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A FEW billion kilometres from here, two cerulean marbles hang in the blackness of space. Neptune and Uranus are not the most distant objects in the solar system. They aren鈥檛 the biggest or the smallest. They don鈥檛 have the brightest colours. But they do hold the greatest mysteries.

We have roved across Mars, we have orbited Jupiter, we have even landed on Venus. But our only close look at the ice giants Neptune and Uranus came more than 30 years ago when the Voyager 2 spacecraft hurtled past on its way out of the solar system. It snapped a few pictures, then the planets faded from view. 鈥淓verything we know up close about these planets and their moons is based on that single fly-by,鈥� says NASA scientist Amy Simon.

Now impetus is building to go back. The little we do know about these frigid planets suggests they are extremely weird. They have crooked magnetic fields, they鈥檙e colder than they should be and we don鈥檛 know why they spin in odd ways. Understanding all this is important, because in the past two decades our sky surveys have started spotting planets around other stars, and worlds about the size of Uranus and Neptune are the most common type. That tells us mid-sized gassy planets are a basic ingredient of the universe. 鈥淭hese are objects as fundamental as a rocky planet or a star,鈥� says Jonathan Fortney at the University of California, Santa Cruz. 鈥淎nd we know almost nothing about this class of object.鈥�

If we want answers any time soon, we had better get moving. There is a small window of opportunity in the late 2020s and early 2030s when the planets will be arranged so that a slingshot around Jupiter would boost a probe to Neptune within six years. If we miss that slot, 2050 is the earliest we could be there.

That鈥檚 why NASA and the European Space Agency are both making plans for missions to learn more about the ice giants. A NASA group is studying concepts including orbiters with atmospheric probes, a Uranus orbiter without an atmospheric probe but with extra instruments, and a fly-by of Uranus. 鈥淎s scientists, we want everything and we want it as soon as possible,鈥� says Adam Masters at Imperial College London, who was appointed to the NASA group.

Whether he will get his wish depends largely on whether NASA dishes the cash in the next decadal survey, an exercise that starts shortly and chooses the next batch of ambitious space missions. If a trip to the ice giants does get the go ahead, here are the biggest questions it could answer.

Why is Uranus too cold?

When Voyager 2 buzzed past Uranus, its heat sensors picked up no signal at all. That doesn鈥檛 make sense if this ice giant emits heat in the way we think planets do.

As far as we know, gas, dust and eventually rocks smash together during planet formation and that locks up heat in their cores. The heat then radiates out over billions of years. How much heat is coming out at any given time depends on what the planet is made of and how old it is.

We can estimate how much heat should be emanating from the ice giants by taking a cue from Saturn and Jupiter, which we have studied more extensively. Heat moves from their cores to their atmospheres of hydrogen and helium, gets shuffled around, and eventually escapes. The ice giants are colder and smaller than these planets, and their atmospheres include methane and ammonia as well as hydrogen and helium. But the same sort of heat dispersion process should apply.

You might suspect Uranus鈥檚 lack of a heat signal is because its interior is different from what we have so far guessed. But here鈥檚 the weird thing: the models based on Jupiter and Saturn predict the heat output of Neptune rather neatly, just not that of Uranus.

Perhaps Neptune and Uranus are just different then? That is possible, but unlikely. They must have formed at around the same time and place to have accumulated the amount of hydrogen and helium we see in their atmospheres, suggesting their innards are similar.

An alternative idea is that Uranus鈥檚 cold heart is related to its odd tilt (see 鈥淲hy does Uranus orbit sideways?鈥�). This could make its poles hotter than the equator and may have sped up the planet鈥檚 loss of heat. Or it could be the opposite; the core is still warm but there is something that stops the heat getting out for us to measure, such as slushy ice in the atmosphere.

