Miraculous Conception: Are Planets Forming Without Stars? | The ...

Miraculous Conception: Are Planets Forming Without Stars?
One of the most remarkable discoveries about exoplanets—planets that exist outside of our solar system—is that some of them appear to be flying through interstellar space on their own. They’re not orbiting a star, nor are they tied to any other obvious companion.
The precise number of these planets—alternately called “free-floating,” “Steppenwolf,” “unbound,” or “rogue” worlds—is debatable, but it could be enormous. Some data gathered from gravitational-microlensing surveys suggests that there could be one to two times as many of these interstellar voyagers as there are stars in the Milky Way, which would be a number in the hundreds of billions.
So where do they come from? Previously, it seemed likely that these unbound planets had parent stars and were formed in solar systems. Stars are born when a globule of interstellar gas and dust condenses as its own weight causes it to collapse and shrink inward. The core accumulates the lion’s share of raw matter, while the rest swirls around in a vast orbiting disk. Over the course of ten million years or so—the blink of a cosmic eye—this central proto-stellar nub becomes so massive and hot that it ignites the nuclear fusion of hydrogen and becomes a bona fide star. However, some of the cooler matter in the surrounding disk also condenses and coagulates, producing planets—detritus and crumbs of unused muck orbiting this new sun. It’s a process that we think is similar to what happened in our own past, some five billion years ago; as a scientific idea, it’s one that goes back to eighteenth-century thinkers like Laplace and Kant.
This birthing procedure can place planets in a curious spot: nature, producing planets in abundance, occasionally leaves them in orbits that are too close to each other. Dynamical instability—gravitational chaos—can eject them out into space, a larger-scale version of the gravitational slingshot we sometimes employ to fling probes deep into space. Indeed, some studies have suggested that at least seventy-five per cent of all youthful planetary systems might go through the sort of orbital rearrangement that squirts objects out to the surrounding void.
Now, though, astronomers observing the Rosette Nebula—a great interstellar cloud some five thousand light-years away—have found that some planet-mass objects might actually form alone, without ever having a parent star. Infrared light and its more penetrating, lower frequency neighbor, sub-millimeter light, let them peer into the thick, gassy stew of the nebula to spot baby planets in an embryonic state. These “globulettes” of dense nebular material appear to be dwarf versions of the globules of molecular gas and dust that we know form stars. Careful measurements reveal that the density of these globulettes is such that gravity ought to squeeze them in on themselves; the estimates of their masses indicate that they can’t be forming stars, but they could, and perhaps are, forming objects the size of large-ish planets.
What’s even more intriguing is that these globulettes are part of an expanding, moving complex of gas and dust that ensures any planet formed this way will speed ever outward from the Rosette Nebula. Right now, they’re zipping along at about fifty thousand miles an hour, more than fast enough to escape the neighborhood and head out to the rest of the galaxy.
The Rosette Nebula is just one of hundreds, if not thousands, of similar clouds of dense interstellar gas and dust spread throughout our galaxy right now—and many more have come and gone over the past ten billion years or so. The globulettes seen there may represent only a fraction of a population that could have cast big planets throughout interstellar space in the kind of numbers that other surveys are pointing to.
This case of apparent miraculous conception overturns some of our most widely held assumptions about the origins of planets, and offers a peek into what may be a veritable underworld of cool, dark objects whizzing around our galaxy. It’s a surprising marker in the long road to understanding the extraordinary degree to which the Milky Way makes planetary bodies, and the incredible diversity of these new worlds.