In 1961, Frank Drake of the National Astronomy And Ionosphere Center devised a mathematical equation for the number of planets in the Milky Way galaxy that could support life. The equation has several drawbacks, including the difficulty of finding the data.
The equation (named after Drake) is N=RxfpxnexflxfixfcxxL, where R is the average rate of star formation in the Milky Way; fp is the ratio of those stars that have planets; ne is average number of planets that can support life per star; fl is the ratio of ne that can develop life; fi is the ratio of fl that can develop intelligent life; fc is the ratio of civilizations that have technology advanced enough to be detected by a radio telescope; and L is the length of time the civilizations can be detected from space by the radio telescopes. N is what the equation equals, which is the number of civilizations with which communication might be possible.
The Circumstellar Habitable Zone (CHZ) is the area around a star that has sufficient atmospheric pressure to maintain water, which is necessary for all known life to exist. The distance from a planet and the star it orbits is considered one important criteria, although there can be exceptions. The planet must have a circular orbit for the best chance of being habitable. The Galactic Habitable Zone, discovered by Guillermo Gonzalez in 1995, is the distance a planet is from the Galactic Center, which is the center of the Milky Way. The Galactic Center is about twenty-seven thousand light years from the Earth.
According to seti.org, a star named TW Hydrae was discovered by Herschel telescope that might be surrounded by a gigantic disk that might be making new planets, but was not expected to be able to. The star is about 175 light years from the Earth. There have been several unidentified object (UFO) reports since ancient times. Although we have not made known contact with any extraterrestrial life, we also have not visited even the closest planets like Mars and Venus. Life can exist on a moon orbiting a planet if the planet is in a habitable zone. Titan, a moon of Saturn, has an atmosphere and is considered to be a habitable moon. Mars is the planet that is considered to be most likely habitable. Venus is currently considered to be too turbulent for life to exist on its surface, but it is millions of miles away.