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Into the Brown Dwarf Desert

At first glance, the object called COROT-exo-3b doesn’t seem all that interesting. True, planets that orbit remarkably close to their stars were a surprise when first detected, but we’ve since found enough of them to know that a gas giant in a four-day orbit, which is about how long this object takes to circle its star, is not a great rarity. We’ve also learned that radial velocity methods are going to detect large, close objects more quickly than planets that orbit farther from their primary. Why is COROT-exo-3b making news?

Then you look a little more closely at the new find. For one thing, the COROT mission depends not upon radial velocity measurements but planetary transits. More significantly, COROT-exo-3b is roughly the size of Jupiter but is fully twenty times as massive. Orbiting an F-class dwarf with metallicity values much like the Sun, the object opens up a new perspective, for we’ve found planets twelve times as massive as Jupiter and stars seventy times as massive, but nothing in the range in between. A brown dwarf or a planet? We’re in the so-called ‘brown dwarf desert’ between star and planet with this one.

Image: Relative sizes of the Sun, COROT-exo-3b and Jupiter, an artist’s impression. Credit: OAMP.

Says Francois Bouchy (Institut d’Astrophysique de Paris), a member of the discovery team:

“COROT-exo-3b might turn out to be a rare object found by sheer luck. But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We’re now beginning to think that the more massive the star, the more massive the planet.”

This discovery reminds us that we’re still trying to draw the line between planets and brown dwarfs, the latter being ‘almost’ stars that cannot sustain fusion at the core. Considered as a planet, COROT-exo-3b is the most massive and the densest yet found, and it raises questions about massive planetary formation in an environment this close to a star. Surely migration from further out in the system is at work here, but we have much study ahead before we can draw confident conclusions.

The paper on this work sums up the situation nicely:

CoRoT-Exo-3b reopens the debate about the existence of a hitherto non-detected brown-dwarf population at short orbital periods but also about the definition of a planet, such as the common one which, in this range of mass, relies on the deuterium burning limit. The exact nature of this new object is therefore still doubtful. Its parameters are in pretty good agreement with the model predictions for brown-dwarfs and if that is that case, it might simply be the first secure and well-characterized object at the lowest mass end of the stellar population. If CoRoT-Exo-3b is indeed a brown-dwarf one should review as well the other massive planets like XO-3b, HAT-P2b and WASP-14b as potential members of this class. An alternative explanation is that CoRoT-Exo-3b belongs to a new, yet unexplored, very massive planet population, widening the variety of exoplanets. The currently ambiguous nature of CoRoT-Exo-3b makes it therefore a very worthwhile object for further deeper studies.

We can expect future COROT finds to aid these investigations. Indeed, the mission seems made to order for planets in short orbital periods. With the capability of monitoring up to 12,000 stars simultaneously in each observing run, covering a span of 150 days of nearly continuous observations, the instrument is obviously sensitive to such worlds, whose study can then be followed up with radial velocity techniques.

The paper is Deleuil et al., “Transiting exoplanets from the CoRoT space mission VI. CoRoT-Exo-3b: The first secure inhabitant of the brown-dwarf desert,” accepted for publication in Astronomy & Astrophysics (abstract).

Comments on this entry are closed.

  • ljk October 7, 2008, 9:22
  • kurt9 October 7, 2008, 13:27

    This thing is 4 times as dense as the Earth. What the hell could it be made of?

  • andy October 7, 2008, 15:55

    This thing is 4 times as dense as the Earth. What the hell could it be made of?

    Actually, standard gas giant mix works quite well here (as noted in the discovery paper, “CoRoT-Exo-3b parameters are in good agreement with the expected mass-radius relationship on the low-mass tail of these substellar objects”): counter-intuitively the radius of a gas giant stops increasing after a certain point (around a couple of Jupiter masses or so), instead increasing compression of the core becomes the more relevant process. (At substantially higher masses, the radii start to decrease, assuming the objects in question have had time to cool their interiors).

  • ljk October 28, 2008, 22:55

    Epsilon Indi Ba, Bb: a spectroscopic study of the nearest known brown dwarfs

    Authors: Robert R. King, Mark J. McCaughrean, Derek Homeier, France Allard, Ralf-Dieter Scholz, Nicolas Lodieu

    (Submitted on 27 Oct 2008)

    Abstract: The discovery of Epsilon Indi Ba and Bb, a nearby binary brown dwarf system with a main-sequence companion, allows a concerted campaign to characterise the physical parameters of two T dwarfs providing benchmarks against which atmospheric and evolutionary models can be tested.

