BREAKING NEWS: Pluto has three moons!

Contributed by
Oct 31, 2005

Pluto Has Three Moons

My good friend Dan Durda just sent me a very exciting email: a team has discovered two more moons of Pluto!

According to the press release, these tiny moons were discovered in May 2005 using images from the Hubble Space Telescope. The two as-yet unnamed moons orbit Pluto at distances of about 50,000 and 65,000 kilometers (for comparison, Charon, the previously-known moon of Pluto discovered ion 1978, is about 20,000 km from Pluto). Their sizes are not known, since they are so very far away, but the larger of the two may be 110-160 km in size, and the smaller about 100-140 km (again, for comparison, Charon is about 1170 km in diameter, so these are pretty small moons). They are extremely faint: the brighter of the two is at a magnitude of 23, meaning that the faintest star you can see with your unaided eye is still about 6 million times brighter than this moon. Pluto itself is about 4000 times brighter than these moons, so it's hard to see them against Pluto's glare. That's why they haven't been firmly detected before.

It's not possible to get much more info about these two moons from the current data. They are too small to directly resolve, so they appear as point-like objects. That means the sizes can only be estimated using their brightness. Also, the orbits are estimates using the limited data, but it seems clear that they are indeed objects orbiting Pluto, and not chance background stars or some other distant objects orbiting the Sun.

As the team themselves say:

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.

So for now, consider this discovery tentative but almost certain to be confirmed.

This is very exciting, and I congratulate the team for their hard-won effort!

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