1. The discovery of two new
satellites of Pluto
Max Mutchler
Space Telescope Science Institute
Open Night
3 January 2006

2. Andy Lubenow

3. 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 (SwRI), S. Alan Stern (SwRI),
Leslie Young (SwRI), John Spencer (SwRI), Marc Buie (Lowell Observatory),
Bill Merline (SwRI), Max Mutchler (STScI), and…Eliot Young (SwRI)

4. 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?

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

6. The discovery of
Pluto in 1930,
and confirmation

7. 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

8. The slowly emerging picture of Pluto

10. Charon
1200 km
Earth Pluto Moon
12,800 km km km

13. Everything we know about Pluto 1
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
1978 Charon discovered; “binary planet” *
1980 Occultation reveals Charon radius to be 600 km
1985 Pluto-Charon mutual events begin
Pluto has not
given up it’s secrets
very easily over
the first 75 years…
Everything we know about Pluto
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.

16. Early Hubble
observations
of Pluto
and Charon

17. 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.

18. 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…

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

20. Calibrating and drizzling ACS images
The Whirlpool Galaxy M51

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

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

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

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

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

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

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

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

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

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

32. 18 May 2005, sum 4 frames

33. “Clean” image
18 May 2005, median 4 frames

34. “Clean” image
15 May 2005, median 4 frames

35. 15 and 18 May 2005, sum 8 frames

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

38. 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)

39. 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

40. 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!

41. 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

42. 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?

43. The “quadruple planet” Pluto New moons are 5000x fainter, 12x smaller, and 3-4x farther out than Charon, with possible 6:4:1 orbital resonances
Visual
magnitude
Diameter
Orbital
radius *
(barycentric)
period *
Pluto
14.2
2328 km
+/- 42 km
6.387 days
Charon
16.2
1208 km
+/- 4 km
S/2005 P 1
22.93
+/- 0.12 km
64,700 km
(3.7x Charon)
38.2 days
(~6x Charon)
S/2005 P 2
23.38
+/- 0.17 km
49,400 km
(2.8x Charon)
25.5 days
(~4x Charon)

44. Relative sizes of Pluto, Charon, and the two new moons (P1 and P2)
~100 km
2300 km km

45. What does a “quadruple planet” look like?

46. 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:

47. “Xena & Gabrielle”
The 10th planet?

48. Xena
Pluto Moon Earth

49. 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”

51. Launch currently set for:
Launch currently set for:
January 17, 2006
2:11 PM EST

52. Good luck to New Horizons, the next great Voyage of Discovery…
Voyagers
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…

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

54. Questions? … AND TWO LITTLE MOONS ! More information: