27Nov2014

Astronomers Ready To Discover One, Or Even Two, New Planets

Our Solar System

Hidden world … Astronomers are examining evidence that there may be one, or two, more planets beyond the orbit of Neptune

PLANET X is back: New evidence suggests one — or possibly two — wayward worlds are circling in the darkness beyond Neptune. As intrepid explorers Voyager’s 1 and 2 can attest to, it’s very dark out there.

Quiet. Lonely. A perfect place for a planet to hide. You’ve seen maps of the Solar System: Eight planets, all neatly circulating around the Sun on a nice, flat, plane. Pluto got the chop as a planet as it dared to deviate — following an ellipse which kicks it up and away from its conformist colleagues.

Put it seems we may soon have a ninth planet once again — and possibly a tenth. A group of astronomers say they’ve found the telltale traces such planets would leave behind. And they’re clues that have Australian professor of Astrophysics Sarah Maddison intrigued.

Celestial family … The eight aligned planets of our solar system, along with a few wayward wanderers such as Pluto and Ceres. Source: NASA Source: Supplied

Astronomers have long been looking at “wobbles” and “bumps” in the orbits of Neptune, Uranus and Pluto for the signs of an as-yet undiscovered planet’s gravitational pull. It’s become known in common culture as “Planet X”. But clues to the existence of such a possible undiscovered deep-space neighbour comes from unusual orbits of “rubble” left over from the construction of the Solar System, the Kuiper Belt.

Earlier this year, astronomers Chad Trujillo and Scott Sheppard found a 450km wide lump of this icy stuff and named it 2012VP. It’s weird: The closest it comes to the Sun is 80AU (one Astronomical Unit (AU) is the distance from the Earth to the Sun) before cannoning back out to 450AU.

But it’s more than just 2012VP. Sedna — another huge 1000km wide chunk of stuff — is also on an elongated orbit which passes some 76AU from the Sun at its closest point before spinning out to a maximum 930AU. “They’re kind of in a no man’s land,” says Carnegie Institution for Science researcher Scott Sheppard. “These objects couldn’t get out there with what we currently know.”

Professor Maddison explains: “So Trujillo and Sheppard obviously got pretty excited, because astronomers were already pretty baffled by the discovery of Sedna back in 2003 — how did this object form and why does it have such an unusual orbit?

Connecting the dots … The orbits of several distant objects, including Sedna and 2012VP, indicate the presence of an as-yet undiscovered planet. Source: Supplied Source: Supplied

“Theories include good old Planet X — some as-yet undiscovered planet in the outer solar system — or a gravitational kick from a passing star. Perhaps it’s even the gravitational tug of the gravity field of the Milky Way galaxy itself.” Each of these three theories can work, she says. “But which is more likely? And how can we “prove” any of these theories?”

Trujillo and Sheppard have now noticed these big Kuiper Belt bodies and ten other large lumps of leftovers mysteriously come closest to the Sun when they pass the elliptical — the plane upon which the planets rotate. In astronomer speak, their “argument of perihelion” are all about zero degrees.

Professor Maddison, of Melbourne’s Swinburne University, explains: “This means we plot all their orbits in 3D space and compare them to the reference frame for the Earth’s orbit around the Sun, when all these outer icy bodies are at their point of closest approach to the Sun they’re pretty much aligned with the Earth’s orbit — so their “height” above or below the Earth’s orbit is about zero.

“So what would cause that? Perhaps that unseen outer planet again.” The discoverers speculate is that it would need to be a planet between twice and 15 times as heavy as Earth, orbiting the sun at some 250AU — so distant that even the biggest and best telescopes would struggle to see such a faint smudge of reflected sunlight.

The idea is that “true” planets — those that follow the ellipse — could have been “kicked out” of the inner solar system four billion years ago as giants such as Jupiter and Saturn jockeyed for space. Now they act as giant gravitational “magnets”, swinging Kuiper Belt rubble on to new tracks. It’s an argument that is gaining strength, with several arguments published in the Monthly Notices of the Royal Astronomical Society Letters.

artists impression of the surface of Pluto and its moon Charon. Now astronomers suspect two more planets lurk beyond the known edge of our solar system. Source: NASA

Researchers for the Carnegie Institution for Science and the Gemini Observatory have reviewed the data and taken their conclusions one step further: Others think the orbits of the Kuiper Belt objects need more than one planet to explain their orbits. “The analysis of several possible scenarios strongly suggests that at least two trans-Plutonian planets must exist,” write Carlos and Raul de la Fuente Marcos of the Complutense University of Madrid.

Another review calculates the orbits could be produced by a planet twice the weight of Earth orbiting at 500Au, or a planet 15 times heavier than our own at 1000AU. Not all astronomers are convinced; with some saying Neptune (at 30AU) may have enough influence at that distance to pull many of the Kuiper Belt objects into line. The rest could be down to coincidence, they say. “Everyone says “we need more data”,” Professor Maddison says. But at least we know roughly what to look for.

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