An exoplanet or extrasolar planet is a planet outside the Solar System. An exoplanet (extrasolar planet) is a planet outside the Solar System. To date, there is no accepted definition of extrasolar planets, or exoplanets. Aries contains several stars with extrasolar planets. Astrophysicists have applied tidal theories to predict the obliquity of extrasolar planets. These were the first extrasolar planets discovered. Brown dwarfs are often discovered in surveys to discover extrasolar planets. Sodium has even been detected in the atmospheres of some extrasolar planets via transit spectroscopy. No secondary characteristics have been observed around extrasolar planets. Fifteen stars in Hercules are known to be orbited by extrasolar planets. She wakes up in her Tracer orbiting an extrasolar planet. Absorption spectroscopy is also employed in the study of extrasolar planets. Current interests include climate of extrasolar planets. The rotational periods of extrasolar planets are not known. There is evidence that extrasolar planets can have an atmosphere.