We now know of almost 5,000 planets outside the Solar System. Imagine what life would be like on one these distant planets or exoplanets. If you are a Star Wars fan, you might include more than one parent star.
But scientists have recently discovered that more planets than we thought are floating through space all by themselves–unlit by a friendly stellar companion. These are FFPs, or icy “free floating planets”. How did they get there? What can they tell us about the formation of such planets?
Exploring more exoplanets has helped us to understand what a planet looks like. In particular, the line between planets and “brown dwarfs”–cool stars that can’t fuse hydrogen like other stars–has become increasingly blurred. It has been disputed for years what determines whether an object is either a planet, or a brown dwarf. Is it a matter of mass? If objects undergo nuclear fusion, do they cease to be considered planets? Is it more important to consider the method in which the object was created?
While about half of the stars and brown dwarfs are isolated, there are many star system ,. We tend to think of planets in orbit around a single star as subordinate objects. Recent technological advances have allowed us to see cooler and smaller isolated objects in space. This includes FFPs, which are objects that have too low temperatures or mass to be called brown dwarfs.
What we don’t yet know is how these objects were created. Stars and brown dwarfs are formed when a region in space of dust and gas begins to collapse in on itself. Gravitational collapse is the process by which this region becomes dense and so more material falls onto it.
Eventually, this ball of gas is dense enough to allow nuclear fusion to begin–hydrogen burning in stars’ cases, and deuterium burning for brown dwarfs . FFPs can form in the same manner, but they will never be large enough to allow fusion to begin. Also, it is possible for a planet to start life around a star but eventually get kicked out into interstellar.
How to spot a wandering planet
Rogue planets can be difficult to spot due to their small size and low temperatures. Their only source for internal heat is the energy that was left after the collapse that caused their formation. The faster that heat is radiated, the smaller the planet.
Cold objects emit less light than other objects in space, and their light that does emit is more reddish. The visible spectrum is where a star like the Sun’s peak emission occurs, while the FFP peak is in the infrared. Many of these planets were discovered indirectly using “gravitational microlensing”, which is when distant stars are in the right place for their light to be gravitationally deformed by the FFP.
However, the downside to detecting planets through a single, singular event is that we won’t be able to observe it again. We don’t have the ability to see the planet in its context, so we are missing vital information.
To observe FFPs in person, it is best to capture them young. This means that they still have some heat from their formation so they can shine brightest. Researchers did this in a recent study.
The team combined images from a large number of telescopes in order to find the faintest objects within a group of young stars, in a region called Upper Scorpius. They used data from large, general purpose surveys combined with more recent observations of their own to generate detailed visible and infrared maps of the area of sky covering a 20-year period. Then they looked for faint objects that moved in a way that indicated their affiliation with the group of stars, rather than background stars further away.
The group found between 70 and 170 FFPs in the Upper Scorpius region, making their sample the largest directly identified so far–though the number has significant uncertainty.
Based upon our current understandings of gravitational collapsing, it seems that there are too many FFPs within this group of stars to have all formed in the same way. The study authors conclude that at least 10% of them must have started out life as part of a star system, forming in a disk of dust and dust around a young star rather than through gravitational collapse. At some point,