(CNN) In order to further explore our solar system, NASA has chosen four proposals for missions that would investigate Venus, Jupiter's moon Io and Neptune's moon Triton. If chosen, NASA would develop the missions.

The four proposals highlight planets and moons that are not currently included in the agency's ongoing projects. They're part of the agency's Discovery Program, which encourages scientists and engineers to help us explore the solar system by designing new, imaginative mission concepts. Since 1992, the program has supported development for more than 20 scientific instruments and missions.

"These selected missions have the potential to transform our understanding of some of the solar system's most active and complex worlds," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate. "Exploring any one of these celestial bodies will help unlock the secrets of how it, and others like it, came to be in the cosmos."

Now, the teams behind each proposal will have nine months and $3 million to develop their ideas, resulting in a Concept Study Report. Final selections, which could include up to two of these missions, will happen in 2021.

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