The universe is filled with weird stuff, in every possible combination. Take planets. We are used to talking about them orbiting stars, and we are used to finding them by looking at how they interact with their star’s light. Planets don’t need stars, however, and during its extended K2 mission, Kepler peered at a field of stars toward the center of the galaxy. This dense region was picked as a high probability location for seeing rogue planets passing in front of background stars.

When two objects line up, the gravity from the front object can bend background light from the rear object – light that was meant for some other part of the universe and that we now get to see. This effect is minimal when one human lines up with another human, but it becomes noticeable when large enough planets align with stars. Planets have been found this way from Earth many times, and now we know Kepler could find planets this way, too. In a new paper in Monthly Notices of the Royal Astronomical Society (MNRAS) and led by Ian McDonald, four new free-floating planets have been found in this one field, and 22 planets were co-identified by Kepler and Earth-based surveys.

Let’s put this in perspective. Kepler looked at a patch of sky that contained many, many stars and an unknown number of rogue planets. All those stars and all those planets are ever so slowly moving through the sky as they orbit the galaxy. In 26 cases, the orbits lined up just right for the planet to magnify the amount of light we get. Since we know how many stars we’re looking at and we know how long Kepler looked, it starts to be possible to figure out how common this kind of alignment is. And the answer is that they appear to be very common. The research team teases that they’ll have more precise estimates in their next paper, but the question at this point is becoming “are there more planets orbiting stars or cast out among the stars?”

This raises all sorts of interesting questions about how many worlds solar systems start with, how many collide and merge, and how many simply get thrown away like so much unwanted rock or ice; the Universe isn’t picky. These worlds are destined to be cold, dark, and generally invisible except through their periodic gravitational interactions. While scientifically we can’t observe these worlds in a satisfying amount of detail, knowing they’re out there is food for thought as I dream of having time to write fiction. Here is to hoping someone dreams up a cool future for these lonesome worlds.

More Information

RAS press release

The University of Manchester press release

Astronomers Spot a Batch of Rogue Planets Near the Galactic Core (Gizmodo)

“Kepler K2 Campaign 9 – I. Candidate short-duration events from the first space-based survey for planetary microlensing,” I McDonald et al., 2021 July 6, Monthly Notices of the Royal Astronomical Society