Apparent Stellar Wobble by a Planet in a Circumstellar Disk: Limitations on Planet Detection by Astrometry

Astrometric detection of a stellar wobble on the plane of the sky will provide us the next breakthrough in searching for extrasolar planets. The Space Interferometry Mission (SIM) is expected to achieve high-precision astrometry as accurate as 1 μas, which is precise enough to discover a newborn Jupiter mass planet around a pre-main-sequence (PMS) star in the Taurus-Auriga star-forming region. PMS stars, however, have circumstellar disks that may be obstacles to the precise measurement of the stellar position. We present results on disk influences on the stellar wobble. The density waves excited by a planet move both the disk's mass center and the photocenter. The motion of the disk mass center induces an additional wobble of the stellar position, and the motion of the disk photocenter causes a contamination in the measurement of the stellar position. We show that the additional stellar motion dynamically caused by the disk's gravity is always negligible but that the contamination by the disk light can interfere with the precise measurement of the stellar position if the planet's mass is smaller than ~10M_J. The motion of the disk photocenter is sensitive to a slight change in the wave pattern and the disk properties. Measurements by interferometers are generally insensitive to extended sources such as disks. Because of this property, SIM will not suffer significant contamination by the disk light, even if the planet's mass is as small as 1M_J.