The discovery of two new satellites of Pluto Max Mutchler Space Telescope Science Institute Open Night 3 January 2006 Andy Lubenow 1956 - 2005 Hubble Pluto Satellite Search Team Overview reporting the discovery to the New Horizons Science Team on November 2, 2005 at the Kennedy Space Center • Discovery of Pluto, Charon, and the Kuiper Belt •Early Hubble observations of Pluto • Hubble mission support for New Horizons: discovery of two more Pluto satellites • Confirming and following-up the discovery • Implications, and recent related discoveries Left to Right: • New Horizons mission update by Hal Weaver Hal Weaver (JHU/APL), Andrew Steffl (SwRI), S. Alan Stern (SwRI), Leslie Young (SwRI), John Spencer (SwRI), Marc Buie (Lowell Observatory), • Questions? Bill Merline (SwRI), Max Mutchler (STScI), and…Eliot Young (SwRI) The discovery of The search for “Planet X” Pluto in 1930, Lowell Observatory, Flagstaff, Arizona and confirmation Percival Lowell Vesto Slipher Clyde Tombaugh 1 The discovery of Pluto’s moon Charon in 1978 James Christy and Robert Harrington, U.S. Naval Observatory, Washington, D.C. • 1950 Kuiper & Humason, didn’t find Charon • 1978 Christy & Harrington, serendipitous discovery of Charon (above) • 1991 Stern: found no more satellites beyond ~100,000 km from Pluto • 2005 Gladman et al. paper -- seemed to doom this Hubble search The slowly emerging picture of Pluto Charon 1200 km Earth Pluto Moon 12,800 km 2300 km 3000 km 2 Everything we know about Pluto 1 • 1930 Pluto discovered; eccentric orbit * • 1955 rotation period 6.4 days Pluto has not • 1965 stable 3:2 resonant orbit with Neptune given up it’s secrets • 1973 obliquity > 90 deg * • 1976 methane ice on surface; size constrained very easily over • 1978 Charon discovered; “binary planet” * the first 75 years… • 1980 Occultation reveals Charon radius to be 600 km • 1985 Pluto-Charon mutual events begin Everything we know about Pluto 2 • 1986 Pluto & Charon radii, albedos, colors • 1987 Pluto density is 2 g/cm3 • 1988 Pluto orbit chaotic; atmosphere, polar caps • 1989 Pluto & Triton similar, structure in atmosphere • 1992 Nitrogen and CO ice, density disparity • 1992 Discovery of the Kuiper Belt • 2001 Binary Kuiper Belt objects (KBOs) • 2005 Two more moons discovered! P1 and P2. Early Hubble observations of Pluto and Charon Discovery of two new moons of Pluto New satellite discovery observations • Hubble proposal designed by Weaver, Stern, et al., initially rejected, then accepted when STIS died • Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) covers entire orbital stability zone • Pluto-Charon near chip gap: peek-a-boo! • 4 long exposures on May Press release image for new moons: the 15 and May 18, 2005, using only 2 orbits discovery was surprisingly easy for Hubble with • Hal’s request, June 13… ACS… but not quite as easy as it looks here. 3 Calibrating and drizzling ACS images Hubble Servicing Mission 3B in March 2002: ACS installed Advanced Camera for Surveys (ACS) The Whirlpool Galaxy M51 Notice the star trails, cosmic rays, chip gap… 15 May 2005, frame 1 Notice the star trails, cosmic rays, chip gap… Dithering across the chip gap now…see anything? 15 May 2005, frame 2 15 May 2005, frame 3 4 Dithering across the chip gap now…see anything? Looking for real objects among all the artifacts… 15 May 2005, frame 4 15 May 2005, sum 4 frames Looking for real objects among all the artifacts… Do it again 3 days later…where are the moons? 15 May 2005, median 4 frames 18 May 2005, frame 1 Do it again 3 days later…where are the moons? Dither across the gap…where are the moons? 18 May 2005, frame 2 18 May 2005, frame 3 5 Do it again 3 days later…where are the moons? 