# Infant Planet Still in Formation

Long before the first exoplanets were found, one speculation about our own Solar System was that a passing star had disrupted the solar nebula so as to promote the formation of planets. We now know that planets form in many ways, but it’s interesting to see that HL Tau, a star discussed yesterday at the Royal Astronomical Society meeting in Belfast, may have been influenced by a recent close pass by XZ Tau, another young star nearby. Did this ‘flyby’ disrupt the circumstellar disk around HL Tau, helping to form a proto-planet that has now been observed?

Whatever the case, we do seem to have interesting processes at work around HL Tau. The newly discovered proto-planet is thought to be only one percent of the age of a planet found last year around TW Hydrae, a world ten times the mass of Jupiter that was once the youngest planet yet detected. That one orbited inside the inner hole of a pronounced circumstellar disk. HL Tau b remains little more than a bright clump within its dusty disk, but one that has undergone extensive computer modeling.

Jane Greaves (University of St Andrews) presented the team’s findings on this unusual object at the RAS session. We’re apparently dealing with the earliest stage of planetary formation:

“We see a distinct orbiting ball of gas and dust, which is exactly how a very young protoplanet should look. In the future, we would expect this to condense out into a gas giant planet like a massive version of Jupiter. The protoplanet is about 14 times as massive as Jupiter and is about twice as far from HL Tau as Neptune is from our Sun.”

HL Tau itself is a youngster, perhaps no more than 100,000 years old. Located in the direction of the constellation Taurus and some 520 light years from Earth, the star’s unusually bright circumstellar disk has been studied before, its bright clump noted, but it took the Very Large Array and the UK’s MERLIN array of radio telescopes to confirm the presence of rocky materials (as opposed to hot gases) in the early stages of planet formation. Computer simulations at the University of Edinburgh suggest that the mechanism at work is gravitational instability in a disk that is half as massive as the star itself, allowing individual structures to form.

Image: HL Tau and its surrounding disk as seen in computer simulation. In the model the dense clump (seen at top right) forms with a mass of about 8 times that of Jupiter at a distance from the star about 75 times that from the Earth to the Sun. Click on the image for a movie showing the growth of this disk (Credit: Greaves, Richards, Rice & Muxlow 2008).

Now we’re back in the bidding war between gravitational instability and core accretion as models for planet formation. The latter suggests that planetary cores form as planetesimals gradually clump together until reaching a mass sufficient to capture gas from the surrounding materials. The two do not, of course, have to be mutually exclusive, with probable examples of gravitational instability in particular cases not ruling out core accretion in other settings. But the more we study young systems like this, the more we will learn about the respective merits of each theory.

Comments on this entry are closed.

• ljk April 3, 2008, 10:15

There was an old theory by Sir James Jeans regarding the formation
of the Sol system that the planets formed when a star passing by
our Sun pulled out material from Sol which condensed around our
yellow dwarf star into the worlds we know. As a result, Jeans
concluded that planetary systems and any life upon them are
quite rare.

Jeans theory was dismissed in favor of the nebula hypothesis of
planetary formation, but wouldn’t it be interesting if what we are
seeing here is an example of Jeans own theory? Assuming the
object in question is a forming planet and not a new star.

• Administrator April 3, 2008, 12:33

Right you are, Larry, and I recall being taught this theory way back in grade school, or rather, taught that it was one possibility. Certainly would cut down on the chances for other civilizations if true!

• andy April 3, 2008, 13:11

Is ljk more than one person, or is someone committing an act of handlejacking?

• Administrator April 3, 2008, 13:22

My mistake, andy, due to some recent reconfiguration here. ljk is indeed a single, hard-working individual, but not the one who wrote the post above yours!

• Kevin April 3, 2008, 15:42

Can someone point me to an online resource that would explain to me the difference(s) between the gravitational instability and the core accretion models?

• Administrator April 3, 2008, 17:27

Kevin, a great place to study this is Greg Laughlin’s systemic site. Greg devotes a number of posts to explaining the difference. I would start with this one on core accretion:

http://oklo.org/?p=72

and then move to this one on gravitational instability:

http://oklo.org/?p=73

If you have access to a good library, Alan Boss describes gravitational instability quite well in “Giant planet formation by gravitational instability,” the details of which are here:

• Kevin April 3, 2008, 21:54

Thank you so much for the links. This is such fascinating stuff!

