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Lucky strike?

In the past decade or so, astronomers have found more than 100 planets around other stars. Now they've apparently found a new one around our own star. 2003 UB313 is a stunningly cold object that spends most of its time far beyond Pluto, in an egg-shaped orbit that's tilted about 45 degrees to the plane of the solar system.

Large white cylinder with lenses, cameras and mirrors for seeing distant objects in space.According to a report in this week's Nature, UB313 is considerably larger than Pluto.

Astronomers have feuded for years over whether Pluto is a planet, or just an icy orbital object that was mistakenly branded as a "planet."

At Palomar Observatory, the long tube on the bottom is the finder scope used to guide the camera. Photo: Caltech

Like UB313, Pluto is part of the Kuiper belt, a swarm of orbiting junk beyond Neptune that is hard to see, let alone understand.

The new data on UB313 gives us two choices. UB313 orbits the sun, like the nine planets, and it's larger than one of them, which should, in a fair and consistent world, make it the tenth planet. But the Kuiper belt may contain many similar objects, and if you don't want to promote an unknown list of other dim, distant objects to planethood, then maybe Pluto does not deserve its vaunted planetary status. "It needs to be a planet if we consider Pluto a planet," says Scott Sheppard of the Carnegie Institution of Washington. "You either drop Pluto, or include this new object."

As our knowledge of the universe and solar system increases, he says, "We see that nature is not being nice to us, it does not put thing into nice bins."

Planet of ice revolves around a very distant, but bright sun
An artist's drawing of the possible appearance of 2003 UB313. The bright star is the Sun. Photo: NASA

Seek and ye shall find
We talked to Frank Bertoldi, a professor of radioastronomy at the University of Bonn (Germany), chief author of the new paper, and he told us that UB313 was first imaged in 2003 during a computerized search for Kuiper belt objects by Michael Brown of the California Institute of Technology. Not until 2005, however, did Brown's group publish the discovery of UB313, which was moving slower across the plates than the closer-in Kuiper-belt objects Brown was originally seeking.

Like the sun, planets and moons, objects in the Kuiper belt condensed when the solar system formed from a whirling disk of gas and dust about 4.5 billion years ago.

UB313 is so distant that it's just a point of light through a telescope. Its distance can be calculated based on its velocity and orbit, but finding its size is more difficult, since a small, shiny object can be just as bright as a big, dull one. "Albedo [reflectivity] can vary from 3 to 4 percent for comets to 60 percent for Pluto," said Bertoldi.

To calculate the size and reflectivity of UB313, you start by assuming that it's in equilibrium with its environment: incoming energy equals outgoing energy. Incoming solar energy (which we know based on distance from the sun) is either reflected as visible light, or absorbed and reradiated as infrared radiation. This reradiated infrared can be measured from Earth, and it indicates that UB313 is a frigid 24 to 27 degrees Kelvin. (That's colder than Pluto, which is about three times closer to the sun and basks in a balmy 36 degrees above absolute zero.)

From these numbers, you can calculate reflectivity and then UB313's diameter, about 3,000 kilometers. Various uncertainties could throw this estimate off by a few hundred kilometers either way, but it's highly unlikely that UB313 is smaller than Pluto, estimated at about 2,300 kilometers, Bertoldi added.

Diagram shows incoming = reflected + reradiated. reflected/incoming = reflectivity
From the incoming solar radiation and the outgoing infrared radiation, you can calculate reflectivity.

Plutonic problem
And that's the problem. For more than a decade, astronomers have thought that Pluto had little in common with the other planets. Instead, it's part of the Kuiper belt, a scattered group of far-flung objects at the edge of the solar system. But there's been no sustained push to delegitimate Pluto.

So if Pluto is indeed the ninth planet, then the somewhat larger UB313 is the "tenth planet."

So far, so good (so long as somebody finds a less monstrous moniker than "2003 UB313"). But the Kuiper belt may have more objects in the same size range. How many new planets are we willing to accept?

Alternatively, we could rebrand Pluto as a "Kuiper-belt object," and shrink the solar system to eight planets. But Bertoldi thinks that's a poor option: "We don't want to demote Pluto, that would be impolite culturally. We would rather promote others."

Although the Kuiper belt may have more UB313-like surprises, he does not expect many. "And I think it's exciting to discover new planets, in our solar system or extrasolar. Why not have growth in our own solar system?"

A time-lapse photo shows a new planet, represented by a small dot of light, revolves around the sun.
This time-lapse image of 2003 UB313, taken by Palomar Observatory in 2003, shows it moving across a field of stars. This movement shows UB313's velocity, which helps determine its distance from the sun. Photo: NASA

Systematic system
The International Astronomical Union will tease that one apart (anyone want to buy naming rights to the 10th planet?), but no matter what it decides, the discovery adds finer detail to our picture of the solar system. Rather than one star and nine planets in largely circular orbits, it now appears to be a smorgasbord of planets, moons, asteroids, comets, and those weird whatchamacallits in the Kuiper belt. The orbiting zoo includes things that are rocky or icy, and big fat bags of liquid gas. It includes solar neighbors, like broiling Mercury, and distant travelers in the Kuiper belt and the even more far-flung Oort Cloud of comets.

A similar diversity appears in telescope images of other solar systems, Bertoldi said. "We have also been looking at middle-aged stars that have dusty disks.... They have planets, and small-object clouds that are the equivalent of the Kuiper belt. There is a direct connection between what we can see [in this solar system], and what we see in other stars with planetary disks. To make that connection is very exciting."

-- David Tenenbaum

A Planet More, a Planet Less? Scott S. Sheppard, Nature, Feb. 2, 2006.
The Trans-Neptunian Object UB313 Is Larger than Pluto, F. Bertoldi et al, Nature, Feb. 2, 2006. .

Related Why Files
Making of a Comet
Lost and Found in Space
Where Do Planets Come From?
Infrared Astronomy
Moon Origins
Hubble's Greatest Hits
Star Formation

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