18 May 2005, frame 4 18 May 2005, sum 4 frames “Clean” image “Clean” image 18 May 2005, median 4 frames 15 May 2005, median 4 frames S/2005 P 1 Charon S/2005 P 2 15 and 18 May 2005, sum 8 frames 15 and 18 May 2005, median 8 frames 6 Initial thoughts • Too easy ?!? Well-designed program: long exposure times (but not too long), two epochs • Surprised there were two new objects: one would have been plenty amazing! • Two objects somewhat validate each other, and our assumptions about their orbits • Surprised they are so close to Pluto: expecting any moons to be much farther out, but they don’t violate dynamical constraints (Stern, 1994) Confirmation and follow-up The “checklist” of possible explanations • Independent discovery in Aug 2005 by Andrew Steffl • Search other existing data: Hubble, Subaru… • Hubble follow-up: impossible until Feb 2006 (2 gyros) • Ground-based attempts to image the new moons in Sep/Oct: Keck, VLT, Gemini (difficult until spring 2006) • Artifacts from the detector or optics: hot • Checklist of alternate explanations: rule them out? pixels, optical ghosts, scattered light, etc • Confident enough to announce on 31 October 2005 • Real foreground/background objects: asteroids? binary KBO? Plutinos? • New moons of Pluto! Preliminary assumptions Pre-discovery observations in 2002 and implications • Hubble program by Buie & Young • Orbits are co-planar with Charon, nearly circular, • ACS High Resolution Channel possibly in stable resonances with each other • Primarily designed to map • Probably formed primordially with Charon surface features of Pluto and (collision), not later (captured) Charon • New moons marginally • No other moons of similar magnitude (unless detected artifacts hid them in June; we’d find them in Feb • Further observations will 2006); very compact system definitively determine orbits, and hopefully confirm these • Pluto first KBO with multiple satellites: implies detections: are the satellites there are probably many more where they should be? 7 The “quadruple planet” Pluto Relative sizes of Pluto, Charon, and the two new moons (P1 and P2) Visual Diameter Orbital Orbital magnitude radius * period * (barycentric) Pluto 14.2 2328 km 6.387 days +/- 42 km Charon 16.2 1208 km 6.387 days P1 +/- 4 km P2 S/2005 P 1 22.93 61-167 km 64,700 km 38.2 days P2 +/- 0.12 (3.7x Charon) (~6x Charon) 2300 km 1200 km ~100 km S/2005 P 2 23.38 46-137 km 49,400 km 25.5 days +/- 0.17 (2.8x Charon) (~4x Charon) New moons are 5000x fainter, 12x smaller, and 3-4x farther out than Charon, with possible 6:4:1 orbital resonances What does a “quadruple planet” look like? Announcement and publications Weaver et al, 2005, IAU Circular 8625 Weaver et al., 2006, Nature (accepted) Stern et al., 2006, Nature (accepted) Steffl et al., Astronomical Journal (submitted) Pre-prints available online at: http://arxiv.org/archive/astro-ph “Xena & Gabrielle” Xena The 10th planet? Pluto Moon Earth 8 Should we call Pluto a planet? • I’m neutral. But some things to consider… • Is Pluto the first of many Kuiper Belt “ice dwarf” planets discovered? • Is larger Xena the 10th planet? • Are slightly smaller Sedna, Quaoar planets? • Ceres was called a planet for ~50 years, then “demoted” to asteroid (a precedent) • Will we have only 8 planets, or hundreds of them? • Is this a problem? Seems like progress to me. • The IAU is working on it…in the meanwhile, it is a harmless and healthy “non-controversy” Voyagers http://pluto.jhuapl.edu Launched in 1977 Good luck to New Horizons, the next great Voyage of Discovery… It’s greatest discoveries will surely be the unexpected ones. It will surely inspire the next generation of math and science students… 13 17 00 00 Launch currently set for: January 17, 2006 2:11 PM EST Questions? Hal Weaver Johns Hopkins University Applied Physics Lab and New Horizons Project Scientist … AND TWO LITTLE MOONS ! More information: http://www.boulder.swri.edu/plutonews http://pluto.jhuapl.edu 9.