Abstract
Very little is known about magnetic fields of extrasolar planets and brown dwarfs. We use the energy flux scaling law presented by Christensen et al. to calculate the evolution of average magnetic fields in extrasolar planets and brown dwarfs under the assumption of fast rotation, which is probably the case for most of them. We find that massive brown dwarfs of about 70 MJup can have fields of a few kilo-Gauss during the first few hundred Million years. These fields can grow by a factor of two before they weaken after deuterium burning has stopped. Brown dwarfs with weak deuterium burning and extrasolar giant planets start with magnetic fields between ~100 G and ~1 kG at the age of a few Myr, depending on their mass. Their magnetic field weakens steadily until after 10 Gyr it has shrunk by about a factor of 10. We use observed X-ray luminosities to estimate the age of the known extrasolar giant planets that are more massive than 0.3 MJup and closer than 20 pc. Taking into account the age estimate, and assuming sun-like wind-properties and radio emission processes similar to those at Jupiter, we calculate their radio flux and its frequency. The highest radio flux we predict comes out as 700 mJy at a frequency around 150 MHz for τ Boo b, but the flux is below 60 mJy for the rest. Most planets are expected to emit radiation between a few Mhz and up to 100 MHz, well above the ionospheric cutoff frequency.
Highlights
The discovery of hundreds of extrasolar planets during the last two decades has enabled us to investigate the physical properties of these objects and to compare them to the known planets of our own solar system
To calculate the mass-loss rate that is required in Eq (3) for host stars of known extrasolar giant planets (EGPs), we parameterize the stellar mass-loss rates using the results from Wood et al (2005) and use X-ray luminosities taken from the NEXXUS database1 (Schmitt & Liefke 2004) in analogy with Stevens (2005) but with the updated parameters from Wood et al (2005)
For stars and brown dwarfs, i.e., objects with masses larger than 13 MJ, we show the average magnetic field of the dynamo, Bdyn, which is probably similar to the average surface field
Summary
The discovery of hundreds of extrasolar planets during the last two decades has enabled us to investigate the physical properties of these objects and to compare them to the known planets of our own solar system. Interesting questions are whether planets, brown dwarfs, and stars share the same physical principles with regard to their magnetic dynamos, and what typical field strengths must be expected at objects for which a positive field detection is missing so far (brown dwarfs and exoplanets). Radioemissions from magnetized planets result from the interaction of their magnetospheres with the stellar wind This accelerates electrons to several keV, which emit radio waves at the local cyclotron frequency (e.g., Zarka 1992, 1998; Farrell et al 1999; Zarka 2007). Sensitive enough to observe the expected weak radio flux (but see Lazio & Farrell 2007) Both the frequency and the total radio flux critically depend on the magnetic field strength of the planet, the latter primarily because it controls the cross section of the magnetosphere interacting with the stellar wind. We revisit the question of magnetic field strength at brown dwarfs and giant planets and the estimate for the radio flux from extrasolar planets, using this scaling law, which we believe to be on more solid grounds both theoretically and observationally than previously suggested scaling laws
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