A point source observed 8 AU in projection from \beta Pictoris in { \mathit { L } ^ { \prime } } ( 3.8 µm ) imaging in 2003 has been recently presented as a planet candidate .
Here we show the results of { \mathit { L } ^ { \prime } } -band adaptive optics imaging obtained at Keck Observatory in 2008 .
We do not detect \beta Pic b beyond a limiting radius of 0.29″ , or 5.5 AU in projection , from the star .
If \beta Pic b is an orbiting planet , then it has moved \geq 0.12 ″ ( 2.4 AU in projection ) closer to the star in the five years separating the two epochs of observation .
We examine the range of orbital parameters consistent with the observations , including likely bounds from the locations of previously inferred planetesimal belts .
We find a family of low-eccentricity orbits with semimajor axes \sim 8 –9 AU that are completely allowed , as well as a broad region of orbits with e \lesssim 0.2 , a \gtrsim 10 AU that are allowed if the apparent motion of the planet was towards the star in 2003 .
We compare this allowed space with predictions of the planetary orbital elements from the literature .
Additionally , we show how similar observations in the next several years can further constrain the space of allowed orbits .
Non-detections of the source through 2013 will exclude the interpretation of the candidate as a planet orbiting between the 6.4 and 16 AU planetesimal belts .