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Brown Dwarf Analysis Offers Exoplanet Implications

Brown dwarfs offer exciting prospects for exoplanet work. Not only are we learning that they can have planets of their own, but brown dwarfs themselves are a useful bridge between planets and stars. We also know that there are a lot of them out there. According to data from the WISE (Wide-field Infrared Survey Explorer) mission, there is about one brown dwarf for every six main sequence stars. That’s plenty of brown dwarfs, but it actually represents a reduction in estimates, which once went as high as a brown dwarf for every star.

Moreover, WISE could identify about 200 brown dwarfs relatively near the Sun, with 33 within 26 light years. 211 stars are found in that same volume, incidentally. If we extrapolate this out to the galaxy at large, we should have about 33 billion brown dwarfs, assuming a galactic total of 200 billion stars. These numbers have a certain flexibility in them, especially given how hard it can be to track the coolest Y-dwarfs. Added to this is the fact that brown dwarfs fade with time.

Once we get below about 75 Jupiter masses, an object cannot ignite hydrogen fusion. That has broad implications for how brown dwarfs evolve. After formation, they continually cool and contract with time, with a concomitant increase in surface gravity. Thus across the age range we find brown dwarfs in heat regimes not that dissimilar from a small star, while others are as cool as a planet. Brown dwarf contraction in this mass range is opposed by electron degeneracy, so despite gradual constriction, the object never reaches the temperature for nuclear fusion.


Image: Brown dwarfs in relation to more familiar celestial objects. Credit: Gemini Observatory/Jon Lomberg.

Brown Dwarfs as Bridge

What we do get is a significant bridge between stars and planets. In a new paper, Jacqueline Faherty (Carnegie Institution for Science) and colleagues analyze the properties of 152 suspected young brown dwarfs, objects that show tremendous diversity in mass, age and chemical composition. Finding where many of the brown dwarfs formed allowed the team to eliminate age and chemical composition as the primary cause of their differences.

Faherty’s kinematic study adds new radial velocities, proper motions and parallaxes for a number of these objects, finding 39 brown dwarfs that appear to be members of nearby moving groups, indicating similar age. Another 92 objects are considered ‘ambiguous members,’ while 21 belong to no moving group. Using the moving group membership as an age calibration, the paper looks at the wide range of color and spectral features, making the case that atmospheric conditions are responsible for many of the observed differences between brown dwarfs.

Objects with masses <∼ 75 MJup cool through their lives with spectral energy distributions evolving as their atmospheric chemistry changes with decreasing temperatures…At the warmest temperatures, the atmosphere is too hot for the condensation of solids…But as the Teff [effective temperature] falls below 2500 K, both liquid (e.g. Fe) and solid (e.g. CaTiO3, VO) mineral and metal condensates settle into discrete cloud layers…

It’s a process that will continue to affect our observations:

As temperatures cool further, cloud layers form at such deep levels in the photosphere that they have little or no impact on the emergent spectrum. This transition between “cloudy” to “cloudless” objects occurs rapidly over a narrow temperature range (1200-1400 K, corresponding to the transition between L-type and T-type spectra) and drives extreme photometric, spectroscopic, and luminosity changes.

Faherty considers young brown dwarfs like these to be siblings of the giant exoplanets we have seen through direct imaging, which means that quantifying their observable properties should help us better understand how young gas giant planets evolve. In many ways we can consider them exoplanet analogs, but exoplanets without the distracting light of their host star. We can study young, warm exoplanets for evidence of the same kind of atmosphere-related changes.

The overlap between brown dwarfs and planets is striking, offering fertile ground for further work:

In general, M, L, T, and Y classifications identify brown dwarfs. If the source is older (> 2 – 3 Gyr), late-type M and early L dwarfs are stars. But if the source is young (< 1 Gyr), even those warmer classifications will describe an object that is < 75 MJup. To date, all directly imaged giant exoplanets have observable properties which lead to their classification squarely in this well-studied regime. Planetary mass companions such as 2M1207 b, 51 Eri b, β Pictoris b, ROXs 42B b, and the giant planets orbiting HR8799, have observables that are similar to L or T brown dwarfs…Furthermore, there exists a population of “classical” brown dwarfs that overlap in effective temperature, age – many in the same moving group – and mass with directly observed planetary mass companions (e.g. PSO 318, SDSS1110, 0047+6803…

These early cuts at deep classification of brown dwarfs are obviously going to get even deeper as we accumulate more data, but the analysis by moving groups offers us a useful way to frame the issue. Pondering Faherty’s work on my morning walk (in an already sweltering 90° F), I found myself recalling an interesting analysis of brown dwarf atmospheres from a few years back. A bit of poking around on the Net later confirmed that this was a study of the brown dwarf ULAS J222711-004547. The work, by Frederico Marocco (University of Hertfordshire) and team, analyzed clouds of mineral dust, all floating in an atmosphere of water vapor, methane and (possibly) ammonia.

ULAS J222711-004547 turns out to be one of the reddest brown dwarfs we’ve ever found. Marocco’s paper refers to it as ‘a peculiar L dwarf,’ and identifies nine other objects in the same category. The researchers consider these unusually red L-dwarfs to be a bridge between brown dwarfs and giant planets atmospheres, thus a useful test bed for the atmospheric models that Faherty so painstakingly analyzes in her later work. In such ways does the analysis of dust clouds in brown dwarfs add insight into our models of young exoplanet atmospheres.

The paper is Faherty et al., “Population Properties of Brown Dwarf Analogs to Exoplanets,” accepted for publication in the Astrophysical Journal Supplement Series (preprint). The Marocco paper is “The extremely red L dwarf ULAS J222711−004547 – dominated by dust,” Monthly Notices of the Royal Astronomical Society, published online February 5, 2014 (full text). I wrote about this one back in 2014 in Unusually Red Brown Dwarfs (and What They Tell Us).


