Astronomers estimate that there are more free roaming planets in our galaxy than there are planets in orbit around stars.
In fact, rogue exoplanets - planets with no star whatsoever - far outnumber all other planets in our galaxy, by 20 times. Trillions of worlds wandering alone.
Astronomers have reached this conclusion based on the results of a nine year survey that looked at special events that occur when an object such as a star or planet comes into near-perfect alignment with an unrelated background star from our vantage point.
Because anything with mass warps the fabric of space-time, light from the distant star bends around the nearer object as it passes close by. The nearer object acts as a natural lens, creating a brief spike in the brightness of the background star’s light that gives astronomers clues about the foreground object that they can’t see any other way.
Known as microlensing events, they are our primary method of observing free-floating planets in our galaxy. Since they do not shine light on their own, planets are usually observed by looking at their reflected light of the star. Even then, seeing exoplanets is hard. We have to block out the light from the star to see anything at all, something we’ve only just been able to do.
Another big way of seeing exoplanets is to infer their existence with dips in brightness from transits, or gravitational wobbling as stars were pulled around by their planets.
All of this means that seeing exoplanets is a difficult business, but it’s not impossible. Microlensing is one more technique in the arsenal of astronomers to detect these small, dark worlds. We are using gravity to discover objects we would have no hope of seeing directly.
The nine year survey recently completed by astronomers found one Earth-sized rogue planet and the second discovery of its kind. The study also analyzed the survey data and concluded that rogue planets are more abundant than all other planets in our galaxy.
Why? Astronomers reason that rogue exoplanets tend to be smaller, Earth-sized or less. Since they are small, they are more weakly tethered to their host star, and could be pulled away easier during gravitational encounters with other objects in the galaxy. Another reason lies with the fact that planet building is a chaotic business that could easily see Earth-sized bodies thrown out of a nascent planetary system.
Finding exoplanets in microlensing events is extraordinarily rare, the stars literally have to line up perfectly for us to be able to see them. Each microlensing event is a one-time occurrence, meaning astronomers can’t go back and repeat the observations once they’re over. So the best thing to do is look at large areas of the sky at once for a long time, to try and detect these small blips.
So to test these results, astronomers are looking forward to the launch of the wide-field Nancy Grace Roman Space Telescope in May 2027. Roman will be sensitive to even lower-mass rogue planets since it will observe from space, and it is estimated that it will find 400 rogue planets when it begins its survey of the heavens.
Working with ground-based telescopes these observations will help scientists measure the masses of rogue planets much more accurately than ever before, deepening our understanding of these dark nomads that grace our galaxy.