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The discovery of two new satellites of Pluto

Max MutchlerSpace Telescope Science Institute

Open Night3 January 2006

Andy Andy LubenowLubenow1956 1956 -- 20052005

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)

Overview• History: discovery of Pluto, Charon, and the

Kuiper Belt• Early Hubble observations of Pluto • Hubble mission support for New Horizons, and

Deep Impact• Discovery of two new satellites with Hubble• Confirming the discovery: checklist, ground-

based and Hubble follow-up• Implications, and recent related discoveries: 10th

planet with a satellite, etc.• Questions? Door prizes!

Percival Lowell Vesto Slipher Clyde Tombaugh

The search for “Planet X”Lowell Observatory, Flagstaff, Arizona

The discovery of Pluto in 1930, and confirmation

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Announcing thediscovery of a“trans-Neptunian”planet (Pluto)

The discovery of Pluto’s moon Charon in 1978James Christy and Robert Harrington, U.S. Naval Observatory, Washington, D.C.

Circular No. 3241

Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION

Postal Address: Central Bureau for Astronomical Telegrams Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. TWX 710-320-6842 ASTROGRAM CAM Telephone 617-864-5758

1978 P 1

Capt. J. C. Smith, U.S. Navy, reports: "Elongation of the photographic image of Pluto has been detected by J. W. Christy on plates taken with the U.S. Naval Observatory's 155-cm astrometric reflector on 1978 Apr. 13, 20 and May 12; 1970 June 13, 15, 16, 17 and 19; and 1965 Apr. 29 and May 1. The maximum elongation is ~0"9 in p.a. 170o-350o. Observed position angles are consistent with a uniform revolution/rotation period equal to the lightcurve period of 6.3867 days. The data suggest that there is a satellite, 2-3 magntiudes fainter than Pluto, revolving around Pluto in this period at a mean distance of ~20 000km; the implied sun-Pluto mass ratio is 140 000 000:1. The other orbital elements are: e ~ 0, Node = 350o, Incl = 105o (with respect to the plane of the sky), T(Node) = 1978 May 12.2 UT. The probable satellite was confirmed on exposures with the 155-cm reflector on 1978 July 2 and 5 and by J. A. Graham with the 400-cm reflector at Cerro TololoInteramerican Observatory on July 6. Further observations are very desirable; a brief ephemeris follows."

1978 UT p.a. Sep. 1978 UT p.a. Sep. July 8.2 357o 0"8 July 12.2 165o 0"9

9.2 342 0.8 13.2 129 0.3 10.2 277 0.2 14.2 5 0.6 11.2 181 0.7 15.2 384 0.9

The designation 1978 P 1 conforms with the temporary nomenclature system for announcing discoveries of new satellites. This system was approved at the meeting in June of the IAU Working Group for Planetary System Nomenclature and has been submitted for endorsement at the forthcoming meeting of the IAU Executive Committee.

Announcing thediscovery of 1978 P 1 (Charon)

The slowly emerging picture of Pluto

Earth Pluto Moon12,800 km 2400 km 3000 km

Charon1200 km

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Pluto has not given up it’s secrets very easily.

Everything learned about itsince 1930 still fits on a couple of 3x5 cards!

* And much of it seems, well, weird!

Everything we know about Pluto 1

Everything we know about Pluto 2

• 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

• 1986 Pluto & Charon radii, albedos, colors• 1987 Pluto density is 2 g/cm3• 1988 Pluto orbit chaotic; occultation reveals

atmosphere and polar caps• 1989 Pluto and Triton similar, thermal structure in

atmosphere• 1992 Nitrogen and CO ice, density disparity• 1994 Discovery of the Kuiper Belt• 2001 Binary Kuiper Belt objects (KBOs)

Is Pluto…

?

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Pluto satellite searches

• Kuiper & Humason, 1950 (didn’t find Charon)

• Stern 1991: beyond 6 arcsec• Christy & Harrington (serendipitous)• Buie & Young (albedo maps, too shallow)• Weaver 2005 (almost rejected!)• DD or GO in 2006?

Early Hubbleobservations of Pluto and Charon

Satellite discovery observations

• HST proposal submitted by Weaver, Stern, et al., was rejected, then accepted when STIS died

• Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) covers entire orbital stability zone, puts Pluto-Charon near interchip gap

• 4 long exposures on May 15 and May 18, 2005• An informal request to inspect the data from Hal

Weaver on June 13, while preparing for Deep Impact Hubble ACS imaging

Advanced Camera for SurveysAdvanced Camera for Surveys

Hubble Servicing Mission 3B

Calibrating and drizzling ACS images

The Whirlpool Galaxy M51

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Pluto WFC image with full orbital stability zone (circle)…

…although subsequent images are cropped to the center box

Chip gap: peek-a-boo!

15 May 2005, frame 1

15 May 2005, frame 2 15 May 2005, frame 3

15 May 2005, frame 4 15 May 2005, sum 4 frames

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15 May 2005, median 4 frames 18 May 2005, frame 1

18 May 2005, frame 2 18 May 2005, frame 3

18 May 2005, frame 4 18 May 2005, sum 4 frames

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18 May 2005, median 4 frames 15 May 2005, median 4 frames

Finding needles in the haystack…

15 and 18 May 2005, sum 8 frames 15 and 18 May 2005, median 8 frames

Charon orbital radius: 19,400 kmP1 orbital radius: 55,000 kmP2 orbital radius: 45,000 km

Charon period: 6.4 daysP1 period: 25.5 daysP2 period: 38.2 days

P1

Charon

P2

Press release image: the discovery was surprisingly easy for Hubble with ACS…but not quite as easy as it looks here.

