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Rethinking Planets and Stellar Metals

We often speak about planets migrating from the outer to the inner system of a star, something that helps us put ‘hot Jupiters’ in context. But what about migration within the galactic disk? It’s an idea under continuing investigation. In the absence of direct observational evidence, we infer migration and assume that older stars often come from regions with significantly different metallicity than stars in their current environment. The presumed origin would be the inner disk, which Misha Haywood defines as that part of the galaxy inside the radius from galactic center to our Sun.

Dave Moore sent me Haywood’s latest paper a few months back and I’ve been slow in getting to it because I wanted to give its conclusions further thought. It’s intriguing stuff. Haywood (Observatoire de Paris) takes note of the fact that we tend to find gas giants around stars that are rich in metals (here a pause to remind newcomers that by ‘metals,’ we mean elements higher than helium). And he wants to answer a key question: How do we know that this higher percentage of Jovian worlds detected around metal-rich stars is the result of metallicity, and not some other factor linked with their origin in the inner disk? The question is relevant, Haywood writes:

…because any measurable property of inner disk stars other than metallicity would be correlated with the presence of planet. The obvious a priori response is that metallicity is a measurable parameter, and intrinsic to the star. But there could be others however, which, although not measurable on the stars, could be no less important, such as, for example, the surface density of molecular hydrogen in the inner galactic disk regions.

The obvious next step is to find exceptions to the giant planet/metallicity correlation, and Haywood notes that we don’t see the same metallicity connection among giant stars hosting planets that we do around smaller stars. Moreover, at intermediate metallicities, giant planets seem to favor thick disk stars rather than thin disk objects.

Here we’re talking about different and distinct star populations. The ‘thin disk’ we see edge-on in images of spiral galaxies is complemented by the more diffuse ‘thick disk,’ containing older stars. The thick disk population, thinks Haywood, comes from migrating stars from the inner disk, while the metal-poor group derives from stars from the outer disk.

This takes us to an interesting place:

We are now facing the following picture: stars that come from the inner disk are noticeably rich in giant planets, while stars that come from the outer disk seems to be less favored in this respect. This new information changes considerably how we envisage the correlation between metallicity and the presence of giant planets. For the surprising point here is not the fact that most host-planet stars are metal-rich, since they come from a region where most stars are metal-rich, but the very fact that most would come from the inner disk. We are led to conclude that the distance to the galactic center must somehow play a role in setting the percentage of giant planets

The italics above are mine, because the statement is the core of the argument. Looked at from this perspective, the correlation between metals and the presence of giant planets turns out to reflect the galactic origin of the stars. It does not imply that metallicity is the necessary cause for the formation of these planets.

But if not metallicity, what other factors can we link to the galactocentric distance of a star? One possibility is dust density, which would favor the development of planetesimals. But Haywood prefers molecular hydrogen as the answer. It is the basic ingredient for the formation of giant planets, the principal consituent of stellar disks. Moreover, we have to think in terms of where it is most abundant:

Its main structure in the Galaxy, the molecular ring, is thought to contain 70% of H2 gas inside the solar circle…, thereby providing a huge reservoir for star (H2 is known to be directly linked to star formation…) and planet formation. The most interesting aspect however, is the fact the molecular ring reaches a maximum density at 3-5 kpc from the sun, corresponding to the distance where stars with metallicity in the range (+0.3,+0.5) dex are expected to be formed preferentially.

Haywood argues that stars hosting ‘super Earths’ or Neptune-class worlds with no accompanying gas giants are less likely to have had an origin in the inner disk, and thus form in an environment less dense in molecular hydrogen. We would, then, expect no predominance of metal-rich stars among this population. Surveying twelve systems that house super-Earths or Neptune-class planets, Haywood finds that the seven with no Jovian planets have low metallicity, fitting his theory, while the five that do contain gas giants indeed show a higher proportion of metals, “…amply confirming the possibility that the first group of stars could be genuine solar radius objects, and the second wanderers from inside the Galaxy.”

