THERE’S something odd going on in the solar system. Our once settled and peaceful home is looking increasingly disturbed and deranged. Bits of it are flying about in eccentric and inexplicable ways. Other bits seemingly shouldn’t be there at all. Meanwhile, the sun is rotating at a rakish angle we are hard-pressed to explain.
, a planetary scientist at the California Institute of Technology in Pasadena, thinks he can explain these strange goings on. There is an unsettling influence in the room: something big and distinctly mobile. Not a poltergeist – but another planet.
Advertisement
It is a controversial claim that, if true, would revolutionise our view of the solar system and go some way to explaining why it looks so peculiar when compared with other solar systems. Brown and other proponents of “Planet Nine” say they now have enough clues to pin down its existence once and for all – or show we must seek another explanation for the solar system’s eccentricities.
It wouldn’t be the first time a planetary interloper had been unmasked in this way. Back in the 1840s, astronomers couldn’t explain the wobbling orbit of Uranus, then the solar system’s seventh and outermost planet. The French mathematician Urbain Le Verrier cracked the nut, suggesting the wobbles revealed a hitherto unseen eighth planet, and pinpointed where it must be. Just a few months later, astronomers found Neptune pretty much just at the right spot.
It’s no sure-fire winner, mind: a decade or so later, Mercury had the collywobbles, and Le Verrier issued the same prescription. An unnoticed planet, dubbed Vulcan, was orbiting between Mercury and the sun and disturbing the cosmic balance, he suggested. But Mercury’s orbital oddity was eventually revealed to be down to something completely different, exposing flaws in the underlying theory of gravity that only Einstein’s general theory of relativity would correct. Still, the principle remained: interesting things come to those who take note of planetary irregularities.
We have a few to take note of now. In 2006, Pluto was controversially declassified as a planet, largely because of the discovery of a swarm of other trans-Neptunian objects (TNOs) orbiting in the Kuiper belt beyond Neptune and even further out. One, Eris, was nearly as big as Pluto – although all are too puny to be full planets as now defined (see “How to make a planet”, below). And some of these objects follow truly bizarre paths. Take Sedna. This 1000-kilometre-wide body takes more than 11,000 years to revolve around the sun, and does so on a highly elliptical, or eccentric, trajectory. Where 1 astronomical unit, or AU, is Earth’s distance from the sun, Sedna varies between 76 AU, about double Pluto’s average distance, and a wild 940 AU.
How to make a planet
The International Astronomical Union’s definition of a planet, adopted in 2006, controversially demoted Pluto to a dwarf planet. To be a fully blown planet, a solar-system body must now fulfil three criteria:
1. It must orbit the sun;
2. Its mass and gravity must be large enough to mould it into an almost round shape;
3. It must have cleared its surrounds of bodies other than those bound to it by direct gravitational influence (such as moons).
As just one of many “trans-Neptunian objects” orbiting in a similar space, Pluto fell foul of the third criterion. Planet Nine, if it exists, is almost certainly sufficiently larger than Pluto for none of the conditions to be a roadblock.
Sedna and a handful of other “extreme” TNOs also orbit at distinct angles to the ecliptic, the plane around the sun’s midriff on which all the major planets lie. There are other similarities in their orbits that are hard to explain with our current models of solar system dynamics.
Many explanations have been put forward for these oddballs. Some suggest, for example, that they are interlopers forced into the solar system by an interaction with a passing star. And in 2012, Rodney Gomes of the National Observatory in Rio de Janeiro proposed that they might be influenced by an as-yet undiscovered “planetary mass solar companion” lurking hundreds of AU out. Each time one of the extreme TNOs came close to it, its orbit would be altered, eventually causing them all to skew in a similar manner (see diagram and video below).