Date: Thu, 16 June 2005 11:22:23 -0400From: Max Mutchler < mutchler@stsci.edu > Subject: Re: Pluto data To: Hal Weaver < hal.weaver@jhuapl.edu >

Hal,

I now have…images for all 10 Pluto-Charon frames…for each epoch…useful for blinking against other images to decide if something is real or just… artifacts ….still need visual verification. I think visually scanning / blinking among the files above is the way to go…However, I found myself wishing Pluto-Charon were not placed so close to the WFC interchip gap.

I saw two interesting candidate near Pluto-Charon in epoch 1: one near 12-o-clock and another near 10-o-clock, then at a similar distance in epoch 2, something at 9-o-clock: could it be an object in orbit around Pluto (orbital period ~36 days)?

Since the gap coincides with the Pluto-Charon orbital plane (and the orbital plane of any additional moons?), unfortunately we are only 2-exposures deep there (easier to be tricked by cosmic rays)…

More later, Max

Excerpts of the e-mail that got things rolling….well, eventually…

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My summer vacation or

“life is what happens to you while you are busy making other plans”

• June 15: Pluto moons discovery• June 27: Supernova SN2005cs in M51• July 4: HST imaging of Deep Impact• Aug: HST Moon observations• Aug 26: return from vacation; e-mail reply

from Hal Weaver• Aug 28: Hubble enters 2-gyro mode!

Confirmation• Independent discovery in Aug 2005 by Andrew

Steffl (SwRI in Boulder, Colorado)• Checklist of alternate explanations (proceed with

extreme paranoia): detector artifacts (show)? Plutinos? KBO? Neptune Trojans?

• Search other existing data: Hubble, Subaru• Our own ground-based attempts to image the

new moons: Keck, VLT, Gemini (bad timing)• Hubble followup in Feb 2006 (2 gyros!)• Confident enough to announce on 31 Oct 2005

Attempts to image the new satellites with ground-based observatories

Gemini (Hawaii) Very Large Telescope (Chile)

The “checklist” of possible explanations

• Artifacts from the detector or optics: hot pixels, ghosts, scattered light (show some; Hartig)?

• Background binary KBOs? Binary KBO?• Plutinos or Neptune Trojans?• Expecting moons much farther out; hiding in

plain view, front and center; MT pipeline!• Predicted small chance of one KBO in entire

WFC field; odds of two vanishingly small• New moons of Pluto!

Preliminary assumptions and conclusions

• Orbits are co-planar with Charon, and nearly circular

• Probably formed primordially with Charon(collision), not later (captured)

• Possible dust arcs• No other moons of similar magnitude

(unless artifacts hid them in June)

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Pre-discovery observations in 2002• Hubble program 9391 by Buie &

Young• ACS / HRC with blue and visual

filters• 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

* These numbers assume co-planar and circular orbits for P1 and P2

23.38+/- 0.17

22.93 +/- 0.12

Visualmagnitude

25.5 days(~4x Charon)

49,400 km(9999 mi; 2.8x

Charon

30-160 km(99 mi)

S/2005 P 2

38.2 days(~6x Charon)

64,700 km(9999 mi; 3.7x

Charon)

30-160 km(99 mi)

S/2005 P 1

6.4 days1200 km(999 mi)

Charon

6.4 days2400 km(9999 mi)

Pluto

Orbitalperiod *

Orbital radius *

(barycentric)

Diameter

What does a“quadruple planet”look like?

Announcing thediscovery ofS/2005 P 1 andS/2005 P 2 on Oct 31, 2005 with an IAU Circular

The name game

• Pluto: god of the underworld, picked by a little girl, Disney’s dog (add cartoon); symbolizes Percival Lowell

• Charon: ferries the dead across the river Styx; mispronounced like James Christy’s wife Sheryl

• P1 and P2: we’ll decide soon; decided not to temporarily use informal names

• IAU guidelines (Greco-Roman mythology); approval takes awhile

Should we call Pluto a planet?• I’m neutral: a semantic “parlor game”. But consider…• Is Pluto just the first of many Kuiper Belt “ice dwarfs”?• Ceres was called a planet for 50 years, then “demoted” to asteroid

(a precedent)• Is larger Xena the 10th planet? • Are slightly smaller Sedna, Quaoar planets too? • Will we have only 8 planets, or hundreds of them?• Is this a problem…or is it progress?• The IAU is working on it…maybe they shouldn’t?• A healthy unresolved “controversy”: let the healthy debate (and

people’s interest) rage on! • A rose by any other name… these objects are fascinating

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Recent discoveries & implications

• Xena & Gabrielle: 10th planet, with a moon• Buffy• Pluto: ugly duckling, or the norm in a new

reality? • Are the other 8 planets the oddballs?

Pluto Moon Earth

XenaP1P1

P2P2

~100 km~100 km

Relative sizes of Pluto, Charon, and the two new moons (P1 and P2)

2400 km 1200 km2400 km 1200 km

Launch currently set for: January 17, 2006

2:11 PM EST

13 17 00 00

Hitch a ride on the fastest rocket ever…http://pluto.jhuapl.edu

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Another Voyage of Discovery to inspire the next generation of math and science students -- for them to literally grow up with --and where we should expect the unexpected…

Good luck New Horizons!

VoyagersLaunched in 1977 Introduce Hal here

• Add his 4-5 slides here

Questions?

… AND TWO LITTLE MOONS !