NASA has officially narrowed its shortlist of Discovery Program candidates down to four, serving up the tantalizing possibility that an upcoming space mission will soon be headed to Venus, Jupiter’s moon Io or Neptune’s moon Triton.

Launched in 1992, the Discovery Program invites scientists and engineers to submit proposals for “small” missions centered on planetary science. These missions are complementary to NASA’s larger, “flagship” solar system explorations, including the New Frontiers and Solar System Exploration missions, which tend to carry heftier price tags, reports Mary Beth Griggs at the Verge. As such, Discovery-class missions can cost no more than $450 million and must come together on shorter timelines.

But budgetary constraints haven’t stopped previous Discovery missions from sparking some serious scientific achievement. Previously greenlit endeavors include the Kepler space telescope, which aided the discovery of more than 2,000 exoplanets during its nine-year tenure; and the Lunar Reconnaissance Orbiter, now mapping the moon’s mysterious surface; and multiple Marsbound missions, including Mars Pathfinder, the first rover to touch down on another planet, and the InSight lander, currently surveying the red planet’s interior. Also backed by Discovery are several missions like Lucy and Psyche that have yet to leave Earth—both of which will get scientists up close and personal with some asteroids that could help us understand how the planets formed.

In other words, the four finalists in this round have some big cosmic shoes to fill.

Two of the missions have their sights set on Venus. First up is DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus), which would parachute down to the inhospitable planet’s surface, bypassing its harsh, acid-rich atmosphere and snapping a series of photos along the way. The gas-focused probe might be able to gather crucial intel on Venus’ formation and evolution, including, perhaps, data on whether liquid water was ever a part of the planet’s past, reports Gizmodo’s George Dvorsky.

The team behind VERITAS (Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy) shares a planetary target with DAVINCI+, but would instead concentrate more intently on Venus’ surface with a sensor-heavy satellite. Aimed at mapping Venus’ topography, the mission might glean some insights into why Earth and its neighbor embarked on such starkly different geologic trajectories.

Next up is the Io Volcano Observer (IVO), which would explore Jupiter’s moon, Io, the most volcanically active body known in the solar system. Through a series of close flybys, the probe would help scientists suss out what’s at work beneath the moon’s surface—and possibly alert them to the existence of a magma ocean in its interior.

Last on deck is TRIDENT, a mission to Neptune’s icy moon Triton, often billed as a potentially habitable world in the chilly outer reaches of the solar system. Though Triton’s surface is icy, NASA’s Voyager 2 probe revealed that it’s highly active, too, and may even boast its own atmosphere. During a single, close flyby, the probe would map the moon’s surface and inspect it for a subsurface ocean, an exciting feature that’s been hinted at by past studies.

“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, says in a statement. “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.”

NASA has awarded the teams behind the four missions with $3 million to embark upon nine-month studies to expand upon their proposals, which they’ll resubmit to Discovery around year’s end. No more than two of the missions will make the cut, and NASA will announce its decision sometime next year.