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NEAR its equator, a storm larger than all of Earth rages. The smaller hurricanes surrounding it are themselves planet-sized. Dive into them and you will be bombarded with water and foul-smelling ammonia, and lower down frigid liquid hydrogen. Descend even further towards this planet’s centre, and you may never find it. That isn’t just because you will be dead – if the heat doesn’t get you, the crushing pressure will – but also because a definite core might not exist at all.

This weird, wild world is Jupiter, the biggest and perhaps most important planet in the solar system. Its movements governed how our planetary neighbourhood formed, and might even ultimately be responsible for life on Earth. Its moons, too, are worlds of superlatives, including the largest in the solar system, the most volcanically active and the one perhaps most likely to hold the solar system’s second cradle of life.

Reasons enough to want to get a better view. And for the past three years, a tiny spinning probe powered by huge solar sails has been giving us a longer, harder look at Jupiter than ever before, skimming around its poles and diving to within just 5000 kilometres of its cloud tops. The first results from this mission are trickling in – revealing both huge surprises and how much we have still to learn.

Jupiter is named after the mightiest of the Roman gods for good reason. At more than 140,000 kilometres across, it is about 11 times Earth’s diameter, and a tenth the size of the sun. It seems unusual in comparison to planets orbiting other stars, too. Although we have found many bigger exoplanets, few are this big and so far out from their stars.

But then Jupiter has always offended established norms. When, in 1610, Galileo Galilei discovered four moons circling Jupiter through his newly invented telescope, they were the first bodies conclusively shown to be orbiting a planet other than Earth. That broke a world view that had persisted for more than 2000 years, and helped get Galileo into a lot of hot water with the religious authorities of his day. They didn’t know the half of it. At last count, Jupiter has 79 moons, more than any other planet. But the four original “Galilean” moons – Io, Europa, Ganymede and Callisto – remain the showstoppers (see boxes).

With such a cornucopia of delight and intrigue, Jupiter was an obvious destination for our first ventures into the outer solar system. Four brief encounters with the planet came in the 1970s, courtesy of the passing probes Pioneer 10, Pioneer 11 and Voyagers 1 and 2. They gave us our first glimpses of its swirling gases, powerful magnetic field and delicate rings, as well as a closer look at its most iconic feature – the “Great Red Spot”, a vast storm that has raged since at least 1830.

In 1995, NASA’s Galileo, the first craft designed to orbit Jupiter, arrived after a six-year journey from Earth. In the following eight years, it gave us our first intimate view of the planet and its moons, including the four discovered by its namesake.

As far as the planet itself was concerned, the deepest insights came from a probe that the Galileo spacecraft dropped into Jupiter’s atmosphere on 7 December 1995. Reaching an entry speed in excess of 170,000 kilometres per hour, it sent data for 57 minutes, detecting winds of up to 500 km/h, a strange absence of lightning compared with similar storms on Earth, and the energy driving the atmospheric convulsions was heat upwelling from its interior.

But the biggest find was that Jupiter’s atmosphere seemed to have far less water in it than we expected for a body at its position in the solar system. It might have been that the Galileo observations were just made at a particularly dry spot. “The big question at the end of the Galileo probe era was whether this was a local lack of water or whether it was global,” says planetary scientist , Boulder.

With its fuel running low, in 2003 the Galileo probe was sent to plunge into Jupiter’s atmosphere and burn up. Since then, however, developments have only increased the planet’s intrigue – leading us to the suggestion that its origin and early history are of huge significance not just for understanding it, but also the wider history of the solar system.

Planetary billiards

One thing we do think we know about Jupiter: it was the firstborn of the solar system planets, and was born small. About 4.6 billion years ago, a huge cloud of dust and gas collapsed to form the sun. Just a million years later, the leftovers of this cloud gave birth to the beginnings of Jupiter. Over the following few million years, this rocky core grew bigger and grabbed hold of the surrounding gas to build the swirling giant that we know today, consisting mainly of hydrogen and helium.

But that is probably not where the story ended. Jupiter’s missing water isn’t the solar system’s only planetary mystery. Take Mars: it is small, only just over half the diameter of Earth, despite orbiting where there should have been plenty of planet-building material. Then there are Uranus and Neptune, the two ice giant planets furthest from the sun. Here we have the opposite problem: they can’t have formed where they are now, because there simply wasn’t enough material there to make worlds that large.

The only way we can explain the size and distribution of the planets as they are now, is if they formed somewhere else and migrated to their current positions. To move whole worlds around, you need something big to give them a gravitational shove – something like Jupiter. “Jupiter was the big mass that pushed things around in the early solar system and shaped what we have,” says Bagenal.