• Adam April 4, 2008, 6:34

Hi All

Jeans’ Tidal theory was modified by Michael Woolfson and John Dormand in the 1960s – in their Capture Theory, the newly formed Sun interacts tidally with the extended cloud of a protostar, pulling filaments out of the cloud tidally and causing gravitational instabilities in the filaments that collapse into the major planets.

Initially the major planets are on very eccentric orbits, and interact tidally with the Sun – this causes them to throw out material that becomes their retinues of moons. The Sun’s dust disk also provides friction that causes the planets’ orbits to precess, and circularise. One consequence of their precession is the possibility of tidal interactions between the planets – thus the tilting of Uranus’ onto its side.

Another consequence is the possibility of a planetary collision – Earth and Venus are the remnant cores of two small gas giants that collided. The rest of the debris became the asteroids and comets, while their several moons become Luna (or Theia), Mars and Mercury. The planetary collision also initiated a nuclear fusion reaction, fuelled by enhanced deuterium levels in one of the planets. This produced most, if not all, of the isotopic anomalies found throughout the meteorites.

While Woolfson and Dormand’s theory has been a minor alternative in cosmogony a lot of recent, high resolution simulations of tidal interactions between protostars confirms their basic argument – that Jeans collapse in tidal filaments can produce large numbers of planets. I’m not convinced of all their theory, but these days there’s nothing novel about the Capture theory’s suggestions anymore – features are found in a huge number of new studies – but Dormand and Woolfson first thought of it ~45 years ago.

Even the current levels of exoplanet discoveries can be accounted for by the observed fact that stars form in an embedded phase at very high humber densities per cubic parsec. This makes tidal interactions between protostars a near certainty – Woolfson’s most recent estimates put planets around ~14% of stars or more.

Alan Boss’s work on gravitational instability initially assume uninteracting stars, but his more recent papers have brought tides into the picture. Other researchers have shown an important role for tidal interactions in triggering gravitational instability – otherwise know as Jeans Collapse, the process that makes planets in the Capture Theory. It’s fascinating watching so many researchers converging on the same process in cosmogony. No one seriously doubts that either process – core accretion and Jeans Collapse – is involved in making planets, just their relative importance.

• andy April 4, 2008, 7:46

A similar explanation has been postulated for the formation of the planet orbiting the pulsar/white dwarf binary PSR B1620-26: the planet may have formed when a circumbinary disc produced during the common-envelope phase of the system’s evolution was disrupted by a passing star. See Beer et al. (2004) for details.

• ljk April 4, 2008, 10:41

The Structure of the DoAr 25 Circumstellar Disk

Authors: Sean M. Andrews, A. M. Hughes, D. J. Wilner, Chunhua Qi

(Submitted on 2 Apr 2008)

Abstract: We present high spatial resolution (< 0.3″ = 40$AU) Submillimeter Array observations of the 865 micron continuum emission from the circumstellar disk around the young star DoAr 25. Despite its bright millimeter emission, this source exhibits only a comparatively small infrared excess and low accretion rate, suggesting that the material and structural properties of the inner disk may be in an advanced state of evolution. A simple model of the physical conditions in the disk is derived from the submillimeter visibilities and the complete spectral energy distribution using a Monte Carlo radiative transfer code. For the standard assumption of a homogeneous grain size distribution at all disk radii, the results indicate a shallow surface density profile,$\Sigma \propto r^{-p}\$ with p = 0.34, significantly less steep than a steady-state accretion disk (p = 1) or the often adopted minimum mass solar nebula (p = 1.5). Even though the total mass of material is large (M_d = 0.10 M_sun), the densities inferred in the inner disk for such a model may be too low to facilitate any mode of planet formation. However, alternative models with steeper density gradients (p = 1) can explain the observations equally well if substantial grain growth in the planet formation region (r < 40 AU) has occurred. We discuss these data in the context of such models with dust properties that vary with radius and highlight their implications for understanding disk evolution and the early stages of planet formation.

