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Brown Dwarf or Planet: The Case of XO-3b

Odd planets seem to be sprouting in our data like mushrooms. Take the case of XO-3b. It’s got the mass of thirteen Jupiters but orbits its star in less than four days, making it the largest, most massive planet ever found in such a tight orbit. But XO-3b also seizes the attention because its orbit is significantly elliptical rather than circular. Is this evidence for the gravitational effects of another object in the same system?

We should be able to learn a lot more about this and other questions because XO-3b is also a transiting world, passing between its star and the Earth. This is the third transiting planet identified by the XO Project, which uses two small telescopes at Haleakala (operated by the University of Hawaii) to identify transit candidates before passing the data on to a network of amateur astronomers for further study. After sufficient evidence is gathered, the work goes back to large telescopes at McDonald Observatory (University of Texas) for confirmation.

Announced at the American Astronomical Society meeting in Honolulu, the new planet is close enough to a brown dwarf in size to make its actual status uncertain. Sustainable hydrogen fusion seems to need about eighty Jupiter masses to occur, marking the upper limit between brown dwarf and star. But XO-3B may be right at the lower mass limit. Christopher Johns-Krull (Rice University), says this:

“The controversy lies at the lower end of the scale. Some people believe anything capable of fusing deuterium, which in theory happens around 13 Jupiter masses, is a brown dwarf. Others say it’s not the mass that matters, but whether the body forms on its own or as part of a planetary system.”

All of which may help us with a mystery. The so-called ‘brown dwarf desert’ is the term coined for the unexpected dearth of brown dwarfs in our exoplanet hunts. Radial-velocity techniques ought to be able to spot objects of XO-3b’s size and larger relatively easily given their mass, but the number of brown dwarfs so identified has remained small. Finding objects on the borderline may help us sort out some of the constraints on brown dwarf formation both in and out of planetary systems.

Let’s linger for a moment on the collaboration that’s happening here. Telescope time on the largest and best instruments is scarce, but time is precisely what planet-hunting astronomers need to firm up their findings. “…[T]hat’s where our amateur partners come in,” says XO Project director Peter McCullough (Space Telescope Science Institute), “culling our long lists of candidates down to more manageable size to observe with the big telescopes.” That kind of synergy is likely to produce a continuing harvest of transits, another confirmation of the role amateurs working with commercially available equipment can play in delivering good data.

Comments on this entry are closed.

  • ljk June 5, 2007, 12:51

    Hubble and Spitzer Observations of an Edge-on Circumstellar Disk around a Brown Dwarf

    Authors: K. L. Luhman, Lucia Adame, Paola D’Alessio, Nuria Calvet, Kim K. McLeod, C. J. Bohac, William J. Forrest, Lee Hartmann, B. Sargent, Dan M. Watson

    (Submitted on 2 Jun 2007)

    Abstract: We present observations of a circumstellar disk that is inclined close to edge-on around a young brown dwarf in the Taurus star-forming region. Using data obtained with SpeX at the NASA Infrared Telescope Facility, we find that the slope of the 0.8-2.5 um spectrum of the brown dwarf 2MASS J04381486+2611399 cannot be reproduced with a photosphere reddened by normal extinction. Instead, the slope is consistent with scattered light, indicating that circumstellar material is occulting the brown dwarf. By combining the SpeX data with mid-IR photometry and spectroscopy from the Spitzer Space Telescope and previously published millimeter data from Scholz and coworkers, we construct the spectral energy distribution for 2MASS J04381486+2611399 and model it in terms of a young brown dwarf surrounded by an irradiated accretion disk. The presence of both silicate absorption at 10 um and silicate emission at 11 um constrains the inclination of the disk to be ~70 deg, i.e. ~20 deg from edge-on. Additional evidence of the high inclination of this disk is provided by our detection of asymmetric bipolar extended emission surrounding 2MASS J04381486+2611399 in high-resolution optical images obtained with the Hubble Space Telescope. According to our modeling for the SED and images of this system, the disk contains a large inner hole that is indicative of a transition disk (R_in~58 R_star~0.275 AU) and is somewhat larger than expected from embryo ejection models (R_out=20-40 AU vs. R_out

  • Sofia Catalini June 9, 2007, 8:03

    I would like to know why Americans never give credit where it’s due. This discovery is merit of two Italians: Gianluca Masi and Franco Mallia with particular attention to Christopher Johns-Krull. America is not alone in these types of discoveries since it has been made public at the convention held in USA (American Astronomy Society) that 15 astronomers participated in the discovery of X0-3B, from countries such as Spain, Belgium, Italy and the USA. European countries have the diplomacy and respect to include truth in their presentation of facts, while Americans like to take credit for everything without acknowledging the participation of various countries who contribute tremendously with their technology and hard work. The world does not rotate around the USA gentlemen. Thank you.

