New Moons around Pluto

Monday, October 31st, 2005 at 03:12 PM by Juan

Pluto and its 3 moons

When astronomers discover new objects in the solar system, the announcements are made through International Astronomical Union (IAU) Circulars. Earlier today I saw a post on the Minor Planet Mailing List noting that IAU Circular 8625 was announcing the discovery of two new moons around Pluto. IAU Circular 8625 reads in part as follows:

The objects, provisionally designated S/2005 P 1 and S/2005 P 2, were clearly detected [] in two different Hubble Space Telescope [] images on May 15 [] and 18 [].
…
Unique orbits cannot be calculated from the available data, but the measured positions are consistent with nearly circular orbits in the orbital plane of [Pluto's previously known moon, Charon].
…
Examination of archival HST [images] taken on 2002 June 14 (PI: M. W. Buie) shows two objects near the locations predicted by these orbits, providing independent support for the satellite detections.

These moons are considerably smaller than Charon, the previously know Plutonian satellite. The plausible diameter of “P 1″ range from 110 to 160 km (68 to 100 miles) whereas “P 2″ should be between 90 km and 140 km (56 to 87 miles) in diameter. For comparison, Pluto, the smallest planet, has a diameter of 2390 km (1485 miles), less than the diameter of Earth’s Moon. Charon, its only previous known moon, has a diameter of 1205 km (750 miles). And as the IAU Circular notes

A search over the entire orbital stability zone (±100″ around Pluto) does not show any other potential satellites to a limiting magnitude of V ~ 27.1.

Pluto and its moons in 2002
(Click on image for full-size version.)
These archival HST images taken on 2002 June 14 show the two newly discovered Plutonian moons, S/2005 P 1 and S/2005 P 2, three years before their discovery. The new moons are barely visible, but present in their expected locations if their orbits are circular.

This means it is unlikely there are any other Plutonian moons with diameters larger than about 20 km (12 miles) in stable orbits around Pluto.

This discovery was made by Alan Stern and his collaborators at the Southwest Research Institute in Boulder, the Principal Investigator of the upcoming New Horizons Pluto mission. Going to this group’s website you find out a couple of interesting things about these moons. They were discovered as the New Horizons Pluto mission team repeatedly applied for Hubbble Space Telescope time to look for potential targets for their spacecraft to image as it passes by Pluto. After several attempts, they were given the time, and the photographs above are the result

As noted in the IAU circular, their orbits can’t be calculated based on just the two HST observations although a previous HST observation in the HST archives shows a circular orbit is a likely fit. If the orbits of these satellites are indeed circular, they suggest that these satellites formed with Pluto and are not “captured” Kuiper Belt Objects, since the gravity of Pluto is too weak to tidally circularize the orbits of these moons. Unfortunately, as noted on the group’s webpage, they put together a proposal for followup HST observations to confirm the orbits of these objects, however

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 [] proposal was rejected the next day.

So it may be some time next year before we have a confirmation of the true orbital orientations of these moons.

This is a neat discovery, very topical for my Astronomy 106 students as we discuss the solar system this week, and shows that there is still a lot to learn about Pluto and the other planets in the outer solar system. Who would have thought these little snowballs would be able to form multiple moon systems?

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Posted in Astronomy News

One Response to “New Moons around Pluto”

  1. SCSU Astronomy » Blog Archive » Pluto’s new moons confirmed wrote on 02/23/06 at 11:21 am :

    [...] NASA and Alan Stern’s research group have announced that new Hubble Space telescope photographs of Pluto shot last week (February 15) have now confirmed the existence of two additional moons. dubbed “S/2005 P 1″ and “S/2005 P 2″ whose discovery was announced last Holloween. There really wasn’t any doubt as to the reality of the moons, but the nice bit is that Anxiously awaited follow-up observations with NASA’s Hubble Space Telescope have confirmed the presence of two new moons around the distant planet Pluto. The moons were first discovered by Hubble in May 2005, but the science team probed even deeper into the Pluto system last week to look for additional satellites and to characterize the orbits of the moons. [T]he team [was] happy to see the moons show up very close to the locations predicted from the earlier Hubble observations. [...]