8.7 Brown dwarfs as planet hosts

Brown dwarfs, objects between stars and planets, offer unique insights into celestial body formation. Their mass ranges from 13 to 75 Jupiter masses, bridging the gap between stars and gas giants. These cosmic in-betweeners emit faint light but can't sustain hydrogen fusion.

Studying planets around brown dwarfs challenges traditional formation models. These systems face unique hurdles, like limited material for accretion and shorter formation timelines. Detection methods must adapt, but discoveries are reshaping our understanding of planetary systems and expanding the search for habitable worlds.

Definition of brown dwarfs

• Brown dwarfs bridge the gap between stars and planets in exoplanetary science
• These objects play a crucial role in understanding the continuum of celestial bodies
• Studying brown dwarfs provides insights into both stellar and planetary formation processes

Mass range of brown dwarfs

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• Spans approximately 13 to 75 Jupiter masses
• Lower limit determined by deuterium fusion threshold
• Upper limit set by minimum mass for sustained hydrogen fusion
• Mass estimation techniques include gravitational microlensing and binary system observations

Comparison to stars vs planets

• Share characteristics with both stars and gas giant planets
• Emit faint light due to gravitational contraction, unlike planets
• Lack sufficient mass to sustain hydrogen fusion like stars
• Atmospheres resemble those of gas giants (Jupiter, Saturn)
• Internal structure includes a degenerate core, similar to white dwarfs

Formation of brown dwarfs

• Brown dwarf formation illuminates the processes of both star and planet formation
• Understanding their origins helps refine models of celestial object evolution
• Formation mechanisms impact the potential for planetary systems around brown dwarfs

Core accretion vs disk instability

• Core accretion involves gradual accumulation of solid particles
  • Forms a rocky core that accretes gas from the surrounding nebula
  • Typically occurs in protoplanetary disks around young stars
• Disk instability results from gravitational collapse of a massive protoplanetary disk
  • Rapid formation process compared to core accretion
  • More likely in massive, cool disks
• Both mechanisms potentially contribute to brown dwarf formation
  • Core accretion favored for lower-mass brown dwarfs
  • Disk instability may explain more massive brown dwarfs

Differences from star formation

• Insufficient mass to initiate and sustain hydrogen fusion
• Formation often truncated before reaching stellar masses
• May form through fragmentation of collapsing molecular clouds
• Ejection from multiple star systems can halt accretion
• Turbulence in star-forming regions may disrupt growth to stellar masses

Brown dwarf characteristics

• Brown dwarfs exhibit unique physical properties distinct from stars and planets
• Their characteristics evolve significantly over time due to lack of stable energy source
• Studying these properties aids in identifying and classifying brown dwarfs in surveys

Temperature and luminosity

• Surface temperatures range from ~250 to 3000 K
• Luminosity decreases steadily over time as they cool
• Emit primarily in infrared wavelengths
• Lack stable energy source from nuclear fusion
• Cooling rate depends on mass and age
  • More massive brown dwarfs cool more slowly
  • Younger brown dwarfs are generally hotter and more luminous

Spectral classification

• Classified using spectral types L, T, and Y
• L dwarfs (1300-2000 K) show strong metal hydride and alkali metal lines
• T dwarfs (700-1300 K) exhibit methane absorption bands
• Y dwarfs (<700 K) display ammonia absorption features
• Spectral features change as brown dwarfs cool over time
• Transition between spectral types not solely dependent on temperature
  • Atmospheric chemistry and cloud formation also play roles

Planet formation around brown dwarfs

• Planetary systems around brown dwarfs expand our understanding of planet formation
• These systems challenge traditional models developed for sun-like stars
• Studying brown dwarf planets provides insights into formation in extreme environments

Protoplanetary disk properties

• Disks around brown dwarfs tend to be less massive than those around stars
• Typical disk masses range from 0.1 to 1 Jupiter mass
• Disk lifetimes may be shorter due to lower surface densities
• Dust grain growth and evolution occur rapidly in these disks
• Gas-to-dust ratio may differ from disks around more massive stars

Challenges in planet formation

• Limited material available for accretion
• Shorter timescales for planet formation before disk dissipation
• Reduced gravitational influence may hinder planetesimal growth
• Migration of forming planets more pronounced in less massive disks
• Radiation pressure and photoevaporation effects more significant

Detection methods for planets

• Detecting planets around brown dwarfs requires adapting existing exoplanet techniques
• Each method presents unique challenges and advantages for brown dwarf systems
• Combining multiple detection methods provides the most comprehensive results

Direct imaging techniques

• Favorable contrast ratio between brown dwarf and planet
• Angular separation often larger than for stellar systems
• Adaptive optics and coronagraphy enhance imaging capabilities
• Infrared observations particularly effective due to brown dwarf and planet temperatures
• Challenges include faintness of both brown dwarf and potential planets

Radial velocity measurements

• Measures doppler shifts in brown dwarf spectra caused by orbiting planets
• More pronounced signal due to lower mass ratio between brown dwarf and planet
• Requires high-resolution spectroscopy in the infrared
• Limited by intrinsic variability of brown dwarf atmospheres
• Precision potentially affected by rotational broadening of spectral lines

