Day: January 14, 2009

A Likely Long-Period Comet from the Inner Oort Cloud

[0901.1690] 2006 SQ372: A Likely Long-Period Comet from the Inner Oort Cloud.

Since Jan Oort and Ernst Opik suggested the existence of the Oort Cloud based on long-period comet statistics we’ve yet to spot its members for sure, until recently anyway. The oddly orbitting Sedna, for example, has been proposed as one Oort Cloud member, and now some others seem to be refugees from it too.

2006 SQ372 is a cometoid on an odd orbit lasting over 8,000 years, flying between 24 and 796 AU from the Sun. Computer simulations show that it isn’t in a permanent orbit either – sooner or later Neptune’s gravity will fling it inwards or outwards. But just where is it from? The Scattered Disk is one possibility – a group of cometoids on inclined orbits, spread out between 30 and 100 AU. The Inner Oort Cloud is another – a flattened inner torus of the main spherical Cloud. The simulations suggest the Inner Oort is more likely because comets are 16 times more likely to evolve inwards into a SQ372 style orbit, than evolve outwards. Comet 2000 OO67 is on a similar orbit, so the Oort Cloud count now seems to be 3.

The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?

[0901.1698] The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?.

A clever use of orbital tidal evolution to constrain the possible structure of the 3 sub-Neptune mass planets around HD 40307. Fast-forwarding the evolution of the system shows it becomes unstable if the planets are too efficient at turning tidal deformations into heat. A terrestrial planet is estimated to turn roughly 1.0%-0.5% of tidal flexing into heat, which doesn’t sound like much, but is enough to liquefy much of Io’s interior, for example. For terrestrial planets very close to their star the situation would be even more dramatic and they’d be largely volcanic. Worse than Io, if you can imagine such a thing.

Ocean planets, for real, which are fluid down to the core, if not beyond, are much more flexible and less able to turn flex into heat. Typically a Jovian or Ice-Giant is estimated to turn 1/20,000th or less of the tidal flex into frictional heating and so the HD 40307 planets, if they were mini-Neptunes, wouldn’t evolve in their orbits as quick.

Sounds like a long chain of inference, I know, but much of the geophysical figuring involved comes from hard-data from the planets we know best, including our own. Nature, however, might know something we don’t…