The image on the right is an artist’s conception of the view of the Pluto/Charon system from the surface of P2, one of Pluto’s recently discovered moons (credit: NASA). While we’ve known for a month or so that Pluto has two “new” moons the paper announcing the discovery was just published Thursday. Their discovery is a truly fine example of observational astronomy, and I thought I would share some top-rate science with you all. Here I examine a pair of papers, one announcing and detailing the discovery of P1 and P2, the other speculating on their genesis and implications for the nature of the Pluto/Charon system.
Citations
- Weaver, H.A. and 8 others (2006), “Discovery of Two New Satellites of Pluto”, Nature 439(7070), pp. 943-945 [online at CiteULike.org]
- Stern, S.A. and 8 others (2006), “A Giant Impact Origin for Pluto’s Small Moons and Satellite Multiplicity in the Kuiper Belt”, Nature 439(7070), pp. 946-948 [online at CiteULike.org]
Synopsis
The observational routine that discovered P1 and P2, the two new moons of Pluto went as follows:
- Take a single short exposure (5s) to get an accurate position of Pluto and Charon
- Take two long exposures (475s) a the same location, this saturates the detector at Pluto and Charon (see the images on the left), but reveals objects down to an apparent magnitude of ~26.
- Move the telescope minutely and take the two exposures again, thus compensating for a slight gap between detectors on the HST
- Repeat this same procedure 3 days later
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Above is a pair of images that reveal the existence of two objects in apparent close proximity to the Pluto/Charon system on May 15th and 18th, 2005. You can see the overspill in the detector around Pluto and Charon, as well as the diffraction spikes from the telescope itself. But faintly surrounding the pair are two small dots that represent what may be two new moons of Pluto. Before Weaver and his co-authors could be sure of this conclusion, several possibilities had to be ruled out, including the possibility that P1 and P2 could be visual artefacts (ruled out by their appearance on two separate days), and that they are merely coincident objects somewhere in front or behind of Pluto and Charon (the authors calculate that there is a 1.6×10-11 chance of this being the case). The brightness of P1 and P2 indicates that they have diameters between 45 and 165 km, depending on the albedo of each.
Given the data collected from the two sessions, the authors calculated that the orbital radii of P1 and P2 are approximately 64,700 km and 49,400 km for P1 and P2, respectively. Compared to the Earth/Moon system then, the entire Pluto system is quite compact. The diagram on the right shows the calculated geometry of the system, notice how circular the orbits of the two new moons are. The orbital periods are 38 and 26 days for P1 and P2, respectively. This puts them at what are known as orbital resonances with Charon (6:1 for P1, and 4:1 for P2). These facts will be important in understanding how P1 and P2 came to be Plutonic satellites.
Before reading on here, scroll down and read Capturing a Moon. If P1 and P2 were captured by Pluto/Charon early in the history of the solar system, they would be in highly eccentric orbits. But instead, they are in near-circular orbits at harmonics of Charon’s orbital period. Charon is suspected of having been formed in a massive collision with Pluto because of its size, proximity, and orbit in the plane of Pluto’s rotation. This collision undoubtedly produced fragments that either coalesced with Pluto and Charon, remained in orbit, or were gravitationally disrupted and ejected from the Plutonic system. Because P1 and P2 appear to be in circular harmonic orbits, they must have originated nearer to Pluto and moved outwards as the system evolved. Therefore, the most likely explanation for all of these facts is that P1 and P2 were originally ejected impact fragments.
Interestingly, the authors take a moment to explore a very poetic question: does Pluto ever have rings? Small impacts on Pluto and Charon are incapable of ejecting dust into orbit because Kuiper Belt Objects interact so slowly (approximately 1-2km/s, as opposed to 30-60 km/s here on Earth). But, since P1 and P2 are so small, even low-speed impacts can eject tremendous dust clouds. Based on statistical models of the Kuiper Belt, the authors estimate that Pluto has an intermittent ring system about as dense as Jupiter’s. Whether or not the New Horizons mission will find a ring system when it arrives is an open question, as the ring system would only last for several hundred thousand years after each collision.