Comments on this entry are closed.

  • Mark August 16, 2016, 14:25

    “Once we get below about 75 Solar masses, an object cannot ignite hydrogen fusion.”

    Does anyone know why it just got very dark outside?

    Shouldn’t that be 75 Jupiter masses? Obviously an object with only one solar mass (our sun) can ignite hydrogen fusion.

  • Robert Leigh August 16, 2016, 14:40

    “Once we get below about 75 Solar masses, an object cannot ignite hydrogen fusion.”

    Jupiter masses?

  • Brendan Steffens August 16, 2016, 15:46

    I’m pretty certain that “Jupiter masses” was intended to be in the following statement,

    “Once we get below about 75 Solar masses, an object cannot ignite hydrogen fusion. ”

    not “Solar masses”.

  • Neil Stahl August 16, 2016, 15:48

    In paragraph 3 you surely don’t mean 75 Solar masses. 75 Jupiter masses?

  • Mike Scott August 16, 2016, 16:06

    I’m going to go out on a limb here and guess you didn’t mean to say that an object with less than 75 times the mass of the Sun can’t ignite fusion, perhaps that should have been < 75 Jupiter masses?

  • George King August 16, 2016, 16:30

    “Once we get below about 75 Solar masses . . .”

    Believe you meant Jupiter masses.

    Yeah, these are fascinating objects to me, too. Hoping that they successfully identify some more close in.

  • Curt Wohleber August 16, 2016, 16:41

    “Once we get below about 75 Solar masses, an object cannot ignite hydrogen fusion.”
    Wouldn’t that mean our own sun should be able to fuse hydrogen? Should that be Jupiter masses? Or am I misunderstanding something?

  • Jens Knudsen August 16, 2016, 16:48

    ‘Once we get below about 75 Solar masses, an object cannot ignite hydrogen fusion.’ – do you mean Jupiter masses?

  • Paul Gilster August 16, 2016, 16:49

    Absolutely! Sorry for the egregious typo. Thanks to all who brought it to my attention; now fixed in the text.

  • Michael August 18, 2016, 6:47

    There are a few problems with exoplanets orbiting BD’s that in order to get enough heat orbiting planets will need to be close to the object which has tidal implications of heating, a rapidly moving magnetic field that can stripe atmospheres and a deep gravity well increasing the energy of impacts.

    Not to friendly in my book.

  • ljk August 18, 2016, 9:42

    Some other very recent brown dwarf news:


    An international team of astronomers reports the discovery of a giant planet and a brown dwarf in a close binary system designated HD 87646. The findings, described in a paper published Aug. 11 on arXiv.org, reveal that HD 87646 is the first close binary system with more than one substellar circum-primary companion known to date.


    An international team of astronomers has found a new substellar mass companion to one of the stars in the Pleiades open cluster. The discovery could contribute to our understanding of stellar and substellar multiplicity as well as formation mechanisms in this cluster. A study detailing the new findings was published Aug. 5 on the arXiv pre-print server.

  • ljk August 25, 2016, 10:47


    Search for exoplanets and brown dwarfs with VLBI

    K. Katarzynski, M. Gawronski, K. Gozdziewski

    (Submitted on 24 Aug 2016)

    The main aim of this work is to estimate possible radio GHz emission of extrasolar planets and brown dwarfs and to check if such radiation can be detected by Very Large Baseline Interferometers (VLBI). In the estimation we assume that the emission may originate in processes similar to those observed in the Jupiter system. The frequency of the radio emission that is produced in this system depends mostly on the magnetic field strength. Jupiter’s magnetic field (∼9 G on average) allows for radiation from kHz frequencies up to 40 MHz. This is well below the frequency range of VLBI.

    However, it was demonstrated that the magnetic field strength in massive and young object may be up to two orders of magnitude higher than for Jupiter, which is especially relevant for planets around short-lived A type stars. This should extend the range of the emission up to GHz frequencies.

    We calculated expected flux densities of radio emission for a variety of hypothetical young planetary systems. We analysed two different emission scenarios, and found that the radiation induced by moons (process similar to Jupiter-Io interactions) appears to be less efficient than the emission generated by a stellar wind on a planetary magnetosphere.

    We also estimated hypothetical emission of planets and brown dwarfs located around relatively young and massive main sequence A-type stars. Our results show that the emission produced by stellar winds could be detected by currently operating VLBI networks.

    Comments: 11 pages, 5 figures

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

    Journal reference: 2016 MNRAS vol. 461 p. 929

    DOI: 10.1093/mnras/stw1354

    Cite as: arXiv:1608.06719 [astro-ph.EP]
    (or arXiv:1608.06719v1 [astro-ph.EP] for this version)

    Submission history

    From: Krzysztof Katarzynski [view email]

    [v1] Wed, 24 Aug 2016 05:32:49 GMT (218kb,D)


  • ljk September 6, 2016, 10:55

    Brown dwarfs hiding in plain sight in our solar neighborhood

    Tuesday, September 06, 2016

    Washington, DC— Cool brown dwarfs are a hot topic in astronomy right now. Smaller than stars and bigger than giant planets, they hold promise for helping us understand both stellar evolution and planet formation. New work from a team including Carnegie’s Jonathan Gagné has discovered several ultracool brown dwarfs in our own solar neighborhood. Their findings are published in The Astrophysical Journal.


    The paper here:


    To quote:

    “Everyone will benefit from the study of brown dwarfs, because they can often be found in isolation, which means that we can more easily gather precise data on their properties without a bright star blinding our instruments,” Gagné said, who is also a collaborator of the Institute for Research on Exoplanets (iREx) at Université de Montréal.