Transit photometry adaptations

• Detects periodic dimming as planet passes in front of brown dwarf
• Larger transit depth due to similar sizes of brown dwarf and giant planets
• Requires precise photometry in infrared wavelengths
• Challenges include intrinsic variability of brown dwarf brightness
• Limited by smaller number of observable systems due to geometric alignment

Known brown dwarf planetary systems

• Discoveries of planets around brown dwarfs reshape our understanding of planetary systems
• These systems provide crucial data points for planet formation theories
• Studying known systems helps refine detection methods and target selection

Notable discoveries

• 2MASS J12073346-3932539 b: first directly imaged planet around a brown dwarf
• MOA-2007-BLG-192L b: super-Earth discovered via microlensing
• OGLE-2012-BLG-0358L b: Jupiter-mass planet orbiting a very low-mass brown dwarf
• VHS 1256-1257 b: wide-orbit planetary-mass companion to a brown dwarf binary
• WISE J085510.83-071442.5: potential free-floating planetary-mass object

Frequency of planets

• Current estimates suggest 10-30% of brown dwarfs host planets
• Giant planets appear less common than around sun-like stars
• Earth-sized and smaller planets may be more prevalent
• Detection biases favor discovery of larger, more massive planets
• Multiplicity of planetary systems around brown dwarfs remains uncertain

Habitability considerations

• Exploring potential habitability of planets around brown dwarfs expands search for life
• Unique characteristics of brown dwarf systems present both challenges and opportunities
• Understanding these factors guides future targeted searches for habitable worlds

Tidal locking effects

• Planets in habitable zone likely tidally locked due to close proximity
• Creates permanent day and night sides with extreme temperature differences
• Atmospheric circulation crucial for heat distribution and potential habitability
• May lead to unique climate patterns and weather systems
• Impacts on planetary magnetic field generation and strength

Radiation environment

• Lower overall radiation output compared to main sequence stars
• Ultraviolet radiation levels generally lower, potentially beneficial for life
• X-ray emission varies with brown dwarf age and activity level
• Flare activity more common in younger brown dwarfs
• Radiation effects on atmospheric chemistry and potential biosignatures

Observational challenges

• Detecting and characterizing brown dwarf planetary systems pushes observational limits
• Overcoming these challenges drives technological advancements in astronomy
• Innovative observation strategies and data analysis techniques continually evolve

Faintness of brown dwarfs

• Intrinsically low luminosity requires long integration times
• Signal-to-noise ratio limitations for spectroscopic observations
• Infrared observations crucial but face higher sky background
• Atmospheric water vapor absorption complicates ground-based observations
• Space-based telescopes provide advantages for studying faint brown dwarfs

Angular resolution requirements

• Close proximity of planets to host brown dwarfs demands high resolution
• Diffraction limit of telescopes challenges direct imaging of close-in planets
• Interferometry techniques push resolution boundaries
• Adaptive optics systems continually improve to meet resolution needs
• Future extremely large telescopes promise breakthrough capabilities

Future prospects

• Ongoing advancements in technology and methodology drive brown dwarf exoplanet research
• Upcoming missions and surveys will significantly expand our knowledge of these systems
• Interdisciplinary collaborations enhance our understanding of brown dwarf planetary science

Upcoming surveys and missions

• James Webb Space Telescope: unprecedented infrared sensitivity and resolution
• Roman Space Telescope: wide-field surveys to discover more brown dwarf systems
• Extremely Large Telescopes (ELT, TMT, GMT): ground-based direct imaging and spectroscopy
• PLATO mission: transit detection of small planets around bright stars and brown dwarfs
• ARIEL mission: atmospheric characterization of exoplanets, including those around brown dwarfs

Technological advancements

• Improved infrared detectors with lower noise and higher quantum efficiency
• Advanced coronagraphs for better contrast in direct imaging
• Machine learning algorithms for automated detection and characterization
• High-precision radial velocity instruments operating in the infrared
• Development of new spectroscopic techniques for brown dwarf and planet atmospheres

Implications for planetary science

• Brown dwarf planetary systems challenge and refine our understanding of planet formation
• These systems provide a unique laboratory for studying extreme planetary environments
• Insights gained from brown dwarf planets inform broader exoplanetary science

Expanding definition of planets

• Blurs distinction between planets and brown dwarfs at the high-mass end
• Raises questions about formation mechanisms and their influence on classification
• Challenges traditional mass-based definitions of planets
• Encourages consideration of formation history in planetary categorization
• Impacts how we define and search for habitable worlds

Insights into planet formation theories

• Tests planet formation models at the low-mass end of host objects
• Provides constraints on disk evolution and planet migration in low-mass systems
• Informs understanding of planet formation efficiency across different environments
• Highlights importance of initial conditions in determining final planetary system architecture
• Contributes to developing a unified theory of planet formation applicable across all stellar types