This starts with a scenario called the grand tack model. This postulates that once Jupiter had grown beyond a certain size, increased friction with the disc of dust and gas that formed all the planets slowed it down. This caused it to fall towards the sun, to around where Mars is now, its huge gravity sweeping planet-building material out of its way. Jupiter itself was saved from a cataclysmic end crashing into the sun only by the slightly later formation of Saturn, the solar system’s second, only marginally smaller, giant: its increasing gravity pulled Jupiter back from the brink.

“Jupiter’s early history is of huge importance for the wider solar system”

Jupiter’s gravitational bulldozing is our best guess for why Mars ended up so small. To explain Uranus and Neptune, we need a second hypothesis. It is called the Nice model, after the French city where the team that came up with it in 2005 was based. It says that the ice giants probably formed at least about 25 per cent closer to the sun than they are now, near Saturn’s current orbit. The Kuiper belt, a region of icy dwarf planets and comets where Pluto resides, was probably much closer then too, about where Neptune is now.

But about 4 billion years ago, Jupiter’s powerful gravity destabilised the orbits of the ice giants, pushing them outwards. This model has a lot going for it besides explaining Uranus and Neptune. The movements of these planets in turn flung rocks from the Kuiper belt back in towards Jupiter, many of which swung around the gassy behemoth and were catapulted back again to far beyond their original positions. This is our best guess for explaining the Oort cloud, a reservoir of rocks thought to encircle the solar system from which lonesome cometary travellers occasionally reach us.

Other rocks flung inwards – along with complex chemicals and water – probably stuck around in the inner solar system, perhaps explaining how planets like Earth got water when the environment they originally formed in was probably too hot. “Jupiter may have kicked water into the inner solar system to make us,” says Bagenal. If so, we might be able to credit the gas giant with life as we know it.

“We’re still not sure what you’d encounter on a trip to the centre of Jupiter”

In 2011, it was time for life as we know it to return the favour, and mount a second mission to the solar system giant. – short for Jupiter Near-polar Orbiter, and also the name of Jupiter’s wife in Roman mythology – blasted off on 5 August that year, and entered orbit around the planet almost five years later, on 5 July 2016.

Its instruments include cameras to take pictures of the planet at optical, infrared, ultraviolet and microwave wavelengths, and so better determine the composition of its atmosphere, what water there is and where, plus devices to measure its magnetic and gravitational fields, providing insights into what’s going on beneath the surface. Powering it all are three solar panels radiating out from the central module. The largest ever deployed on an interplanetary space probe, they give it a total wingspan of 20 metres.

The plan is for Juno to spend until mid-2021 looping over the planet’s poles. Its 32 orbits will slightly spiral in relation to the planet’s surface, while the probe itself is continuously tumbling over itself, to allow it to survey as much of the planet from as many angles as possible. Already it has come up with surprises. The first ever pictures of Jupiter’s poles have revealed strange cyclones there, nine in the north, with eight in a circle around one in the middle, and six in the south in a similar configuration. Why do these storms form in such odd, organised rings? “Good question. Don’t know yet,” says Bagenal, who is on the Juno team. Passing over the Great Red Spot, Juno’s measurements have shown that the storm has , 80 times the average depth of Earth’s oceans.

Comparatively speaking, that is nothing. The roots of some storms seem to extend down about 3000 kilometres into the gas giant, where the jet streams that cause its rotating bands of colour appear to originate.

Beneath that lie the real mysteries still to be revealed, however. Galileo measurements had led us to believe that Jupiter’s innards were neatly arranged in layers. A shallow “crust” of liquid hydrogen lies above a much deeper layer of liquid metallic hydrogen, formed when hydrogen is subjected to the intense pressures rising to millions of times the atmospheric pressure at sea level on Earth coupled with temperatures of thousands of degrees. That is probably shiny like liquid metallic mercury, says Bagenal, and heavier elements would dissolve into it.

All this overlies a small core that is about 70,000 kilometres down. Juno’s initial gravity measurements cast doubt on whether it would be solid, however, pointing to an ill-defined, fuzzy core that mingles with the metallic hydrogen above. But we still aren’t sure what you would encounter on a trip to the centre of Jupiter. “Would it just get denser and denser and denser and before you realised it you’d be compressed to nothing or would you eventually find a surface that you could walk on or swim in? That’s not something that we know yet,” says Bagenal. “We know it gets nasty quickly as you go in.”

Only once Juno has completed its loop the loops, and we have looked at the full data set, will we hopefully glean more insights. Beyond that, a planned mission centred on the moon Europa and its possibly life-harbouring internal ocean may provide additional clues. It is a giant voyage of discovery. We are learning a lot about how Jupiter works, says Bagenal – “but it’s raising even more questions”.