SOLAR SYSTEN FAMILY-FACTS ABOUT JUPITER, THE MIGHTIEST PLANET

Among all the planets in our solar system, there is one who is itself a spectacle to behold. Its humongous size, intense gravity, numerous violent storms and a plethora of natural satellites, really makes it one of a kind. Back in 1610, when Galileo Galilei trained his telescope towards this bright celestial object, it revolutionized our understanding of the whole cosmos. The subsequent observation of a planet surrounded by four natural satellites, provided the greatest support in favour of the Copernican model of our universe. And thus, the mighty Jove or rather Jupiter paved the way for the beginning of a new chapter in the field of astronomy.

A Bit of Mythology- Jupiter has been known since ancient times as a bright star-like object. The ancient Romans used to call it the ''star of Jupiter'' as they considered Him to be the most sacred and  a ''God of the Sky''. Being an extremely powerful mythological being, He wielded lightning bolts and brought violent thunderstorms. His Greek counterpart was Zeus, a god of thunder and the king of the Olympian gods. Just like Jupiter, Zeus' principal weapon was the thunderbolt and was like a father, a ruler and a protector to all gods and men. In ancient India, Jupiter was known as ''Lord Brihaspati'' or ''Dev Guru'' which can be translated as ''the mighty one''.

Basic Facts: Jupiter, being a gas giant, is completely different from the terrestrial or the rocky planets viz., Mercury, Venus, Earth and Mars. Gas giants are a distinct class of celestial objects and are composed of the same elements that make our Sun i.e., hydrogen and helium. Had Jupiter been more massive(some 75-80 times its present mass) then our solar system would have gained a second star! The thick Jovian(from the Roman 'Jove') atmosphere has a beautiful region called the Great Red Spot, which is a violent storm going on for 350 years. Apart from these, Jupiter's tremendous gravity has played a significant role in the evolution of our solar system, the formation of the asteroid belt and the terrestrial planets.

Full Disk View of Jupiter/Image Credits: NASA, ESA, and A. Simon (Goddard Space Flight Center), Public domain, via Wikimedia Commons

Planetary Characteristics:

• Orbital & Rotational Characteristics: Jupiter is situated at an average distance of  778 million km from the Sun or about 5.2 AU and has an orbital period nearly equal to 12 Earth years. Because of being located beyond the main asteroid belt, Jupiter along with Saturn, Uranus and Neptune fall under the category called the ''Outer Planets of the solar system''. The Jovian orbit has an eccentricity of 0.048 and as a result, the aphelion distance is 816.62 million km, while the perihelion distance is about 740.52 million km. Jupiter has the fastest rotational velocity and spins about its axis once every 10 hours. This high velocity has given the planet a visually recognizable equatorial bulge, and the equatorial diameter is 9275 km longer than the polar diameter.

• A Huge Planet: For comparison, Jupiter has a mean radius of 69,911 km whereas Earth has a mean radius of 6371 km. Although the planet occupies a volume equal to 1321 Earths, it is however only a fifth as dense. This is because gases are not as dense as the rocky materials of a typical terrestrial planet. The former has a mass equal to 500 Earths and is 2.5 times more massive than all the planets combined. 

• Internal Structure and Composition: Jupiter is made up of nearly 89% hydrogen, 10 % helium and trace amounts of methane, ammonia, hydrogen deuteride, ethane, carbon, neon, oxygen, sulfur, phosphine and water. As the planet is mostly gas, scientists cannot say for sure whether there is some solid surface to land on for an hypothetical explorer. This is because deep below the thick atmosphere, an exponential rise of temperature and pressure forces the hydrogen gas to transform into a super-critical fluid where it becomes capable of conducting electrical currents and behaves neither like a gas, nor like a liquid. Some theoretical models suggest that the planet might have a solid, rocky core as it is not probable for it to be entirely made up of gas. Nevertheless, it is quite evident that deep inside that beautiful atmosphere, temperatures are expected to rise above 20,000 degrees Kelvin and pressures to be equal to 4500 Gigapascals. 

• Atmosphere and the Great Red Spot: Jupiter has the deepest(altitude of 5000 km) and the most dynamic atmosphere in our solar system. The Jovian atmosphere is divided into four distinct layers-the troposphere, stratosphere, thermosphere and the exosphere. In absence of a solid surface, the lower atmospheric layers gradually merges with the fluid interior, where the gas and the liquid phases coexist as an electrically conducting liquid material. Without a solid interior to provide any friction, wind speeds can easily exceed 500kmph. The atmosphere is however composed of mostly molecular hydrogen and helium with traces of methane, ammonia, hydrogen sulfide, nitrogen, sulfur, and some noble gases. The outer region of the atmosphere, i.e., the visible surface is made up of several dark and light colored bands formed as a result of the different elements picked up from the deep interior by the violent winds. These bands indicate different layers of clouds, turbulent air circulations, vortices, cyclones and anticyclones that have been relatively stable over long periods of time. One of the most important features of the Jovian atmosphere is the Great Red Spot, an active anticyclone known to have existed for 350 years. The Great Red Spot is so large that it can easily accommodate 2 or 3 Earths. Apart from the Great Red Spot, there are other storm systems throughout the atmosphere which are of considerable importance to scientists. 