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.0437v1 [astro-ph]

Submission history

From: Sean Andrews [view email]

[v1] Wed, 2 Apr 2008 21:10:28 GMT (50kb)

http://arxiv.org/abs/0804.0437

• ljk April 4, 2008, 10:44

`Tail-end’ Bondi-Hoyle accretion in young star clusters: Implications for disks, planets, and stars

Authors: Henry B. Throop, John Bally

(Submitted on 3 Apr 2008)

Abstract: Young stars orbiting in the gravitational potential well of forming star clusters pass through the cluster’s dense molecular gas and can experience Bondi-Hoyle accretion from reservoirs outside their individual protostellar cloud cores. Accretion can occur for several million years after the stars form, but before the cluster disperses. This accretion is predominantly onto the disk and not the star. N-body simulations of stars orbiting in three young model clusters containing 30, 300, and 3000 stars are presented. The simulations include the gravitational potential of the molecular gas which smoothly disperses over time. The clusters have a star formation efficiency of 33% and a radius of 0.22 pc. We find that the disks surrounding solar-mass stars in the N=30 cluster accretes ~0.01 M_sol (~1 minimum-mass solar nebula, MMSN) per Myr. The accretion rate scales as M^2.1 for stars of mass M. The accretion rate is ~5 times lower for N=3000 cluster, due to its higher stellar velocities and higher temperature. The Bondi-Hoyle accretion rates onto the disks are several times lower than accretion rates observed directly onto young stars (e.g., Muzerolle et al 2005): these two accretion rates follow the same M^2 behavior and may be related.

The accreted disk mass is large enough that it may have a substantial and unappreciated effect on disk structure and the formation of planetary systems. We discuss a variety of implications of this process, including its effect on metallicity differences between cluster stars, compositional differences between a star and its disk, the formation of terrestrial and gas-giant planets, and isotopic anomalies observed in our Solar System.

Comments: 44 pages, 12 figures, 2 tables. Accepted by AJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.0438v1 [astro-ph]

Submission history

From: Henry Throop [view email]

[v1] Thu, 3 Apr 2008 01:29:26 GMT (1232kb)

http://arxiv.org/abs/0804.0438

• Adam April 5, 2008, 5:10

Hi andy

Another researcher to look out for is Anthony Whitworth, who has his name attached to a lot of papers about forming brown dwarfs and planets through tidal interactions between protostar disks. His work has used smoothed particle hydrodynamics (SPH) to simulate large scale disk processes and the results are generally consistent with what Dormand and Woolfson determined using far more primitive computer programs.

A number of other researchers have produced papers with similar processes being key to forming planets – tidal interactions, Jeans collapse and so forth. I suspect that as exoplanet statistics become more complete we’ll find planets made by either accretion or collapse – both are physically plausible, and neither seems able to explain all the data. So why not both?

The other feature of Dormand and Woolfson, planetary collisions, has also appeared in a few papers for various reasons, most recently the idea that Venus formed from two equally massive protoplanets colliding. What’s unique to their idea is the view that the collision triggered fusion reactions – exothermic deuterium fusion, because proto-Venus already had enhanced levels of deuterium. There are competing theories for the origin of the meteoritic isotopic anomalies, but no overarching single process like planetary collision ignited fusion reactions.

• ljk April 21, 2008, 15:04

The Exceptionally Large Debris Disk around \gamma Ophiuchi

Authors: K. Y. L. Su, G. H. Rieke, K. R. Stapelfeldt, P. S. Smith, G. Bryden, C. H. Chen, D. E. Trilling

(Submitted on 18 Apr 2008)

Abstract: Spitzer images resolve the debris disk around \gamma Ophiuchi at both 24 and 70 um. The resolved images suggest a disk radius of ~520 AU at 70 um and >=260 AU at 24 um. The images, along with a consistent fit to the spectral energy distribution of the disk from 20 to 350 um, show that the primary disk structure is inclined by ~50 degree from the plane of the sky at a position angle of 55+/-2 degree. Among a group of twelve debris disks that have similar host star spectral types, ages and infrared fractional luminosities, the observed sizes in the infrared and color temperatures indicate that evolution of the debris disks is influenced by multiple parameters in addition to the proto-planetary disk initial mass.

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.2924v1 [astro-ph]

Submission history

From: Kate Y. L. Su [view email]

[v1] Fri, 18 Apr 2008 01:40:11 GMT (608kb)

http://arxiv.org/abs/0804.2924

• ljk April 28, 2008, 23:07

Multiwavelength studies of the gas and dust disc of IRAS 04158+2805

Authors: A. M. Glauser, F. Ménard, C. Pinte, G. Duchêne, M. Güdel, J.-L. Monin, D. L. Padgett

(Submitted on 22 Apr 2008)