  • Administrator June 9, 2007, 8:40

    Although I take your point, Sofia, I’m hard pressed to find anything in this article that is untruthful or, indeed, chauvinistic. In any case, sorry to have offended you.

  • Gianluca Masi June 19, 2007, 10:22

    Hi there,

    the discovery of this intriguing object is the result of a wonderful joint effort (not the first for the XO team), not an exclusive result by us!

    We observed the object one or two times (I do not have the log handy).

    It is also important to underline that the principal investigator is Dr. Krull.

    I would invite interested people to check this abstract, properly listing the people involved in the discovery:


    Kind Regards,
    Gianluca Masi and Franco Mallia

  • Administrator June 19, 2007, 10:49

    Thank you, Gianluca and Franco, for this clarification. And congratulations to you and the entire XO team for your excellent and continuing work!

  • Shaun June 29, 2007, 5:22

    Quote: “…Others say it’s not the mass that matters, but whether the body forms on its own or as part of a planetary system.”

    While I’ll agree with Marcy that it is likely that a myriad of processes are involved in producing planets and perhaps even BDs, I share the same misgivings with Gibor Basri in that using cosmogony as a yardstick to separate planets from BDs is problematic if not impossible. i.e. how in the world is it possible to irrefutably (i.e. without an iota of doubt whatsoever) confirm how an object form i.e. direct collapse; gravitational instability or from accretion?

  • ljk July 19, 2007, 11:47

    A high resolution spectral atlas of brown dwarfs

    Authors: A. Reiners, D. Homeier, P.H. Hauschildt, F. Allard

    (Submitted on 18 Jul 2007)

    Abstract: We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf system, spectral classes at which most of the objects are brown dwarfs. Our atlas covers the optical region from H$\alpha$ up to the near infrared at 1 $\mu$m. We present spectral details of ultra-cool atmospheres at very high resolution ($R \sim 33 000$) and compare the spectra to model calculations. Our comparison shows that molecular features from VO and CaH, and atomic features from Cs and Rb are reasonably well fit by current models. On the other hand, features due to TiO, CrH, and water, and atomic Na and K reveal large discrepancies between model calculations and our observations.

    Comments: 17 pages, 11 figures, accepted by A&A, reduced figure quality for arXiv

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.2652v1 [astro-ph]

    Submission history

    From: Ansgar Reiners [view email]

    [v1] Wed, 18 Jul 2007 07:07:40 GMT (652kb,D)


  • ljk July 26, 2007, 2:02

    16-20 Jupiter mass RV companion orbiting the brown dwarf candidate ChaHa8

    Authors: V. Joergens, A. Mueller (Max-Planck Institute for Astronomy Heidelberg)

    (Submitted on 25 Jul 2007)

    Abstract: We report the discovery of a 16-20 Jupiter mass radial velocity companion around the very young (~3 Myr) brown dwarf candidate ChaHa8 (M5.75-M6.5). Based on high-resolution echelle spectra of ChaHa8 taken between 2000 and 2007 with UVES at the VLT, a companion was detected through RV variability with a semi-amplitude of 1.6 km/s. A Kepler fit to the data yields an orbital period of the companion of 1590 days and an eccentricity of e=0.49. A companion minimum mass M2sini between 16 and 20 Jupiter masses is derived when using model-dependent mass estimates for the primary. The mass ratio q= M2/M1 might be as small as 0.2 and, with a probability of 87%, it is less than 0.4. ChaHa8 harbors most certainly the lowest mass companion detected so far in a close (~ 1 AU) orbit around a brown dwarf or very low-mass star.

    From the uncertainty in the orbit solution, it cannot completely be ruled out that the companion has a mass in the planetary regime. Its discovery is in any case an important step towards RV planet detections around BDs. Further, ChaHa8 is the fourth known spectroscopic brown dwarf or very low-mass binary system with an RV orbit solution and the second known very young one.

