After probing Jupiter with Juno, what next for Nasa?

Elliptical orbits of large asteroids beyond Pluto suggest a massive heavenly body is out there, but because it receives little or no light, it’s going to be a tough one to find.

The success of the Juno probe in arriving at Jupiter prompts another question – where next for Nasa? The next search could be in deep space for what has become known as Planet Nine. Nasa / AP Photo

Nasa’s triumph in putting a lorry-sized probe in orbit around Jupiter this month masks an inconvenient truth for the space agency: it’s running out of places to go.

Even before Juno arrived at the giant planet, Nasa had sent probes to all the key destinations of the solar system at least once, completing its bucket list last summer when the New Horizons mission zipped past Pluto.

So where next? Fortunately, astronomers think they may have found a stunning new destination for Nasa to aim for – a planet far beyond Pluto and one much bigger than the Earth.

Code-named Planet Nine, hints of its existence have been building for more than a decade. Now some of the world’s biggest telescopes are being employed to scour the region of the night sky where it is thought to lie.

They have been led there by a series of clues littered about the dark wilderness of space far beyond Pluto, once regarded as the furthest outpost of the solar system.

Discovered in 1930, Pluto was itself thought to be the ninth planet, with a mass roughly equal to that of the Earth.

But as more data emerged it became clear Pluto was smaller even than our own Moon, prompting its official demotion to the status of dwarf planet in 2006.

But by then hints of a worthy successor to the title of ninth planet had begun to emerge.

Pluto’s demotion had been prompted by the discovery during the 1990s that it is just one of a vast belt of similar moon-like bodies that lies beyond Neptune.

Known as Kuiper Belt Objects, or KBOs, more than 1,000 have been identified, their vast orbits round the sun strongly influenced by the huge gravitational pull of Neptune. One KBO broke the mould. Sedna, discovered in 2003 by a team led by Prof Mike Brown, of the California Institute of Technology, has a bizarre, elliptical orbit that took it out more than 140 billion kilometres from the Sun.

That made Sedna a long-range probe of whatever lay out in the frigid void far beyond Neptune. Studies of its orbit suggested it was under the gravitational influence of something other than Neptune.

These suspicions grew in 2014, when another Sedna-like object was found on a barely less extreme orbit.

Combining its orbital data with that of Sedna and three other long-range KBOs, Prof Brown and his Caltech colleague and theoretician Konstantin Batygin found signs of gravitational “herding” by some unknown source of gravity.

To find out more, they turned to one of the standard tools in modern-day science – computer simulations.

Starting from a random collection of KBOs spread out beyond Neptune, the two tried to recreate their current arrangement by introducing a hypothetical planet with different masses and orbits.

This showed that Planet Nine had to be pretty hefty, about 10 times the mass of Earth, and followed a highly elliptical orbit going even deeper into space than Sedna.

The simulations also revealed a telltale sign pointing to the reality of Planet Nine – it would lead to some KBOs following strange paths tilted almost vertically relative to the Earth’s orbit.

Describing their work in the current issue of Physics World, Prof Brown and Prof Batygin report how they checked for the existence of such KBOs last autumn, and were stunned to find that five were already known to astronomers.

Even so, the professors remain cautious about the evidence: “Until Planet Nine is caught on camera, it remains a theoretical prediction.”

So what are the chances of Planet Nine being seen? They say the object is probably about three times the diameter of the Earth and is about 16 times further from us than Pluto.

Being so far from the Sun, Planet Nine will thus be extremely faint and certainly thousands of times dimmer even than Pluto.

Nevertheless, astronomers believe telescopes are well capable of spotting the new planet, if they are looking in the right place.

Calculations suggest it may lie in the constellation of Orion and, given it probably takes 19,000 years to complete one orbit of the Sun, Planet Nine is not moving out of that region any time soon.

Among the telescopes already recruited to the hunt for Planet Nine is the colossal, 8.2-metre Subaru Telescope in Hawaii, which can scour large areas of the night sky for extremely faint objects in one pass.

Meanwhile, other astronomers are trying to answer the next big question about Planet Nine – if it does exist, why is it so far away from the other eight?

Until recently, the standard theory of the solar system pointed to a pretty sedate origin for the planets that would make Planet Nine a huge conundrum.

Along with the Sun, they supposedly condensed out of a primordial cloud of dust and gas about 4.5 billion years ago, with gravity sculpting them and leaving them in pretty much the same orbits they started in.

But more recently, computer simulations have suggested the early solar system was a far more violent place, with the major planets locked in gravitational tussles that shunted them around.

Some may have changed places – or even been thrown out altogether.

Prof Brown and Prof Batygin are among those who think Planet Nine may be a case in point, ejected from the solar system billions of years ago. But now a new theory is doing the rounds that our Sun stole Planet Nine from another star.

A study recently published by the UK Royal Astronomical Society suggests that the “theft” could have taken place when the Sun was still close to other newly formed stars 4.5 billion years ago.

As it left its galactic birthplace, it could have snatched Planet Nine from one of its neighbours.

This would make Planet Nine a truly extraordinary member of our solar system – an “exoplanet” from another star.

With even the nearest stars being 10,000 times further away than Pluto, Planet Nine thus presents a potentially unique opportunity.

As study leader Dr Alexander Mustill, of Lund University, Sweden, put it: “This is the only exoplanet that we realistically would be able to reach using a space probe.”

Using the technology that took Juno to Jupiter, it would take a probe decades to get to Planet Nine. But to Nasa’s engineers, that will sound like a challenge too good to pass up.

Robert Matthews is Visiting Professor of Science at Aston University, Birmingham.