We can鈥檛 disprove any of the hypotheses because we know almost nothing about what鈥檚 going on inside Uranus or Neptune. We know their size and mass, which tells us their density 鈥� but there are lots of ways to bake that. 鈥淵ou just think up new stories,鈥� says Fortney. 鈥淭here鈥檚 no way to test any of this stuff with the information we鈥檝e got.鈥� Instead, we must go there.

Why are the magnetic fields lumpy?

A lack of heat wasn鈥檛 the only strange reading Voyager 2 picked up. It also managed a single measurement of the ice giants鈥� magnetic fields. They were like nothing we have ever seen.

Earth, Jupiter and Saturn all have magnetic fields with an alignment that roughly matches their spinning axis. Earth鈥檚 magnetic field is like a bar magnet, the north pole is only 11 degrees off from the geological north pole, and it is a similar story at the south pole. Uranus鈥檚 magnetic field is more complex. It has lumps and bumps pointing in different directions, and is totally skew-whiff, sitting at about a 45 degree angle from the spin axis (see diagram). It is the same with Neptune. Even stranger, Voyager measurements showed that, for both planets, the centre of the magnetic field doesn鈥檛 seem to sit in the middle of the planet. 鈥淭hese magnetic fields are crazy,鈥� says Carol Paty at the Georgia Institute of Technology.

Earth鈥檚 magnetic field is generated by a swirling spherical skin of molten iron in its outer core, the motion of which is driven by the core鈥檚 heat and corralled by the planet鈥檚 rotation. It is hard to see how a process as symmetrical as this would create an off-centre field.

The dynamos that give rise to the ice giants鈥� fields are probably very different, then. Our best guess is that the planets鈥� interiors are made of layers of electrically conducting ices. Each layer may have different properties that mean they flow around each other in complex ways. In other words, the thing generating the magnetic field is not a single uniform sphere, but several interacting shells. 鈥淭he composition, the thermal gradients, all the drivers of the dynamo are going to be different because of the colder temperatures,鈥� says Paty.

鈥淯ranus鈥檚 magnetic field seems to slam open and shut every day鈥�

If that鈥檚 true, it may help explain another bizarre feature of Uranus鈥檚 magnetic field: that it seems to slam open and shut every day.

We can鈥檛 even be sure that the magnetic fields are quite as they appear. We have only one measurement of each, and it is possible that they are constantly shifting and just happened to be off-centre when Voyager 2 whistled past.

Fields opening and shutting isn鈥檛 unheard of. It happens at Earth鈥檚 poles occasionally. When it does, it lets in particles from the solar wind that create super intense auroras. The chance of glimpsing something like that over Uranus is another tempting reason to visit.

Why does Uranus orbit sideways?

Most of the planets in our solar system rotate on an axis that is approximately at right angles to the plane of their orbits. They are like a series of spinning tops skittering on a table, all circling the sun in the same plane and spinning in the same direction. Even Neptune follows this pattern reasonably well.

Not Uranus. The spin of this frigid world is almost perpendicular to its orbital direction, with its spin axis close to the plane of its orbit. Most of its moons orbit in the same direction that it rotates, at a right angle to the plane of the solar system (see diagram, above). Something must have happened when Uranus was forming to make it so crooked.

Most planetary scientists believe that it was an epic collision. For a single crash to knock a planet as large as Uranus off-kilter, the other object would have to be several times more massive than Earth. Or it could have been a series of collisions with smaller objects.

That doesn鈥檛 explain why the moons are tilted too. 鈥淭he Uranian satellite system looks perfectly normal, just tilted over on its side,鈥� says Elizabeth Turtle at Johns Hopkins University in Baltimore.

Maybe the moons grew out of the rubble tossed into space by the collision, in a similar manner to how Earth鈥檚 moon is believed to have formed. Then again, of the 27 moons, 18 of them orbit normally, around the planet鈥檚 equator, and 9 follow less regular orbits. It is possible that the normal moons were around Uranus before the collision, whereas the others grew out of collision debris.

