More Moons Around Pluto? Options

[JRehling]

Press Release Source: NASA


NASA's Hubble Reveals Possible New Moons Around Pluto
Monday October 31, 12:30 pm ET


WASHINGTON, Oct. 31 /PRNewswire/ -- Using NASA's Hubble Space Telescope to view the ninth planet in our solar system, astronomers discovered Pluto may have not one, but three moons.
If confirmed, the discovery of the two new moons could offer insights into the nature and evolution of the Pluto system; Kuiper Belt Objects with satellite systems; and the early Kuiper Belt. The Kuiper Belt is a vast region of icy, rocky bodies beyond Neptune's orbit.

"If, as our new Hubble images indicate, Pluto has not one, but two or three moons, it will become the first body in the Kuiper Belt known to have more than one satellite," said Hal Weaver of the Johns Hopkins Applied Physics Laboratory, Laurel, Md. He is co-leader of the team that made the discovery.

Pluto was discovered in 1930. Charon, Pluto's only confirmed moon, was discovered by ground-based observers in 1978. The planet resides about 3 billion miles from the sun in the heart of the Kuiper Belt.

"Our result suggests other bodies in the Kuiper Belt may have more than one moon. It also means planetary scientists will have to take these new moons into account when modeling the formation of the Pluto system," said Alan Stern of the Southwest Research Institute, Boulder, Colo. Stern was co-leader of the research team.

The candidate moons, provisionally designated S/2005 P1 and S/2005 P2, were observed approximately 27,000 miles away from Pluto. The objects are roughly two to three times as far from Pluto as Charon.

The team plans to make follow-up Hubble observations in February to confirm the newly discovered objects are truly Pluto's moons. Only after confirmation will the International Astronomical Union consider names for S/2005 P1 and S/2005 P2.

The Hubble's Advanced Camera for Surveys observed the two new candidate moons on May 15, 2005. The candidates are roughly 5,000 times fainter than Pluto. Three days later, Hubble looked at Pluto again. The two objects were still there and appeared to be moving in orbit around Pluto.

The team looked long and hard for other potential moons around Pluto. "These Hubble images represent the most sensitive search yet for objects around Pluto," said team member Andrew Steffl of the Southwest Research Institute. "It is unlikely that there are any other moons larger than about 10 miles across in the Pluto system," he said.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. The Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.


For detailed information and images about this research on the Web, visit:

http://hubblesite.org/news/2005/19

For information about NASA and agency programs on the Web, visit:

http://www.nasa.gov/home




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Source: NASA

[Rob Pinnegar]

Just a couple of points concerning this exciting new discovery:

(1) The orbital periods given for the satellites, 38.2 days and 25.5 days, are really close to the Charon 6:1 and 4:1 resonances. (Of course, those periods have some error associated with them; when more precise values are available, we'll have a better idea of this.)

Here's a question for the classical dynamicists in the audience: since Pluto and Charon are both significantly displaced from their mutual barycentre, would an object orbiting that barycentre in the Charon 4:1 or 6:1 resonances have a stable non-elliptical orbit due to three-body effects? What I mean is, could its distance from the system's barycentre reach several maxima and minima per orbit, instead of just one, if the overall ellipticity of the orbit were low enough to start with?

(2) Just running through some quick Matlab calculations: Seen from Pluto's surface, the two new moons should both have total magnitudes in the neighbourhood of -1 or so. So they should be quite visible. If they're in the neighbourhood of 100 to 150 km across, then, as seen from Pluto, they'd both show disks maybe a fifth to a third the size of the Moon as seen from Earth.

If one of them is 150 kilometres across, it could appear as large as the Moon in the Earth's sky as seen from the other, during mutual closest approach (~15000 km).

[Edit: Corrected an ambiguity.]

[Alan Stern]

See www.boulder.swri.edu/plutonews for a great deal more info.

-Alan

[Rob Pinnegar]

I'm confused here: On the website cited above, the estimates for the diameter of S/2005 P1 are 160 km if its albedo is 0.04, and 110 km if its albedo is 0.35.

But shouldn't the estimated diameters of the new moons vary roughly as 1/sqrt(albedo)? I would've thought that a ninefold increase in reflectively would decrease the estimated diameter by a factor of about three, since the total brightness has to stay the same.

Using 16.8 as apparent M_v for Charon, and 23.0 as apparent M_v for S/2005 P1 as given on the website, and assuming both bodies have the same albedo, I get a diameter about one-seventeenth of Charon's, or ~70 km, for 2005 P1. Have I got something wrong?

[john_s]

I'm confused here: On the website cited above, the estimates for the diameter of S/2005 P1 are 160 km if its albedo is 0.04, and 110 km if its albedo is 0.35.

But shouldn't the estimated diameters of the new moons vary roughly as 1/sqrt(albedo)? I would've thought that a ninefold increase in reflectively would decrease the estimated diameter by a factor of about three, since the total brightness has to stay the same.

Using 16.8 as apparent M_v for Charon, and 23.0 as apparent M_v for S/2005 P1 as given on the website, and assuming both bodies have the same albedo, I get a diameter about one-seventeenth of Charon's, or ~70 km, for 2005 P1. Have I got something wrong?

No, you don't have anything wrong- we goofed on the web site, though we got the diameter right for the lowest albedo. Thanks for catching the error- we'll fix it! The correct numbers should be something like 160 km for an albedo of 0.04 and 52 km for an albedo 0.35.