91av

IN THE dark reaches of the solar system lurk swarms of hidden worlds. Too small and too distant to reflect sunlight, they have remained under the cover of darkness for billions of years. But now the outer solar system is giving up its secrets. And with them comes an astonishing claim: “It’s quite possible that there is a halo of planets surrounding our solar system, just waiting to be found,” says Eugene Chiang, an astronomer at the University of California, Berkeley.

What makes Chiang’s claim so surprising is the sheer number and size of these planets. Weighing more than Mars, they dwarf Sedna and Quaoar, the largest rocky bodies spotted circling the sun beyond Pluto.

The evidence comes from a source closer to home: the discovery of a 100-kilometre lump of ice and rock circling the sun in the same orbit as Neptune. This frigid asteroid was chalked up as a mere curiosity when it was first spotted in 2001. Now astronomers realise it could point to an undiscovered world on the edge of the solar system.

Chiang is not the only one to think that the nine planets we know of are only half the story. Alan Stern, a planetary astronomer at the Southwest Research Institute in Boulder, Colorado, is convinced there are more sizeable planets out there. “I think it is a 100 per cent certainty,” he says. “Definitely there are Earth-sized objects out there and some will be larger than the Earth.”

Dust bunnies

Such discoveries would revolutionise our largely two-dimensional image of the solar system. We think of the planets and the accompanying swarm of icy rocks as being trapped in a flat disc that extends roughly 50 times further from the sun than the Earth. The halo, if it exists, would surround the sun with planets orbiting at all sorts of crazy angles, at 1000 or even 10,000 astronomical units (AU) from the sun – that’s up to 10,000 times the distance from the sun to Earth.

Just a decade ago, most astronomers had backed away from the idea of a 10th planet beyond Pluto. Today’s change of mind has come about thanks to the latest fashionable theory to explain the creation of the solar system: oligarchic planet formation. Adherence to this theory appears to demand a second population of planets surrounding our solar system (see Diagram).

In the oligarchic scenario, planets form from dust particles that gradually accumulate matter. “It’s the same process that makes dust bunnies under your bed,” says Scott Kenyon, a planet formation expert from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

These particular dust bunnies grow to the size of asteroids, and then sometimes keep on growing until they are big enough to develop appreciable gravitational fields. They start to pull in smaller pieces of matter, rapidly putting on weight until each one is the size of a planet. These objects are called oligarchs because of the control their gravity exerts on the surroundings. “In the inner solar system alone, simulations suggest that there were 20 to 30 Mars-sized objects,” says Kenyon of a time around 4.5 billion years ago. “In the outer solar system there may have been the same number, or even more, Earth-sized bodies.”

According to the theory, our solar system was born without gas giant planets such as Jupiter, and was home to 60 or so rocky planets. While a few of these became the planets we know today, some were cast out to become the halo planets. But what could hurl a planet out to the edge of the solar system?

The answer lies with the oligarchs’ gravitational tug. If oligarchs drew close to each other, their gravity would cause subtle changes in the shape and size of their orbits. These changes built up until, suddenly, things went out of control. A chaotic period ensued in which oligarchs were flung all over the solar system by the force of their mutual gravity. Many collided and coalesced to become the familiar planets of today. In the outer solar system they grew so large and generated so much gravity that they attracted large gaseous atmospheres.

But according to computer simulations by Kenyon and others, not all of the oligarchs in the outer solar system coalesced into gas giants. Any oligarch that narrowly avoided colliding with a growing giant planet as it swung past experienced a huge tug of gravity that dramatically altered its orbit. These planets were thrown into exile.

“We can’t predict what happens to these oligarchs with any certainty at present, because our models are not powerful enough,” says Kenyon. “But we certainly see about 10 to 20 per cent being slung out of their orbits.” That equates to between 6 and 12 Earth or Mars-sized oligarchs.