Gomes didn’t get much attention, but in 2014, Chadwick Trujillo of the Gemini Observatory in Hawaii and Scott Sheppard of the Carnegie Institution for Science in Washington DC . In January 2016, Brown and his Caltech colleague Konstantin Batygin used the orbits of six extreme TNOs to pin down how big Planet Nine must be and what its orbit must look like.
They came up with a Neptune-sized world in a highly elliptical orbit at an average of 400 to 500 AU from the sun. If this planet exists, in an orbit tilted from the ecliptic by about 18 to 25 degrees, it has probably not yet completed a single revolution of the sun since woolly mammoths and sabre-toothed tigers roamed Earth 10,000 years ago.
Such a misaligned giant might not be so shocking, given what we have learned about planets orbiting other suns in the past couple of decades. In exoplanetary systems, inclined, eccentric orbits are the rule rather than the exception. Our sun, with its retinue of planets orbiting on neat, near-circular orbits all in the same plane, looks rather anomalous.
“Planet Nine wouldn’t have completed an orbit since woolly mammoths roamed Earth”
Even another long-standing mystery, that of the sun’s misaligned spin axis (see “Sun skew”), is less perplexing if we expect in our own solar system something we see in others: that planets orbit not just in two dimensions, but three. “With exoplanets we have a wide variety of alignments,” says , a planetary scientist at the University of Arizona. “It was when it was just our solar system that we thought everything is aligned.”
That still doesn’t mean Planet Nine exists. Because we don’t know where exactly on its wide orbit it might be, the planet hunters must still search a several-degree-wide swathe amounting to about 8 per cent of the sky. That’s a lot less than the whole sky, admittedly, but it is still a challenge.
Planet Nine’s highly elliptical orbit means it is also statistically much more likely to be at the far end, a long way from the sun, so you would need a big telescope to see it. Brown and his team have just collected what he describes as a “nice data set”, which they are currently evaluating using the 8-metre Subaru Telescope atop Mauna Kea in Hawaii.
Others have joined the hunt, too. One team is looking at images from a smaller telescope in Chile that were collected for the international Dark Energy Survey. The goal of that study is to patrol the sky for far-off supernovae and measure the distance to them, the better to understand how the universe is expanding. But you can also spot things closer by as they trundle across the foreground of the images.
So far, the Dark Energy Survey team has found a tidy 200 or so TNOs besides the 1867 . Most of these new discoveries aren’t in orbits relevant to the search for Planet Nine, and none is big enough to be the suspected planet. At the Division of Planetary Sciences meeting of the American Astronomical Society this October, however, at the University of Michigan in Ann Arbor announced that the team had found another data point in the hunt for Planet Nine: a 500-kilometre-diameter misfit TNO her team nicknamed “Caju”, after the Portuguese for cashew. “We’ll fold it into our predictions,” says Brown. “That will inevitably move our search region a little bit.”
Becker’s team has also found some TNOs in orbits so elongated that at their closest approach to the sun they come close to Neptune’s orbit – so close that the planet’s gravity should long ago have sent them plunging into the sun, crashing into another planet or on a fast-track exit from the solar system. Becker thinks they are still there because of Planet Nine. “Instead of getting kicked out, they hop into a new orbit,” she says. “Planet Nine enhances the dynamic stability of these objects.”
Meanwhile, Matthew Payne, an astrophysicist at Harvard University, thinks Planet Nine could also help to explain the elongated trajectories of another class of outer solar system bodies, the centaurs, whose orbits cross those of one or more of the giant planets: Jupiter, Saturn, Uranus and Neptune. In the hope that they too might strike lucky in the hunt, Payne’s group is combing data from the two 1.8-metre telescopes of the Pan-STARRS system in Hawaii, which were primarily designed to find near-Earth asteroids.
All I want for Christmas…
Not everyone is leaping on the bandwagon, however. In July, a team involved in a decade-long sky survey called the Outer Solar System Origins Survey (OSSOS), which has discovered more than 800 TNOs, published a statistical analysis. This showed that if you point your telescope in certain directions – to avoid the clutter of known planets and asteroids in the ecliptic, for example, or away from the brightness of the Milky Way – at times of the year when important telescopes are likely to have the best viewing conditions, you are clustered on oddball orbits that might make you think Planet Nine exists.