The paper is Haywood, “On the Correlation Between Metallicity and the Presence of Giant Planets,” accepted at Astrophysical Journal Letters and available as a preprint.


Comments on this entry are closed.

  • Adam August 27, 2009, 16:29

    Provocative piece. Does that mean protoplanetary disks out in our part of the Galaxy are relatively gas-poor? Got to be some worthwhile research pointers in that!

  • Administrator August 27, 2009, 18:15

    Yes, Haywood’s bibliography is worth a look — he’s been working on this topic for a while. Interesting paper indeed, though one that causes me to wonder just how much we really know about galactic migration in the first place. Haywood lists some references on the possible dynamics involved.

  • Ronald August 28, 2009, 7:39

    Interesting, so metallicity would then be an indicator rather than a causal factor in giant planet formation. But I wonder: are there any implications in this theory with regard to the formation of terrestrial class planets?
    Is this still considered to be primarily metallicity dependent? Or simply unknown?

  • Thomas August 28, 2009, 17:37

    @ Ronald, there aren’t many known terrestrial exoplanets due to their difficulty to detect, but most of the ones known do orbit M dwarfs of low metallicity. It could be that this is simply an observational bias though, as terrestrial exoplanets are easier to find around brigher M dwarfs (M dwarfs have a lot less noise in the radial velocity data than do FGK stars).

  • andy August 29, 2009, 4:32

    The question of what is going on with the giant stars and metallicity is an interesting one. However it seems that the hot Jupiters are more strongly affected by metallicity than gas giants in longer period orbits, and the evolution of the giant stars would have destroyed any hot Jupiters in orbit around them.

  • David Merchant August 29, 2009, 18:01

    Could this have a bearing on Dr. Gerhardt Meurer of Johns Hopkins University in Baltimore research into why the ratio of high-mass to low-mass newborn stars differs between galaxies? Dwarf’ galaxies have a higher ratio of low-mass stars than large galaxies, possibly because dwarf galaxies are lower pressure environments than large galaxies. Could this mean that dwarf galaxies also have fewer stars rich with gas giants? See for more information about Dr. Meurer’s research.

  • Kea August 31, 2009, 8:36

    Of course, this was predicted by the Riofrio cosmology, in which black holes (dark matter) seed stellar system formation.

  • ljk September 25, 2009, 13:01

    Long-Lived Planetesimal Discs

    Authors: Kevin Heng, Scott Tremaine

    (Submitted on 21 Sep 2009)

    Abstract: We investigate the survival of planetesimal discs over Gyr timescales, using a unified approach that is applicable to all Keplerian discs of solid bodies — dust grains, asteroids, planets, etc.

    Planetesimal discs can be characterized locally by four parameters: surface density, semi-major axis, planetesimal size and planetesimal radial velocity dispersion. Any planetesimal disc must have survived all dynamical processes, including gravitational instability, dynamical chaos, gravitational scattering, physical collisions, and radiation forces, that would lead to significant evolution over its lifetime. These processes lead to a rich set of constraints that strongly restrict the possible properties of long-lived discs.

    Within this framework, we also discuss the detection of planetesimal discs using radial velocity measurements, transits, microlensing, and the infrared emission from the planetesimals themselves or from dust generated by planetesimal collisions.

    Comments: 31 pages (single column, font size 10), 10 figures, 2 tables. Accepted by MNRAS

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

    Cite as: arXiv:0909.3850v1 [astro-ph.EP]

    Submission history

    From: Kevin Heng [view email]

    [v1] Mon, 21 Sep 2009 20:00:32 GMT (1423kb)


  • ljk September 28, 2009, 8:50

    A Uniform Analysis of 118 Stars with High-Contrast Imaging: Long Period Extrasolar Giant Planets are Rare around Sun-like Stars

    Authors: Eric L. Nielsen, Laird M. Close (Steward Observatory)

    (Submitted on 24 Sep 2009)

    Abstract: We expand on the results of Nielsen et al. (2008), using the null result for giant extrasolar planets around the 118 target stars from the VLT NACO H and Ks band planet search (Masciadri et al. 2005), the VLT and MMT Simultaneous Differential Imaging (SDI) survey (Biller et al. 2007), and the Gemini Deep Planet Survey (Lafreniere et al. 2007) to set constraints on the population of giant extrasolar planets.

