Free Astronomy Magazine September-October 2015

SPACE CHRONICLES large majority of exoplanets cata- loged so far are very close to their host stars because several current planet-hunting techniques favor finding planets in short-period or- bits. But this is not the case with the microlensing technique, which can find more distant and colder plan- ets in long-period orbits that other methods cannot detect. Microlensing occurs when a fore- ground star amplifies the light of a background star that momentarily aligns with it. If the foreground star has planets, then the planets may also amplify the light of the back- ground star, but for a much shorter period of time than their host star. The exact timing and amount of light amplification can reveal clues to the nature of the foreground star and its accompanying planets. The system, cataloged as OGLE- 2005-BLG-169, was discovered in 2005 by the Optical Gravitational Lensing Experiment (OGLE), the Mi- crolensing Follow-Up Network (Mi- croFUN), and members of the Micro- lensing Observations in Astrophysics (MOA) collaborations — groups that search for extrasolar planets through gravitational microlensing. Without conclusively identifying and characterizing the foreground star, however, astrono- mers have had a difficult time determining the properties of the accom- panying planet. Using Hubble and the Keck Ob- servatory, two teams of astronomers have now found that the system consists of a Uranus-sized planet orbiting about 370 million miles from its par- ent star, slightly less than the distance between Ju- piter and the Sun. The host star, however, is about 70 percent as mas- sive as our Sun. "These chance alignments are rare, occurring only about once every 1 million years for a given planet, so it Uranus-sized planet discovered through microlensing by NASA & Keck Observatory N ASA's Hubble Space Telescope and the W. M. Keck Observa- tory in Hawaii have made in- dependent confirmations of an exo- planet orbiting far from its cen- tral star. The planet was discovered through a technique called gravita- tional microlensing. This finding opens a new piece of discovery space in the extrasolar planet hunt: to uncover planets as far from their central stars as Jupiter and Saturn are from our sun. The A rtist's rendering of a super-Earth similar to OGLE-2005-BLG-169 b. [MicroFUN Collabora- tion, CfA, NSF]