We normally think of planets as being synonymous with gas giant or rocky worlds orbiting a parent star. And as far as stars go, the Milky Way is lined with hundreds of billions of them, each with their own unique and varied stories of birth and history. Some are massive and bright, some are smaller and dim; some were born only a few million years ago, others are nearly as old as the Universe itself. But there is one thing that practically all of them are expected to have in common: solar systems. As the Kepler mission and other exoplanet studies have shown, if you want to find planets, simply pick a star and look around it: you’re bound to find not just one but a whole system of planets.

But beyond that — in addition to the stars and all the bodies that orbit them — there ought to be a huge number of planets with no central stars at all: the rogue planets of our galaxy. We think this is true everywhere in the Universe, from small star clusters to interstellar space to the cores of giant galaxies. As best as we can tell, there are at least as many starless planets wandering the cosmos as there are stars, and probably many more than that. This means that, for every point of light you see, there are a great many more massive points that exist that you don’t see, since they emit no visible light of their own.

Observationally, we’ve recently discovered a number of possible rogue planet candidates. “Candidate” is an important word; we can’t be certain these are true planets because there’s no good verification technique. They’re so difficult to detect even with our best modern equipment (and, even at that, are only visible from their very faint heat signatures in the infrared) that we fully expect that there must be many, many more than what we’ve seen so far. Still, the fact that they’re so difficult to find combined with the fact that we’ve still found a good number of likely ones is promising. If you’re at all curious, you can’t help but wonder where these rogue planets come from!

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