Free Astronomy Magazine September-October 2015

SPACE CHRONICLES T his graphic illustrates how a star can mag- nify and brighten the light of a background star when it passes in front of the distant star. If the foreground star has planets, then the planets may also mag- nify the light of the background star, but for a much shorter period of time than their host star. Astronomers use this method, called gravitational microlen- sing, to identify plan- ets. [NASA, ESA, and A. Feild (STScI)] our solar system. Astrono- mers often rely on two indi- rect techniques to hunt for extrasolar planets. The first method detects planets by the subtle gravitational tug they give to their host stars. In another method, astron- omers watch for small dips in the amount of light from a star as a planet passes in front of it. Both of these techniques work best when the planets are either extremely mas- sive or when they orbit very close to their parent stars. In these cases, astronomers can reliably de- termine their short orbital periods, ranging from hours to days to a cou- ple years. But to fully understand the architec- ture of distant planetary systems, astronomers must map the entire distribution of planets around a star. Astronomers, therefore, need to look farther away from the star − from about the distance of Jupiter is from our Sun, and beyond. "It's important to understand how these systems compare with our solar system," said team member Jay Anderson of the Space Telescope was thought that a very long wait would be required before the plan- etary microlensing signal could be confirmed," said David Bennett of the University of Notre Dame, In- diana, the lead of the team that analyzed the Hubble data. "Fortu- nately, the planetary signal predicts how fast the apparent positions of the background star and planetary host star will separate, and our ob- servations have confirmed this pre- diction. The Hubble and Keck Obser- vatory data, therefore, provide the first confirmation of a planetary mi- crolensing signal." In fact, microlens- ing is such a powerful tool that it can uncover planets whose host stars cannot be seen by most tele- scopes. "It is remarkable that we can detect planets orbiting unseen stars, but we'd really like to know something about the stars that these planets orbit," explained Vir- ginie Batista of the Institut d'Astro- physique de Paris, France, leader of the Keck Observatory analysis. "The Keck and Hubble telescopes allow us to detect these faint planetary host stars and determine their pro- perties." Planets are small and faint com- pared to their host stars; only a few have been observed directly outside