[X] My Assistant

[Rob Pinnegar]

Possibly all planets/worlds/worldoids formed with moons, but in the outer solar system the perturbing effect of Jupiter and the other giants was sufficiently slight for orbits to remain stable over very long periods - far more than in the inner solar system.

I don't know about that -- it might be true, but my first guess is that the number-one consideration is proximity to the Sun, not proximity to Jupiter or the other giants. The inner planets have low masses and are very close to the Sun. Thus they have two strikes against them where moon-building is concerned.

The third strike, I suppose, might be the velocities of impacting objects. A large comet on a near-parabolic orbit will be travelling a lot faster when it crosses Mercury's orbit than it did crossing Neptune's orbit. An impactor like that would do a good job of pulverizing any hapless moons of Mercury, and anything left in Mercury's neighbourhood could lose mass to fragments being snatched away by the Sun, perhaps enough to negate what was gained from the impactor. Repeat that process a few times and... no more moon.

Mars, being farther away, might have been able to re-accrete Phobos or Deimos if they got smashed up. And for the Earth's Moon, comets would be merely an annoyance. But why hasn't Venus got any moons? Bad luck? Or maybe good luck (no giant impact).

[Guest_BruceMoomaw_*]

Pinnegar: "This leads to the possibility that Pluto might have other small satellites, and this has actually been considered. I vaguely remember reading, a while back, that at least one of the teams that have discovered all those distant satellites of the Jovians in the past few years, did at one time propose doing a similar search for Pluto. Can't remember whether it was carried out, though."

There has indeed been at least one detailed search for small, distant moons of Pluto. I can't remember the details, except that it turned up nothing down to a size of (I think) about 30 km diameter.

[Rob Pinnegar]

There has indeed been at least one detailed search for small, distant moons of Pluto.  I can't remember the details, except that it turned up nothing down to a size of (I think) about 30 km diameter.

Thanks, Bruce. I thought that was the case, but wasn't sure.

One of these years the IAU is going to have to decide where to draw the line at numbering and/or naming satellites of planets. The giant planets probably have thousands, or even tens of thousands, of satellites in the 10- to 100-metre size range. Likely Pluto has a few as well. I guess we'll get round to it when it becomes a problem.

[ljk4-1]

Paper: astro-ph/0510029
Date: Mon, 3 Oct 2005 03:57:40 GMT (53kb)

Title: Satellites of the largest Kuiper belt objects

Authors: M.E. Brown, M.A. van Dam, A.H. Bouchez, D. Le Mignant, R.D.
Campbell, J.C.Y. Chin, A. Conrad, S.K. Hartman, E.M. Johansson, R.E. Lafon,
D.L. Rabinowitz, P.J. Stomski, Jr., D.M. Summers, C.A. Trujillo, P.L.
Wizinowich

Categories: astro-ph
\\
We have searched the four brightest objects in the Kuiper belt for the
presence of satellites using the newly commissioned Keck Observatory Laser
Guide Star Adaptive Optics system. Satellites are seen around three of the four
objects: Pluto (whose satellite Charon is well-known), 2003 EL61, and 2003
UB313. The object 2005 FY9, the brightest Kuiper belt object after Pluto, does
not have a satellite detectable within 0.4 arcseconds with a brightness of more
than 0.5% of the primary. The presence of satellites to 3 of the 4 brightest
Kuiper belt objects is inconsistent with the fraction of satellites in the
Kuiper belt at large at the 99.1% confidence level, suggesting a different
formation mechanism for these largest KBO satellites. The satellites of 2003
EL61 and 2003 UB313, with fractional brightnesses of 5% and 2% of their
primaries, respectively, are significantly fainter relative to their primaries
than other known Kuiper belt object satellites, again pointing to possible
differences in their origin.

\\ ( http://arXiv.org/abs/astro-ph/0510029 , 53kb)

[Guest_BruceMoomaw_*]

"One of these years the IAU is going to have to decide where to draw the line at numbering and/or naming satellites of planets. The giant planets probably have thousands, or even tens of thousands, of satellites in the 10- to 100-metre size range. Likely Pluto has a few as well. I guess we'll get round to it when it becomes a problem."

