Why Study Jupiter?

Unlike Earth and the other inner planets, which are made of rocky material, Jupiter and the other gas giants are mostly – if not entirely – huge balls of gas. Jupiter’s enormous mass allows it to continue holding onto all of the gases it accumulated when it was forming. Since its gases haven’t changed in four billion years, studying its composition is a way to investigate our solar system’s history.

With the exception of its solid core, Jupiter’s interior is probably well mixed, meaning that the composition of its outer atmosphere is likely a good indication of what’s deeper in the planet. By measuring the amount of water in its atmosphere, we can estimate the amount of oxygen – a key component of water – inside Jupiter, a vital step in understanding the planet’s formation.

For example, knowing how much oxygen Jupiter has will help us determine how far away from the sun it was when it formed. Jupiter was initially thought to have been born roughly where it orbits today. But when NASA’s Galileo spacecraft visited the gas giant in the 1990s, it dropped a probe into the planet’s clouds and discovered evidence that suggested otherwise. The probe found more heavy elements – carbon, nitrogen, sulfur, argon, krypton, and xenon – than expected. This finding was a surprise because chemicals with these elements could only have formed in extremely low temperatures, and they were mixed with materials that form in warmer conditions.

One possible explanation for the abundance of heavier elements is that Jupiter actually formed farther away from the Sun than its present orbit. There, it was able to collect these materials that had condensed in the frigid regions beyond the orbit of Neptune. Then, Jupiter migrated inward to its present orbit. Another theory allows for Jupiter to have formed where it is now. In this scenario, the heavier materials were trapped inside ice crystals that populated Jupiter’s neighborhood. As the planet formed, it gobbled up these crystals.

It turns out that the two theories predict different amounts of water in Jupiter. Juno’s Microwave Radiometer and JIRAM instruments will measure this water content and determine which theory is correct – or if we have to come up with entirely new ideas to explain Jupiter’s composition.

Because the existence of Earth and of life depends on the presence of oxygen and these other heavier elements, learning how Jupiter acquired these materials can also tell us something about our very own origin.

In other planetary systems, we see evidence that giant planets like Jupiter can migrate from where they originally formed, spiraling inward to an orbit closer to their stars. When these giants wander toward their stars, any small, rocky planets that stand in the way can be swallowed up or, due to the giants’ strong gravity, flung out of the star system altogether.

But if Jupiter-like planets remain distant from their stars, they can serve as the gatekeepers to their planetary systems. They protect their fellow planets on inner orbits, allowing them to maintain nearly circular orbits that provide stable climates over extended periods of time. Long, elliptical orbits cause extreme climate shifts for an Earth-like planet, possibly preventing any sort of sustained life from evolving.

In our solar system, Jupiter can eat up any asteroid or comet that ventures near, earning the nickname “vacuum cleaner of the solar system.” The asteroid belt in between the orbits of Mars and Jupiter is another example of the gas giant’s influence. Its gravity likely prevented the asteroids from combining into a planet.

Jupiter can also radically alter the orbits of small bodies that stray close, hurling them on long orbits that take hundreds or even thousands of years for those bodies to return. We think this is how comets got the extreme orbits that carry them to the far-flung reaches of the solar system. They spend most of their time out there, forming a cometary collection called the Oort cloud, which may extend as far as halfway to the nearest star.

While Jupiter often protects Earth and the other inner planets by deflecting comets and asteroids, sometimes it sends objects on a collision course straight toward the inner planets. Earlier in the solar system’s history, when there were more objects flying around, the increased amount of impacts would have brought to Earth water and other ingredients for life. Of course, other collisions would have been disastrous, such as the impact that likely led to the extinction of the dinosaurs 65 million years ago.

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According to most theories, Jupiter has a dense core of heavy elements that formed during the early solar system. The solid core of ice, rock, and metal grew from a nearby collection of debris, icy material, and other small objects such as the many comets and asteroids that were zipping around four billion years ago. These bits of matter clumped together due to their mutual gravity, becoming larger chunks called planetesimals, which, in turn, collided and stuck together to form Jupiter’s core.

Soon, the core grew big enough so that it had enough gravity to attract even hydrogen and helium, the lightest elements that exist. More and more gas accumulated until it became what we now know as Jupiter. Although most scientists agree on this general story, many details remain unknown. For example, we’re still not sure where all the icy matter comes from.

Another theory, however, suggests that there’s no core at all. Instead, Jupiter formed from the large cloud of gas and dust that surrounded the Sun soon after its birth. As this cloud cooled and condensed, gas and dust particles lumped together so that some regions were denser than others. One of these dense splotches was able to gravitationally pull more and more gas and dust together, swelling into a full-fledged planet.

By measuring Jupiter’s gravitational and magnetic fields, Juno will be able to determine whether a core exists. If it does, exactly what the fields look like will depend on how big it is. Different theories make different predictions about the core, and knowing the size will help determine which theory – if any – is more likely to be correct.

If Juno finds no evidence of a core, then that could strengthen the condensed-cloud theory. Another possibility is that Jupiter once had a core, but it has since eroded away. It could also be that whatever Juno finds won’t fit any theory, and scientists will have to come up with completely new ideas.

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There are still many basic, unanswered questions about Jupiter. With Juno, we’ll be able to learn about Jupiter’s composition, inner structure, how its swirling clouds are connected to its dynamic interior, and how it formed. By learning about Jupiter, we can better understand the early history of the solar system and the conditions in which Earth was born.

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With the exception of the Sun, Jupiter is the most dominant object in the solar system. Because of its size and the fact that it was the first of the gas-giant planets to form, it has profoundly influenced the formation and evolution of all the other planets. For example, Jupiter is the reason why there’s an asteroid belt – and not another planet – between it and Mars. Jupiter has also catapulted countless comets out to the edge of the solar system. Like a gatekeeper, Jupiter has safeguarded Earth from many comet impacts.

The planets are the leftovers from the star-forming process, and Jupiter accounts for the bulk of that material – more than twice that of all the other planets combined. Its atmosphere – predominantly hydrogen and helium – is similar to the composition of the sun and other stars as well as the clouds of gas and dust in our galaxy.

When the sun was born – when it accumulated enough mass for nuclear fusion to ignite – it generated a wind that blew away most of the gas and dust that still remained. The fact that Jupiter’s composition is similar to that of the original cloud suggests that it formed early on, before the wind could clear away that material.

To give you an idea of how dominant Jupiter is, an alien observing our solar system through a telescope would see an average yellow star and Jupiter with three other large planets. Earth and the inner planets would appear merely as debris.

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