Context
The discovery of P1 and P2 revealed a surprisingly rich planetary system that is likely to be relatively common throughout the Kuiper Belt. In fact, 20% of Kuiper Belt Objects (KBOs) imaged so far have been shown to have large companion moons. Smaller moons like P1 and P2 are even more likely. The exact likelihood of moons of KBOs is uncertain, and determining it would tell us a lot about the origin of our solar system. One of the primary uncertainties in modeling solar system evolution is the density distribution of the original dust disk from which we formed. Refining our understanding of the density distribution out into the Kuiper Belt will reduce the uncertainty in this quantity greatly. This will then help us to model extrasolar planetary systems, which will allow us to understand how common planets are around other stars. This then will constrain how many planets exist in habitable zones around stars. Since life as we know it can arise only in habitable zones, the probability of life arising around other stars depends on how often planets exist there. So, by studying KBOs, we can learn more about life in the universe (to me this says that science is not a collection of separate disciplines, but a deeply unified study about the nature of everything).
The discovery of P1 and P2 was made possible using the Hubble Space Telescope on two observing sessions last May. While their apparent magnitude (at ~23, for those familiar with the brightness magnitude system) makes them a difficult target for most ground-based scopes, HST could have found these two satellites at any point in its illustrious 15-year career. It’s a shame that there is only one of these amazing orbital facilities because were there two or three, the pace of astronomical discovery would be, well, astronomical.
General Explanations
Capturing a Moon
A number of moons of the gas giants, and the two moons of Mars, are suspected to be captured rogue objects. When an object on a highly eccentric (non-circular) orbit through the solar system passes near a planet, there is a slight chance the moon will be captured by the planet. When the object is captured, it will have a highly eccentric orbit that over eons becomes more circular. This circularization occurs through the transfer of orbital velocity into tidal heating of the planet and its new moon. Each time the moon reaches its closest approach to the planet (periapsis), it is tidally deformed, thus reducing the orbital velocity of the moon and cutting the maximum distance that it reaches on each orbit (apoapsis). Every orbit becomes more and more circular, until after millions of close approaches, the separation between the periapsis and apoapsis is virtually eliminated.
An additional effect of the tidal deformation of the moon is that its rotational period is slowed until it eventually matches its orbital period. At this point, the moon is “tidally locked”, as is Luna. Some moons never become tidally locked, but instead rotate at a harmonic of their orbital period, such as 2:1, or 4:1. Mercury (a “moon” of Sol) is an example of this, it rotates exactly 3 times for every 2 orbits about the sun, so it is at a 3:2 resonance (the orbital period is 87.96934 days, and its rotational period is 58.6462 days, which works out to exactly 3:2).
If the moon is tiny, stiff, and relatively distant from its host planet, circularization takes a very long time. For the case of P1 and P2, the authors estimate that the process would require between 65 and 500 Gyr, roughly 6-90 times the age of the universe. On the other hand, if the moon is a loose rubble pile, and is quite close to its planet (or the planet has an extremely high gravity, such as Jupiter or Saturn), the evolution of the captured moon’s orbit is much more rapid.
While your explanation of tidal locking makes sense, I don’t understand the mechanism
which could create orbital resonance for satellites in circular orbits. For elliptical orbits
interactions at periapsis can have an effect, but for circular orbits there would seem to
have to be an asymmetric interaction with the fixed stars to cause satellite locations
to be preferentially synchronized in a particular (fixed) direction. Perhaps the
synchronization is in the Sun-radial direction?
Toby
The resonances can be of a few types:
1) In the case of Mercury, the resonance is one between the orbital period and the rotational period. This results, despite the near-circular orbit, from tidal distortions and may eventually lead to complete tidal locking.
2) For circular orbits of satellites that are large compared to their host planet, the actual point of rotation is known as the barycenter of the orbit, and for the case of Pluto and Charon is far enough away from Pluto so as to make Pluto noticeably orbit the barycenter. In this case, with two planets orbiting their barycenter the rotation can easily reach resonance.
3) In the case of P1 and P2, the orbits are resonant probably because they were formed initially from the same impact that formed Charon, or so the authors speculate. So in this case I think the resonance was there from the very beginning. Also, the orbits were initially non-circular very likely, but because P1 and P2 were so close to Pluto/Charon their apoapsis quickly decayed. Then, as the Pluto/Charon system evolved, the transfer of angular momentum allowed P1 and P2 to move further out.
I hope this addresses your question. If not, perhaps you could clarify it for me a bit.