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Uranus & Neptune: A Planetary Switch?

How long did it take for the planets in our Solar System to form? Much depends upon the surface density of the solar nebula protoplanetary disk, the gas and dust from which the planets emerged. And the problem with surface density — mass per area — in these settings is that it’s hard to observe with our current instrumentation. Looking at distant systems in the process of formation, we see mostly dust and miss larger objects.

Thus an estimate based on known factors is called into play. It produces the so-called minimum mass solar nebula. Using it, scientists can estimate solar nebula mass by starting with the rocky components of each planet, adding hydrogen and helium until the composition resembles that of the Sun. Spread that mass over the area of each planet’s orbit and you get disk masses that look like what we see in systems around other stars.

But there’s a problem. The low surface densities this model produces aren’t sufficient to allow the planets to form in a reasonable period of time. This is where Steve Desch (Arizona State University) goes to work. Here’s the problem he sees:

“I was thinking about planet formation and noticing that all the current models failed to predict how Jupiter could grow to its current size in the life time of the solar nebula. Given Jupiter’s composition and size, models predicted it would take many millions of years for it to form, and billions of years for Uranus and Neptune – but our solar system isn’t that old.”

Desch turned to a different model for considering the solar nebula. The ‘Nice model’ (named after the French city whose open markets — fabulous olives! — still have my head spinning) operates according to fundamentally different assumptions. For one thing, the model assumes the giant planets formed much closer together than their present position suggests, with Neptune forming at around 15 AU. Using this model to assume a much more tightly packed solar nebula, Desch was able to simulate a nebula with smooth variations in surface density with distance from the Sun, although that density falls off sharply at the outer edge.

In fact, using this model, all the planets could be accounted for, assuming you switched Uranus and Neptune in their places. Desch now thinks that for the first 650 million years of the Solar System, Neptune was closer to the Sun than Uranus. Planet formation, meanwhile, now seems to fit the kind of time restrictions imposed by the early nebula. Desch again:

“The surface density of the solar nebula isn’t what we originally thought – it is actually much higher – and this has implications for where we formed and for how fast planets grow. A higher surface density of the solar nebula means that Uranus and Neptune formed closer and faster, in only 10 million years instead of billions.”

Just as significant, Desch thinks he can explain why the solar nebula falls off so sharply in density at large distances from the Sun. The process at work is photoevaporation, with material at the outer edge of the disk being constantly removed because of the effect of ultraviolet radiation from nearby massive stars. The paper examining all this is Desch, “Mass Distribution and Planet Formation in the Solar Nebula,” Astrophysical Journal 671 (December 10, 2007), pp. 878–893 (abstract). Dr. Desch has also made the full text available via his Web site.

Comments on this entry are closed.

  • ljk December 17, 2007, 12:08

    On the Solar System-Debris Disk Connecction

    Authors: Amaya Moro-Martin

    (Submitted on 14 Dec 2007)

    Abstract: This paper emphasizes the connection between solar and extra-solar debris disks: how models and observations of the Solar System are helping us understand the debris disk phenomenon, and vice versa, how debris disks are helping us place our Solar System into context.

    Comments: 8 pages, Exoplanets: Detection, Formation and Dynamics Proceedings IAU Symposium No. 249 2008

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.2294v1 [astro-ph]

    Submission history

    From: Amaya Moro-Martin [view email]

    [v1] Fri, 14 Dec 2007 06:49:56 GMT (866kb)


  • thohry December 20, 2007, 18:32

    It’s interesting !. But I think Uranus and Nepture are quite similar in side and mass, so why Neptune was tugged out faster than Uranus?

  • ljk January 9, 2008, 0:56

    Constraints to Uranus’ Great Collision. IV. The Origin of Prospero

    Authors: Gabriela Parisi (IAR-LaPlata), Giovanni Carraro (ESO-Santiago), Michele Maris (OATS), Adrian Brunini (LaPlata)

    (Submitted on 8 Jan 2008)

