According to a new study published in the journal Proceedings of the National Academy of Sciences, gravitational tugs from the planets Jupiter and Venus gradually affect Earth's climate and life forms. The phenomenon occurs every 405,000 years and has been going on for at least 215 million years. USA Today reports: Jupiter and Venus are such strong influences because of their size and proximity. Venus is the nearest planet to us -- at its farthest, only about 162 million miles -- and roughly similar in mass. Jupiter is much farther away, but is the Solar System's largest planet. The study says that every 405,000 years, due to wobbles in our orbit caused by the gravitational pulls of the two planets, seasonal differences here on Earth become more intense. Summers are hotter and winters colder; dry times drier, wet times wetter. At the height of the cycle, more rain falls in the tropics, allowing lakes there to fill up. This compares to the other end of the cycle, when seasonal rains in the tropics "are less and lakes have much less of a tendency to become as full," [study lead author Dennis] Kent said. The results showed that the 405,000-year cycle is the most regular astronomical pattern linked to the Earth's annual turn around the sun, he said. Right now, we are in the middle of the cycle, as the most recent peak was around 200,000 years ago.