I like to keep my mental model of the universe updated as much as possible. One of the things I learned not too long ago about the universe is the frequency of “rogue” planets. These are planets that are not bound to any star. The dominant current theory is that most of these rogue planets formed in stellar discs just like bound planets but then at some point in their history interacted with another large body and were flung out of their stellar system. In fact, most planets that form in a stellar system may have unstable orbits that will doom them over the cosmological short term to either collide with another body, fall into their parent star, or get flung out of the system. Only those planets with stable orbits remain, and so eventually systems will settle down with a relatively few planets in stable orbits.

How many rogue planets (also called FFPs for free floating planet) are there in the Milky Way? We don’t know, and estimates vary wildly. The most cited recent estimate based upon computer simulations suggest that there are at least 50 billion rogue planets in the Milky Way. This is not much, actually, as there are between 100 and 400 billion stars, so that’s less than on per star. Still, that is a lot of rogue planets wandering the vast darkness of interstellar space. But other estimates of the number of rogue planets are much higher. At the high end one estimate put the number at 100,000 times greater than the number of stars. That seems implausible. But it suggests the number could be much higher than 50 billion. Even if on average there are several rogue planets for each stellar system, that would create hundreds of billions of such planets in our galaxy. And we have to include all the dead stars whose planets are still roaming interstellar space.

As an aside, any such estimate has to include a size parameter – how many FFPs of size greater than or equal to X. The estimates above use Pluto as the lower end of the size range. The lower you go, however, the greater the number.

What we need is some observational data to supplement the theoretical simulations. If we can do a survey looking for rogue planets then the number of such worlds we find can be plugged into our simulations to come up with better estimates for the total number. But how, you may be wondering (if you don’t know already) can we find a cold dark planet at the vast distances of interstellar space? Right now we find exoplanets by using a few methods. One method is the transit methods – we look at the dip in light output from a star as a planet passes in front of it. Another is the wobble method – we look for the wobble in the path of a star that indicates it is being tugged by a large planet going around it in its orbit. We might also be able to directly image a planet by looking at the reflected light off of it and removing the glare from the parent star. None of these methods will work for a rogue planet.

The solution, in a word, is microlensing. Einstein once again comes to the rescue with his theory of general relativity. When any large mass passes in front of a more distant light source, like a star, the gravity of that mass will bend the light from that star (like a gravitational lens). We can see this effect dramatically when we image distant galaxies that are “behind” (relative to our perspective) a large gravitational source. We might see what is called “Einstein’s cross” as the more distant light source is bent around the gravitational lens producing multiple images that our telescopes see.

A microlensing effect is much more subtle, and can be produced by the mass of even something as small as a rogue planet. Such microlensing effects, however, require that a rogue planet pass right in front of a distant star. The alignment has to be very exact to work, and will only happen once. Nothing is orbiting, so this will not be a recurrent event. Since individually these events are so rare, in order to find them we must look at millions of stars at once and wait for a rogue planet to pass directly in front of one them. This is exactly what we are now doing, and the hope is we will get a much better estimate of the number of rogue planets out there.

One such survey, the OGLE survey, has recently bore fruit, finding a rogue planet that is Earth-sized:

This is why modern surveys hunting for gravitational microlensing events are monitoring hundreds of millions of stars in the Milky Way center, where the chances of microlensing are highest. The OGLE survey – led by Warsaw University astronomers – carries out one of such experiments. OGLE is one of the largest and longest sky surveys, it started operations over 28 years ago. Currently, OGLE astronomers are using a 1.3-meter Warsaw Telescope located at Las Campanas Observatory, Chile. Each clear night, they point their telescope to the central regions of the Galaxy and observe hundreds of millions of stars, searching for those which change their brightness.

There most recent discovery is the Earth-sized FFP. But the real goal of this survey is to estimate the number of Rogue planets in the Milky Way. I can’t wait to update my mental map of the universe with this number. But beyond just knowing this fact, I also have to wonder what the potential is for rogue planets to harbor life. At first it seems that the probability is quite low, since they are not near any star and so would be frigidly cold, too cold for organic reactions. But this does not mean the odds are zero. First, many such worlds, especially if they are large enough, will have internal heat (like the Earth), which can be maintained for billions of years, especially if they have radioactive decay (like Earth). Rogue gas giants may have lots of internal heat, and large magnetic fields. I don’t know how plausible it is for a rogue gas giant to have retained a large moon, but if so those moons could be warmed by their giant parent, and tidal forces could keep them quite balmy.

These would be exotic worlds, no doubt, and not have an environment anything close to Earth. But life, as someone famously said, finds a way. It is not implausible that we might find life on a rogue world, but I suspect it would mostly be subterranean bacteria-like life, but who knows.