    Abstract: It is widely accepted that the large obliquity of Uranus is the result of a great tangential collision (GC) with an Earth size proto-planet at the end of the accretion. We attempt to constraint the GC scenario as the cause of Uranus’ obliquity as well as on the mechanisms able to give origin to the Uranian irregulars. Different capture mechanisms for irregulars operate at different stages on the giant planets formation process. The mechanisms able to capture the uranian irregulars before and after the GC are analysed. Assuming that they were captured before the GC, we calculate the orbital transfer of the nine irregulars by the impulse imparted by the GC. If their orbital transfer results dynamically implausible, they should have originated after the GC. We investigate and discuss the dissipative mechanisms able to operate later. In particular Prospero could not exist at the time of the GC. Different capture mechanisms for Prospero after the GC are investigated. Gas drag by Uranus’envelope and pull-down capture are not plausible mechanisms. Capture of Prospero through a collisionless interaction seems to be difficult. The GC itself provides a mechanism of permanent capture. However, the capture of Prospero by the GC is a low probable event. Catastrophic collisions could be a possible mechanism for the birth of Prospero and the other irregulars after the GC. Orbital and physical clusterings should then be expected. Either Prospero had to originate after the GC or the GC did not occur. In the former case, the mechanism for the origin of Prospero after the GC remains an open question. In the latter case, another theory to account for Uranus’ obliquity and the formation of the Uranian regular satellites on the equatorial plane of the planet would be needed.

    Comments: 12 pages, 1 eps figure, accepted for publication in A&A. Abstract rephrased to fit in

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.1258v1 [astro-ph]

    Submission history

    From: Giovanni Carraro dr [view email]

    [v1] Tue, 8 Jan 2008 15:46:28 GMT (186kb,D)


  • ljk January 30, 2008, 13:29

    Long-term observations of Uranus and Neptune at 90 GHz with the IRAM 30m telescope – (1985 — 2005)

    Authors: C. Kramer, R. Moreno, A. Greve

    (Submitted on 29 Jan 2008)

    Abstract: The planets Uranus and Neptune with small apparent diameters are primary calibration standards. We investigate their variability at ~90 GHz using archived data taken at the IRAM 30m telescope during the 20 years period 1985 to 2005. We calibrate the planetary observations against non-variable secondary standards (NGC7027, NGC7538, W3OH, K3-50A) observed almost simultaneously. Between 1985 and 2005, the viewing angle of Uranus changed from south-pole to equatorial. We find that the disk brightness temperature declines by almost 10% (~2sigma) over this time span indicating that the south-pole region is significantly brighter than average. Our finding is consistent with recent long-term radio observations at 8.6 GHz by Klein & Hofstadter (2006).

    Both data sets do moreover show a rapid decrease of the Uranus brightness temperature during the year 1993, indicating a temporal, planetary scale change. We do not find indications for a variation of Neptune’s brightness temperature at the 8% level. If Uranus is to be used as calibration source, and if accuracies better than 10% are required, the Uranus sub-earth point latitude needs to be taken into account.

    Comments: accepted for publication in A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.4452v1 [astro-ph]

    Submission history

    From: Carsten Kramer [view email]

    [v1] Tue, 29 Jan 2008 09:28:31 GMT (268kb)


  • ljk March 14, 2008, 16:09

    Revealing the night sides of Uranus’ moons


  • ljk April 15, 2008, 12:27

    On the Dynamical Stability of the Solar System

    Authors: Konstantin Batygin, Gregory Laughlin

    (Submitted on 11 Apr 2008)

    Abstract: A long-term numerical integration of the classical Newtonian approximation to the planetary orbital motions of the full Solar System (sun + 8 planets), spanning 20 Gyr, was performed. The results showed no severe instability arising over this time interval. Subsequently, utilizing a bifurcation method described by Jacques Laskar, two numerical experiments were performed with the goal of determining dynamically allowed evolutions for the Solar System in which the planetary orbits become unstable.

    The experiments yielded one evolution in which Mercury falls onto the Sun at ~1.261Gyr from now, and another in which Mercury and Venus collide in ~862Myr. In the latter solution, as a result of Mercury’s unstable behavior, Mars was ejected from the Solar System at ~822Myr. We have performed a number of numerical tests that confirm these results, and indicate that they are not numerical artifacts. Using synthetic secular perturbation theory, we find that Mercury is destabilized via an entrance into a linear secular resonance with Jupiter in which their corresponding eigenfrequencies experience extended periods of commensurability. The effects of general relativity on the dynamical stability are discussed.

    An application of the bifurcation method to the outer Solar System (Jupiter, Saturn, Uranus, and Neptune) showed no sign of instability during the course of 24Gyr of integrations, in keeping with an expected Uranian dynamical lifetime of 10^(18) years.

    Comments: 37 pages, 18 figures, accepted for publication in the Astrophysical Journal

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.1946v1 [astro-ph]

    Submission history

    From: Konstantin Batygin [view email]

    [v1] Fri, 11 Apr 2008 18:49:18 GMT (725kb,D)


  • ljk July 2, 2008, 15:24

    Ice Lines, Planetesimal Composition and Solid Surface Density in the Solar Nebula

    Authors: Sarah E. Robinson (1), Karen Willacy (2), Peter Bodenheimer (1), Gregory Laughlin (1), Neal J. Turner (2), C. A. Beichman (3) ((1) UCO/Lick Observatory, (2) Jet Propulsion Laboratory, (3) Michelson Science Center)

    (Submitted on 23 Jun 2008)

    Abstract: To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2-3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula.