Kenyon says that some will probably have been given enough of a kick to be thrown clean out of the solar system to drift around the galaxy. These outcasts are often called planetars. Most, however, should still be in the sun’s gravitational grip, forming a halo of planets that loop the sun in giant orbits taking between tens of thousands and millions of years to complete. Because the gravitational kick given to these oligarchs alters the size, shape and orientation of their orbits, they will almost certainly be outside the disc containing the rest of the planets (see “A lot of gravity”).

Not everyone agrees with the oligarchic theory of planet formation. Alan Boss of the Carnegie Institution in Washington DC thinks that the giant planets formed in the blink of a celestial eye when dust and gas collapsed together in a process known as disc instability, rather than through coalescence alone.

According to Boss’s model, a 10th planet is unlikely to have formed far beyond Pluto’s orbit. The dusty disc from which the solar system formed was no bigger than 100 AU across, so a halo of planets 1000 AU away is virtually out of the question.

“There is a halo of planets surrounding the solar system just waiting to be found”

Crucial evidence

Which of the two models is correct is hard to say, as both claim to predict the planets we see today. The oligarchic method requires two steps, whereas Boss’s disc instability model is a one-shot deal. The key difference is the stage containing the 60 or so oligarchs, and that is the part whose existence is most difficult to prove. “The final chaotic phase of formation erases all the evidence of the original oligarchic population. That presents a problem in trying to test this theory,” says Kenyon.

Enter the icy asteroid found orbiting Neptune in 2001. It could break the deadlock because it is seen as a crucial piece of evidence for oligarchic formation. Its very existence suggests that a miniature version of oligarchic planet formation happened around Neptune, with the ice giant and its companion asteroid playing the roles of the sun and an oligarch.

Asteroid 2001 QR322 turned up serendipitously in a telescopic survey conducted three years ago by Chiang. It is the first known member of Neptune’s “Trojan” asteroid population, named after a similar family of captured asteroids that tag along in front of and behind Jupiter as it orbits the sun. At these points on Jupiter’s orbit, the gravitational force of the sun and the planet are balanced, so that rocks there remain trapped in a gravitational no-man’s land. And at 100 kilometres wide, QR322 is about the same size as Jupiter’s largest Trojan, Hektor.

But that’s where the resemblance with Jupiter ends. Chiang now thinks that QR322 formed very differently from Hektor. His computer models show that it is almost impossible for Neptune to capture asteroids because the planet is only half the size of Jupiter and so possesses a much weaker gravitational field. This proved a stumbling block until Chiang realised that QR322 could have formed in situ. If dusty debris fell into Neptune’s gravitational no-man’s land after the planet had formed, dust bunnies could have gone through a rerun of planetary formation, albeit on a much smaller scale.

So Chiang tooled up a mini-version of the oligarchic planet formation scenario on his computer and set about recreating QR322 from scratch. The ease with which he could do it gave him confidence that he was on the right track. From his model, Chiang expects 10 to 20 asteroids like QR322 near Neptune. And last month, the Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, announced the tentative identification of a second one.

Chiang had identified a region in which a completely independent version of oligarchic formation had taken place. And because the asteroids’ gravity was so much weaker, the chaotic process that kicked the planets out of their orbits in the oligarchic scenario never happened.

“They push each other around a little but are not big enough to throw each other out. So, sitting there next to Neptune should be an unadulterated population of miniature oligarchs,” Chiang says. “Find them and it should give us a better idea of planet formation and allow us to predict a little more accurately what to expect from the formation of our solar system.” If Chiang’s predicted number of Neptune Trojans is found, confidence in the oligarchic theory – and in the halo planets – will get a big boost.

Some claim circumstantial evidence for the second planetary population already exists. Stern points out that Uranus spins on its side, unlike the rest of the planets, implying that a massive collision occurred between Uranus and an Earth-sized object. Such a chance encounter is unlikely in today’s solar system, but if a large number of Earth-sized planets passed by Uranus on their way out to the scattered halo, it becomes more likely that one of them struck home.