Earth and the Great Red Spot/Image Credits: Photo by form PxHere

• Magnetosphere & Magnetic Field: The mighty planet deserves an equally intense magnetosphere, extending up to 7 million km towards the Sun on one side and to the orbit of Saturn on the other side. Scientists estimate that this strong magnetosphere is generated because of the intense magnetic field resulting from the circulation of liquid metallic hydrogen. The Jovian magnetic field interacts with the solar wind plasma, and in addition to further magnetic activity from one of its moons i.e., Io, produces an intense region of highly energized particles which conduct electrical currents and generate some of the spectacular aurorae at the polar regions. 

Aurorae at the Polar regions/Image Credits: NASA, ESA, and J. Nichols (University of Leicester), Public domain, via Wikimedia Commons  

• Natural Satellites: Due to its strong gravitational influence, Jupiter has attracted countless objects towards itself and is surrounded by a swarm of 79 known moons, out of which only 56 have been officially recognized. Of all the moons, four deserve special mention. They are called the famous Galilean Moons viz., Io, Europa, Callisto and Ganymede which are also the largest moons of our solar system. 

• Influence on the Solar System: From the very beginning, Jupiter's intense gravity has played an important role in the development of our solar system and the formation of the inner, rocky planets. It has been found that except the orbit of Mercury, all other planetary orbits lie closer to Jupiter's equatorial plane. The mighty planet controls the orbit of several asteroids and is the main reason for the formation of the asteroid belt. In addition to asteroids, it is also responsible for the origin of the short period comets(whose orbits lie within the orbit of Jupiter). Finally, Jupiter is so massive that its barycentre(the centre of gravity between it and the Sun) lies above the Sun's surface. For all other planets, their barycentre lies below the Sun's surface.

The Four Galilean Moons/Image Credits: NASA/JPL/DLR, Public domain, via Wikimedia Commons 

Life and Biological Considerations: Based on our present understanding of Earth life, it can be safely assumed that no form of living organisms can survive the harsh Jovian environment. Although chances of finding some exotic Jovian life is rather slim, it is probable that whatever exotic alien beings are out there, they must float or fly through the thick atmosphere. In fact, scientists are more optimistic about detecting life on some of the Jovian moons and quite recently it has been found that Europa might possess the necessary elements required to support microbial or unicellular life forms.

Observations and Explorations-

• Observations: Jupiter(at opposition) is the third brightest object in the sky, preceded by the Moon and Venus. As mentioned earlier, when Galileo observed Jupiter with his telescope, he undoubtedly provided the greatest support in favour of Copernicus' Heliocentric Theory(Earth revolves around the Sun), contrary to the Geocentric Theory(Every celestial object revolves around Earth) prevalent at that time. However, the Galilean moons were also independently discovered by the astronomer Simon Marius who gave them their names- Io, Europa, Ganymede and Callisto. Giovanni Cassini was the first to observe the colorful bands in the atmosphere and also the Great Red Spot, in addition to other astronomers who gathered data via terrestrial observations until the middle of the 20th century. By mid 20th century, Jupiter was observed with the help of radio telescopes and it was found that the former is a good emitter of radio waves. 

• Explorations: Since the 1970s, many spacecraft have visited the Jovian system, sometimes as flyby missions and other times as an orbiter. The Jovian missions are as follows:

1. Pioneer Missions: Pioneer 10(launched in 1972) became the first spacecraft to explore the Jovian system, and was soon followed by the Pioneer 11(launched in 1973). Both the crafts obtained close up images of the Galilean moons, in addition to data on the planetary atmosphere, its magnetic field, radiation belts, polar regions and internal composition. 
2. Voyager Missions: Voyager-1 and Voyager-2, launched in 1977 performed close flybys of Jupiter at a distance of 349,000 km from the planet's centre and 570,000 km from the planet's cloud tops respectively. Although the flybys were of short duration, it was enough to gather some important information of which discovery of a thin system of rings deserve special mention.
3. Other Flyby Missions: These include the Ulysses Probe which used Jupiter's gravity to attain a polar orbit around the Sun, the Cassini spacecraft en route to Saturn and the New Horizons en route to Pluto.
4. Galileo Mission: The first mission dedicated for the study of Jupiter was the Galileo Mission launched by NASA in October 1989. The Galileo spacecraft had an atmospheric entry probe, and an orbiter which remained operational for seven years. The entry probe gathered immense data about the violent thunderstorms and lightning, intense radiation, and atmospheric composition before being vaporised because of extreme atmospheric pressure and temperature deep below the visible cloud layers. The orbiter observed the thin ring system, its magnetic field, gravitational field and also did an extensive study on the four moons.
5. Juno Mission: Juno was launched by NASA in August 2011 and it entered a polar orbit of Jupiter in July 2016. The orbiter probe has sent back some high resolution images of the polar regions which are being seen for the first time. At present, Juno is on an extended mission till September 2025 and the data collected within this period will help scientists gain more insight into the nature of the planetary core, its history of formation and subsequent evolution, and other unanswered questions.
6. Some future missions include the Jupiter Icy Moon Explorer(JUICE) by ESA scheduled to be launched in 2022 and the Europa Clipper by NASA, also scheduled to be launched in 2024.

Jupiter as observed by Juno/Image Credits: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill, Public domain, via Wikimedia Commons

Some Quick Facts:

References:

1. https://en.wikipedia.org/wiki/Jupiter
   
2. https://solarsystem.nasa.gov/planets/jupiter/in-depth/
   
3. https://solarsystem.nasa.gov/planets/jupiter/overview/