5 modern revolutions in planetary science

I’m a planetary scientist who was born the year of Sputnik’s launch. Over the course of my 40-year career, I’ve witnessed not one but at least five separate revolutions that fundamentally transformed this field of study. 

Before I tell you about these pivotal moments, let’s look at two examples of even earlier revolutions that set the stage for the field I entered.

Early revolutions

To me, the first “modern” transformation in planetary science occurred in the early 19th century when it was discovered that it wasn’t just a single body (1 Ceres, discovered in 1801) orbiting in the gap between Mars and Jupiter, but a whole zoo of such worlds. These ultimately came to be collectively known as the asteroid belt or main belt. 

This addition rocked both the geography of the solar system and also our knowledge of its content, forcing the realization that most solar system bodies are not planets, but small bodies. 

Another early but utterly fundamental transformation in the field was the development of spaceflight in the mid-20th century. This epochal advancement turned planetary science from a backwater of observational astronomy into a major field of its own. Spacecraft enabled us to study solar system bodies with in situ techniques like magnetic sounding and atmospheric and surface sampling. It also delivered imagery with resolution orders of magnitude beyond what is possible from Earth. This, in turn, opened up the fields of planetary geology, planetary meteorology, and other research domains that had been unable to advance via remote sensing from ground-based observatories alone.

I entered grad school and began regularly publishing research results in the mid-1980s. Using that as a start date, I count five truly epochal transformations that have washed over planetary science since then. All five shook the field to its core. 

Here’s how I’ve ranked them, in order of importance.

Tell us your top five

You might be thinking that your list of the top revolutions in planetary science over the past few decades is not the same as mine. That’s just fine! In fact, we’d like to hear from you so we can compile a “readers’ choice” top five and runners-up list, too. Head over to www.astronomy.com/poll and take part in our poll, which we’ll use to create the final reader’s choice list. Have at it, Astronomy hive mind!

1. The discovery of the Kuiper Belt

The greatest single revolution in planetary science during during my time happened in the early 1990s with the discovery of the Kuiper Belt, the third zone of our solar system after the terrestrial planet and giant planet regions.

I rank this at the top because it transformed the field in so many important ways. Like the much earlier discovery of the asteroid belt, the discovery of the Kuiper Belt redrew the map of our planetary system — but in this case, even more fundamentally. The Kuiper Belt enlarged the planetary region dramatically and revealed the source of almost all short-period comets. It also showed that Pluto was not a misfit planet that didn’t fit in with the terrestrial and giant planets, but instead was the harbinger of a whole new class of so-called dwarf planets, which we now know outnumber the terrestrial and giant planets combined. And the Kuiper Belt’s orbital distribution of bodies crucially also showed us unequivocal evidence that the giant planets migrated dramatically from where they formed to where we see them today. 

The discovery of the Kuiper Belt transformed both the architecture and population of our solar system from the quaint 20th-century model of nine planets and an asteroid belt to a far larger, far richer, and far less static solar system that no one had anticipated.

2. The discovery of exoplanets

My second-place planetary science game-changer in the past four decades also occurred in the 1990s: the discovery of exoplanets. 

Thanks to the rapid advance of this research area since then, we have now identified nearly 6,000 exoplanets. But more importantly, we now know that exoplanets are common to virtually all stars and that planets far outnumber the galaxy’s stars. 

We also now know that the architecture of our solar system — with four terrestrial planets near the Sun, followed by an asteroid belt and then four giant planets, with a surrounding primordial debris belt (the Kuiper Belt) harboring a variety of dwarf planets — seems to be far from typical. In fact, as more and more exoplanets have been discovered, we’ve learned that there are many new types of planets not anticipated from studies of our own solar system. These include planets circling pulsars, Jupiter-like worlds close to their stars, “fluffy” low-density exoplanets (as light as balsa wood!), super-Earths, and more.

Many of you and my planetary science colleagues might consider the discovery of exoplanets and their ubiquity to be the No. 1 planetary science game-changer of the past few decades. But it lands at my No. 2 because these bodies lie outside our solar system and can only be studied by astronomical techniques (as opposed to by spacecraft that can visit and study them up close).

3. The discovery of ocean worlds

When 20th-century planetary astronomers looked out across our solar system using the tools of their time, they saw no other worlds with oceans and concluded that Earth was unique for this feature. This remained the paradigm for more than a generation.

Until it crumbled. Beginning in the late 1990s, a series of spacecraft missions revealed that the solar system does, in fact, contain other ocean worlds. But these oceans are not like ours because they lie underneath (rather than on top of) their world’s crust. 

This revolution began when NASA’s 1990s Galileo orbiter found unequivocal evidence for a water ocean beneath the surface of Jupiter’s moon Europa, based on induced magnetic fields in the moon. By the mid-2000s, NASA’s Cassini mission had also found saltwater plumes emerging from Saturn’s moon Enceladus, irrefutable evidence for a subsurface ocean there as well. Today, we see strong evidence for subsurface oceans in many other icy satellites and also among the dwarf planets, ranging from Titan to Triton, Ceres to Pluto,
and others. 

With the discovery of so many ocean worlds, the possibility of the development of life in them has driven astrobiologists to place these worlds high on the priority list for more detailed study. This push has led to missions like NASA’s Europa Clipper and the European Space Agency’s JUICE, which will explore the Galilean satellites.

The discovery of ocean worlds across our solar system also challenged the notion that life can exist only within a Goldilocks-like, narrow habitable zone near our star, turning that paradigm on its head as we discover potentially habitable worlds at least as far out as the Kuiper Belt. The 20th-century view that Earth was our solar system’s only ocean world has now been shattered; all these other ocean worlds with water beneath their surfaces simply escaped detection until spacecraft found them through in situ investigations. As a result, we now know that ocean worlds are common in our planetary system, and we suspect the same is true across the universe. 

