Background Information Regarding Our Two Newly Discovered Satellites of Pluto

Alan Stern (SwRI), Hal Weaver (JHU APL), Max Mutchler (STScI), Andrew Steffl (SwRI), Bill Merline (SwRI), Marc Buie (Lowell Observatory), John Spencer (SwRI), Eliot Young (SwRI), and Leslie Young (SwRI)

Names for Pluto's Moons S/2005 P1 and S/2005 P2 Nominated By the Discovery Team to the IAU.     Word Doc (500k)

For more information about the HST discovery of Pluto's second and third moons, see:

For the SwRI press release on the discovery, see

Nature paper on the discovery by Weaver, et al.: PDF or astro-ph

Nature paper on the characteristics and origin of the quadruple system by Stern, et al.: PDF or astro-ph

For the text of an International Astronomical Union Circular (IAUC) on follow-up HST observations CLICK HERE.

For more information on NASA's New Horizons mission to Pluto and the Kuiper Belt, see:

For the text of the official International Astronomical Union Circular (IAUC) communicating the HST discovery of Pluto's second and third moons, CLICK HERE.

For an animation depicting the architecture of the Pluto system and its three moons, CLICK HERE.

For information on Pluto, see

Figure Caption: The Pluto system as we understand it today, with Charon orbiting in the innermost position, and the two new satellites orbiting somewhat farther out. Positions and orbits are shown as seen from Earth for 15 May 2005.

How and when were the new satellites found?

Both satellites were detected using the images obtained by Hubble Space Telescope’s (HST’s) Advanced Camera for Surveys (ACS) in May, 2005. Our project has its roots in work stretching back to 1990, and our discovery is the result of teamwork involving all of us. We think the success of this project illustrates both the value of teamwork and the value of persistence.

Although some of us, and others, had been involved in earlier work to search for satellites of Pluto, this team’s quest to use Hubble to search for satellites of Pluto stretches back almost 3 years.

We began shortly after NASA’s selection of the New Horizons (NH) Pluto mission in November 2002 (see for more information on NH). We began when the NH Principal Investigator, Alan Stern, initiated an effort to win Hubble time to search for small satellites that may be lurking undiscovered in the Pluto-Charon system.

NH science team member John Spencer then led an HST proposal submitted in April 2003 for Director’s Discretionary time in support of NH, but that program was not approved. Despite this, we remained convinced that there might be undiscovered satellites around Pluto, and we proposed a regular, peer reviewed HST science program in January 2004, this time with Hal Weaver as the Principal Investigator. This program was also turned down. However, when HST’s STIS spectrograph failed in early August of 2004, resulting in some valuable HST observing time becoming available, the Space Telescope Science Institute (STScI) awarded time to a number of highly ranked proposals that has just missed being accepted. As a result, on September 28th, 2004, we received notification from the STScI that our Pluto companion search program had been accepted after all!

The Pluto satellite search observations were scheduled for the spring of 2005 when Pluto was in a good position for observation. As a result, HST ACS images were made on May 15th and May 18th of 2005. By design, the observations were very sensitive—being able to detect objects as faint as 27th magnitude in some portions of the images; this is approximately 100,000 times fainter than Pluto itself, and about half a billion times fainter than the human eye can see.

The images covered enough sky to encompass the entire region that Pluto’s gravity controls—this stretches out over two million kilometers (1.25 million miles) from Pluto. This was very important to us, because we wanted to completely survey all locations where satellites of Pluto might lurk.

Our expectation going in to the project was that Pluto might have one or more faint, distant satellites that orbit very far from it, as some Kuiper Belt Objects (KBOs) do. No objects like this were found. However, to our surprise and joy, two faint bodies fairly close to Pluto were detected.

Max Mutchler first spotted the two satellite candidates, on June 15th, during an examination of the HST images requested by Principal Investigator Hal Weaver. He then discussed the possible find with Hal Weaver, but the potential find needed a lot of follow up work, but they agreed to defer the necessary follow-up work because both of them were committed to supporting the ongoing Hubble observations of Comet 9P/Tempel 1, the target of NASA’s Deep Impact mission. Their work on the Hubble observations of the Deep Impact event kept Hal and Max completely occupied through the second week of July.

