Hubble exoplanet search field in Sagittarius [full ACS view]
This is an image of one-half of the Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). The field contains approximately 150 000 stars, down to 30th magnitude. The stars in the Galactic disk and bulge have a mixture of colours and masses. The field is so crowded with stars because Hubble was looking across 26 000 light-years of space in the direction of the centre of our Galaxy.
Half of these stars are bright enough for Hubble to monitor for any small, brief and periodic dips in brightness caused by the passage of an exoplanet passing in front of the star, an event called a transit. Hubble took approximately 520 pictures of this field, at red and blue wavelengths, from 22 to 29 February 2004. Hubble identified 16 candidate stars that are orbited by planets with periods of a few days. Planets so close to their stars with such short orbital periods are called hot Jupiters.
These are considered candidate exoplanets because most of them are too faint to allow for spectroscopic observations that would allow for a precise measure of the planet's mass and confirmation of their nature.