Hidden Planets of the Solar System

The far extremes of the solar system have been probed with modern technology in recent years and are yielding their secrets. One astronomer, Eugene Chiang talks of the possibility of a halo of planets surrounding our solar system. Chiang's claim is surprising for the sheer number and size of planets in this halo, weighing more than Mars.

The discovery in 2001, of a 100km asteroid of ice and rock in the same orbit as Neptune has indicated the possibility of an undiscovered world on the edge of the solar system. Chiang is not alone in his beliefs, Alan Stern, a planetary astronomer at the Southwest Research Institute thinks it is a 100% certainty, "Definitely there are Earth sized objects out there and some will be larger than the Earth" he says.

Any discovery would revolutionise our largely two-dimensional view of the solar system. We think of the planets and asteroids as orbiting within a flat disc that extends 50AU (AU=astronomical unit, the distance from the Sun to the Earth). The halo, if it exists, would be out at 1000AU to 10,000AU with the planets orbiting at all angles.

Only a decade ago, most astronomers dismissed the idea of a 10th planet beyond Pluto (would be 9th now as Pluto is classified a dwarf planet ). Today's change of thinking is due to the latest fashionable theory to explain the creation of the solar system: oligarchic planet formation. This theory appears to demand a second population of planets surrounding our solar system.

In the oligarchic model, planets form from dust particles that gradually accumulate matter. These accumulations of dust grow to the size of asteroids, sometimes continuing to grow until they are big enough to develop appreciable gravitational fields. They then pull in smaller pieces of matter, growing rapidly until they are the size of a planet. The objects are called oligarchs because of the control their gravity has on the surroundings. Simulations suggest that around 4.5 billion years ago there were 20 to 30 Mars-sized objects in the in the inner solar system alone. In the outer system there may have been the same number, or more, Earth-sized bodies.

The theory says that the solar system was born without gas giant planets and was home to 60 or so rocky planets. Some of these became the planets we know today and the others were cast out to become the halo planets.

It was the oligarchs' gravitational tug that caused these planets to be cast out. If the oligarchs drew close to each other, their gravity would cause changes in the shape and size of their orbits. An accumulation of these changes eventually caused the system to go haywire. In the ensuing period of chaos oligarchs were flung all over the solar system by the force of their mutual gravity. Many collided and coalesced to become the planets of today. In the outer solar system they grew large and attracted big gaseous atmospheres.

But according to computer simulations, not all of the oligarchs in the outer system coalesced into gas giants. Any oligarch that narrowly avoided colliding with a growing giant planet experienced the slingshot gravity that dramatically altered its orbit. These planets were thrown into exile in the halo.

The present models are not powerful enough to predict what happens to oligarchs with any certainty. But certainly about 10 to 20 per cent of the oligarchs were slung out of their orbits - between 6 and 12 Earth or Mars sized oligarchs.

Some of the oligarchs will have probably been thrown clear of the solar system to drift around the galaxy, called planetars. Most, however, should still be in orbit around the sun forming a halo of planets that loop the sun in giant orbits taking between tens of thousands and millions of years to complete.

Oligarchic theory does have its opponents. Alan Boss of the Carnegie Institution in Washington DC thinks that the giant planets formed in an instant when the dust and gas collapsed together in a process known as disc instability.

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