What is uncommon in our solar system, it now turns out, are worlds that wear their oceans on the outside like Earth — another idea never anticipated before this revolution.

4. The rise of PI-led missions

It’s worth saying that not all of the planetary science revolutions I have witnessed were game-changing discoveries or shifts in scientific paradigms. There have also been two major changes in how planetary science is done.

The first of these was the rise of missions led by a PI — a principal investigator, or scientist. This is now the dominant mission type in U.S. planetary science, but prior to the 1990s, no planetary scientist led a space mission to any world, no matter how close or far, no matter how small or large. Instead, all planetary missions were led by project managers, or PMs. The science team — usually led by a single project scientist — reported to the PMs, who were not scientists but instead were engineers or even professional managers. This often put science in the back seat because the mission’s science objectives were throttled by non-scientists at the helm. 

However, starting in the late 1990s, NASA began developing a new kind of mission, led by a PI scientist to whom the PM (and everyone else) reported. It was seen as a smarter way to do science missions. NASA has now selected nearly 30 PI-led missions in our solar system, all but one of which have been successful. Compare that with just four non-PI-led NASA planetary missions chosen in the same time frame. 

Through NASA’s Discovery and New Frontiers programs, the PI-led paradigm has almost completely replaced older PM-led missions. Putting a scientist in charge has transformed how planetary science is both led and accomplished.

5. The rise of commercial spaceflight

The other factor that has changed how planetary science is done is the rise of commercial spaceflight. This has already transformed the launch business by making launcher reusability routine, significantly lowering price points and dramatically raising launch rates. These changes have opened up a wide variety of new use cases in the space economy.

Commercial involvement is also transforming the way space missions are tracked and how they communicate with Earth, by developing ground- and space-based tracking and communications networks that are replacing traditional government networks like NASA’s Tracking and Data Relay Satellites and ground stations. This has vastly increased network bandwidth capacity while also reducing mission costs. 

Equally important are the first glimmers of commercial interplanetary space missions. So far, we have seen the first small commercial missions planned or operated to the Moon, Venus, and asteroids while NASA has called for innovative commercial solutions to restructure its dysfunctional Mars Sample Return megaproject. 

Simply put, the commercial space revolution has sparked a sea change in how missions are completed by replacing the older government-only mode that opened space in the first era of exploration beyond Earth. The development of commercial spaceflight, in many ways, parallels the revolution in computing that occurred when old-school mainframes gave way to proliferated, less expensive, and more capable PCs. We can’t really predict how commercial spaceflight will transform planetary exploration in the coming decades, but I suspect this revolution is only in midstride. 

The runners-up

There were seven runners-up to my top five list. Here they are, in no particular order.

• In the 1980s, we discovered that Saturn’s rings are not unique. In fact, rings are common among all the giant planets of our solar system, and even (as we discovered in the 2010s) extant around small bodies like Centaurs and dwarf planets. 

• Also in the 1980s came the general recognition that Earth’s Moon was formed by a giant impact with a planet-sized intruder that struck Earth to spall off Luna. Moreover, we accepted that impacts like this also helped shape Mercury’s present-day structure, formed the Pluto-Charon binary, and may have tilted Uranus and Neptune’s poles. 

• We have discovered numerous satellites around asteroids, not one of which was known before the Galileo mission’s groundbreaking discovery of the moon Dactyl orbiting the small main-belt asteroid 243 Ida in 1991. 

• There is now strong forensic evidence that the giant planets of our solar system migrated considerably into their current positions after forming, and that a fifth giant planet might even have formed among the four giant planets we see today. 

• I also considered the discovery that the Moon’s poles possess large quantities of water ice, something conjectured but not proven until the 1990s and 2000s. 

• Scientists rarely travel to use big telescopes anymore, even though there are now many more of these facilities than in the past. Instead, most large telescope work is done remotely, while sitting at home! 

• We’ve discovered that small planets like Pluto can remain wildly geologically active more than 4 billion years after their formation. This finding broke the paradigm that isolated smaller worlds generally cool off and all but die, geologically speaking, not long after forming.

What’s next? 

This retrospective now begs the question of what the biggest discoveries of the next 40 years might be. Experience teaches us that any such predictions are likely to be wrong, or at least naïve. Nonetheless, I’ll venture to suggest some possibilities.

In terms of scientific discoveries, perhaps we will find life on one or more of the solar system’s ocean worlds. Or maybe we will uncover multiple planets within the outer Oort Cloud, which lies far beyond the Kuiper Belt. Or both.

The way we do research will likely change as well. Just as most terrestrial geologists are now employed by private sector resource and energy companies, commercial space could eventually employ more planetary scientists than academia and government agencies combined.

Other changes to come could be that advanced propulsion dramatically shortens trip times across the solar system, making its worlds more quickly and routinely accessible. And we might find that humans will be working in many locales around the solar system within the next few decades, and that artificial intelligence is doing much of the scientific discovery that was once only the provenance of humans.

Looking back over the time I’ve spent in this field, it’s hard to believe how much our view of the solar system and the way we should explore it have changed. It’s also hard to believe how the field of planetary science has grown. In the mid-1980s, there were perhaps 1,500 practicing planetary scientists in the world. Today, there are well over 10,000! 

The march of technology, the much greater workforce size, and the proliferation of countries and companies involved in spaceflight have all accelerated the pace of change in planetary science over the decades. I expect these trends to continue. And I can’t wait to see what the next few decades of discovery will bring!