How many rogue planets are in the Milky Way? The Roman Space Telescope will give us an answer

Over the past decade or so, astronomers have speculated about the characteristics of rogue planets in the Milky Way galaxy. These “free-floating” worlds don’t orbit stars, but instead roam the spaceways. They’re hard to spot with current technology, but the upcoming Nancy Grace Roman Space Telescope (Roman) will be a perfect instrument to find them and give insights into the history and features they may have in common with solar system worlds.

A recent paper has focused on interstellar planetary wanderers and looked at ways the Roman telescope will advance the scientific understanding of how they form, where they exist, and what types of planets they are. In particular, the paper looks at what’s called the “free-floating mass function” as a way to understand the origins of rogue planets. The planetary mass function describes the distribution of planetary masses around stars. The free-floating planet mass function does the same for rogue planets wandering the galaxy.

The study is published on the arXiv preprint server.

The authors suggest that—among other things—Roman could vastly improve on the existing measurements of the abundance of these types of planets smaller than Earth. It could also help characterize the number of higher-mass rogue planets and their distribution throughout space. Finally, its data could improve astronomers’ understanding of the processes that create and then eject these worlds from their birthplaces.

What are rogue planets?

The term “rogue planet” conjures up visions of such places as Hoth, Alderaan, and Endor (from the “Star Wars” universe). Those places orbit stars, whereas free-floating planets don’t do that anymore. A newly forming planetary system is a busy place. Bits and pieces of worlds (essentially protoplanets) move around and experience gravitational effects from other bits and pieces. Eventually, they combine with each other to form planets. The dynamics of formation are energetic enough that some newly formed planets get ejected into space, never to return to their parent systems.

Astronomers estimate that the Milky Way could contain millions or billions of rogue planets. If that’s true, then there could be many more rogue worlds than there are worlds bound to their parent stars. The rogues aren’t warmed by nearby stars, so astronomers suspect that most (if not all of them) are frozen, icy, empty, and uninhabitable.

Rogue planets aren’t very easy to spot, since they’re basically traveling “in the dark.” At the moment, one of the best ways to spot them is through microlensing. That happens as the planet passes in front of a star from our point of view here on Earth. The rogue’s gravity “bends” the light of the background star. That produces a little “wobble” in our view of the star and tells us something has just passed through the field of view.

How can Roman help find free-floating planets?

The Nancy Grace Roman Space Telescope will improve on the discovery of rogue planets by such instruments as the James Webb Space Telescope (JWST), which has discovered other rogue planets. The Roman telescope will perform a search called the Galactic Bulge Time Domain Survey, which will help astronomers detect several hundred to several thousand free-floating “rogue” planets. That should give researchers a good population to study to understand their masses and distributions throughout the galaxy.

That, in turn, should provide a better understanding of how and where rogue planets are born. Essentially, Roman’s data will fill in the “demographics” of these worlds.

Although all rogues are tough to spot, the population of low-mass planets would be particularly tough to find. These are worlds smaller and less massive than Earth. They probably form in protoplanetary disks as their larger siblings do, but are small enough that it wouldn’t take much of a kick to eject them into space.

Larger and more massive worlds would be less likely to get kicked out early in the process. The existence of rogue “big brothers and sisters” says something about the processes needed to eject them into space, since they’re more massive and theoretically require more energetic “pushes” to interstellar space.

Roman’s search for exoplanets will use the transit method as well as microlensing. The transit method is a fancy way to describe what we see when one object in space passes in front of another one. What we see is a ‘dimming’ of the light from the background object. The length of the dimming, as well as the depth of it (how much light is blocked), give clues to the nature of the foreground object. Microlensing also involves an “eclipse” of one object by another, but astronomers then detect the gravitational warping of the light from the background object.

Roman is nearly complete, but won’t be launched for a couple of years. Even so, astronomers are already predicting that its observations of rogue planets as well as other non-luminous bodies (primordial black holes, anyone?) will revolutionize our understanding of the objects populating our galaxy.