Free Astronomy Magazine November-December 2016

bending 19 NOVEMBER-DECEMBER 2016 SPACE CHRONICLES circumbinary planets — planets or- biting two stars — have been dis- covered to date. Most of these circumbinaries have been detected by NASA’s Kepler mission, which uses the transit method for detec- tion. During a transit an exoplanet moves between its parent star and the observer. As a result a small fraction of the star’s light is blocked and the star becomes fainter. This newly discovered planet, how- e ver, is very unusual. “The exoplan- et was observed as a microlensing event in 2007. A detailed analysis revealed a third lensing body in ad- dition to the star and planet that were quite obvious from the data,” says David Bennett from the NASA Goddard Space Flight Center, USA, lead author of the study. The event, OGLE-2007-BLG-349, was detected during the Optical Grav- itational Lensing Experiment (OGLE), a Polish astronomical pro- ject that was established in 1992, with the primary intention of in- vestigating dark matter using gra- vitational microlensing. OGLE searches for and observes effects from small distortions of spacetime, caused by stars and exoplanets, which were predicted by Einstein in his theory of General Relativity. These small distortions are known as microlensing. However, the OGLE observation could not confirm the details of the OGLE-2007-BLG-349 event on its own, especially the nature of the third, unknown lensing body. A number of models could have ex- plained the observed light curve. The additional data from Hubble were essential to enable the scien- tists to pin down a circumbinary planet as the only possible expla- nation for both OGLE’s light curve and the Hubble observations. “OGLE has detected over 17,000 microlensing events, but this is the first time such an event has been caused by a circumbinary planetary system,” explains Andrzej Udalski from the University of Warsaw, Poland, co-author of the study. This pioneering discovery suggests some intriguing possibilities. While Kepler is more likely to detect planets with small orbits — and in- deed all the circumbinary planets it discovered are very close to the lower limit of a stable orbit — mi- crolensing allows planets to be found at distances far from their host stars. “This discovery, suggests we need to rethink our observing strategy when it comes to stellar binary lens- ing events,” explains Yiannis Tsa- pras, co-author of the study from the Astronomisches Recheninstitut in Heidelberg, Germany. “This is an exciting new discovery for micro- lensing” . Now that the team has shown that microlensing can suc- cessfully detect events caused by circumbinary planets, Hubble could provide an essential role in this new realm in the continued search for exoplanets. T his artist’s impression shows a gas giant planet circling the two red dwarf stars in the system OGLE-2007-BLG-349, located 8,000 light-years away. The planet — with a mass similar to Saturn — orbits the two stars at a distance of roughly 480 million kilometres. The two red dwarf stars are a mere 11 million kilometres apart. The artist's impression is based on observations made with Hubble that helped astronomers confirm the existence of a planet orbiting the two stars in the system.The system is too far away for Hubble to take an image of the planet. Instead, its presence was inferred from gravitational microlens- ing. This phenomenon occurs when the gravity of a foreground star bends and amplifies the light of a background star that momentarily aligns with it. The particular character of the light magnification can reveal clues to the nature of the foreground star and any associated planets. The Hubble observations re- present the first time such a three-body system has been confirmed using the gravitational microlensing technique. [NASA, ESA, and G. Bacon (STScI)] n