Identification of Exoplanet Host Star
This is a Hubble Space Telescope view of a small region of our galaxy where the host star to a gravitationally lensed planet (catalogued as OGLE-2003-BLG-235L/MOA-2003-BLG-53L) is located. The star is identified by the crosshatch at frame center. The planet was first identified in ground-based microlensing observations in July 2003. Gravitational microlensing happens when a foreground star-planet system slightly amplifies the light of a background star that momentarily aligns with it.
A blowup of the target (lower left) reveals the light of two stars: a foreground star and a background star superimposed on each other. The background star is the brighter, solar type star, and the foreground star is the fainter star. The motion of the foreground star, as it drifts past the more distant background star is apparent in the Hubble image taken in 2005, even though it is below Hubble's resolution. The light from each star is progressively more offset, year after year. This gives rise to a color difference effect because the foreground star turns out to be a different color from the background star. By observing the stars though a red and blue filter, astronomers were able to enhance the visibility of the offset. The relative offset is 0.7 milliarcseconds (the angular width of a dime seen 3,000 miles away) from the source star. The deduced positions of the two stars in 2005 are shown with red and blue crosshatches.