    By combining a viscously evolving protostellar disk with a kinetic model of ice formation, we calculate the solid surface density in the solar nebula as a function of heliocentric distance and time.

    We find three effects that strongly favor giant planet formation: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) recent lab results (Collings et al. 2004) showing that the ammonia and water ice lines should coincide, and (3) the presence of a substantial amount of methane ice in the trans-Saturnian region.

    Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. (1996) by a factor of 3 to 4 throughout the trans-Saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.

    Comments: 12 figures, 7 tables, submitted to Icarus

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.3788v1 [astro-ph]

    Submission history

    From: Sarah Robinson [view email]

    [v1] Mon, 23 Jun 2008 23:35:41 GMT (2806kb,D)


  • ljk July 10, 2009, 7:53

    Did Galileo Discover Neptune? Galileo’s Notebooks May Reveal Secrets of New Planet


    “Galileo knew he had discovered a new planet in 1613, 234 years before its official discovery date, according to a new theory by a University of Melbourne physicist.”

    Though I distinctly recall it from a book published in 1988 on the outer planets.

  • ljk September 4, 2009, 13:14

    Origin and Dynamical Evolution of Neptune Trojans – I: Formation and Planetary Migration

    Authors: P. S. Lykawka, J. Horner, B. W. Jones, T. Mukai

    (Submitted on 2 Sep 2009)

    Abstract: We present the results of detailed dynamical simulations of the effect of the migration of the four giant planets on both the transport of pre-formed Neptune Trojans, and the capture of new Trojans from a trans-Neptunian disk.

    We find that scenarios involving the slow migration of Neptune over a large distance (50Myr to migrate from 18.1AU to its current location) provide the best match to the properties of the known Trojans. Scenarios with faster migration (5Myr), and those in which Neptune migrates from 23.1AU to its current location, fail to adequately reproduce the current day Trojan population. Scenarios which avoid disruptive perturbation events between Uranus and Neptune fail to yield any significant excitation of pre-formed Trojans (transported with efficiencies between 30 and 98% whilst maintaining the dynamically cold nature of these objects). Conversely, scenarios with periods of strong Uranus-Neptune perturbation lead to the almost complete loss of such pre-formed objects. In these cases, a small fraction (~0.15%) of these escaped objects are later recaptured as Trojans prior to the end of migration, with a wide range of eccentricities (<0.35) and inclinations (<40 deg).

    In all scenarios (including those with such disruptive interaction between Uranus and Neptune) the capture of objects from the trans-Neptunian disk (through which Neptune migrates) is achieved with efficiencies between ~0.1 and ~1%. The captured Trojans display a wide range of inclinations (<40 deg for slow migration, and <20 deg for rapid migration) and eccentricities (<0.35), and we conclude that, given the vast amount of material which undoubtedly formed beyond the orbit of Neptune, such captured objects may be sufficient to explain the entire Neptune Trojan population. (Shortened version)

    Comments: 25 pages, 6 figures, MNRAS (in press)

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0909.0404v1 [astro-ph.EP]

    Submission history

    From: Patryk Sofia Lykawka [view email]

    [v1] Wed, 2 Sep 2009 12:12:41 GMT (1486kb)


  • ljk September 4, 2009, 13:19

    Speed limit on Neptune migration imposed by Saturn tilting

    Authors: Gwenaël Boué, Jacques Laskar, Petr Kuchynka

    (Submitted on 2 Sep 2009)

    Abstract: In this Letter, we give new constraints on planet migration. They were obtained under the assumption that Saturn’s current obliquity is due to a capture in resonance with Neptune’s ascending node. If planet migration is too fast, then Saturn crosses the resonance without being captured and it keeps a small obliquity. This scenario thus gives a lower limit on the migration time scale tau.

    We found that this boundary depends strongly on Neptune’s initial inclination. For two different migration types, we found that tau should be at least greater than 7 Myr. This limit increases rapidly as Neptune’s initial inclination decreases from 10 to 1 degree. We also give an algorithm to know if Saturn can be tilted for any migration law.

    Comments: 5 pages, 4 figures, published in ApJL

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Journal reference: ApJL 702 (2009), L19-L22

    DOI: 10.1088/0004-637X/702/1/L19

    Cite as: arXiv:0909.0332v1 [astro-ph.EP]

    Submission history

    From: Gwenaël Boué [view email]

    [v1] Wed, 2 Sep 2009 06:31:34 GMT (229kb)