Free Astronomy Magazine November-December 2015

ASTROBIOLOGY Then, between day 1510 and 1570, the same star underwent a series of drops in brightness ranging from a few hundredths of magnitude to a drastic peak of 22%, i.e. more than 1/5 th of the surface covered by some- thing. To understand the extent of this event, it must be considered that a large planet such as Jupiter transit- ing across the disk of a star as big as the Sun dims the light of the latter by just 1%. And KIC 8462852 is ac- tually 1.5 times larger than the Sun and it is even brighter (spectral type F3, surface temperature 6750 ± 120 K). Unfortunately, in 2013 a critical mal- function put an end to Kepler’s pri- mary mission, making no longer pos- sible to observe the star with this in- strument. The available data were, however, sufficient to Boyajian and her large team of collaborators for a first series of evaluations, aimed at excluding as many causes as possible behind the amazing drops in light. In this task were also involv- ed Jason Wright, astronomer at Penn State University, known for his theoretical re- search on signs of alien civilizations, and Andrew Siemion, director of the SETI (Search for Extraterrestrial Intelligence) Research Center at UCLA, Berkeley. Strong of its variety of expertise and expe- rience, the team of researchers reviewed all the possible known causes that could ex- plain KIC 8462852 behaviour. Let us briefly look at them. 1) If the star was very young or very old it could display in- trinsic variability of different na- ture, instead it appears to be a quiet main sequence star, nei- ther particularly young nor par- ticularly old. 2) If for some ab- normality the photosphere was to be occasionally affected by giant spots (of the solar type but much larger), there may ac- tually be deep drops in light, but the more these formations are large, the more slowly they evolve, generating recognizable light curves related to the star’s project of citizen science, dubbed Planet Hunters, was launched in 2011 as part of the larger Zooniverse programme, and en- trusted to the supervision of Tabetha Boya- jian, an astronomer at Yale University. Shortly after Planet Hunters was started, some volunteers reported something un- usual: a star called KIC 8462852 (KIC stands for Kepler Input Catalog) showed an ab- surd light curve, with drops in light never observed before. This star, that during phases of normalcy has a magnitude of 11.7, after about 800 days of monitoring by Kepler it dimmed by 15% in nearly a week, after which it return- ed to its previous brightness. T op, infrared image taken with UKIRT, in which KIC 8462852 shows a suspicious left bulge (the inter- val between lines is 10 arcseconds). Right, the star recorded with Keck, which re- veals the presence of a possible companion. [T. Boyajian et al.]