Should geologists care what kind of planet Pluto is?
The subject of defining planets captured the attention of astronomers 200 years ago. Ceres, discovered in 1801, was briefly considered to be a planet of the same rank as Mars, Jupiter and the rest of the familiar tribe. But the next year Pallas was found in the same part of the solar system. William Herschel, the world's foremost astronomer, dubbed these small objects asteroids, unworthy of being considered planets.
That was that until a few years ago, when new objects discovered in the outer solar system raised new questions. Then everyone started talking about Pluto. When Pluto was discovered in 1930, people remarked that it was a puny little thing compared to Neptune, but soon schoolkids learned to recite its name. Seventy years later we began finding other Pluto-like, Pluto-sized bodies in the same part of the solar system. With no modern-day Herschel weighing in, the International Astronomical Union had to name them something.
But geologists don't care. Well, the handful who work on planetary formation care, because they need to know the terminology. The planetary bodies geologists are interested act like Earth in some way, and many of those are outside the bounds of the proper planets.
The Modern Planet
No one ever thought of Earth as a planet until Copernicus set it circling the Sun in the mid-1500s. Before that, Earth was simply the world. Though geology emerged as a discipline around 1800, geologists knew little about Earth-the-planet for another century. But astronomers knew next to nothing about the other planets either.
Happily, geologists and astronomers got serious at the same time: the 1960s. Spacecraft were sent to survey the Moon, Venus and Mars; in 1962 a planetology section was established in the American Geophysical Union. In 1965 Mariner 4 returned its epochal photos of craters on Mars. On Earth, a year later, plate tectonic theory began to coalesce. With the ability to examine planets from space and a new set of questions to ask about them, modern planetology was born all at once.
Leading Geologic Questions
Until recently, geologists had the most fruitful research questions in planetology, and naturally they involved the solid, terrestrial planets Mercury, Venus, Earth and Mars:
- What do these planets' surfaces tell us about their interiors?
- What geologic processes do they share and which are unique?
- What do the planets tell us about the history of the solar system?
As space probes went farther, other solid bodies came under scrutiny: the large satellites of the outer planets, the asteroids and the comets. Many of these bodies have undergone internal processes that can only be called planetary: differentiation, convection, volcanism, faulting, hydrothermal activity. Titan has lakes, Europa an ocean, Ganymede has an iron core, little Enceladus has geysers. The asteroid Vesta is now often called the smallest terrestrial planet. When it comes to geologic processes, the traditional planets do not have a monopoly on "planetness."
Geologists don't have a formal preference for what constitutes a planet, but let me borrow an idea from the field of fossil studies. If Earth can be taken as the type example of the planet, then planets are bodies with solid surfaces and relatively thin or absent atmospheres. They are round, predominantly shaped by gravity. And inside, they are or have been active in some sense, whether that activity is melting, convection or differentiation. That notion would extend from the larger asteroids and moons up to something short of Neptune, which is better thought of as a gas-dominated object—or gassy planet, I'll grant that.
Astronomy Resurgent
If geologists no longer observe a strict boundary between planets and nonplanets, astronomers have opened up a new arena of science where the terminology matters again. Just as in 1801, observers have found many new bodies in the solar system. Just as the asteroids are relatively tiny bodies just beyond the terrestrial planets, so the new trans-Neptunian objects are relatively tiny icy bodies just beyond the gas giants. Many workers lumped Pluto in with these and called them all plutinos. The trouble is that "relatively tiny" compared to Neptune is larger than some of the inner planets.
At the same time, cutting-edge observations have been detecting planets around other stars—well, we all call them planets. There are now hundreds of them on the books. The favorite name for them is exoplanets. So far, all of them are unlike anything in our own family of planets. But eventually our techniques will begin to uncover smaller, more ordinary planets—there I am, using that word again.
The astronomical community had a big debate in 2006 about how to label the new spectrum of objects. In the end they made Pluto a "dwarf planet," lumped in with the asteroids and other distant objects. Between boulders and brown dwarfs is a wide range of cold bodies. As far as geologists are concerned, the best solution is to call most of them some type of planet.