In early July, Alan Stern directed his postdoc Andrew Steffl to sift carefully through all of the HST images in order to see whether or not Pluto has any faint satellites. Hal and Alan discussed this, and Hal agreed to familiarize Andrew with the data and to acquaint Andrew with a detailed search strategy during a trip Andrew would make to visit Hal in early August. So they could perform a completely independent evaluation of the images, Hal did not tell Alan or Andrew of what Max and he thought they might have found.

After a couple of weeks of work, in mid-August, Steffl reported to Stern on August 18th that he thought he’d found two objects the day prior. After a few days of follow up, Stern and Steffl became convinced that the two candidate satellites were very possibly the real deal. Among the follow up tests, we performed were quick look calculations showing that the odds of the possible satellites being Kuiper Belt Objects or image artifacts were low.

On August 24th Stern contacted Weaver about the potential satellites finds. In that phone call, Hal told Alan that Max Mutchler and he had found two satellites themselves back in June. Within an afternoon, it became clear that both groups had found the same objects in the same images.

Weaver and Stern then put together a Director’s Discretionary (DD) time proposal to use HST to immediately confirm the potential discoveries. The proposal was submitted to HST on August 30th. Unfortunately, HST had just entered “2-gyro” observing mode on August 29th (which had been done to extend HST’s useful lifetime). In 2-gyro mode, HST is limited as to what parts of the sky it can look at, at any given time. As luck would have it, observations of Pluto would not be technically feasible until mid-February 2006, so our DD proposal was rejected the next day.

At this point, the team embarked on two parallel paths: one was to more carefully scrutinize the HST data to ensure that no camera artifacts (e.g., optical ghosts, bad pixels, etc.) were fooling us, and the other was to seek confirming data from any other (i.e., non-HST) available resource.

Another issue that had to be addressed was whether the objects, even if real, could be background or foreground KBOs not associated with Pluto. Our calculations showed that the odds of even a single KBO of the same brightness as the candidate satellites being anywhere in our HST ACS images is <0.1%, and the probability of any given KBO being so close to Pluto is far smaller still: less than 1 in 100,000. The odds of two KBOs lying so near Pluto in a single image are about 1 in 1 billion.

In September, with the great assistance of Bill Merline, we attempted to instead confirm the discovery using some of the largest, best equipped groundbased observatories in the world: the Keck and Gemini telescopes in Hawaii, and the ESO-VLT telescope in Chile. However, despite the best efforts of all three observatories and their staff, the groundbased efforts were not able to reach faint enough magnitudes to be able to detect the satellite candidates we found. The problem wasn’t that these telescopes can’t detect the new satellites—we are convinced they eventually can—it was just that by September, Pluto was barely visible before setting each night after twilight.

Merline also discovered that these objects must be real by demonstrating that the new moons are moving in orbits that seem to be in the same plane as Charon, and with low orbital eccentricity—like Charon.

Figure Caption: Although a full orbital solution for the satellites cannot be determined from only two Hubble measurements, their paths closely follow that expected for objects orbiting Pluto (or, more correctly, the Pluto system's barycenter) in a perfect circle in the same plane as Charon's orbit. In this diagram, the barycenter (i.e., the gravitational balance point of the system) is the dot in the center, Pluto's orbit around it is the smallest ellipse, Charon's orbit is the next ellipse outward (its positions on 2005 May 15 and May 18 are indicated by the filled and open circles, respectively), an orbit that is consistent with P2's measured positions is next, followed by an orbit that is consistent with P1's measured positions. For both the orbit of P1 and P2, the filled squares are the detected positions of P1 and P2 on May 15 and open squares are the detected positions of P1 and P2 on May 18. Note that circular orbits look elliptical when they are projected onto the plane of the sky to mimic what we would see from HST. The result shown here means that the orbits of P1 and P2 are likely to be essentially circular, and in the plane of Pluto’s equator, where Charon’s orbits.