    Some recent observations suggest the models at low mass and intermediate age may not reflect reality with, however, few conclusive tests.

    We are carrying out a comprehensive characterisation of these, the nearest known brown dwarfs, to allow constraints to be placed upon models of cool field dwarfs. We present broadband photometry from the V- to M-band and the individual spectrum of both components from 0.6-5.1 microns at a resolution of up to R=5000. A custom analytic profile fitting routine was implemented to extract the blended spectra and photometry of both components separated by 0.7 arcsec.

    We confirm the spectral types to be T1 and T6, and notably, we do not detect lithium at 6708A in the more massive object which may be indicative both of the age of the system and the mass of the components.

    Comments: 4 pages, 2 figures, to appear in proceedings of Cool Stars 15

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0810.4927v1 [astro-ph]

    Submission history

    From: Robert King [view email]

    [v1] Mon, 27 Oct 2008 20:07:27 GMT (1668kb)


  • ljk November 18, 2008, 0:54


    Date: Sat, 15 Nov 2008 17:18:24 GMT (28kb)

    Title: Brown Dwarfs as Galactic Chronometers

    Authors: Adam J. Burgasser

    Categories: astro-ph

    Comments: 8 pages, contribution to IAU 258: Ages of Stars, eds. E. Mamajek, D. Soderblom & J. Valenti

    Brown dwarfs are natural clocks, cooling and dimming over time due toinsufficient core fusion. They are also numerous and present in nearly allGalactic environments, making them potentially useful chronometers for avariety of Galactic studies.

    For this potential to be realized, however,precise and accurate ages for individual sources are required, a prospect made difficult by the complex atmospheres and spectra of low-temperature brown dwarfs; degeneracy between mass, age and luminosity; and the lack of useful agetrends in magnetic activity and rotation.

    In this contribution, I review five ways in which ages for brown dwarfs are uniquely determined, discuss their applicability and limitations, and give current empirical precisions.

    http://arxiv.org/abs/0811.2508 , 28kb

  • ljk July 17, 2009, 16:23

    Structure and evolution of the first CoRoT exoplanets: Probing the Brown Dwarf/Planet overlapping mass regime

    Authors: J. Leconte (ENS-Lyon), I. Baraffe (ENS-Lyon), G. Chabrier (ENS-Lyon), T. Barman (Lowell), B. Levrard (ENS-Lyon)

    (Submitted on 15 Jul 2009)

    Abstract: We present detailed structure and evolution calculations for the first transiting extrasolar planets discovered by the space-based CoRoT mission. Comparisons between theoretical and observed radii provide information on the internal composition of the CoRoT objects.

    We distinguish three different categories of planets emerging from these discoveries and from previous ground-based surveys: (i) planets explained by standard planetary models including irradiation, (ii) abnormally bloated planets and (iii) massive objects belonging to the overlapping mass regime between planets and brown dwarfs.

    For the second category, we show that tidal heating can explain the relevant CoRoT objects, providing non-zero eccentricities. We stress that the usual assumption of a quick circularization of the orbit by tides, as usually done in transit light curve analysis, is not justified a priori, as suggested recently by Levrard et al. (2009), and that eccentricity analysis should be carefully redone for some observations.

    Finally, special attention is devoted to CoRoT-3b and to the identification of its very nature: giant planet or brown dwarf ? The radius determination of this object confirms the theoretical mass-radius predictions for gaseous bodies in the substellar regime but, given the present observational uncertainties, does not allow an unambiguous identification of its very nature.

    This opens the avenue, however, to an observational identification of these two distinct astrophysical populations, brown dwarfs and giant planets, in their overlapping mass range, as done for the case of the 8 Jupiter-mass object Hat-P-2b. (abridged)

    Comments: 6 pages, 5 figures, accepted for publication in Astronomy and Astrophysics

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0907.2669v1 [astro-ph.EP]

    Submission history

    From: Isabelle Baraffe dr [view email]

    [v1] Wed, 15 Jul 2009 17:46:19 GMT (60kb)