    Comments: 8 pages, 1 figure, ApJ Letter in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.3744v1 [astro-ph]

    Submission history

    From: Viki Joergens [view email]

    [v1] Wed, 25 Jul 2007 13:50:26 GMT (29kb)


  • ljk December 31, 2007, 3:19

    XO-3b: A Massive Planet in an Eccentric Orbit Transiting an F5V Star

    Authors: Christopher M. Johns-Krull, Peter R. McCullough, Christopher J. Burke, Jeff A. Valenti, K. A. Janes, J. N. Heasley, L. Prato, R. Bissinger, M. Fleenor, C. N. Foote, E. Garcia-Melendo, B. L. Gary, P. J. Howell, F. Mallia, G. Masi, T. Vanmunster

    (Submitted on 27 Dec 2007)

    Abstract: We report the discovery of a massive (Mpsini = 13.00 +/- 0.64 Mjup; total mass 13.24 +/- 0.64 Mjup), large (1.92 +/- 0.16 Rjup) planet in a transiting, eccentric orbit (e = 0.219 +/- 0.035) around a 10th magnitude F5V star in the constellation Camelopardalis. We designate the planet XO-3b, and the star XO-3, also known as GSC 03727-01064. The orbital period of XO-3b is 3.1915426 +/- 0.00014 days. XO-3 lacks a trigonometric distance; we estimate its distance to be 260 +/- 23 pc. The radius of XO-3 is 2.13 +/- 0.21 Rsun, its mass is 1.41 +/- 0.08 Msun, its vsini = 18.54 +/- 0.17 km/s, and its metallicity is [Fe/H] = -0.177 +/- 0.027. This system is unusual for a number of reasons. XO-3b is one of the most massive planets discovered around any star for which the orbital period is less than 10 days. The mass is near the deuterium burning limit of 13 Mjup, which is a proposed boundary between planets and brown dwarfs. Although Burrows et al. (2001) propose that formation in a disk or formation in the interstellar medium in a manner similar to stars is a more logical way to differentiate planets and brown dwarfs, our current observations are not adequate to address this distinction. XO-3b is also unusual in that its eccentricity is large given its relatively short orbital period. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. Analysis of the transit light curve of XO-3b suggests that the spectroscopically derived parameters may be over estimated. Though relatively noisy, the light curves favor a smaller radius in order to better match the steepness of the ingress and egress. The light curve fits imply a planetary radius of 1.32 +/- 0.15 Rjup, which would correspond to a mass of 11.71 +/- 0.46 Mjup.

    Comments: 26 pages, 10 figures. Accepted by ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.4283v1 [astro-ph]

    Submission history

    From: Christopher M. Johns-Krull [view email]

    [v1] Thu, 27 Dec 2007 21:11:58 GMT (521kb)


  • ljk April 30, 2008, 11:40

    The Transit Light Curve Project. IX. Evidence for a Smaller Radius of the Exoplanet XO-3b

    Authors: Joshua N. Winn, Matthew J. Holman, Guillermo Torres, Peter McCullough, Christopher M. Johns-Krull, David W. Latham, Avi Shporer, Tsevi Mazeh, Enrique Garcia-Melendo, Cindy Foote, Gil Esquerdo, Mark Everett

    (Submitted on 28 Apr 2008)

    Abstract: We present photometry of 13 transits of XO-3b, a massive transiting planet on an eccentric orbit. Previous data led to two inconsistent estimates of the planetary radius. Our data strongly favor the smaller radius, with increased precision: R_p = 1.217 +/- 0.073 R_Jup.

    A conflict remains between the mean stellar density determined from the light curve, and the stellar surface gravity determined from the shapes of spectral lines. We argue the light curve should take precedence, and revise the system parameters accordingly. The planetary radius is about 1 sigma larger than the theoretical radius for a hydrogen-helium planet of the given mass and insolation. To help in planning future observations, we provide refined transit and occultation ephemerides.

