All kinds of worlds exist outside our solar system, and distant alien planets, called exoplanets, can be giant gas creatures like Jupiter, rock spheres about the size of our planet, or even "superjets." Christiansen's latest research suggests that these worlds shrink because radiation from the planets' cores pushes their atmospheres away into space.The study, published in The Astronomical Journal on Wednesday, may solve the mystery of lost exoplanets.The planets themselves may be pushing their atmospheres away, and shrinking exoplanets may lack mass (and therefore gravity) to keep their atmospheres close.The new study supports one hypothesis scientists call "loss of mass by energy." The statement said that the loss of mass with basic energy is not a modern new exercise plan, as it occurs when the planet's core emits radiation that pushes its atmosphere from below, causing it to separate from the planet over time.The other hypothesis, called photoevaporation, says that the planet's atmosphere dissipates due to the radiation of its host star.To test both hypotheses, Christiansen's team looked at data from NASA's Kepler Space Telescope.They examined star clusters more than 1 million years old. Because planets are thought to be about the same age as their host stars, the planets in these clusters would be large enough to undergo photoevaporation, but not large enough to lose mass with central energy.