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Planetary Systems in Miniature

‘Planemos’ are planetary mass objects not much larger or heavier than Jupiter. The emerging technical term for them is ‘isolated planetary mass objects’ (IPMO), although the nomenclature is still evolving. Back in 2006, Ray Jayawardhana (University of Toronto) challenged the American Astronomical Society’s Calgary meeting to consider how our definition of ‘planet’ is blurred by planemos that act much like little solar systems. Consider Jupiter itself, a small system doubtless born with its own disk of dust and gas that produced the raw materials for its larger moons.

Backing up such thinking was the brown dwarf 2M1207, known to have a planetary companion eight times the mass of Jupiter and now shown to be surrounded by a disk of its own. Thus it comes as no surprise that Jayawardhana, following up this work with Alexander Scholz (University of St Andrews), has been using the Spitzer Space Telescope to study eighteen planemos in a star cluster in Orion. At three million years old, young stars tend to be surrounded by gas and dust that glows in the infrared, a marker of the raw materials for planetary formation. About one-third of the planemos under study show similar disks.

So ‘planetary’ systems may form even in the presence of a planemo instead of a central star. What’s left hanging is the question of where the planemos come from in the first place. They’re smaller than brown dwarfs (with masses close to or below the deuterium-burning limit) and may well be planets expelled from young planetary systems. Or perhaps, say the scientists, they’re stellar embryos ejected from mini-clusters or multiple star systems. Whatever the case, the σ Orionis cluster offers the largest population of these objects yet identified.

From the paper (internal references deleted for brevity):

“…our results fit into previous claims for a T Taurilike phase in the planetary mass regime… Disk fractions and thus lifetimes are similar for objects spanning more [than] two orders of magnitude in mass…possibly indicating that stars, brown dwarfs, and IPMOs share a common origin. Star formation theory thus has to account for a number of objects with masses below the Deuterium burning limit.”

Interesting work, because in recent years the study of brown dwarfs has shown that they have what Jayawardhana and Scholz call ‘circum-sub-stellar disks’ with life times between five and ten million years, which makes them not dissimilar to other kinds of stars. We’re deep into what’s known as the initial mass function (IMF), which describes the distribution of stellar masses in a formation event in a specified volume of space. Just how that IMF is extended to encompass findings like these, moving down into the realm of the giant planets, will help us to probe its apparent universality in relation to star formation theory.

Thus the similarities between objects of vastly different sizes has to be factored into our thinking about how stars form. Again we note, in the presence of those gas and dust disks, the apparently ubiquitous phenomenon of planetary formation. Are we really looking at a universe that seems to seed almost every kind of star and even giant planets with companions? The paper, to be published in The Astrophysical Journal Letters, is “Dusty disks at the bottom of the IMF,” available online.

Comments on this entry are closed.

  • Christopher L. Bennett November 24, 2007, 22:18

    If full-sized star systems form out of nebulae as we know them, is it possible that there could be much smaller “micronebulae” out there, too small and dim for us to see, that can collapse into planemos? I actually proposed this in my recent STAR TREK tie-in novel THE BURIED AGE (sorry, I still don’t know how to do italics on this board). The Trek universe is full of “nebulae” that are closer to Earth, far denser, and usually much smaller than any known nebula. So I suggested that those fictitious nebulae are actually a new class of micronebulae that we haven’t discovered yet (just like there are still red dwarf stars within a few dozen light-years that are so dim we still haven’t discovered them all). Given the existence of planemos, I’ve been wondering if this bit of fictional rationalization might actually have some truth to it.

    But I guess the next question would be, where would the micronebulae come from? I suggested in the book that they might be dense clumps left over when a supernova dissipated most of a larger nebula — maybe clumps that were on their way to becoming stellar embryos but got heated by the supernova, decreasing their density and blowing away a lot of their material.

  • dad2059 November 25, 2007, 15:33

    The formation of planemos and brown dwarf pseudo-stars with their own planets can be very useful in the exploration of interstellar space, especially if there are any between the Solar System and Alpha Centauri. Sci-fi author Karl Schroeder posits such possibilities in his novels Ventus and Permanence using interstellar cyclers.