A few years ago Michael Swanwick wrote a short-short SF story about the lonely government functionary whose only function is to officially identify and name every single solitary ring particle of Saturn. One day, sitting in his dingy office, he views a photo of one such particle, realizes that it must be an ancient derelict alien starship, names it "Youshouldhavepaidmemore", and then continues his work, secure in the knowledge that no one will ever look at that photo of that moonlet again.

[Rob Pinnegar]

Umm. That story sounds fun, but... I'm a dope for not considering ring particles and ring-embedded moonlets in my previous post. I was thinking more of outer eccentrics like Himalia. Naming ring particles would be a grim task indeed.

Anyways, here's another question regarding "Xena" that might be interesting for someone to answer eventually: Does it appear on any of the images Clyde Tombaugh examined during the search for Pluto?

My first guess is that it doesn't. He states in chapter 16 of "Out of the Darkness" that he could have picked up a Pluto-sized object out to about 60 AU. Thus it seems unlikely that "Xena" is on any of those plates. Still, it'd be interesting to check, if anyone has the time and the inclination to do so.

[SigurRosFan]

Detection of Six Transneptunian Binaries with NICMOS: A High Fraction of Binaries in the Cold Classical Disk

http://fr.arxiv.org/abs/astro-ph/0510130

[JRehling]

Anyways, here's another question regarding "Xena" that might be interesting for someone to answer eventually: Does it appear on any of the images Clyde Tombaugh examined during the search for Pluto?

My first guess is that it doesn't. He states in chapter 16 of "Out of the Darkness" that he could have picked up a Pluto-sized object out to about 60 AU. Thus it seems unlikely that "Xena" is on any of those plates. Still, it'd be interesting to check, if anyone has the time and the inclination to do so.

Xena's pretty far off the ecliptic. I read that Tombaugh's searches in the 40s were limited to +/- 15 degrees of the ecliptic, so no chance -- Xena doesn't move fast enough to have been there then and where it is now.

[JRehling]

Detection of Six Transneptunian Binaries with NICMOS: A High Fraction of Binaries in the Cold Classical Disk

http://fr.arxiv.org/abs/astro-ph/0510130

One thing that makes this fascinating is the speculative argument regarding habitability, axial obliquity, and the Drake Equation. As the argument goes, Earth benefits greatly from the gyroscopic stability that the Moon provides to the Earth-Moon system. Mars, lacking this, goes through extreme cycles in axial tilt due to the influence of Jupiter. Thereby, in epochs when the martian pole is near the plane of its orbit, it undergoes extreme seasons, which impair the prospects for life (or more advanced life forms).

Given only Mercury, Venus, Mars, and Pluto as unbiased data points (can't count ourselves -- we wouldn't be here if things weren't favorable!) for "terrestrial" worlds, we saw that 1/4 had a sizable moon. It's hard to conclude from that whether the proportion in the universe is near 50% or much lower. Assuming that TNOs play by the same rule as rocky, warmer terrestrial worlds, this data could be giving us an answer of universal scope, and the percentage is fairly high (11% within NICMOS's threshold of detection -- we may suppose a bit larger value given better resolving power). Granted, the argument that this has much bearing on, eg, extraterrestrial life is oblique, but it is hard data of a kind.

[Rob Pinnegar]

Xena's pretty far off the ecliptic. I read that Tombaugh's searches in the 40s were limited to +/- 15 degrees of the ecliptic, so no chance -- Xena doesn't move fast enough to have been there then and where it is now.

Tombaugh searched almost all of the sky from -50 to +50 degrees, according to the caption of one of the figures in the plate section of "Out of the Darkness".

[Guest_BruceMoomaw_*]

One thing that makes this fascinating is the speculative argument regarding habitability, axial obliquity, and the Drake Equation. As the argument goes, Earth benefits greatly from the gyroscopic stability that the Moon provides to the Earth-Moon system. Mars, lacking this, goes through extreme cycles in axial tilt due to the influence of Jupiter. Thereby, in epochs when the martian pole is near the plane of its orbit, it undergoes extreme seasons, which impair the prospects for life (or more advanced life forms).