Hard confirmation of P1 and P2 came just last week, on October 24th, when Marc Buie and Eliot Young found the two objects in several images of an HST dataset they and their collaborators had taken for a Pluto mapping project in 2002.

Figure Caption: A careful search of HST images taken with the ACS/HRC on June 14, 2002, reveals two objects that are consistent with the expected locations of the newly-discovered satellites. One image (left) was taken in yellow light (555 nm) and the other (right) was taken in blue light (475 nm). The ellipse shows the orbital path of the new satellites derived from the May 2005 Hubble observations. Based on the 2005 HST detection dataset, satellites P1 and P2 should lie somewhere along this ellipse. And indeed, there are two objects along the predicted path (near the red dots), confirming the detections made with the 2005 HST dataset.

In related news, just 10 days ago, Space Telescope Science Institute director Dr. Matt Mountain has graciously granted us 2 additional HST orbits to re-image the Pluto system, reconfirm the satellites, and improve our knowledge of their orbits. These observations must wait until February of next year, when Pluto’s position is farther from the glare of the Sun.

What led us to believe the objects near Pluto in the HST images are satellites of Pluto?

Several factors:

First, both satellites appear to be moving through space with Pluto, and they also appear to be moving around Pluto, as one can see from the images. Since we commanded the Hubble Space Telescope to track Pluto during the imaging, objects not moving with Pluto (like stars and asteroids) appear as streaks in the images, rather than a point-like source moving with Pluto. It is highly improbable that an object would appear to be moving with Pluto unless it was really in orbit around it.

Second, both objects appear to be true point sources in the images, which is evidence that they are real, physical objects in space as opposed to optical glints, stray reflections, or other instrumental signatures in the instrument. HST ACS camera experts like George Hartig who examined our images do not believe that any known instrumental effects could generate signatures that mimic the satellite candidates we had found

Third, the information we have about the orbits of the satellite candidates is consistent with their orbits being in the same plane as Charon’s, and also nearly circular. This is extremely important, because it is very highly unlikely that any image artifact or other astronomical body would mimic such motion while also appearing to travel with Pluto.

Fourth, we determined that the new objects are small enough that they would not have been detectable in previous (less sensitive) Pluto satellite searches quoted in the literature, and that their gravitational effects on Charon’s orbit would not have previously indicated their presence.

Finally, and perhaps most importantly, Marc Buie and Eliot Young located faint images of both satellites in HST ACS data taken for a Pluto mapping project they spearheaded in 2002.

How are the two new satellites of Pluto currently designated?

Both satellites have official, provisional designations from the Central Bureau for Astronomical Telegrams of the International Astronomical Union (IAU). The brighter satellite is dubbed S/2005 P 1; the fainter one is dubbed S/2005 P 2. They will receive names at a later date.

What are the properties of the two new Plutonian satellites?

Very little is known about these bodies at this time. We know their brightnesses, but not their rotation periods, their colors, their surface reflectivities, or their surface compositions.

Both satellites appear to stray no farther than 3 arcseconds from Pluto; for reference, Charon orbits no farther than 0.9 arcseconds from Pluto.

By comparing the positions of the satellites in different images on different dates, we find that the positions are consistent with both S/2005 P 1 and S/2005 P 2 having orbits that are nearly circular and which lie in Pluto’s equatorial plane. This is a preliminary conclusion, but if it holds up, then the orbital semi-major axes of P1 and P2 are near 64,700 +/- 850 and 49,500 +/- 600 km, respectively. These semi-major axes in turn correspond to orbital periods of approximately 38.2 +/- 0.8 and 25.5 +/- 0.5 days, respectively.

The brighter satellite, S/2005 P 1, has a V-band magnitude of about 23.0. Because we do not yet know its surface reflectivity, we cannot definitively calculate its exact size. But by estimating surface reflectivities and using its distances from the Earth and Sun at the time of the HST observations, we can determine the range of plausible sizes it might have. Assuming surface reflectivities ranging from 4% (like the darkest known KBOs), to 15% (like many KBOs), to 35% (like Charon), this is what we find:

Assumed ReflectivityApproximate Diameter
04%160 km
15%80 km
35%52 km

S/2005 P 2 is about 25% fainter than S/2005 P 1, so could be a 10% to 15% smaller than S/2005 P 1, assuming they have the same surface reflectivity.