    Our analysis is extended to include the planet luminosity models of Fortney et al. (2008), as well as the correlation between stellar mass and frequency of giant planets found by Johnson et al. (2007). Doubling the sample size of FGKM stars strengthens our conclusions: a model for extrasolar giant planets with power-laws for mass and semi-major axis as giving by Cumming et al. (2008) cannot, with 95% confidence, have planets beyond 65 AU, compared to the value of 94 AU reported in Nielsen et al. (2008), using the models of Baraffe et al. (2003). When the Johnson et al. (2007) correction for stellar mass (which gives fewer Jupiter-mass companions to M stars with respect to solar-type stars) is applied, however, this limit moves out to 82 AU.

    For the relatively new Fortney et al. (2008) models, which predict fainter planets across most of parameter space, these upper limits, with and without a correction for stellar mass, are 182 and 234 AU, respectively.

    Comments: 67 pages, 16 figures, accepted to ApJ

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

    Cite as: arXiv:0909.4531v1 [astro-ph.EP]

    Submission history

    From: Eric Nielsen [view email]

    [v1] Thu, 24 Sep 2009 20:59:34 GMT (253kb)


  • ljk October 4, 2009, 23:13

    Photospheric parameters and C abundances in solar-like stars with and without planets

    Authors: Ronaldo Da Silva, André Milone

    (Submitted on 1 Oct 2009)

    Abstract: We have been analyzing a large sample of solar-like stars with and without planets in order to homogeneously measure their photospheric parameters and Carbon abundances. Our sample contains around 200 stars in the solar neighborhood observed with the ELODIE spectrograph, for which the observational data are publicly available.

    We performed spectral synthesis of prominent bands of C$_{2}$ and C I lines, aiming to accurately obtain the C abundances. We intend to contribute homogeneous results to studies that compare elemental abundances in stars with and without known planets.

    New arguments will be brought forward to the discussion of possible chemical anomalies that have been suggested in the literature, leading us to a better understanding of the planetary formation process. In this work we focus on the C abundances in both stellar groups of our sample.

    Comments: 2 pages, 2 figures, to be published in the proceedings of the IAU Symposium No. 265, 2009

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0910.0261v1 [astro-ph.SR]

    Submission history

    From: Ronaldo da Silva [view email]

    [v1] Thu, 1 Oct 2009 20:17:21 GMT (52kb)


  • ljk October 31, 2010, 12:43

    The melting curve of iron at extreme pressures: implications for planetary cores

    Authors: G.Morard, J.Bouchet, D.Valencia, S.Mazevet, F.Guyot

    (Submitted on 25 Oct 2010)

    Abstract: Exoplanets with masses similar to that of Earth have recently been discovered in extrasolar systems. A first order question for understanding their dynamics is to know whether they possess Earth like liquid metallic cores. However, the iron melting curve is unknown at conditions corresponding to planets of several times the Earth’s mass (over 1500 GPa for planets with 10 times the Earth’s mass (ME)). In the density-temperature region of the cores of those super-Earths, we calculate the iron melting curve using first principle molecular dynamics simulations based on density functional theory.

    By comparing this melting curve with the calculated thermal structure of Super Earths, we show that planets heavier than 2ME, have solid cores, thus precluding the existence of an internal metallic-core driven magnetic field. The iron melting curve obtained in this study exhibits a steeper slope than any calculated planetary adiabatic temperature profile rendering the presence of molten metallic cores less likely as sizes of terrestrial planets increase.

    Subjects: Materials Science (cond-mat.mtrl-sci); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1010.5133v1 [cond-mat.mtrl-sci]

    Submission history

    From: Johann Bouchet [view email]

    [v1] Mon, 25 Oct 2010 14:10:02 GMT (301kb)