Abstract: We present a study of the circumstellar environment of IRAS 04158+2805 based on multi-wavelength observations and models. Images in the optical and near-infrared, a polarisation map in the optical, and mid-infrared spectra were obtained with VLT-FORS1, CFHT-IR, and Spitzer-IRS.
Additionally we used an X-ray spectrum observed with Chandra. We interpret the observations in terms of a central star surrounded by an axisymmetric circumstellar disc, but without an envelope, to test the validity of this simple geometry. We estimate the structural properties of the disc and its gas and dust content. We modelled the dust disc with a 3D continuum radiative transfer code, MCFOST, based on a Monte-Carlo method that provides synthetic scattered light images and polarisation maps, as well as spectral energy distributions. We find that the disc images and spectral energy distribution narrowly constrain many of the disc model parameters, such as a total dust mass of 1.0-1.75×10^-4 sollar masses and an inclination of 62-63 degrees. The maximum grain size required to fit all available data is of the order of 1.6-2.8 microns although the upper end of this range is loosely constrained. The observed optical polarisation map is reproduced well by the same disc model, suggesting that the geometry we find is adequate and the optical properties are representative of the visible dust content. We compare the inferred dust column density to the gas column density derived from the X-ray spectrum and find a gas-to-dust ratio along the line of sight that is consistent with the ISM value. To our knowledge, this measurement is the first to directly compare dust and gas column densities in a protoplanetary disc.

Comments: 8 figures, 11 pages, accepted by A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.3483v1 [astro-ph]

Submission history

[v1] Tue, 22 Apr 2008 13:41:20 GMT (865kb)

http://arxiv.org/abs/0804.3483

• ljk April 28, 2008, 23:10

Towards planetesimals: dense chondrule clumps in the protoplanetary nebula

Authors: Jeffrey N. Cuzzi, Robert C. Hogan, Karim Shariff

(Submitted on 21 Apr 2008)

Abstract: We outline a scenario which traces a direct path from freely-floating nebula particles to the first 10-100km-sized bodies in the terrestrial planet region, producing planetesimals which have properties matching those of primitive meteorite parent bodies. We call this “primary accretion”. The scenario draws on elements of previous work, and introduces a new critical threshold for planetesimal formation. We presume the nebula to be weakly turbulent, which leads to dense concentrations of aerodynamically size-sorted particles having properties like those observed in chondrites. The fractional volume of the nebula occupied by these dense zones or clumps obeys a probability distribution as a function of their density, and the densest concentrations have particle mass density 100 times that of the gas. However, even these densest clumps are prevented by gas pressure from undergoing gravitational instability in the traditional sense (on a dynamical timescale). While in this state of arrested development, they are susceptible to disruption by the ram pressure of the differentially orbiting nebula gas. However, self-gravity can preserve sufficiently large and dense clumps from ram pressure disruption, allowing their entrained particles to sediment gently but inexorably towards their centers, producing 10-100 km “sandpile” planetesimals. Localized radial pressure fluctuations in the nebula, and interactions between differentially moving dense clumps, will also play a role that must be allowed for in future studies. The scenario is readily extended from meteorite parent bodies to primary accretion throughout the solar system.

Comments: Astrophys. J accepted April 11, 2008

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.3526v1 [astro-ph]

Submission history

From: Jeffrey N. Cuzzi [view email]

[v1] Mon, 21 Apr 2008 23:02:09 GMT (451kb)

http://arxiv.org/abs/0804.3526

• ljk May 7, 2008, 6:39

Disk Dispersal and Planet Formation Time Scales

Authors: Lynne A. Hillenbrand

(Submitted on 3 May 2008)

Abstract: Well before the existence of exo-solar systems was confirmed, it was accepted knowledge that most — if not all — stars possess circumstellar material during the first one-to-several million years of their pre-main sequence lives, and thus that they commonly have the potential to form planets. Here I summarize current understanding regarding the evolution of proto-planetary dust and gas disks, emphasizing the diversity in evolutionary paths.

Comments: Similar to version that will be published by IOP in 2008, Physica Scripta (Special Issue on Nobel Symposium 135: Physics of Planetary Systems)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.0386v1 [astro-ph]

Submission history

From: Lynne Hillenbrand [view email]

[v1] Sat, 3 May 2008 22:03:27 GMT (39kb)

http://arxiv.org/abs/0805.0386

• ljk May 30, 2008, 15:31

Size-sorting dust grains in the surface layers of protoplanetary disks

Authors: C. P. Dullemond, C. Dominik

(Submitted on 28 May 2008)

Abstract: Aims: We wish to investigate what the effect of dust sedimentation is on the observed 10 mum feature of protoplanetary disks and how this may affect the interpretation of the observations.

