Brown dwarfs are considered to be ‘failed stars’ in the sense that they are born with masses between the least massive stars (0.072 solar masses, M⊙)1 and the most massive planets (∼0.013M⊙)2; they therefore serve as a critical link in our understanding of the formation of both stars and planets3. Even the most fundamental physical properties of brown dwarfs remain, however, largely unconstrained by direct measurement. Here we report the discovery of a brown-dwarf eclipsing binary system, in the Orion Nebula star-forming region, from which we obtain direct measurements of mass and radius for these newly formed brown dwarfs. Our mass measurements establish both objects as brown dwarfs, with masses of 0.054 ± 0.005M⊙ and 0.034 ± 0.003M⊙. At the same time, with radii relative to the Sun's of 0.669 ± 0.034R⊙ and 0.511 ± 0.026R⊙, these brown dwarfs are more akin to low-mass stars in size. Such large radii are generally consistent with theoretical predictions for young brown dwarfs in the earliest stages of gravitational contraction4,5. Surprisingly, however, we find that the less-massive brown dwarf is the hotter of the pair; this result is contrary to the predictions of all current theoretical models of coeval brown dwarfs.
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We are grateful to I. Baraffe, G. Basri, D. Golimowski, and D. Weintraub for discussions, and to D. Gudehaus and A. Prsa for software used in our analyses. This work is supported by grants to K.G.S. and R.D.M. from the National Science Foundation. The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (USA), the Particle Physics and Astronomy Research Council (UK), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
The geometry of the orbit is shown both as it appears from Earth and as it would appear if seen from above, at four different times in the orbit. Also shown is the relationship between the orbital geometry and the diminutions of light that occur when the brown dwarfs eclipse one another. (PDF 142 kb)
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Stassun, K., Mathieu, R. & Valenti, J. Discovery of two young brown dwarfs in an eclipsing binary system. Nature 440, 311–314 (2006). https://doi.org/10.1038/nature04570
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