detection of extrasolar planets
The distances to other stars and the faintness and low mass of planets in comparison to their host stars make the detection of extrasolar worlds difficult. Direct imaging may be possible in the near future through nulling interferometry, although it may already have been achieved by conventional means in the unusual case of TMR-1C. In addition, a variety of indirect detection techniques are available, some of which have already achieved positive results. A planet orbiting a distant star can make its presence known by causing small, regular variations in the line-of-sight velocity (see radial velocity method) or position (see astrometry) of its host star, by slightly dimming the light from its host star if it happens to pass in front of it (see photometry), by contributing to a gravitational lens effect (see microlensing), or, if the host star is a pulsar, by altering the period of the pulsar's cyclic pulses. Several dozen searches for extrasolar planets are in progress or under development using these various techniques (see exoplanets, searches).