  • ljk November 27, 2007, 0:42

    Comparison of cloud models for Brown Dwarfs

    Authors: Ch.Helling, A.Ackerman, F.Allard, M.Dehn, P.Hauschildt, D.Homeier, K.Lodders, M.Marley, F.Rietmeijer, T.Tsuji, P.Woitke

    (Submitted on 26 Nov 2007)

    Abstract: A test case comparison is presented for different dust cloud model approaches applied in brown dwarfs and giant gas planets. We aim to achieve more transparency in evaluating the uncertainty inherent to theoretical modelling. We show in how far model results for characteristic dust quantities vary due to different assumptions. We also demonstrate differences in the spectral energy distributions resulting from our individual cloud modelling in 1D substellar atmosphere simulations

    Comments: 5 pages, Proceeding to “Exoplantes: Detection, Formation, Dynamics”, eds. Ferraz-Mello et a

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.3993v1 [astro-ph]

    Submission history

    From: Christiane Helling [view email]

    [v1] Mon, 26 Nov 2007 13:02:07 GMT (169kb)


  • ljk November 30, 2007, 11:05

    Correlated spectral variability in brown dwarfs

    Authors: C.A.L. Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg)

    (Submitted on 28 Nov 2007)

    Abstract: Models of brown dwarf atmospheres suggest they exhibit complex physical behaviour. Observations have shown that they are indeed dynamic, displaying small photometric variations over timescales of hours. Here I report results of infrared (0.95-1.64 micron) spectrophotometric monitoring of four field L and T dwarfs spanning timescales of 0.1-5.5 hrs, the goal being to learn more about the physical nature of this variability. Spectra are analysed differentially with respect to a simultaneously observed reference source in order to remove Earth-atmospheric variations. The variability amplitude detected is typically 2-10%, depending on the source and wavelength. I analyse the data for correlated variations between spectral indices. This approach is more robust than single band or chisq analyses, because it does not assume an amplitude for the (often uncertain) noise level (although the significance test still assumes a shape for the noise power spectrum). Three of the four targets show significant evidence for correlated variability. Some of this can be associated with specific features including Fe, FeH, VO and KI, and there is good evidence for intrinsic variability in water and possibly also methan. Yet some of this variability covers a broader spectral range which would be consistent with dust opacity variations. The underlying common cause is plausibly localized temperature or composition fluctuations caused by convection. Looking at the high signal-to-noise ratio stacked spectra we see many previously identified spectral features of L and T dwarfs, such as KI, NaI, FeH, water and methane. In particular we may have detected methane absorption at 1.3-1.4 micron in the L5 dwarf SDSS 0539-0059.

    Comments: MNRAS, in press. 14 pages. Movies of the spectral time series are available from this http URL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.4464v1 [astro-ph]

    Submission history

    From: Coryn Bailer-Jones [view email]

    [v1] Wed, 28 Nov 2007 15:15:49 GMT (269kb)


  • ljk December 13, 2007, 10:47

    Chemical Evolution in VeLLOs

    Authors: Jeong-Eun Lee

    (Submitted on 12 Dec 2007)

    Abstract: A new type of object called “Very Low Luminosity Objects (VeLLOs)” has been discovered by the Spitzer Space Telescope. VeLLOs might be substellar objects forming by accretion. However, some VeLLOs are associated with strong outflows, indicating the previous existence of massive accretion. The thermal history, which significantly affects the chemistry, between substellar objects with a continuous low accretion rate and objects in a quiescent phase after massive accretion (outburst) must be greatly different. In this study, the chemical evolution has been calculated in an episodic accretion model to show that CO and N2H+ have a relation different from starless cores or Class 0/I objects. Furthermore, the CO2 ice feature at 15.2 micron will be a good tracer of the thermal process in VeLLOs.

    Comments: Accepted for publication in JKAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.1866v1 [astro-ph]

    Submission history

    From: Jeong-Eun Lee [view email]

    [v1] Wed, 12 Dec 2007 04:14:44 GMT (38kb)


  • ljk January 9, 2008, 18:22