This artist’s concept shows an icy  Earth-mass rogue planet drifting through space alone, without any relationship to a star. (NASA’s Goddard Space Flight Center)

In our Earthling minds, planets exist in solar systems with a Sun in the middle and objects large and small orbiting around it.   This is hardly surprising since planets are pretty much exclusively illustrated in solar systems and, until the onset of the 21st century, no other kind of planet had been identified.

That changed in the last two decades with the discovery of “rogue planets” very large and now quite small — all apparently isolated object speeding through interstellar space and unattached to any Sun or solar system.

The earliest rogue planets identified were large Jupiter-type planets or even larger brown dwarfs, which have masses between that of a large Jupiter and a small star.

But since then smaller planets have been discovered while the estimated population size of rogue planets has ballooned.

Now, new research from NASA and Japan’s Osaka University suggests that rogue planets untethered to a star far outnumber planets that orbit stars in the Milky way.

These results imply that NASA’s Nancy Grace Roman Space Telescope, set to launch by 2027 with a goal to identify rogue planets, could find hundreds of the Earth-mass variety. Indeed, this new study has already identified one such candidate.

“We estimate that our galaxy is home to 20 times more rogue planets than stars – trillions of worlds wandering alone,” said David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a co-author of two papers describing the results.

“We found that Earth-size rogues are more common than more massive ones,” said Takahiro Sumi, a professor at Osaka University in Japan and lead author of the paper with a new estimate of our galaxy’s rogue planets.

The roughly Earth-mass rogue planet the team found marks the second discovery of its kind. The paper describing the finding will appear in a future issue of The Astronomical Journal. A second paper, which presents a demographic analysis that concludes that rogue planets are six times more abundant than worlds that orbit stars in our galaxy, will be published in the same journal.

Because of their location outside of solar systems and away from a warming Sun, rogue planets are considered quite unlikely to harbor life.  But with that diverse population of rogue planets, Sumi said,  they can play an important role in understanding planets generally.… Read more