Free Astronomy Magazine September-October 2015

SPACE CHRONICLES Science Institute in Baltimore, Maryland. "So we need a complete census of planets in these systems. Gravitational microlensing is critical in help- ing astronomers gain insights into planetary formation the- ories." The planet in the OGLE-2005-BLG-169 system is probably an example of a "failed-Jupiter" planet, an object that begins to form a Jupiter-like core of rock and ice weighing around 10 Earth masses, but it doesn't grow fast enough to accrete a sig- nificant mass of hydrogen and helium. So it ends up with a mass more than 20 times smaller than that of Ju- piter. "Failed-Jupiter planets, like OGLE-2005-BLG-169Lb, are predicted to be more common than Jupiters, espe- cially around stars less massive than the Sun, according to the preferred theory of planet formation. So this type of planet is thought to be quite common," Bennett said. Microlens- ing takes advantage of the random motion of stars, which are generally too small to be noticed without pre- cise measurements. If one star, how- ever, passes nearly precisely in front of a farther background star, the gravity of the foreground star acts like a giant lens, magnifying the light from the background star. A planetary companion around the foreground star can produce a var- iation in the brightening of the background star. This brightening fluctuation can reveal the planet, which can be too faint, in some cases, to be seen by telescopes. The duration of an entire microlensing event is several months, while the variation in brightening due to a planet lasts a few hours to a couple of days. The initial microlensing da- ta of OGLE-2005-BLG-169 had indi- cated a combined system of fore- system from Earth. The fore- ground and background stars were observed in several dif- ferent colors with Hubble's Wide Field Camera 3 (WFC3), allowing independent confir- mations of the mass and dis- tance determinations. The observations taken with the Near Infrared Camera 2 (NIRC2) on the Keck 2 tele- scope more than eight years after the microlensing event, provided a precise measure- ment of the foreground and background stars' relative motion. "It is the first time we were able to completely resolve the source star and the lensing star after a micro- lensing event. This enabled us to discriminate between two models that fit the data of the microlensing light curve," Batista said. The Hubble and Keck Observatory data are providing proof of concept for the primary method of exoplan- et detection that will be used by NASA's planned, space-based Wide- Field Infrared Survey Telescope (WFIRST), which will allow astrono- mers to determine the masses of planets found with microlensing. WFIRST will have Hubble's sharpness to search for exoplanets using the microlensing technique. The telescope will be able to ob- serve foreground, planetary host stars approaching the background source stars prior to the microlens- ing events, and receding from the background source stars after the microlensing events. "WFIRST will make measurements like we have made for OGLE-2005- BLG-169 for virtually all the plane- tary microlensing events it observes. We'll know the masses and dis- tances for the thousands of planets discovered by WFIRST," Bennett ex- plained. T his simulation shows the 22-year journey of a star moving through space and passing directly in front of a more distant background star. All stars drift through space. Occasionally, a star lines up perfectly in front of a more distant star. The momentary alignment magnifies and brightens the light from the background star, an effect called gravitational microlensing. [NASA, ESA, D. Bennett (University of Notre Dame), Wig- gle Puppy Productions, and G. Bacon (STScI)] n ground and background stars plus a planet. But due to the blurring ef- fects of our atmosphere, a number of unrelated stars are also blended with the foreground and back- ground stars in the very crowded star field in the direction of our gal- axy's center. The sharp Hubble and Keck Obser- vatory images allowed the research teams to separate out the back- ground source star from its neigh- bors. Although the Hubble images were taken 6.5 years after the len- sing event, the source and lens star were still so close together on the sky that their images merged into what looked like an elongated stel- lar image. Astronomers can measure the brightness of both the source and planetary host stars from the elongated image. When combined with the informa- tion from the microlensing light curve, the lens brightness reveals the masses and orbital separation of the planet and its host star, as well as the distance of the planetary