    Comments: To appear in ApJ [22 pages]

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.4475v1 [astro-ph]

    Submission history

    From: Joshua N. Winn [view email]

    [v1] Mon, 28 Apr 2008 20:04:09 GMT (204kb)


  • ljk May 14, 2008, 11:33

    Theoretical Radii of Extrasolar Giant Planets: the Cases of TrES-4, XO-3b, and HAT-P-1b

    Authors: Xin Liu, Adam Burrows, Laurent Ibgui

    (Submitted on 12 May 2008)

    Abstract: To explain their observed radii, we present theoretical radius-age trajectories for the extrasolar giant planets (EGPs) TrES-4, XO-3b, and HAT-P-1b. We factor in variations in atmospheric opacity, the presence of an inner heavy-element core, and possible heating due to orbital tidal dissipation. A small, yet non-zero, degree of core heating is needed to explain the observed radius of TrES-4, unless its atmospheric opacity is significantly larger than a value equivalent to that at 10$\times$solar metallicity with equilibrium molecular abundances. This heating rate is reasonable, and corresponds for an energy dissipation parameter ($Q_p$) of $\sim10^5$ to an eccentricity of $\sim$0.04, assuming 3$\times$solar atmospheric opacity. For XO-3b, which has an observed orbital eccentricity of 0.26, we show that tidal heating needs to be taken into account to explain its observed radius.

    Furthermore, we reexamine the core mass needed for HAT-P-1b in light of new measurements and find that it now generally follows the correlation between stellar metallicity and core mass suggested by \citeauthor{burrows07b}. Given various core heating rates, theoretical grids and fitting formulae for a giant planet’s equilibrium radius and equilibration timescale are provided for planet masses $M_p=$ 0.5, 1.0, and 1.5 $M_J$ with $a =$ 0.02-0.06 AU, orbiting a G2V star. When the equilibration timescale is much shorter than that of tidal heating variation, the “effective age” of the planet is shortened, resulting in evolutionary trajectories more like those of younger EGPs. Motivated by the work of \citet{jackson08a,jackson08b}, we suggest that this “clock-reset” effect could indeed be important in better explaining some observed transit radii.

    Comments: 11 pages, 4 figures, 3 tables; submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.1733v1 [astro-ph]

    Submission history

    From: Xin Liu [view email]

    [v1] Mon, 12 May 2008 20:10:14 GMT (64kb)


  • ljk May 19, 2008, 23:28

    XO-5b: A Transiting Jupiter-sized Planet With A Four Day Period

    Authors: Christopher J. Burke, P. R. McCullough, Jeff A. Valenti, Doug Long, Christopher M. Johns-Krull, P. Machalek, Kenneth A. Janes, B. Taylor, Michael L. Fleenor, Cindy N. Foote, Bruce L. Gary, Enrique Garcia-Melendo, J. Gregorio, T. Vanmunster

    (Submitted on 15 May 2008)

    Abstract: The star XO-5 (GSC 02959-00729, V=12.1, G8V) hosts a Jupiter-sized, Rp=1.15+/-0.12 Rjup, transiting extrasolar planet, XO-5b, with an orbital period of P=4.187732+/-0.00002 days. The planet mass (Mp=1.15+/-0.08 Mjup) and surface gravity (gp=22+/-5 m/s^2) are significantly larger than expected by empirical Mp-P and Mp-P-[Fe/H] relationships.

    However, the deviation from the Mp-P relationship for XO-5b is not large enough to suggest a distinct type of planet as is suggested for GJ 436b, HAT-P-2b, and XO-3b. By coincidence XO-5 overlies the extreme H I plume that emanates from the interacting galaxy pair NGC 2444/NGC 2445 (Arp 143).

    Comments: 10 pages, 9 Figures, Submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.2399v1 [astro-ph]

    Submission history

    From: Christopher J. Burke [view email]

    [v1] Thu, 15 May 2008 23:08:38 GMT (647kb)


  • ljk May 19, 2008, 23:32

    Thermal Emission of Exoplanet XO-1b

    Authors: Pavel Machalek, Peter R. McCullough, Christopher J. Burke, Jeff A. Valenti, Adam Burrows, Joseph L. Hora

    (Submitted on 16 May 2008)

    Abstract: We estimate flux ratios of the extrasolar planet XO-1b to its host star XO-1 at 3.6, 4.5, 5.8 and 8.0 microns with the IRAC on the Spitzer Space Telescope to be 0.00086 +/- 0.00007, 0.00122 +/- 0.00009, 0.00261 +/- 0.00031 and 0.00210 +/- 0.00029, respectively. The fluxes are inconsistent with a canonical cloudless model for the thermal emission from a planet and suggest an atmosphere with a thermal inversion layer and a possible stratospheric absorber.

