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Zadoc
2011-Jan-29, 05:52 AM
Correct me if I'm wrong, but the general theory goes like this. In the protoplanetary disc, the heavier materials like silica and metals are closer to the star, gas is in the middle, and water and ammonia are at the far edges. That's how you get Mercury at the front, Jupiter in the middle, and the TNOs way in the back.

Once these super hot super Jupiters were discovered, the theory became that they formed in the manner described above, however they spiraled inward and likely gobbled up small rocky planets along the way.

But doesn't it make more sense that these objects were not formed in a protoplanetary disc at all, but rather in a stellar nursery like so many objects being discovered recently that are referred to as free-floating planets, planetary brown dwarfs, isolated planetary-mass objects, cluster planets, superplanets, or a planetars?

Jeff Root
2011-Jan-29, 12:50 PM
Hey Zadoc!

There's no reason to think they were formed independently, as
isolated bodies. I'm not as familiar with the evolution of planetary
systems as some people here, but I also favor an explanation for
hot Jupiters that differes from the migration theory.

The reasons materials are differentiated across the planetary
disk are that 1) the heavier materials stick together more readily,
especially at warmer temperatures, clumping into planetoids
while the lighter materials remain gaseous, and 2) the lighter
materials, especially gases which have not been incorporated
into planetoids or planets, are blown away by the solar wind.
The solar wind comes about after the central star has become
sufficiently large and hot. If a gas giant planet forms close to
the central star before that star begins putting out significant
heat and solar wind, it could be massive enough to first collect
and then hold on to lighter gases even after the wind kicks in.

-- Jeff, in Minneapolis

Romanus
2011-Jan-29, 02:46 PM
1.) It's actually quite difficult (perhaps impossible) for one star to capture another without a third involved (in which one is ejected); if a gas giant planet were involved, it would almost certainly be the one given the boot.

2.) Hot jupiters have too many commonalities to be easily explained by capture: the clustering at low masses, strong correlation with high metallicity (which holds true for most exoplanets of any mass so far discovered), and very fact that they're relatively common, more so than one would expect from isolated capture events (though this probably happens from time to time, to be fair).

Jeff Root
2011-Jan-29, 03:05 PM
Romanus,

Since you appear to know something about it, how do the
metallicities of the exoplanet's parent stars compare with
the metallicity of the Sun?

-- Jeff, in Minneapolis

Hungry4info
2011-Jan-29, 06:14 PM
Gas giant planets form further out from the star and migrate in. They don't necessarily "gobble up" terrestrial planets, but are more likely to scatter them or eject them out of the system.
This migration can occur through a few methods.

Type I migration: planets drive spiral density waves in the surrounding disk. An imbalance occurs between the strength of the interaction with the spirals inside and outside the disk, usually with the outer wave exerting more torque on the planet than the interior wave. This causes a loss of orbital energy, resulting in migration toward the star that are short, relative to the Myr life time of the disk. The scattering of planetesimals also helps the planet lose orbital energy.

Type II migration: A planet clears a gap in the disk, which will end type I migration, but materials continue to enter the gap on the timescale of the accretion disk lifetime, moving the planet and gap inward on the accretion timescale of the disk.

Type III migration: Rather massive disks may have asymmetric density distribution leading to a large torque allowing a planet to migrate inward on a timescale much shorter than is available to Type II migration.

Migrations usually keep on going inward until caught into a resonance (frequently 2:1) with an interior planet, at which point the two will evolve together. Type III migration can blow through 2:1 resonance and continue on to be caught in 2:3 or higher resonances.

Planet-Planet Scattering: Multiple massive planets form, the system is unstable, and planets get thrown around. Some may have periastrons close to the star, where tidal circularisation of their orbit leads to circular, short period Jupiters.

Kozai mechanism: Influence from a distant third body in a non-coplanar orbit (such as a secondary member in a binary star system, or an additional, outer massive planet) can cause the inner planet's orbit to cycle between low-eccentricity, low inclination states to high-eccentricity, high-inclination states. Other planets in the system become prime targets for scattering. During the high-eccentricity states, the planet spends enough time near the star for tidal circularisation to shorten the planet's orbital period some before returning to the low-eccentricity state. After returning to the high eccentricity state, the process repeats. This continues until the planet is left in a circular orbit very close to the star.
This animation shows the Kozai mechanism. http://www.youtube.com/watch?v=xVFRYkux0bQ

It isn't known how frequent each of these situations are. There's still a lot about planet formation that we don't know. The fact that many hot Jupiters have been found to be poorly aligned or retrograde with respect to the stellar spin axis shows that scattering or the Kozai mechanism may be more frequent than we thought originally. Almost certainly though, not all systems evolve the same way. Some will surely experience peaceful migration, others, violent scattering.


Since you appear to know something about it, how do the metallicities of the exoplanet's parent stars compare with the metallicity of the Sun?

Stars with giant planets are typically metal rich, frequently more so than the Sun. This correlation seems to disappear for intermediate mass stars. Furthermore, there doesn't seem to be a metallicity correlation with stars that have sub-Jupiter planets (ice giants, super-Earths).