A computer-generated simulation of TrES-4, with its host star on the right. The planet's home star is bigger and hotter than the Sun, and is about ten times larger than the planet. Astronomers speculate that the large size and low density of TrES-4 may cause a small fraction of its outer atmosphere to escape from the planet's gravitational pull and form an envelope, or a comet-like tail around the planet. Credit: Jeffrey Hall, Lowell Observatory