“Every time you observe the Kuiper belt, you are subject to selection effects,” says Volk, who is a member of the OSSOS team. “For our data, the clustering is completely explainable by observation biases.”
On the other hand, a team of Spanish astronomers concluded this October that the nodes of 28 TNO orbits – the points at which they cross the plane of the ecliptic – are non-randomly distributed . The distribution of orbital nodes shouldn’t be subject to the type of observation biases exposed by the OSSOS team. And at the Division of Planetary Sciences meeting, the Dark Energy Survey team presented an analysis of the distribution of eight extreme TNO orbits using a procedure similar to that used by the OSSOS team, finding only a 4 per cent chance that the apparent clustering is due to selection bias.
While that’s far from a definitive statement either way, as long as there is any hope that Planet Nine is out there, astronomers are going to hunt for it any way they can (see “Shift and add”). It could, admittedly, be a never-ending quest. If no one sees it, is that because it doesn’t exist or because we somehow missed it again – we were looking in the wrong direction, perhaps, or it happened to be so close to a bright star in the sky that it disappeared into the glare?
That’s a tad problematic, says of the University of British Columbia, Vancouver, who leads the OSSOS team. Science works by disproving hypotheses, and it is unclear how the Planet Nine hypothesis could ever definitively be proved incorrect. “But don’t get me wrong,” he says. “I’d love there to be another planet out there. It would be cool and exciting.”
Brown, for his part, is bullish. He has more telescope viewing time scheduled this month. “If we find it in this chunk of data, I would not be surprised,” he says. “I’m thinking it would make a nice Christmas present.”
Sun skew
Planet Nine, if it exists, could help explain a long-standing mystery about the sun. Since the 1850s, we have known that our star rotates on an axis tilted six degrees from the average plane of its retinue of planets. With the exception of Mercury, however, those planets are not offset from each other by more than about a degree. “It’s this small, little-known mystery,” says Elizabeth Bailey at the California Institute of Technology.
Over the years, astronomers have proposed explanations ranging from magnetic interactions with the primordial disc from which the sun and planets formed to the disruptive influence of an ancient stellar companion that somehow got lost to interstellar space.
But Planet Nine might also have done it. In 2016, Bailey and her supervisor, Mike Brown, showed that gravitational torque from Planet Nine, in its inclined orbit, would have slowly tilted the orbits of the other planets by just the right amount until, from our perspective, it looks like it is the sun that’s leaning sideways. “The sun hasn’t tilted, the solar system has tilted,” says Brown. “But if you live on Earth, you think the solar system is straight.”
Shift and add
Could a small telescope and a large supercomputer help the hunt for Planet Nine? That’s the hope of , a graduate student at the University of California, Berkeley.
Specks of light corresponding to Planet Nine would ordinarily be too faint to show up in snaps taken by the 1.23-metre telescope at the Palomar Observatory on the outskirts of San Diego in California. But by using hundreds of thousands of images taken over almost a decade, and laying those that feature the same parts of the sky on top of each other, Medford hopes to bring them out.
A similar “stacking” method is used to find faint, unmoving stars or galaxies. The difference is that Planet Nine would be moving in our field of vision. Instead of stacking, Medford must “shift and add”, moving each image by a few pixels along every possible trajectory along which Planet Nine could be travelling, looking for the magic formula that causes a planet to suddenly hop out of the data.
This is, to put it mildly, a staggeringly difficult task, one that tests the limits of the US Department of Energy’s supercomputers at the Lawrence Berkeley National Laboratory. But Medford thinks he is on the cusp of pushing the “go” button, with results possible soon.
This article appeared in print under the headline “The invisible hand”
