Direct Imaging Discovery of a Fourth Planet at 15AU in the HR 8799 Planetary System

We present the imaging discovery of a fourth planet at 0.376 arcsec (14.8 AU) projected separation from the young nearby star HR8799. This new object, designated HR8799e, is detected at Ks- and L-band wavelengths using the ADI observing technique with adaptive optics and the Keck II telescope on Mauna Kea. Over a 1-year baseline the object is confirmed as co-moving with the primary, revolving counter-clockwise like the three other known planets of this system, and it has similar brightness at Ks and L to HR8799c and d and hence likely similar mass, 10 times the mass of Jupiter. HR8799 has now been shown to possess the same number of gas giant planets as our Solar System, further adding to the various similarities between the two systems.