Will alien life be left undiscovered by humans for another 1,500 years, as a new study by a group of astronomers suggest, or will that day come much sooner? There are several good reasons to believe that intelligent life exists elsewhere in the galaxy — and it may be found fairly close to our home planet.

If advanced life-forms are common throughout the galaxy, it increases the chances such a civilization will be found near Earth. Here are the top five reasons to believe aliens may soon be discovered by our own species.

First, the sheer number of exoplanets is remarkable. The Kepler spacecraft alone has recorded 2,326 planets circling other stars. It would appear alien stars are brimming with worlds.

In addition, water is often considered to be a requirement for life, at least of the type found on Earth. Once thought to be rare on alien worlds, water has now been seen on Europa, Mars and Enceladus. This suggests the universal solvent may also be common on planets circling other suns.

"Life finds a way" is not just a quote from Jurassic Park, but is also a statement on the resiliency of life. Here on Earth, life-forms evolved in areas without light, oxygen and other necessities for most species.

Many precursors of life, including proteins, fats and carbohydrates, are being spotted on comets, moons and planets. These complex organic molecules are seen from the atmosphere of Saturn's moon Titan to the Orion Nebula.

In 1977, astronomer Jerry Ehman was working at the Big Ear radio telescope, searching for signals from intelligent extraterrestrial civilizations. Suddenly, a signal was received that closely resembled a call from outer space. The burst last for 72 seconds, but was never repeated. Current studies suggest this reading may have been the result of a comet tumbling through space. Still, this signal remains the most likely candidate yet seen for a potential call from an alien species.

All these factors may result in human beings contacting alien civilizations for the first time in the next few decades, some researchers contend. As time goes on, the amount of evidence supporting the presence of life on other worlds is becoming greater.

"I think we're going to have strong indications of life beyond Earth within a decade, and I think we're going to have definitive evidence within 20 to 30 years. We know where to look. We know how to look. In most cases we have the technology, and we're on a path to implementing it. And so I think we're definitely on the road," said Ellen Stofan, NASA chief scientist.

During the year 2020, NASA will launch a new rover to Mars, which is specifically designed to find evidence of life, however primitive, on or just beneath the surface of the red planet.

The Drake equation, created by astronomer Frank Drake, was developed to estimate the number of civilizations in the Milky Way. If enough of the factors in the equation — including the number of exoplanets and the fraction that may support life - favor life, then aliens may be living next door.

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