New research hints at vast, unseen planet orbiting sun

Astronomers believe Planet Nine could be three times bigger than Earth

Illustration of the solar system, showing the paths of the eight major planets as they orbit the Sun. The four inner planets are, from inner to outer, Mercury, Venus, Earth and Mars. The four outer planets are, inner to outer, Jupiter, Saturn, Uranus and Neptune.
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Astronomers believe they now have compelling evidence for a vast unseen planet orbiting our sun.

Estimated to lie at least 500 times further from the sun than the Earth, calculations suggest it may be as much as three times as big as our planet, and 10 times its mass.

Code-named simply Planet Nine, a search for it is now underway using telescopes that could find it at any moment.

Confirmation of its existence would mark the first major addition to the sun’s family since the discovery of Neptune over 170 years ago.

And in a case of history repeating itself, the strongest evidence for Planet Nine comes from the discovery of strange behaviour among other objects in the solar system.

The existence of Neptune was first revealed by its gravitational pull on its nearest neighbour, Uranus, which kept straying from its predicted orbit.

Now astronomers Michael Brown and Konstantin Batygin of the California Institute of Technology have used the same idea to predict the existence of Planet Nine.

Analysis of the paths of small icy bodies known as Kuiper Belt Objects (KBOs) far beyond Neptune has revealed that some of their orbits seem to cluster in space.

The effect has been seen in only a handful of the most distant KBOs. As such, it could be just a fluke.

But there is another possibility: that they are being herded together by the gravity of some huge as-yet unseen planet.

To investigate this possibility, Brown and Batygin used a computer to simulate the effect of such an object on a random collection of KBOs scattered in space beyond Neptune.

They found that the herding effect could be explained by the existence of a planet around 10 times the mass of the Earth on a vast, elliptical path around the sun taking around 20,000 years to complete.

But the simulations had an unexpected bonus: they predicted that if the planet existed, it would force some KBOs to follow orbits tilted sharply relative to those of the other known planets.

Sure enough, when Brown and Batygin searched the catalogue of known KBOs, they found five following such orbits.

There remains another possible explanation for the clustering effect, however: so-called observational bias. By coincidence, the telescopes that found the KBOs may have caught them when they happened to lie in the same part of the night sky, creating the appearance they had been herded there somehow.

Now Brown and Batygin have investigated that possibility and their findings, just published in The Astronomical Journal, suggest the odds of such bias explaining the clustering are just 1 in 500.

With the existence of Planet Nine now the most plausible explanation, the pair are focusing on pinning down its whereabouts in the night sky.

Brown and Batygin have joined forces with astronomers at the University of Michigan to estimate the planet’s likely appearance – and thus the chances of telescopes being able to see it.

In research about to appear in Physics Reports, the team now think Planet Nine could be smaller and less massive than originally thought: around 30,000 km across and five times the mass of the Earth.

Surprisingly, this would make Planet Nine easier to find. That’s because a lower mass means the planet would need to come closer to the sun to herd the KBOs – making it brighter.

According to the team, there’s a chance that the planet might already have been spotted by Pan-STARRS, a pair of Nasa-funded telescopes on Hawaii.

With their huge field of view, these can scan the entire sky once a week, looking for the tell-tale shift in position typical of objects orbiting around the sun.

Such a shift may lie as-yet unrecognised in the image files of Pan-STARRS. At Planet Nine’s colossal distance, the shift would be tiny: equivalent to the width of a single hair seen from a distance of 10 metres.

Even so, it’s within the capabilities of Pan-STARRS, which already has some impressive discoveries to its name – including ‘Ouamuamua, the first-ever object from another star system.

But calculations suggest Planet Nine may currently lie in the constellation Orion – close to the Milky Way, which is packed with stars. Finding a new planet there would be like looking for a celestial needle in a haystack.

It is also possible that the planet is larger, more massive and thus fainter – requiring a bigger telescope.

Fortunately, the perfect instrument for spotting Planet Nine is currently being built in the Andes in northern Chile. Due for completion next year, the Large Synoptic Survey Telescope (LSST) has a gigantic 8.4-metre mirror that will scan the entire sky every few nights.

The team thinks the LSST has the power to settle the mystery one way or the other: “Planet Nine – if it exists as described here – is likely to be discovered within a decade," they have claimed.

But the discovery of so vast a planet on so strange an orbit raises a whole new mystery: how did it get there?

Intriguingly, the most obvious answer – that Planet Nine has always been there – is almost certainly wrong.

Planets are thought to be created from the dust and gas that surrounds new stars, with collisions and gravity pulling the debris together into ball-shaped worlds.

But orbiting so far from the sun, Planet Nine would travel too slowly through the debris to acquire sufficient mass.

This has led the team to conclude the planet probably formed elsewhere.

One possibility is that it was once part of the debris that formed the giant planets Uranus and Neptune, but was hurled into deep space in the chaos that reigned in the early solar system.

But there is another possibility: that Planet Nine was captured from another solar system.

Over billions of years, the sun has had close encounters with many other stars – some of which have their own family of planets.

According to the team, there is a chance the sun may have kidnapped Planet Nine from one of its neighbours, and has kept it hostage ever since.

Whatever the truth, it seems we are on the brink of a truly historic event: the discovery of a vast new world on our celestial doorstep.

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