All this depends on Uranus having a solid core 鈥� otherwise it is hard to see how the collision narrative would work. We assume that is does, but we have no evidence. One way to tell would be to measure the planet鈥檚 gravity from orbit 鈥� any areas with a strong pull could signify lumps and bumps on something solid.

Why do the ice giants even exist?

As if the curious magnetic fields and oddball temperatures weren鈥檛 enough, there鈥檚 another issue with the ice giants. 鈥淭heir mere existence is a problem,鈥� says Fortney.

The trouble comes down to the window in which the ice giants could have formed. There鈥檚 a period of about 3 million years after the birth of the sun when there was enough gas in the solar system鈥檚 protoplanetary disc to build up the ice giants鈥� thick atmospheres 鈥� after that, the solar wind had blown most of it away. But Neptune and Uranus both probably have a solid core and these might have taken as much as a billion years to accrete. After that, there would not have been enough gas left around to form their atmospheres.

One possibility is that the ice giants didn鈥檛 form gradually, but in a short burst, with some gravitational glitch causing a cloud of dust to collapse quickly. The conditions under which anything like that might happen are unclear though.

A more probable explanation is that the ice giants were born closer to the sun before migrating to their current home. They would have grown more quickly there, because the cloud of dust was thicker. It is an attractive idea, seeing as we already think the early solar system involved planets pinballing around. Popular models of planet formation involve Jupiter either being born close to the sun or moving inwards in its youth. It could have absorbed or tossed aside huge rocks in the process, clearing the way for planets like Earth to survive unscathed, before moving outwards again.

It wouldn鈥檛 be too difficult to get a handle on what happened if we sent a probe to the ice giants to sample the various chemicals in the atmosphere. The ratio of isotopes, variants of chemical elements with different masses, is a dead giveaway of where the planets formed and would help us reconstruct their movements.

We have never quite managed to measure Jupiter鈥檚 isotopes, despite several missions there. But if we pulled it off for Neptune or Uranus, it would help narrow down the possible ways in which Jupiter moved around too; its awesome gravity would have been a major driver for the way the ice giants moved. In that sense, the mysteries of the ice giants aren鈥檛 merely parochial oddities 鈥� they go to the heart of what makes our solar system the way it is.

Mad moons

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Miranda

Of Uranus鈥檚 27 moons, this one is a particular puzzle. Miranda is only about 470 kilometres in diameter, yet its surface is riddled with canyons up to 12 times as deep as the Grand Canyon. It also has three weird regions called coronae, each at least 200 kilometres wide. These look relatively crater-free and young. They are separated from the rest of the surface, which is older and cratered, by concentric belts of rolling hills. One idea is that these hills formed out of ice rising up from within the moon.

Triton

Neptune has only 14 moons, less than Jupiter, Saturn or Uranus, and it is probably because of Triton. It orbits in the opposite direction to the planet鈥檚 spin and the other moons, so it might have knocked others out of the way.

This retrograde orbit indicates that Triton didn鈥檛 form at the same time as Neptune. Instead, it was probably captured in the planet鈥檚 gravity. It is a little bigger than Pluto, and its surface is made of similar materials, so it may well have come from the dwarf planet鈥檚 backyard, the Kuiper belt.

Part of its surface its textured like the skin of a cantaloupe melon. It also has plumes of nitrogen gas and dust that spurt kilometres high. These plumes may come from areas where sunlight heats nitrogen ice, causing it to sublimate into the atmosphere. Or they could be caused by ice expanding underground and blasting holes in the moon鈥檚 surface, like the plumes on Saturn鈥檚 moon Enceladus. Having said that, the plumes on Enceladus seem to crop up all along a pattern of linear depressions known as tiger stripes. 鈥淥n Triton we saw about four of them,鈥� says planetary scientist Elizabeth Turtle at Johns Hopkins University in Baltimore. 鈥淪o even if they are connected to some sort of deeper body of liquid, there鈥檚 still some difference in the plumbing.鈥�

This article appeared in print under the headline 鈥淭he Call of the Ice Giants鈥