Methods: Using a combination of modeling tools, we simulated the sedimentation of a dust grain size distribution in an axisymmetric 2-D model of a turbulent protoplanetary disk, and we used a radiative transfer program to compute the resulting spectra.

Results: We find that the sedimentation can turn a flat feature into a pointy one, but only to a limited degree and for a very limited set of particle size distributions. Only if we have a bimodal size distribution, i.e. a very small grain population and a bigger grain population, do we find that the transformation from a flat to a pointy feature upon dust sedimentation is strong.

However, our model shows that, if sedimentation is the sole reason for the variety of silicate feature strengths observed in protoplanetary disks, then we would expect to find a correlation such that disks with weak mid- to far-infrared excess have a stronger 10 mum silicate feature than disks with a strong mid- to far-infrared excess. If this is contrary to what is observed, then this would indicate that sedimentation cannot be the main reason for the variety of 10 mum silicate features observed in protoplanetary disks.

Comments: Astronomy and Astrophysics, in press

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.4376v1 [astro-ph]

Submission history

From: C. P. Dullemond [view email]

[v1] Wed, 28 May 2008 15:44:54 GMT (68kb)

http://arxiv.org/abs/0805.4376

• ljk June 11, 2008, 23:49

Planetesimal formation around the snow line in MRI-driven turbulent protoplanetary disks

Authors: F. Brauer, Th. Henning, C.P. Dullemond

(Submitted on 10 Jun 2008)

Abstract: The formation of planetesimals in protoplanetary disks due to collisional sticking of smaller dust aggregates has to face at least two severe obstacles, namely the rapid loss of material due to radial inward drift and particle fragmentation due to destructive collisions.

In this Letter we present a scenario to circumvent these two hurdles. Our dust evolution model involves two main mechanisms. First, we consider a disk with a dead zone. In a nearly laminar region around the midplane, relative turbulent particle velocities are comparatively small decreasing the probability for destructive particle collisions. Second, turbulence is not the only source for violent relative particle velocities, because high radial drift speeds can also lead to boulder fragmentation. For this reason, we additionally focus on the snow line. Evaporation fronts can be associated with gas pressure maxima in which radial drift basically vanishes. This means that particle fragmentation becomes even less likely.

Our simulation results suggest that particles can overcome the fragmentation barrier. We find that boulders of several 100 m can form within only a few thousand years.

Comments: Letter accepted for publication in A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0806.1646v1 [astro-ph]

Submission history

From: Frithjof Brauer [view email]

[v1] Tue, 10 Jun 2008 12:48:08 GMT (216kb)

http://arxiv.org/abs/0806.1646

• ljk June 20, 2008, 14:08

Lavas from Hawaiian volcano contain fingerprint of planetary formation

“… a precision analysis of lava samples taken from the crater is giving
scientists a new tool for reconstructing planetary origins. The results of
the analysis, by the University of Chicago’s Nicolas Dauphas and his
associates, will be published in the June 20 issue of the journal Science.”

http://www.spaceref.com/news/viewpr.html?pid=25714

• ljk July 2, 2008, 12:28

Circumstellar Disks in the Outer Galaxy: the Star-Forming Region NGC 1893

Authors: M. Caramazza, G. Micela, L. Prisinzano, L. Rebull, S. Sciortino, J. R. Stauffer

(Submitted on 1 Jul 2008)

Abstract: It is still debated whether star formation process depends on environment. In particular it is yet unclear whether star formation in the outer Galaxy, where the environmental conditions are, theoretically, less conducive, occurs in the same way as in the inner Galaxy.

We investigate the population of NGC1893, a young cluster ~3-4 Myr in the outer part of the Galaxy (galactic radius >11 Kpc), to explore the effects of environmental conditions on star forming regions. We present infrared observations acquired using the IRAC camera onboard the Spitzer Space Telescope and analyze the color-color diagrams to establish the membership of stars with excesses. We also merge this information with that obtained from Chandra ACIS-I observations, to identify the Class III population. We find that the cluster is very rich, with 242 PMS Classical T-Tauri stars and 7 Class 0/I stars. We identify 110 Class III candidate cluster members in the ACIS-I field of view.

We estimate a disk fraction for NGC1893 of about 67%, similar to fractions calculated for nearby star forming regions of the same age. Although environmental conditions are unfavorable, star formation can clearly be very successful in the outer Galaxy, allowing creation of a very rich cluster like NGC1893.