSE / Lynette Cook
This motion,
along with the planet's brightness, would've helped to mark Jupiter as the king
of the planets.
Revolutions
around Jupiter
In the year
1610, from late January through to early March, the astronomer Galileo used his
new telescope to observe Jupiter. He easily identified and tracked first three,
and then four bright points of light. They formed a straight line either side
of Jupiter, but their positions were constantly and steadily changing relative
to the planet.
Galileo in
his publication Sidereus Nuncius, confidently and quite correctly
explained this motion as the objects being in orbit around Jupiter. Here was
proof that everything in heaven did not orbit the Earth, which at the time led
to conflict between Galileo and the Catholic Church.
Galileo had
discovered Jupiter's four major moons – Io, Europa, Ganymede and Callisto –
each worlds within their own right and often referred to as the Galilean moons.
Since that
time numerous more moons have been discovered and in the past few decades the
count has jumped to 67 known satellites, the greatest number of any
planet. It's no wonder Jupiter is often called a mini-system of its own.
The Great
Red Spot
Saturn has
its rings, Earth has its blue oceans and Jupiter has its vibrant and swirling
bands of clouds. Jupiter rotates very quickly, spinning once every
10 hours. This drives the dynamic weather patterns seen in Jupiter's cloud
tops.
One of the
big questions regarding Jupiter is just how deep do these clouds descend?
Furthermore,
in recent times Jupiter's iconic Great Red Spot has been found to be shrinking.
This massive storm system measuring about twice the size of Earth, has been a
feature of the planet since it was first observed in 1664.
But recent
observations from the Hubble Space Telescope show that the storm may now be less than half the size of some historical
measurements. Since the 1930s astronomers have tracked this downsizing but more
recently, the shrinking appears to be happening more rapidly.
The
shrinking Great Red Spot over 20 years. Credit: NASA
Radiation
Hazard
Jupiter has
the strongest magnetic field of all the planets. At Jupiter's
poles the magnetic field is 20,000 times stronger than Earth's and the field
extends for millions of kilometres in space, even reaching past the orbit of
Saturn.
The dynamo
powering Jupiter's magnetic field is thought to be a layer of liquid hydrogen
deep within the planet. Hydrogen is under such high pressure inside Jupiter
that it becomes a liquid. As such, it takes on the characteristics of a metal;
it can conduct electricity because the electrons within the hydrogen atoms are
able to move around.
With the
addition of Jupiter's fast spin, it's the perfect combination to create the
powerful magnetic field.
This
magnetic field traps charged particles - electrons, protons and ions - some
originating from the solar wind but also flying in from Jupiter's Galilean
moons, particularly volcanic Io.
Some of the
particles are funnelled towards Jupiter's poles, creating Jupiter's impressive
aurora that are 100 times brighter than the aurora experienced on
Earth.
Jupiter’s
inner magnetosphere and the bright radiation belts. Credit: NASA/JPL
Other
particles become trapped by the magnetic field giving rise to Jupiter's
radiation belts, an insanely intense version of the Earth's van Allen belts. Jupiter's magnetic field
accelerates these particles to such a degree that they zip up and down the belt
at nearly the speed of light, creating the most hazardous radiation zone in the
solar system.
Breaking all
the records
Jupiter is
the largest, most massive, fastest rotating, most hazardous planet of the solar
system. It has the strongest magnetic field and the greatest number of known
satellites. It has held onto the pristine gas from the interstellar cloud that
gave rise to our sun.
Its strong
gravitational influence has helped to move material around our solar system,
potentially scattering ice, water and organic molecules from the outer cold
regions of the solar system into the inner solar system where it could be
captured by the Earth.
The first planets
to be found orbiting around other stars were hot Jupiters, an apt description
for exoplanets with masses similar to Jupiter that orbit very close to their
stars and therefore have high surface temperatures.
One of the
most long-standing questions that the Juno spacecraft will answer is how did
Jupiter form? Did it begin with a rocky core that then attracted an enormous
atmosphere or was Jupiter's origin more akin to a star and could it possibly
have collapsed directly out of the solar nebula remaining gaseous right through
to its core?
The next 18 months are
set to be very interesting as the Juno spacecraft increases our
understanding of the great gas giant, Jupiter.
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