There is also a swarm of much smaller objects that were scattered beyond Pluto by gravity. Astronomers call this swarm the scattered disc, and they think it is full of icy asteroids similar in size and composition to those found in the Kuiper belt, between the orbits of Neptune and Pluto. Researchers have already spotted around 100 objects in the scattered disc, the largest of which is the 1700-kilometre-wide rock, Sedna. Unlike the rocks in the Kuiper belt, which follow roughly circular paths around the sun, the scattered disc objects all have extremely elliptical orbits. This suggests that they have been disturbed by something as large as a planet.

But not everyone thinks that Uranus’s spin and the scattered disc are smoking guns of oligarchic formation. Boss points out that disc instabilities could have led to two ice giants colliding, leaving one spinning on its side. And the rapidly growing icy planets could have kicked passing comets off course and into the scattered disc. So the deciding factor between the two models could well come back to the existence or otherwise of the halo planets.

Despite their size, the new planets are going to be tough to find. “They will be reflecting nothing but starlight and that’s going to make them very faint indeed,” Stern warns.

Compounding the problem is the gravitational scattering that sent the planets out there in the first place. Being a chaotic process, it robs astronomers of any way to predict where in the scattered halo the planets are likely to be hiding. The only way to search for them is to trawl the whole sky using telescopes. This extraordinarily time-consuming venture is way beyond the scope of most observatories, which usually allocate time to astronomers in chunks of just two or three nights.

But the research has received a boost from an unexpected source. Concerns about the threat to Earth from asteroid impacts are spurring the development of several dedicated telescopes that will repeatedly map the whole sky. Among them is the 4.2-metre Discovery Channel Telescope (DCT), due for completion at the Lowell observatory in Flagstaff, Arizona, in 2009. Aiming for a similar completion date is Pan-STARRS, a series of four 1.8-metre telescopes that will be sited in Hawaii. Both will search for worrisome asteroids and for Kuiper belt objects, and will be able to detect objects 10 times fainter than any telescope available for the task today.

But will they be able to see halo planets? “I wouldn’t put it past them,” says Stern, “but it is going to be a tough job.”

Stern thinks that another project called the Large Synoptic Survey Telescope has a better chance. LSST is still on the drawing board and has yet to be fully funded. The plan is to site it in Chile or Mexico and for it to have a wide-angle field of view so that it can survey the entire visible sky in three days (see Chart). With a mirror measuring 8.4 metres across, it will be roughly 10 times as sensitive as DCT or Pan-STARRS, putting it 100 times ahead of anything available today.

“The hidden planets could be orbiting at all sorts of crazy angles beyond Pluto”

Hunt the planets

Zeljko Ivezic at the University of Washington in Seattle is confident that LSST will be up to the job. “A conservative estimate is that we could detect Earth-sized planets at approximately 500 AU,” he says. Tweaking the telescope’s performance and using computer techniques to combine images could improve the reach of the telescope to 1000 AU and beyond for an Earth-sized planet. Stern thinks the search will be fruitful: “I’m expecting planets 10, 11, 12 and many more to be found in the distant outer solar system, all larger than Mars and possibly even than the Earth.”

LSST itself will be unable to identify anything it discovers. Automated software will simply recognise moving celestial objects and post the coordinates onto a public website for others to follow up. And that alone will be no mean feat. “We will generate 30 terabytes of information a night,” says Ivezic. That’s enough to fill the hard discs of several hundred PCs.

Professional observatories will be working hard to stay on top of the follow-up observations. But the decision to make the information available to everyone opens the door for amateur astronomers to discover a new planet in the solar system. Would-be planet spotters should keep an eye peeled for unusually slow moving, faint objects, preferably far away from the orbital plane of the solar system. So, if you fancy going down in history, all you need is a telescope, an internet link, and an awful lot of patience and good luck.