Why some planets are the size they are remains something of a mystery. I’m looking at the discovery paper for a planet called WASP-17b, which is said to be twice Jupiter’s size but only half its mass. That raises questions about the mechanisms at work, for you can’t explain the bloated nature of this world with the models of planetary evolution we’re now working with without factoring in massive tidal effects.
In one sense, WASP-17b is completely anomalous. In addition to its size, it orbits its star in retrograde fashion, opposite the direction of the star’s spin. But in other important respects, the new planet joins the ranks of other bloated worlds like HD209458b (the first such world to be discovered), and a flock of other huge planets that includes TrES-4, WASP-12b, XO-3b and HAT-P-1b. TrES-4 shows a density about fifteen percent that of Jupiter, with a radius 1.78 times larger than Jupiter’s.
Image: Orbiting close to its parent star, WASP-17b may look something like this, a huge world bloated by the tidal forces produced by its highly unusual orbit. Credit: NASA/Hubble.
But WASP-17b now seems to take the prize in terms of size. Its primary, about 1000 light years away, is an F6 star in Scorpio, which it transits every 3.7 days. So bloated is this object that Coel Hellier (Keele University, UK) says it is “…only as dense as expanded polystyrene, seventy times less dense than the planet we’re standing on.” That makes it the least dense planet known, and tidal effects seem to be the only way to account for the fact. From the paper (internal references deleted for brevity):
It has been proposed… that tidal dissipation associated with the circularisation of an eccentric orbit is able to substantially inflate the radius of a short-orbit, giant planet. If a planet is in a close (a < 0.2 AU), highly eccentric (e > 0.2) orbit then planetary tidal dissipation will be significant and will shorten and circularise the orbit. Orbital energy is deposited within the planet interior, leading to an inflated planet radius. This process is accelerated by higher atmospheric opacities: as the planet better retains heat, shrinking of the radius is retarded, and a larger radius causes greater tidal dissipation.
This planet’s tidal effects — compression and stretching — must be remarkably powerful. The evidence that WASP-17b is in a retrograde orbit is strong, and the authors note that the angular momenta of star and protoplanetary disk (and thus the planets that emerge from that disk) all derive from the same parent molecular cloud. By all rights, this planet should be orbiting in the same direction as its star.
The theory, then, is that WASP 17-b started out in a prograde orbit and migrated to its current separation of 0.05 AU, being flung in the process into its unusual orbital arrangement. A disruptive brush with another planet or star is about the only thing that could explain the result. The authors note that radial velocity measurements could look for planets further out in this system that might have been involved in this early event, but it is possible that such worlds were ejected from the system in the process. WASP-17b may be the only giant planet remaining.
The paper is Anderson et al., “WASP-17b: an ultra-low density planet in a probable retrograde orbit,” submitted to the Astrophysical Journal and available as a preprint.
Dust-Dust Collisional Charging and Lightning in Protoplanetary Discs
Authors: Takayuki Muranushi
(Submitted on 11 Aug 2009)
Abstract: We study the role of dust-dust collisional charging in protoplanetary discs. We show that dust-dust collisional charging becomes an important process in determining the charge state of dust and gas, if there is dust enhancement and/or dust is fluffy, so that dust surface area per disc volume is locally increased.
We solve the charge equilibrium equations for various disc environments and dust condensation $\eta$ (dust number density of the considered region divided by the fiducial value), using general purpose graphic processors (GPGPU) and {\sc cuda} programming language. We found that as dust condensation $\eta$ increases, the charge distribution experience four phases.
In one of these phases the electrostatic field $E$ caused by dust migration increases as $E \propto \eta^4$. As a result, macroscopic electric discharge takes place typically at $\eta = 30 \sim 300$. We present a model that describes the charge exchange processes in the discs as an electric circuit.
