Can We Call Pluto and Charon a 'Binary Planet' Yet?

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The debate as to whether Pluto is a planet or a dwarf planet rumbles on, but in a new animation of the small world, one can’t help but imagine another definition for Pluto.

NEWS: Radio Telescope Pinpoints Pluto for Spacecraft Buddy

As NASA’s New Horizons spacecraft continues its epic journey into the outer solar system, its Kuiper Belt target is becoming brighter and more defined. Seen through the mission’s Long Range Reconnaissance Imager (LORRI) camera, this new set of observations clearly shows Pluto and its biggest moon Charon locked in a tight orbital dance separated by only 11,200 miles. (Compared with the Earth-moon orbital separation of around 240,000 miles, you can see how compact the Pluto-Charon system really is.)

Close-up of the Pluto-Charon system as imaged by NASA's New Horizons spacecraft. Click to enlarge.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

“The image sequence showing Charon revolving around Pluto set a record for close range imaging of Pluto—they were taken from 10 times closer to the planet than the Earth is,” said New Horizons mission Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colo. “But we’ll smash that record again and again, starting in January, as approach operations begin.

“We are really excited to see our target and its biggest satellite in motion from our own perch,” he added in a New Horizons mission update on Thursday, “less than a year from the historic encounter ahead!”

ANALYSIS: Pluto and Moon Charon May Share Same Atmosphere

We’ve seen fuzzy photographs of both Pluto and Charon before, but this animation shows something more.

Over the course of 5 days, LORRI snapped 12 images of the Pluto-Charon system, tracking nearly one full (6.5 day) Charon orbit. New Horizons was between 267 million to 262 million miles from Pluto at the time. As Charon orbits, however, there’s a very clear wobble in Pluto’s position — Charon’s mass (nearly 12 percent that of Pluto’s) has a powerful gravitational influence on Pluto, yanking it “off center.” Therefore, both masses orbit an imaginary point above Pluto’s surface. This point is called the “barycenter” of the Pluto-Charon system.

This is unique for any planetary body in the solar system — only binary asteroids have been found to have barycenters outside of either mass’ body. This fact alone has led calls for Charon to be recognized as a planetary body in its own right, or that the Pluto-Charon system should be redefined as a “binary planet” — much like the definition for binary stars that are common throughout the galaxy.

Animation of Pluto and Charon. Both bodies orbit a common center of mass known as a "barycenter." Click to enlarge.

OPINION: Not a Dwarf: Is Pluto a Binary Planet?

In a thought-provoking article in 2012, Discovery News’ Ray Villard investigated this possibility, pointing out that the other 4 moons discovered so far around Pluto don’t orbit Pluto. They follow Keplarian orbits about the Pluto-Charon barycenter — the center of mass of Pluto and Charon. They are satellites of Pluto and Charon, not just Pluto!

The international body that defines celestial classifications, however, has yet to address this fact. The International Astronomical Union (IAU) will likely need to re-examine the case of Pluto and Charon, especially after we get a close-up view of Pluto next year.

As New Horizons quickly approaches the Pluto-Charon system for its historic flyby in July 2015, we’re going to see more detail in the Pluto-Charon system; evidence for surface dynamics, detailed geological features and probably more minor satellites (that may or may not prove hazardous for the NASA probe). This mission will put a new spotlight on Pluto and its “dwarf planet” status, potentially highlighting its current classification as a woefully inadequate description of such a dynamic and interesting binary system.

Perhaps re-re-defining Pluto as a binary planet or a dwarf planet binary isn’t such a bad idea after all. What do you think?

Source: New Horizons mission page

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