Given only Mercury, Venus, Mars, and Pluto as unbiased data points (can't count ourselves -- we wouldn't be here if things weren't favorable!) for "terrestrial" worlds, we saw that 1/4 had a sizable moon. It's hard to conclude from that whether the proportion in the universe is near 50% or much lower. Assuming that TNOs play by the same rule as rocky, warmer terrestrial worlds, this data could be giving us an answer of universal scope, and the percentage is fairly high (11% within NICMOS's threshold of detection -- we may suppose a bit larger value given better resolving power). Granted, the argument that this has much bearing on, eg, extraterrestrial life is oblique, but it is hard data of a kind.

Robin Canup's simulations actually suggest that about 1/3 of terrestrial planets may undergo glancing impacts which give them a large moon. However, the fact remains that such a moon could end up stabilizing its primary's obliquity at ANY tilt all the way up to 90 degrees -- suggesting that Earth's main stroke of luck here is not that it has a Moon, but that it has a Moon which stabilized its axial tilt at a low enough angle to prevent the huge seasonal temperature extremes and lengthy periods of darkness which would have resulted from its having a stable extreme tilt.

[helvick]

Robin Canup's simulations actually suggest that about 1/3 of terrestrial planets may undergo glancing impacts which give them a large moon.
...
but that it has a Moon which stabilized its axial tilt at a low enough angle to prevent the huge seasonal temperature extremes and lengthy periods of darkness which would have resulted from its having a stable extreme tilt.

So what is the current best guess on the liklihood of a system having a habitable terrestrial type planet around a star system?

[mike]

Given the number of stars that exist, inevitable.

[JRehling]

So what is the current best guess on the liklihood of a system having a habitable terrestrial type planet around a star system?

Of course, it depends on what is meant by habitable, and while there are several variables known pretty well, others are in great doubt.

If we limit the definition simply to a world that is at least the size of Mars, not so large as to be a gas giant, and receiving such heat that water is a liquid, then the biggest constraint seen in most known planetary systems is a gas giant in an eccentric orbit that would make any such terrestrial planet unstable ("Bad Jupiters"). Furthermore, in systems where a gas giant orbits very closely to the star, it is likely that the process of orbital migration swept out any terrestrial planets that might have started in the habitable zone ("Hot Jupiters"). We could make a three-way division of all mature stellar systems:

1) No planets to speak of
2) Gas giants that would have eliminated habitable planets
3) Either no gas giants or only in "remote" orbits ("Good Jupiters")
4) No gas giants at all -- but terrestrial planets

The problem is that current methods largely fail to distinguish categories 1, 3, and 4 from each other, leaving us with 2s vs everything else. However, arguments pertaining to the metallicity of stars suggest that 4s may be quite unusual, and that most systems will either be 1, 2, or 3. Since we can detect 2s readily, the question becomes the breakdown of 1s and 3s, which more sensitive and varied detection schemes might address.

If we accept that 4s are rare, we also know that 2s comprise about 8% of stars, and the question is how the other 93% turn out. If "good Jupiters" are rare in comparison to "hot/Bad Jupiters", then we are talking about a fraction of a percent to a couple of percent of all planetary systems. Hopefully, there are enough that we have some near enough to Earth that future telescopic surveys give us a look at some -- the overwhelming majority of the galaxies' stars are way too far for us to ever get a good look.

http://www.astrobio.net/news/modules.php?o...rticle&sid=1222

[Guest_BruceMoomaw_*]

That's why Kepler and SIM are so crucial -- Kepler to get an overall count of the numbers and sizes of planets in the habitable zones of a typical star sample, and SIM to get a more precise survey of ALL planets down to large terrestrial ones orbiting our nearby stars. (A gravity-microlensing survey would also be useful, to get a count of the frequency and size of planets modestly outside the habitable zone as well as inside it.) Only after these missions have been flown can we decide which version of Terrestrial Planet Finder to fund first -- since, if habitable planets are scarce, it would be better to fly the Interferometer version of TPF before the cheaper but less sensitive Coronagraph version.