We estimate the total energy, intensity and event ratio of such discharges (`lightning’). We discuss the possibility of observing lightning and sprite discharges in protoplanetary discs by Astronomically Low Frequency ({\em ALF}) waves, {\em IR} images, {\em UV} lines, and high energy gamma rays. We also discuss the effects of lightning on chondrule heating, planetesimal growth and magnetorotational instability of the disc.
Comments: 23 pages, 34 figures, submitted to MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:0908.1575v1 [astro-ph.EP]
Submission history
From: Takayuki Muranushi [view email]
[v1] Tue, 11 Aug 2009 20:20:22 GMT (4497kb)
http://arxiv.org/abs/0908.1575
On fitting planetary systems in counter-revolving configurations
Authors: Julie Gayon-Markt (1 and 2), Eric Bois (2) ((1) NASA Ames Research Center, (2) University of Nice Sophia-Antipolis / CNRS / Observatoire de la Cote d’Azur)
(Submitted on 10 Aug 2009)
Abstract: In Gayon & Bois (2008) and Gayon etal (2009), (i) we studied the theoretical feasibility and efficiency of retrograde mean motion resonances (i.e. two planets are both in orbital resonance and in counter-revolving configuration), (ii) we showed that retrograde resonances can generate interesting mechanisms of stability, and (iii) we obtained a dynamical fit involving a counter-revolving configuration that is consistent with the observations of the HD73526 planetary system.
In the present paper, we present and analyze data reductions assuming counter-revolving configurations for eight compact multi-planetary systems detected through the radial velocity method. In each case, we select the best fit leading to a dynamically stable solution. The resulting data reductions obtained in rms and chi values for counter-revolving configurations are of the same order, and sometimes slightly better, than for prograde configurations.
In the end, these fits tend to show that, over the eight studied multi-planetary systems, six of them could be regulated by a mechanism involving a counter-revolving configuration.
Comments: 4 pages, 1 figure, 2 tables, accepted for publication in MNRAS letters (August 10, 2009)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0908.1292v1 [astro-ph.EP]
Submission history
From: Julie Gayon-Markt [view email]
[v1] Mon, 10 Aug 2009 18:56:34 GMT (44kb)
http://arxiv.org/abs/0908.1292
Hot off the arXiv, the previously-known planet HAT-P-7b may also be in a retrograde orbit. There’s evidence of a second planet too.
HAT-P-7: A Retrograde or Polar Orbit, and a Second Planet
First Evidence of a Retrograde Orbit of Transiting Exoplanet HAT-P-7b
Has anyone noticed the Exoplanet Counter is up to 373!
The rate of exoplanet discoveries is around 25 per year. Kepler, of course, is supposed to find hundreds and possibly a thousand exoplanets in the next 3 years.
Hi Folks;
WASP-17b is a freak but a not uncommon one. But it is a fascinating freak to say the least.
Even though we will soon be probing sub-atomic and sub-nuclear physics anew with the resumption of the operations of the LHC this November, and a whole host of other planned accelerators, it is nice to know that Newtonian Classical Mechanics and thermodynamics can still be used to describe planetary scale systems with great accuracy. As computers become even more powerful, CFD, Finite Element Analysis, and other excellent numerical methods combined with Newtonian scale classical mechanics will continue to play an important role in the study of systems like WASP-17b.
WASP-17b is so close to its parent star that it must be a real scorcher.
Ah, but is it? We have a very small sample of planets whose inclinations relative to their star’s equatorial planes are known with any degree of certainty (the second of the HAT-P-7b papers I linked earlier gives a total of 15). Of those 15, there are 2 which show retrograde motion (WASP-17b and HAT-P-7b). Apparently this is not vanishingly rare. We should expect that a fair few multiplanet systems are going to have substantial misalignments, particularly ones containing eccentric planets which may indicate a history of planet-planet scattering.
IIRC someone proposed that part of the thermal energy that bloats this (and possibly other worlds) in young systems comes from a comparable size impactor. And that this impactor can be partly or fully (?) responsible for the retrograde motion.
