The discovery of two new satellites of Pluto

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 reporting the discovery to the New Horizons Science Team on November 2, 2005 at the Kennedy Space Center Left to Right: Hal Weaver (JHU/APL), Andrew Steffl (Sw. RI), S. Alan Stern (Sw. RI), Leslie Young (Sw. RI), John Spencer (Sw. RI), Marc Buie (Lowell Observatory), Bill Merline (Sw. RI), Max Mutchler (STSc. I), and…Eliot Young (Sw. RI)

Overview • 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 • New Horizons mission update by Hal Weaver • Questions?

The search for “Planet X” Lowell Observatory, Flagstaff, Arizona Percival Lowell Vesto Slipher Clyde Tombaugh

The discovery of Pluto in 1930, and confirmation

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

Everything we know about Pluto • 1930 Pluto discovered; eccentric orbit • • * 1955 rotation period 6. 4 days 1965 stable 3: 2 resonant orbit with Neptune 1973 obliquity > 90 deg * 1976 methane ice on surface; size constrained 1 Pluto has not given up it’s secrets very easily over the first 75 years… • 1978 Charon discovered; “binary • • planet” * 1980 Occultation reveals Charon radius to be 600 km Everything we know about Pluto 1985 Pluto-Charon mutual events begin • 1986 Pluto & Charon radii, albedos, colors • 1987 Pluto density is 2 g/cm 3 • 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)

Early Hubble observations of Pluto and Charon

Discovery of two new moons of Pluto Press release image for new moons: the discovery was surprisingly easy for Hubble with ACS… but not quite as easy as it looks here.

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 15 and May 18, 2005, using only 2 orbits • Hal’s request, June 13…

Hubble Servicing Mission 3 B in March 2002: ACS installed Advanced Camera for Surveys (ACS)

Calibrating and drizzling ACS images The Whirlpool Galaxy M 51

Notice the star trails, cosmic rays, chip gap… 15 May 2005, frame 1

Notice the star trails, cosmic rays, chip gap… 15 May 2005, frame 2

Dithering across the chip gap now…see anything? 15 May 2005, frame 3

Dithering across the chip gap now…see anything? 15 May 2005, frame 4

Looking for real objects among all the artifacts… 15 May 2005, sum 4 frames

Looking for real objects among all the artifacts… 15 May 2005, median 4 frames

Do it again 3 days later…where are the moons? 18 May 2005, frame 1

Do it again 3 days later…where are the moons? 18 May 2005, frame 2

Dither across the gap…where are the moons? 18 May 2005, frame 3

Do it again 3 days later…where are the moons? 18 May 2005, frame 4

18 May 2005, sum 4 frames

“Clean” image 18 May 2005, median 4 frames

“Clean” image 15 May 2005, median 4 frames

15 and 18 May 2005, sum 8 frames

S/2005 P 1 Charon S/2005 P 2 15 and 18 May 2005, median 8 frames

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 • • 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) • Checklist of alternate explanations: rule them out? • Confident enough to announce on 31 October 2005

The “checklist” of possible explanations • Artifacts from the detector or optics: hot pixels, optical ghosts, scattered light, etc • Real foreground/background objects: asteroids? binary KBO? Plutinos? • New moons of Pluto!

Preliminary assumptions and implications • Orbits are co-planar with Charon, nearly circular, possibly in stable resonances with each other • Probably formed primordially with Charon (collision), not later (captured) • No other moons of similar magnitude (unless artifacts hid them in June; we’d find them in Feb 2006); very compact system • Pluto first KBO with multiple satellites: implies there are probably many more

Pre-discovery observations in 2002 • Hubble program by Buie & Young • ACS High Resolution Channel • Primarily designed to map surface features of Pluto and Charon • New moons marginally detected • Further observations will definitively determine orbits, and hopefully confirm these detections: are the satellites where they should be?

The “quadruple planet” Pluto Visual magnitude Diameter Orbital radius * (barycentric) Orbital period * Pluto 14. 2 2328 km +/- 42 km 6. 387 days Charon 16. 2 1208 km +/- 4 km 6. 387 days S/2005 P 1 22. 93 +/- 0. 12 61 -167 km 64, 700 km 38. 2 days (3. 7 x Charon) (~6 x Charon) S/2005 P 2 23. 38 +/- 0. 17 46 -137 km 49, 400 km 25. 5 days (2. 8 x Charon) (~4 x Charon) New moons are 5000 x fainter, 12 x smaller, and 3 -4 x farther out than Charon, with possible 6: 4: 1 orbital resonances

Relative sizes of Pluto, Charon, and the two new moons (P 1 and P 2) P 1 P 2 2300 km 1200 km ~100 km

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” The 10 th planet?

Xena Pluto Moon Earth

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 10 th 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”

http: //pluto. jhuapl. edu 13 17 00 00 Launch currently set for: January 17, 2006 2: 11 PM EST

Voyagers 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… Launched in 1977

Hal Weaver Johns Hopkins University Applied Physics Lab and New Horizons Project Scientist

Questions? … AND TWO LITTLE MOONS ! More information: http: //www. boulder. swri. edu/plutonews http: //pluto. jhuapl. edu