Planet Nine from outer space
Astronomers are confident they are on the cusp of locating a giant, lurking at the fringes of the Solar System. RICHARD A. LOVETT reports.
GENERATIONS OF SCHOOLCHILDREN were taught the Solar System contains nine planets. Perhaps they weren’t misled after all. Pluto may have been banished from the planetary pantheon but astronomers are now almost certain the system really does have a ninth planet hiding in its outer reaches; and this one’s a giant.
With the same mounting excitement that attended the search for Pluto a century ago, 10 astronomical teams are training giant telescopes on the likely orbit of Planet Nine. The planet is believed to be Neptune-sized, circling the Sun in a highly elliptical orbit 200 to 1,200 times more distant than that of the Earth (Pluto’s orbit, by comparison, is never more than 49 times more distant). A full revolution around the Sun is estimated to take about 16,000 years.
Unless the models are totally wrong, somebody is going to find it soon. Meanwhile speculation about the possible origin of Planet Nine is running rife. One of the most intriguing theories is that the Sun roped it in as a stray from interstellar space.
Arguments for the existence of Planet Nine hark back to the same logic that was used to deduce the existence of Neptune and Pluto. It was the skewed orbit of Uranus that suggested the tug of an unseen outer planet that proved to be Neptune. Oddities in Neptune’s orbit then helped lead astronomers to Pluto. Now the skewed orbits of objects in the Kuiper Belt (where Pluto resides) suggest the influence of a gigantic outer planet.
Rodney Gomes of the National Observatory of Brazil first made this argument at the 2012 meeting of the American Astronomical Society’s Division on Dynamical Astronomy. He had observed half a dozen distant objects in the Kuiper Belt with orbits that appeared perturbed by something very big in the dim recesses of the outer Solar System.
Gomes’ peers thought his presentation was interesting but not yet planet-shifting. I was the only journalist in the room at the time, and had trouble finding science editors interested in the story. It would take another year or two, after other researchers began reporting the discovery of more orbital oddities, before the astronomical community really began to get excited.
One of those who got excited was Mike Brown at the California Institute of Technology. Brown is
known as “the man who killed Pluto” because of his team’s discovery of Eris – the distant mini-world, only slightly smaller than Pluto, that showed the Kuiper Belt was potentially home to many such bodies. This led astronomers to lift the bar on what qualifies as a “planet”, bumping Pluto off the list.
In a paper published in July 2016, Brown and colleagues declared in favour of the likely existence of Planet Nine, arguing it would solve a 150-yearold mystery of the Solar System: why it is that the orbits of the known planets all fall within a single plane, give or take a degree, while the spin axis of the Sun is tilted six degrees away from that plane.
According to all that is known about solar system formation, if the Sun and the planets arose from the same primordial disc then their rotations should be aligned. The existence of a large, distant planet with a tilted orbit whose gravitational tug has tilted the orbits of the other planets would explain the perplexing misalignment.
Studies like these, Brown says, have narrowed the hunt for Planet Nine to a portion of space small enough to be “pretty sure” the elusive body will be found within the year.
WHICH BRINGS US BACK to the question of how something that big wound up so far away. Theories vary. One idea is that the planet began its life much closer to the Sun, before being flung out into the far reaches of the Solar System billions of years ago.
Today’s models of Solar System formation are no longer premised on the belief that planets stayed put once they were formed. Instead the models begin with thousands of small objects circling the Sun in an orderly disc. As time passes, these protoplanets start colliding. Some collisions result in mergers that form planets; on other occasions, close encounters fling proto-planets into interstellar space. It is also possible for planets’ orbits to shift while remaining in the Solar System.
One increasingly accepted model for the formation of the Solar System, called the Nice model (because it was first proposed in Nice, France), suggests the system’s four known giant planets – Jupiter, Saturn, Uranus and Neptune – formed closer to the Sun than their locations now, then migrated outward – possibly with Neptune and Uranus exchanging positions in the process.
The Nice model works even better if the Solar System originally had five giant planets rather than four, says Alessandro Morbidelli of Observatoire de la Cote d’azur in Nice. Planet Nine might be that fifth planet.
Another theory is that Planet Nine is a “rogue
planet”, booted out of some other solar system and somehow captured by ours.
Evidence for the existence of such rogues comes not only from computer models but from a 2011 study by a Japan astronomical team that identified Jupiter-sized planets floating in interstellar space, spotted by the way their gravity bent the light from distant stars. Such planets might be as common as stars – and if Jupiter-sized rogues exist, there are almost certainly many smaller-sized ones.
Another solar system is not the only possible source of such a rogue. In a paper presented at a January 2017 meeting of the American Astronomical Society, Eden Girma of Harvard University described how stars passing too close to the giant black hole at the centre of our galaxy can be “spaghettified” into long streamers of matter. These streamers can then recondense into planetsized “spitballs” that are flung out from the galactic centre at speeds of 1,000 to 10,000 km per second.
According to Girma’s models, these spitballs would be too big and fast to be captured by the Sun. While that means a spitball is not a likely explanation for the origin of Planet Nine, her findings nonetheless bolster the idea the galaxy might be teeming with rogue worlds, cruising the darkness of interstellar space.
At the same meeting, James Vesper of New Mexico State University presented modelling of a slower-moving stray planet passing through the Solar System. In 156 simulations of rogues of various sizes, about 60% were simply sling-shotted into interstellar space. In some cases, however, it was possible for the rogue to be captured in a far-flung orbit. “Is Planet Nine a captured rogue?” Vesper asks. “It’s plausible.”
Once Planet Nine is found, its origin may become more clear, Brown says: “My hope is that something about it will make the answer obvious, though I will admit it is not obvious what that something is. Chemical composition? Isotopic differences? A big sign that says where it came from?”
One part of the answer, Morbidelli says, may lie with the planet’s mass. “The ejection process during planet dynamics always ejects the lighter planets and keeps the heavier ones,” he says. “It would be unrealistic to think a [larger-than] Neptune planet was ejected, if Uranus and Neptune stayed behind.” In other words, if Planet Nine is bigger than Uranus and Neptune, it is probably a captured rogue.
“The future will tell,” says Morbidelli. Right now, it seems the future is close. It took decades for speculations about another planet influencing the orbit of Uranus to end with the discovery of Pluto in 1930. This time around, the search for the new ninth planet should take just years; and once found, Planet Nine will be here to stay. The only truly unknown is what we will call it.
‘ IF PLANET NINE IS BIGGER THAN URANUS AND NEPTUNE, IT IS PROBABLY A CAPTURED ROGUE’