Interactions of the magnetospheres of stars and close-in giant planets
Authors: O. Cohen, J.J. Drake, V.L. Kashyap, S.H. Saar, I.V. Sokolov, W.B. Manchester IV, K.C. Hansen, T.I. Gombosi
(Submitted on 16 Sep 2009)
Abstract: Since the first discovery of an extrasolar planetary system more than a decade ago, hundreds more have been discovered. Surprisingly, many of these systems harbor Jupiter-class gas giants located close to the central star, at distances of 0.1 AU or less.
Observations of chromospheric ‘hot spots’ that rotate in phase with the planetary orbit, and elevated stellar X-ray luminosities,suggest that these close-in planets significantly affect the structure of the outer atmosphere of the star through interactions between the stellar magnetic field and the planetary magnetosphere.
Here we carry out the first detailed three-dimensional MagnetoHydroHynamics (MHD) simulation containing the two magnetic bodies and explore the consequences of such interactions on the steady-state coronal structure.
The simulations reproduce the observable features of 1) increase in the total X-ray luminosity, 2) appearance of coronal hot spots, and 3) phase shift of these spots with respect to the direction of the planet. The proximate cause of these is an increase in the density of coronal plasma in the direction of the planet, which prevents the corona from expanding and leaking away this plasma via a stellar wind.
The simulations produce significant low temperature heating. By including dynamical effects, such as the planetary orbital motion, the simulation should better reproduce the observed coronal heating.
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0909.3093v1 [astro-ph.SR]
Submission history
From: Ofer Cohen Dr. [view email]
[v1] Wed, 16 Sep 2009 20:00:06 GMT (250kb)
http://arxiv.org/abs/0909.3093
Thermal Response of A Solar-like Atmosphere to An Electron Beam from A Hot Jupiter: A Numerical Experiment
Authors: Pin-Gao Gu, Takeru K. Suzuki
(Submitted on 25 Sep 2009)
Abstract: We investigate the thermal response of the atmosphere of a solar-type star to an electron beam injected from a hot Jupiter by performing a 1-dimensional magnetohydrodynamic numerical experiment with non-linear wave dissipation, radiative cooling, and thermal conduction.
In our experiment, the stellar atmosphere is non-rotating and is modelled as a 1-D open flux tube expanding super-radially from the stellar photosphere to the planet. An electron beam is assumed to be generated from the reconnection site of the planet’s magnetosphere. The effects of the electron beam are then implemented in our simulation as dissipation of the beam momentum and energy at the base of the corona where the Coulomb collisions become effective.
When the sufficient energy is supplied by the electron beam, a warm region forms in the chromosphere. This warm region greatly enhances the radiative fluxes corresponding to the temperature of the chromosphere and transition region. The warm region can also intermittently contribute to the radiative flux associated with the coronal temperature due to the thermal instability. However, owing to the small area of the heating spot, the total luminosity of the beam-induced chromospheric radiation is several orders of magnitude smaller than the observed Ca II emissions from HD 179949.
Comments: 4 figures, accepted for publication in The Astrophysical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0909.4602v1 [astro-ph.SR]
Submission history
From: Pin-Gao Gu [view email]
[v1] Fri, 25 Sep 2009 04:37:53 GMT (73kb)
http://arxiv.org/abs/0909.4602
NASA Hubble Space Telescope Daily Report #4938
http://www.spaceref.com/news/viewsr.html?pid=32494
“Is the Atmosphere of the Hottest Known Transiting Exoplanet Evaporating?
WASP-12 is the hottest and the largest currently known transiting exoplanet. It has the shortest orbital period and is the closest to its host star.
Previous spectacular HST observations revealed that the atmosphere of HD 209458b appears to be evaporating away, though this interpretation has recently been questioned.
We propose ultraviolet observations of WASP-12 to learn whether it is in a state of hydrodynamic ‘blow-off’ as the work on HD 209458b would suggest.
We will obtain a precise radius for the planet, free from systematic errors caused by the earth’s atmosphere. We will use our data to hone models of exoplanet atmospheres.”