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Unique Kuiper Belt Binary Has Unscathed Orbit

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Figure 1: Gemini Multi-Object Spectrograph (on Gemini North, GMOS-N) observation of the “Plutino” binary system 2007 TY430, which is located nearly 40 astronomical units (AU) from the Sun. (The average distance between the Earth and Sun is 1 AU.) The pair is separated by about 42,000 km, which appears here at maximum separation of only 0.7 arcsecond on the sky.

Figure 2: The smooth line shows the model of the orbit of the 2007 TY430 binary, displayed as the motion of one body around the other. Individual observations are marked with crosses (the Subaru discovery and subsequent Gemini observations) and a circle (from the Hubble Space Telescope).

Small icy bodies are the remnant leftovers from the formation of planets in the Solar System. Ongoing observations made with Gemini of an extremely red pair of such Kuiper Belt objects (KBOs) in an orbiting binary system offer an indirect glimpse into the past. Scott Sheppard (Carnegie Institution of Washington), Darin Ragozzine (Harvard-Smithsonian Center for Astrophysics), and Chad Trujillo (Gemini Observatory) obtained nearly monthly observations of the pair, named 2007 TY430, to yield precise measurements of their orbital motion. Uniquely, compared with other Kuiper Belt binaries, this pair’s current mutual orbit is likely primordial, unchanged since the formation of the system. The primordial orbit reveals the binary’s formation mechanism, and therefore provides hints of past conditions during the formation of the Solar System itself. The research team concludes that this system may have formed in a more complex interaction involving yet another body. The composition of the bodies’ ultra-red material is unknown, but it may be associated with organic material and it depends upon the pair’s formation site.

The members of the pair otherwise have the characteristics similar to ordinary KBOs. They are roughly equally sized (at a radius of about 50 km each), with a total mass of nearly 1018 kg, and their mutual orbits are nearly circular. However, the overall location of the pair is somewhat closer to the Sun than the classical Kuiper Belt boundaries. The system likely moved out of the classical Kuiper Belt and then got stuck in their current location, which is favored because of a resonance with Neptune’s gravitational pull. For every three orbits around the Sun that Neptune makes, 2007 TY430 will complete two. This so-called “3:2 resonance” is the same relationship Pluto has with Neptune, so such objects are also known as “Plutinos.”

A widely separated binary pair like 2007 TY430 is not very stable, and indeed this is the first wide, equal-sized binary found in this resonance with Neptune. Most other similar systems have likely disintegrated. Thus, 2007 TY430 is possibly one of the few remaining examples of the type.

Members of the team previously reported the discovery of this system based on observations from the Subaru Telescope. The new work appearing in the March 2012 Astronomical Journal takes advantage of Gemini’s capabilities, using the Gemini Multi-Object Spectrograph to measure the system precisely, despite the apparent separation of the bodies by only 0.7 arcseconds or less on the sky (Figures 1 and 2). This corresponds to the apparent size of a dime at a distance of 3 miles. Capturing such fine measurements requires excellent image quality, which Gemini delivers.

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