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A Free-Floating ‘Planemo’ Binary

We know all too little about planetary-mass objects — planemos, as Centauri Dreams is learning to call them — that are not associated with a star. But we’ve learned a bit more with the discovery of an unusual binary system. Discovered in optical imagery taken by the European Southern Observatory’s 3.5 meter instrument in La Silla (Chile), Oph162225-240515 (Oph1622 for short) is a 14-Jupiter mass planemo apparently orbited by a companion of about half that mass.

“This is a truly remarkable pair of twins – each weighing some hundred times less than our sun,” says Ray Jayawardhana, an associate professor of astronomy and astrophysics at the University of Toronto. “Their mere existence is a surprise, and their origin and fate a bit of a mystery.”

Following up the find with optical spectra and infrared work, Jayawardhana and ESO’s Valentin Ivanov established that both members of the pair are at the same distance from the Sun and far too cool to be stars. They’re also young, perhaps a million years old, and separated by about six times the Pluto/Sun distance. Their place of birth is the Ophiuchus star-forming region some 400 light years away.

Centauri Dreams‘ take: This discovery, announced in the August 3 Science Express (abstract here), again puts the investigation of binaries into sharp focus. Evidently half of all Sun-like stars are binary, and current estimates are that about one sixth of all brown dwarfs come in pairs. These planemos are far below the brown dwarf ‘cut-off,’ which at 75 Jupiter masses is the level below which an object cannot sustain nuclear fusion. They are, in fact, the same mass as many of the extrasolar planets detected thus far, but they circle no nearby star.

Do free-floating planetary mass objects commonly occur as binaries? The difficulty in detecting them means we have no real idea, but the answer could tell us much about how such objects form. The current thinking is that this pair emerged from a contracting gas cloud that fragmented, creating a tiny version of a stellar binary. The weak gravitational interaction between the two members of Oph1622 puts constraints on other possibilities, including ejection from a developing multi-star system.

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  • ljk September 9, 2009, 12:59

    Candidate free-floating super-Jupiters in the young sigma Orionis open cluster

    Authors: G. Bihain, R. Rebolo, M. R. Zapatero Osorio, V. J. S. Béjar, I. Villó-Pérez, A. Díaz-Sánchez, A. Pérez-Garrido, J. A. Caballero, C. A. L. Bailer-Jones, D. Barrado y Navascués, J. Eislöffel, T. Forveille, B. Goldman, T. Henning, E. L. Martín, R. Mundt

    (Submitted on 4 Sep 2009)

    Abstract: Free-floating substellar candidates with estimated theoretical masses of as low as ~5 Jupiter masses have been found in the ~3 Myr old sigma Orionis open cluster. As the overlap with the planetary mass domain increases, the question of how these objects form becomes important. The determination of their number density and whether a mass cut-off limit exists is crucial to understanding their formation.

    We propose to search for objects of yet lower masses in the cluster and determine the shape of the mass function at low mass. Using new- and (re-analysed) published IZJHKs[3.6]-[8.0]-band data of an area of 840 arcmin2, we performed a search for LT-type cluster member candidates in the magnitude range J=19.5-21.5 mag, based on their expected magnitudes and colours.

    Besides recovering the T type object S Ori 70 and two other known objects, we find three new cluster member candidates, S Ori 72-74, with J=21 mag and within 12 arcmin of the cluster centre. They have theoretical masses of 4 (-2,+3) M_Jup and are among the least massive free-floating objects detected by direct imaging outside the Solar System.

    The photometry in archival Spitzer [3.6]-[5.8]-band images infers that S Ori 72 is an L/T transition candidate and S Ori 73 a T-type candidate, following the expected cluster sequence in the mid-infrared. Finally, the L-type candidate S Ori 74 with lower quality photometry is located at 11.8 arcsec (~4250 AU) of a stellar member of sigma Orionis and could be a companion.

    After contaminant correction in the area complete to J=21.1 mag, we estimate that there remain between zero and two cluster members in the mass interval 6-4 M_Jup.

    Our result suggests a possible turnover in the substellar mass spectrum below ~6 Jupiter masses, which should be investigated further by deeper photometric surveys.

    Comments: 15 pages, 13 figures, 5 tables, and appendix containing 5 figures; Accepted for publication in AA

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

    Cite as: arXiv:0909.0802v1 [astro-ph.SR]

    Submission history

    From: Gabriel Bihain [view email]

    [v1] Fri, 4 Sep 2009 13:56:07 GMT (1643kb)