Because of its immense practical value for agricultural societies, many early cultures developed something that resembled a science: the observational study of the motion of bodies around the solar system. Four hundred years ago, planetary science also became the first to have a solid theoretical underpinning, as Kepler produced a model of planetary motion that accounted for observations and was predictive. But, according to a review of planetary science published in Nature, the actual science languished for centuries until work in an unrelated field spawned the electronics revolution.

The review's author, Joseph Burns of Cornell University, suggests the key contributor to the stagnation was in the limitations of ground-based observatories, which couldn't resolve detailed features on most of the solar system's other bodies. Even as telescope technology progressed, the only object we could study in any detail was the Moon, which was cold, dead, and lifeless, with a rugged geology dominated by impacts. Venus' surface was hidden by clouds, while Mars, at this distance, seemed to many to be a reddish version of the Moon, although others engaged in flights of imagination, seeing the planet as a water-covered Eden.

So for many centuries after the first discoveries of early astronomers, Burns says that telescope technology ruled the day, with better optics (and a bit of orbital mechanics) aiding the identification of Uranus and Neptune, the recognition that comets were orbiting bodies, and the identification of the first asteroids. Although these discoveries helped refine Kepler's ideas, they shed little light on the nature of the bodies, and the lack of a coherent picture left us without anything resembling a theory of how the whole thing could have originated.

That left us in a rather bewildering place in the middle of the last century, as Burns recounts: "As late as 1966, reputable scientists argued over whether vegetation might cover Mars. Ten years before, scientific opinion was split on whether Venus was covered by a desert, a swamp or an ocean. Lunar craters, the only such structures observed in the Solar System but for a handful of terrestrial examples, were believed to be volcanoes until 1950."

Oddly, planetary scientists didn't clear up this mess—in fact, Burns argues that the field didn't really exist until NASA decided it needed to create the discipline in order to have someone understand the data its spacecraft were producing. Instead, a combination of technology and geopolitics changed our perspective on the solar system.

The Space Race

Geopolitics fostered the Cold War Space Race, which quickly sent US and Russian probes to orbit, the Moon, and the nearby planets. These missions were enabled by technology that came from the electronics and computing revolutions, which enabled unmanned probes to give humanity an indirect presence at many planets. But computing power played another key role: as various probes sent back data from different planets, the burgeoning community of planetary scientists fed it into increasingly sophisticated computer models, which grounded many findings in theory (a great example of this is the formation of the moon). The same computing power has also allowed the construction of massive compound telescopes with adaptive optics, providing a better view of the other planets, even when there is no hardware present in orbit.

In some cases, the first of these probes was a revelation. Mariner 2 flew past Venus in 1962, and provided the first measurements of the planet's temperature, which ultimately changed our understanding of how atmospheres and geology can interact. For others, knowledge took decades to build, like the realization that Mars was a dynamic planet that had experienced a water-filled past.

Even as the Space Race ended and NASA's budget became an exercise in political compromise, the agency launched what may be one of its greatest successes ever: the grand tour of the outer planets performed by the Voyager spacecraft. The stunning finds of the Voyagers—volcanoes on Io, possible liquid water on Europa, the dynamic rings and moons of Saturn—both captivated the public and overturned many expectations among the scientific community. The combination was critical for the launching of two follow-up missions, Galileo and Cassini.

Big surprises

Burns includes some of the biggest surprises that came with the new discoveries, and two of these stand out. The first is the finding that small bodies, which were once thought have cooled rapidly and then remained static since the birth of the solar system, have turned out to come in a huge variety of shapes and compositions, and bear clear evidence of repeated remodeling. These small bodies have provided information about different areas of the solar system, and preserved chemicals that were present at its origin.

The other is the realization that the solar system isn't a static, immutable collection of spheres. Instead, the solar system is chaotic: an early Earth collided with a Mars-sized object, destruction in the form of impacts by comets and asteroids still buffet all the planets, and the axes of rotation of the planets feel the pull of their peers, driving changes in planetary climates.

Planetary science may now be a mature field, but Burns suggests that its perspectives have continued to expand in recent years. The discovery that there may be liquid water on a number of solar system bodies has combined with the finding of organisms that thrive in extreme conditions on Earth to completely revise our understanding of the prospects for extraterrestrial life. The rapidly expanding catalog of extrasolar planets is finally enabling planetary scientists to leave the confines of our own solar system behind, and to consider all the probabilities and possibilities for planets throughout our galaxy.

Not bad for a field that NASA had to foster in order to meet its political imperatives.

Nature, 2010. DOI: 10.1038/nature09215 (About DOIs).

Listing image by NASA/JPL/SSI