    A newly emerging correlation between the presence of a thermal inversion layer in the planetary atmosphere and stellar insolation of the planet (Burrows et al. 2007b) is refined. The sub-stellar point flux from the parent star at XO-1b of ~ 0.49 x 10^9 erg cm^-2 s^-1 sets a new lower limit for the occurrence of a thermal inversion in a planetary atmosphere.

    Comments: 7 pages, 4 figures, 2 tables, accepted for publication to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.2418v1 [astro-ph]

    Submission history

    From: Pavel Machalek [view email]

    [v1] Fri, 16 May 2008 04:04:22 GMT (51kb)


  • ljk June 5, 2008, 8:47

    Misaligned spin-orbit in the XO-3 planetary system?

    Authors: G. Hebrard, F. Bouchy, F. Pont, B. Loeillet, M. Rabus, X. Bonfils, C. Moutou, I. Boisse, X. Delfosse, M. Desort, A. Eggenberger, D. Ehrenreich, T. Forveille, A. M. Lagrange, C. Lovis, M. Mayor, F. Pepe, C. Perrier, D. Queloz, N.C. Santos, D. Segransan, S. Udry, A. Vidal-Madjar

    (Submitted on 4 Jun 2008)

    Abstract: The transiting extrasolar planet XO-3b is remarkable, with a high mass and eccentric orbit. The unusual characteristics make it interesting to test whether its orbital plane is parallel to the equator of its host star, as it is observed for other transiting planets.

    We performed radial velocity measurements of XO-3 with the SOPHIE spectrograph at the 1.93-m telescope of Haute-Provence Observatory during a planetary transit, and at other orbital phases. This allowed us to observe the Rossiter-McLaughlin effect and, together with a new analysis of the transit light curve, to refine the parameters of the planet. The unusual shape of the radial velocity anomaly during the transit provides a hint for a nearly transverse Rossiter-McLaughlin effect. The sky-projected angle between the planetary orbital axis and the stellar rotation axis should be lambda = 70 +/- 15 degrees to be compatible with our observations.

    This suggests that some close-in planets might result from gravitational interaction between planets and/or stars rather than migration due to interaction with the accretion disk. This surprising result requires confirmation by additional observations, especially at lower airmass, to fully exclude the possibility that the signal is due to systematic effects.

    Comments: 8 pages, 8 figures, 3 tables, to be published in A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.0719v1 [astro-ph]

    Submission history

    From: Guillaume Hebrard [view email]

    [v1] Wed, 4 Jun 2008 09:16:59 GMT (704kb,D)


  • ljk February 24, 2009, 1:19

    The Spin-Orbit Misalignment of the XO-3 Exoplanetary System

    Authors: Joshua N. Winn, John Asher Johnson, Daniel Fabrycky, Andrew W. Howard, Geoffrey W. Marcy, Norio Narita, Ian J. Crossfield, Yasushi Suto, Edwin L. Turner, Gil Esquerdo, Matthew J. Holman

    (Submitted on 19 Feb 2009)

    Abstract: We present photometric and spectroscopic observations of the 2009 Feb. 2 transit of the exoplanet XO-3b. The new data show conclusively that the planetary orbital axis and stellar rotation axis are misaligned. We thereby confirm the previous finding by Hebrard and coworkers, although we find a significantly smaller angle (37.3 +/- 3.7 deg) between the sky projections of the two axes.

    XO-3b is the first exoplanet known to have a highly inclined orbit relative to the equatorial plane of its parent star, and as such it may fulfill the predictions of some scenarios for the migration of massive planets into close-in orbits. We revisit the statistical analysis of spin-orbit alignment in hot-Jupiter systems.

    Assuming the stellar obliquities to be drawn from a Rayleigh distribution, we find the mode of the distribution to be 13^{+5}_{-2} deg. It remains the case that a model representing two different migration channels–in which some planets are drawn from a perfectly-aligned distribution and the rest are drawn from an isotropic distribution–is favored over a single Rayleigh distribution, but with reduced significance.

    Comments: Submitted to ApJ (17 pg.)

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

    Cite as: arXiv:0902.3461v1 [astro-ph.EP]

    Submission history

    From: Joshua N. Winn [view email]

    [v1] Thu, 19 Feb 2009 21:01:42 GMT (74kb)


  • ljk June 24, 2009, 9:59