Venus, Jupiter's supervolcanic moon Io and Neptune's big satellite Triton are in NASA's crosshairs.

Those three cosmic objects are the focus of the four finalists for NASA's next round of Discovery missions. The Discovery Program develops relatively low-cost robotic-exploration efforts; each one is capped at $500 million, excluding costs for the launch vehicle and mission operations.

"These selected missions have the potential to transform our understanding of some of the solar system's most active and complex worlds," Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate, said in a statement Thursday (Feb. 13), when the finalists were announced.

"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," he added.

Related: Our solar system: A photo tour of the planets

The teams behind the four mission concepts will each receive $3 million to continue maturing their ideas over the next nine months, culminating in the submission of study reports to NASA. Agency officials will then evaluate these reports, eventually choosing up to two missions for continued development toward flight.

Two of the missions could end up getting off the ground. Indeed, that happened during the previous Discovery round; in January 2017, NASA announced that both the Lucy and Psyche missions would proceed to launch (in 2021 and 2022, respectively).

Return to Venus

Two of the four newly announced finalists target Venus . DAVINCI Plus (Deep Atmosphere Venus Investigation of Noble gases, Chemistry and Imaging Plus) would send a probe down through the thick Venusian air, gathering data that would help scientists better understand how the hellishly hot planet's atmosphere has changed over time.

The VERITAS (Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy) mission would map Venus' surface in detail from orbit. This probe's observations would shed considerable light on the planet's geologic history and help confirm if volcanism and plate tectonics are active on the planet today, NASA officials said.

Volcanic Io

The Io Volcano Observer (IVO) mission, meanwhile, would study the incredibly active Jupiter moon up close during a series of flybys.

IVO could help researchers better understand the ways that powerful tidal forces shape the evolution of rocky bodies in our solar system and beyond, NASA officials said. ( Io's extreme volcanism is driven by tidal forces; Jupiter and Io's fellow Galilean moons tug hard on the rocky moon, churning its insides.)

Neptune's Trident

Then, there's the Trident mission, which would study the Neptune moon Triton in detail during a single flyby. Trident would map Triton's surface and attempt to determine if the moon does indeed host a subsurface ocean, as many scientists believe.

You can learn more about all four finalists here .

Proposals for this Discovery round were submitted last year. More than a dozen were made public, including two concept missions that would have studied the strange emissaries from the Kuiper Belt known as Centaurs .

Mike Wall's book about the search for alien life, " Out There " (Grand Central Publishing, 2018; illustrated by Karl Tate ), is out now. Follow him on Twitter @michaeldwall . Follow us on Twitter @Spacedotcom or Facebook .