Watching the Wanderers
published during a waning gibbous moon.

NOW.SPACE is pleased to announce a new monthly column by planetary scientist and New Horizons mission researcher, Michael Summers. This month, Dr. Summers invites you on a tour through history to learn about how our knowledge of planets has evolved with each new discovery.


Ancient Greek astronomy was founded by such pre-Socratic philosophers as Empedocles, Anaximander, and Anaximenes, developed brilliantly by Plato, Aristotle, and the mathematician Hypparchus, and reached its most systematic expression in the Almagest, written in the second century CE by the Alexandrian scientist, Ptolemy.

The word “planet” is traced to the ancient Greeks. The term was used to describe a few very special stars in the sky that appeared to move relative to the other stars, which seemed fixed in space. A more specific term, asters planetai (meaning “wandering stars”) was coined for a special set of planets: Mercury (when it could be seen near the setting or rising sun), Venus, Mars, Jupiter and Saturn. It was clear to the early astronomers that these “wanderers” were special, so they mapped their motions in detail. Little did they know that these objects were special beyond anything they could imagine at the time, and in fact, were much more plentiful than the stars themselves.

By the time of Kepler and Galileo, it was clear that the Earth too is a planet, and so it too is a wanderer. With the invention of the telescope, the planetary list grew to include Uranus, Neptune and eventually Pluto.

I consider myself fortunate to live at this special time in human history when we have just finished the first reconnaissance of the planets in the solar system with robotic spacecraft. I feel amazingly blessed to have participated in this exploration as a planetary scientist working with the NASA space program. For me this has been a dream come true. I cannot imagine a career more interesting than studying the wonderfully diverse planets in our solar system.

In the past half-century of space exploration, we have found that planets are not simple. The origin and evolution of the Earth to its present epoch has been a complex dance of the laws of physics and chemistry acting on the matter surrounding the sun, and punctuated by unimaginably catastrophic events. We have found that each planet in our solar system is a unique world with its own distinct history. Yet other planets have enough similarity to the Earth to invite the question: Is there life on any of these planets?

As we have explored the planets we have been surprised at every spacecraft encounter. We have become so addicted to the thrill of planetary exploration that we can’t help but expect to be surprised with every space mission. The experiences we enjoy in planetary exploration are teaching us the full force of the idea that the universe is not only more complex than we understand, but also more complex than we can understand!

Over the past two decades we have now found thousands of planets around other stars. From this we can estimate the number of planets in our galaxy. That number is jaw-dropping, no matter how many times it’s stated. There are many more planets in our galaxy than the number of humans that have lived on Earth in its entire history. That’s just our galaxy alone! In the visible universe there are more planets than the number of heartbeats of all of the people that have ever lived on Earth.

The planets we have discovered outside our solar system have been far stranger than anything we expected, and we are only just beginning to sample the breadth of their incredible diversity.

For example, we’ve found numerous planets made of rock and metals like Earth, but are much larger than the Earth. We’ve found planets made mostly of water, and some with average densities that appear to be less than that of water. We’ve found planets that are mostly carbon (diamond mantles, and perhaps liquid diamond cores!), and planets that are mostly metal. We’ve found planets cold enough to be superconducting, and some planets hot enough to have vaporized rock for an atmosphere. We’ve found planets around pulsars – the cinders of past supernovae—and planets shooting out of the central regions of our galaxy moving at speeds of thousands of kilometers per second.

And there is even a hint that most planets are not bound to stars. These “stealth worlds” wander about the galaxy on lonely paths. Whether these rogue planets are ejected cousins to the bound planets, or have been formed by some unknown process, it would be grossly incorrect to assume they are geologically dead. Stealth planets may too harbor their own flavor of emergent complexity such as life on Earth.

We are all interested in the possibility that we are not alone in this universe, and the habitability of planets is a central theme in the science of astrobiology. It is now obvious that all life, at least as we know it on Earth, will likely exist on planets. Stars are much too hot for the organic biomolecules needed for life’s processes, and the vastly more numerous comets and asteroids are too cold for liquid water – an essential ingredient for life. As far as we know, only planets have the vast amounts of raw materials and various types of available energy necessary to generate life as we know it on Earth.

We have found that planets and their moons are much more habitable than we ever expected. We know that three of the large moons of Jupiter have subsurface oceans of liquid water; in fact, Europa alone has more liquid water than in all the Earth’s oceans. At least two of Saturn’s moons have subsurface oceans. One of them, Titan, has dunes made of frozen hydrocarbon crystals that may lie on a surface floating above an ocean of water. Another of its moons, Enceladus, spews salty water into space via numerous geysers. This icy brine contains organic compounds processed by hydrothermal vents, which may be similar to Earth’s hydrothermal vents in the deep ocean where we believe Earth life to have originated. Even distant Pluto may harbor a subsurface ocean of liquid water. Surprisingly, most of our solar system’s stores of liquid water aren’t found on the terrestrial planets, but far outside the orbit of Mars!

These recent discoveries are coming at a fast and furious pace. This is forcing us to reconsider many things once thought obvious. What is a planet? What is life? What are life’s requirements? Is asking the question, “Is there life elsewhere?” even the right question? Maybe life as we know it is just one example of an infinite variety of emergent complexity in the universe.

And it’s clear that all of this that we’ve recently discovered is just the beginning. We are now discovering new planets at a rate of about one every two days. That rate will accelerate with the new telescopes coming online over the next decade. We are now routinely searching for life’s signatures in the atmospheres of planets around distant stars. Detecting that signature would be one of the greatest discoveries in human history.

We are learning about these numerous diverse worlds that the Greeks termed “wanderers” at an exponential rate. Major discoveries about exoplanets are announced almost weekly. If there is life on one of these worlds, we may find it soon!