Prospects for exomoons

Kyplanet really is a good Youtube, less excitable than even Anton Petrov whose 'tube is also good. Last weekend Kyplanet linked Zoltan Dencs, Vera Dobos, and Zsolt Regaly on exomoons. Which Petrov once thought we'd found which we hadn't; but it is generally believed we will find them. D.D.R. count a dozen candidates if no confirmations.

In advance of confirmations, D.D.R. make some up. They've run simulations on Jupiter-plus possibilities in the 1-5 AU range, looking for habitability. This range suggests stellar heat: a G star maybe late-F, where oxygenation might happen before red-giant-game-over. The G/F might be chosen because lighter stars, like K, don't tend to own Jovians, which, you know, we do. (K runs the table on Neptunians and Superearths.) This is a real study where, a couple years ago, this blog just eyeballed it.

I should further note in fairness that one example given is HD 114386 in Centaurus - which is K, 28% luminosity. Its Jovian b orbits cold: the 2017 Gaia dataset says 937.7 days, with a beyond-Martian 1.73 AU and eccentricity 0.23. (Wiki sux.) The paper claims msini 1.36 M_J although I read Gaia knocked that back to 1.14. The planet is brought because although any moons are assuredly frozen, they may own tides under the crust. The lower mass in the Gaia DR should shrink the space allowed for tidal-heating effect.

To be sure, few such wide-semimajor planets will transit us. Their moons might be difficult to spot as transits, likewise. I do not think they overlap with the present candidates; indeed D.D.R. rates all twelve too small or hot. Hence, the need for models.

For scope, nobody here is talking about captures in retrograde like Triton, just eccentricity. One might also consider the vanilla resonances like the three inner Galileans' also the less-known moons' around Saturn.

We prospective sattelitical colonials want volcanism running, allowing for atmo replenishment and an internal magnet. The magnet must be stronger than Ganymede's with respect to distance from the Jovian; we also, don't forget, need to protect against the star. So we're looking for moons much more massive than Ganymede, Titan, or the rest - heavier indeed than Mars if possible.

When they say "habitable" they mean by heat. Colonists must worry about moons whose heatsource is tidal/volcanic. Solar irradiance puts constraints upon chloroplasts - on photosynthesis. Abiotic oxygen, meanwhile, stands to poison everything else. Life, perhaps; but Not As We Know It (purple?) and we're not breathing its oxygen. "Luckily" moons as form out where HD 114386 b is at should be too small anyway - and not just for Ks. They're Europae. Not home for humans.

The D.D.R. model points to fewer but larger moons close to the star - so, 1 AU; further, the moons will be more plentiful but smaller, like what's around Jupiter. I guess Saturn's the outlier because it has Titan. (Neptune and Uranus have violent pasts.)

Their model is basically a two-body problem such as ignores migration. A migrate now in the HZ should be bringing its moons with it - small icy moons, according to the paper. Little damp marbles. But for our target of superJovians it may be fair to ignore migration in low-eccentricity unistellar systems without outer browndwarfs.

The masses of these "habitable" moons tend to about half Earth's and I don't know how metallic they can be. On the plus side, this post does not demand a "Cambrian" here; our bias is to younger systems, so if the dynamo dies after 3.5Gy that's fine (an F star might itself begone by then).

The moons should expect to be tidally-locked, although one does - where eccentric without extralunar resonances - ponder the 3:2 Mercurylike spin:orbit. The tidal locking would affect, most, interactions with the Jovian's magnetic-field, which should start extreme. But this post (if not DDR's) does allow for billions of years. Maybe the magnets retract before the atmo and ocean be lost.