Despite the various uncertainties in what we know about P1 and P2 so far, and regardless of their surface reflectivities, these bodies are clearly very small compared to both Pluto (2284 km diameter) and Charon (1192 km diameter). Indeed, it is unlikely that either object has a mass larger than 3/10,000th of Pluto’s or 3/1000th of Charon’s.

Might Pluto have other satellites?

Possibly, but because we searched the space around Pluto very thoroughly, we are confident that if Pluto has any other satellites, they must be much fainter, and therefore much smaller, than either S/2005 P 1 or S/2005 P 2. Our best estimate is that Pluto has no other satellites larger than 20 km in size.

What is the significance of these bodies if they are confirmed to be satellites of Pluto?

To begin, we now know that the Pluto system is in reality a quadruple system.

With not one but three satellites, Pluto has become the first body in the Kuiper Belt known to have more than one satellite. This indicates that many other bodies in the Kuiper Belt may be multiple-satellite systems.

The discovery will also provide new probes of the Pluto system. Examples of the information these satellites will soon yield include improved estimates of the masses and densities of Pluto and Charon, the possibility of a first estimate of the J2’s of Pluto and/or Charon, new constraints on the tidal evolution and origin of the system. When their colors, rotation periods, and sizes are better known, they will shed more light on their own nature. And of course, a better understanding of the Pluto system will by example shed light on the nature and context of other KBO satellite systems.

Moreover, our discovery means that NASA’s New Horizons spacecraft will be even busier than originally planned when it arrives at the Pluto system, since it will have not two, but four bodies to explore!

What formal scientific publications are planned to report these discoveries?

Our first formal scientific communication consisted of a bulletin released on 31 October 2005 by the International Astronomical Union (IAU). This bulletin, called an IAU Circular can be found at this link. We have written drafts of three scientific papers that we plan to soon submit to refereed journals.

These papers can be summarized as follows:


We as a team want to thank the people who built and the people who operate the Hubble Space Telescope. It is an amazingly capable tool for astronomy. We thank NASA for supporting the space sciences in many ways, including the funding that keeps HST operating and the funding that has built New Horizons and made it ready for launch to the Pluto system. We thank Matt Mountain and his staff at the Space Telescope Science Institute for working with us to confirm the discoveries of S/2005 P 1 and S/2005 P 2, and for granting Director’s Discretionary time for follow up detection observations this coming February. We thank the directors and observing personnel of the Keck, Gemini, and VLT observatories for responding to our requests for observing time on short notice, and for working valiantly to detect P1 and P2 under technically difficult circumstances this fall. We thank colleagues Keith Noll and Joel Parker for useful conversations as we worked to confirm P1 and P2, and we thank Jennifer Wiseman, Colleen Hartman, Melissa McGrath, and Andy Dantzler, all of whom at NASA Headquarters, for their support of our high standards for confirmation before releasing news. Finally, we thank the Public Affairs Offices of NASA Headquarters and Space Telescope Science Institute for their assistance in preparing the news release and accompanying materials describing our discoveries of Pluto’s second and third moons.

Caption: Pluto and Charon to scale with each other and the United States; the new satellites, S/2005 P 1 and S/2005 P 2 are about half as large as the letter that spells "P" in "Pluto." As Charon orbits Pluto, it is actually about 19,400 kilometers (about 12,000 miles) away.

Caption: Some previous highlights in the study of the Pluto system.

For more information about the HST discovery of Pluto's second and third moons, see:

For more information on NASA's New Horizons mission to Pluto and the Kuiper Belt, see:

For the text of the official International Astronomical Union Circular (IAUC) communicating the HST discovery of Pluto's second and third moons, CLICK HERE.

For an animation depicting the architecture of the Pluto system and its three moons, CLICK HERE.

For information on Pluto, see