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jjjnettie
12-07-2012, 12:08 AM
It's official, Pluto now has 5 moons. :D

Breaking News.....NEW SATELLITE OF PLUTO: P5 - new Moon

M. R. Showalter (SETI Institute), H. A. Weaver (Applied Physics Laboratory,
Johns Hopkins University), S. A. Stern, A. J. Steffl, M. W. Buie, W. J. Merline
(Southwest Research Institute), M. J. Mutchler, R. Soummer (Space Telescope
... Science Institute) and H. B. Throop (NASA Headquarters) report the discovery of
a fifth satellite of Pluto. The object, provisionally designated S/2012 (134340)
1 and referred to as "P5", was detected in 14 separate sets of images taken by
the Hubble Space Telescope WFC3/UVIS. Each image set comprises 11-12
three-minute exposures. Upon co-adding, S/N = 5-8 in five sets and S/N = 3-5 in
nine sets where the detection was somewhat degraded by P5's close proximity of
Pluto II (Nix). Times and positions are as follows:

June 26.51-26.67 UT, 3 sets, 1".99 from Pluto at p.a. 158 deg
June 27.78-27.94 UT, 3 sets, 1".71 from Pluto at p.a. 182 deg
June 29.64-29.80 UT, 3 sets, 1".44 from Pluto at p.a. 219 deg
July 7.42- 7.58 UT, 3 sets, 1".76 from Pluto at p.a. 352 deg
July 9.41- 9.51 UT, 2 sets, 1".42 from Pluto at p.a. 31 deg

The satellite's mean magnitude is V = 27.0 +/- 0.3, making it 4 percent as
bright as Pluto II (Nix) and half as bright as S/2011 (134340) 1. The diameter
depends on the assumed geometric albedo: 10 km if p_v = 0.35, or 25 km if p_v =
0.04. The motion is consistent with a body traveling on a near-circular orbit
coplanar with the other satellites. The inferred mean motion is 17.8 +/- 0.1
degrees per day (P = 20.2 +/- 0.1 days), and the projected radial distance from
Pluto is 42000 +/- 2000 km, placing P5 interior to Pluto II (Nix) and close to
the 1:3 mean motion resonance with Pluto I (Charon).

spacezebra
12-07-2012, 08:20 AM
This is great news - just saw the Hubble images!

Cheers Petra d.

Varangian
12-07-2012, 08:54 AM
Man other than the ability to clear its orbit this thing is turning out to be more planet-like than Mercury!

Dave2042
12-07-2012, 04:26 PM
I suspect (intuitively) that distance from the sun is a big driver of the ability of a small 'planet' to retain moons. Here's my (Newtonian) reasoning.

Remember that while a 2-body system (ie one planet, one moon) is stable, any more is not. In the long term, non-linearities eventually build up to large effects.

Now consider putting a 2-body system (planet and moon) in orbit around a larger body (the sun). The 2-body system 'sees' the sun as a changing gravitational field, getting weaker away from the sun. That change is the 3'rd body distortion which ulitmately destabilises the system.

Now, the closer the 2-body system is to the sun, the more dramatic the change in that back-ground field, and consequently the more pronounced the distorting effect and consequently the faster instabilities develop and potentially the 2-body system is disrupted. In a sense the further from the sun, the more the planet and moon look like an isolated 2-body system.

Intuitively I'd say this has to hold for more than one moon.

On this basis, I'd say it is unsurprising that despite their similarity Mercury has no moons and Pluto has a bunch. Pluto is so much further away from the sun.

Obviously that's a pretty rough and ready analysis. Anyone able to either poke a hole in my reasoning, or alternatively point to someone having produced a more rigorous version?

bojan
12-07-2012, 05:10 PM
Dave,
your intuitive reasoning is quite correct.
See more on this here:
http://en.wikipedia.org/wiki/Roche_limit

However, it also the influence of other (bigger and closer to Pluto) planets, that play role in Pluto's ability to hold/acquire moons, as well as the availability of moons that can be captured

Dave2042
12-07-2012, 05:23 PM
D'oh! Here's me with an Honours degree in Astrophysics forgetting the Roche limit, which is indeed a specific example of exactly this.

Just goes to show what 15+ years in finance does to you.

Much obliged.

Zhou
23-07-2012, 10:04 PM
Dave said:
"I suspect (intuitively) that distance from the sun is a big driver of the ability of a small 'planet' to retain moons. Here's my (Newtonian) reasoning."

Also check out: http://en.wikipedia.org/wiki/Hill_sphere

Even though the gravitational attraction of Pluto is far more feeble than even our moon it is able to hold on to more orbiting bodies due to the simple fact that the gravitational effect of the Sun is also weaker in the outer periphery of the Solar System compared the inner Solar System.

Dave2042
25-07-2012, 01:40 PM
Thanks. This is fascinating. I've made a decision in the last few months to revisit my old physics texts and get back up to speed for my own amusement, and this is a great addition to the program.