Prodigal Planets

Astronomers have made many exciting discoveries in recent years. For example, we now know—thanks to NASA's Kepler space telescope—that our Milky Way galaxy hosts hundreds of billions of planets; a fact that was mere speculation just a couple of decades ago. We also know that far beyond Pluto, our own little solar system contains hundreds of thousands of proto-planets that orbit the sun. (In fact, Pluto itself was recently downgraded to a “dwarf planet,” losing its role in our solar system's eight-member planetary family.)

Many other recent discoveries are being added to the list. For example, the Juno spacecraft now orbiting Jupiter is giving us a far greater understanding of that giant planet. The Cassini mission, now in its 12^th year flying around Saturn, is revealing many discoveries that are changing our understanding of how planetary systems form and what conditions might lead to life in the universe. Black holes have been discovered in far-off galaxies. Gravity waves have been measured. The list goes on. It's an exciting time in the cosmological world.

A new discovery that I will focus on here is the recent finding of free planets that wander alone in our galaxy—unattached to any star; not a part of any solar system. Until recently, the reality of these prodigal planets was, like planets around other stars, mere speculation. But now a few have been observed. It's very difficult to spot these dark wanderers through interstellar space. They cruise through a black void, nearly invisible. Recent advances in telescope technology are allowing us to locate them.

So now we've spotted several of these solitary worlds—enough to give astronomers the confidence that these free-floating planets are quite common. In fact, there are probably hundreds of billions of them wandering alone through the Milky Way.

The mystery: how did they get out there? Aren't planets formed only around stars? In fact, these lonely planets were created in a similar manner to the planets in our own solar system: through a gradual accumulation of bits of rock and dust and gas surrounding a newly-formed star. But then something like a gigantic slingshot action occurred, as one of them gravitationally encountered a huge planet in its solar system, flinging the smaller planet out into space—destined to become a lonely body free of its originating solar system; forlornly meandering through the ages.

Are these prodigal planets destined to be forever lonely and relegated to being just a cold pile of rocks? Could they ever possibly host life? As improbable as it seems, they theoretically could harbor life. Some scientists speculate that, despite having no star to provide warmth, conditions just may be that warm-seeking life could have come into existence in such a forbidden environment. The scenario is completely conjectural, but radioactive decay deep within such a planet could provide the required warmth. Or a thick covering of ice could hold in the necessary heat.

It's all guesswork, but the point is that, before the discovery of these prodigal planets, we had no idea that they even existed. Now we now they do. The fascinating thing is that they conceivably could harbor life. What's more, they are far more accessible to spacecraft from Earth than planets around distant stars, since they float alone, and some of them might be closer than planets still accompanying their star. We would just need to locate a couple of them and send spacecraft out there to investigate.

Who knows what we might find next in outer space? Just a few years ago we had no confirmation that billions of planets are out there in our galaxy. We had no idea that moons of Saturn or Jupiter just might offer conditions conducive to life. Cosmology is advancing at a breathtaking pace. Stay tuned.