  Vega may have a massive companion in a wide orbit, as evidenced by structure
in its cold dust debris. We have tested this hypothesis by direct imaging with
adaptive optics in the M band. The observations were made with a newly
commissioned thermal infrared camera, Clio, on the 6.5 MMT AO system with
low-background deformable secondary. The observations constrain a planet to be
less than 7 M$_J$ at the approximate position angle expected from the dust
structure and at a radius $>$ 20AU (2.5 arcsec) . This result is more stringent
than similar previous near-infrared observations of Vega, that achieve limits
of 20 and 10 M$_J$ at separations of 7 arcsec. The higher sensitivity is due
both to the more favorable contrast of gas giant planets at M band and to the
higher Strehl and more stable point spread function at longer wavelengths.
Future L' or M band observations could provide a powerful approach for wide
separation planet detection, especially for cooler, and thus older or less
massive planets. The natural best targets are nearby stars where planets in the
range of 5-15 M$_J$ and as old as several Gyr are expected to be detectable
with this technique.
