Less than a hundred of these objects have been discovered so far, but it is estimated that there are several billion of them inside the Milky Way.
It is not difficult to imagine why we know so few of them today: the main methods for discovering exoplanets, radial velocities and transits, are in fact based on slight variations observed in the mother star, which in the case of the interstellar planets is not present. These can therefore only be observed directly, through the radiation they emit due to the residual heat of their formation. But this requires very powerful tools and only the most massive planets can be discovered. Very often, planets with masses greater than those of Jupiter are found, at the limit of the classification between planets and brown dwarfs.
These are icy "free-floating planets," or FFPs. But how did they end up all on their own and what can they tell us about how such planets form?
Finding more and more exoplanets to study has, as we might have expected, widened our understanding of what a planet is. In particular, the line between planets and "brown dwarfs" — cool stars that can't fuse hydrogen like other stars — has become increasingly blurred. What dictates whether an object is a planet or a brown dwarf has long been the subject of debate — is it a question of mass? Do objects cease to be planets if they are undergoing nuclear fusion? Or is the way in which the object was formed most important?
While about half of stars and brown dwarfs exist in isolation, with the rest in multiple star systems, we typically think of planets as subordinate objects in orbit around a star. More recently, however, improvements in telescope technology have enabled us to see smaller and cooler isolated objects in space, including FFPs — objects that have too low a mass or temperature to be considered brown dwarfs.
What we still don't know is exactly how these objects formed. Stars and brown dwarfs form when a region of dust and gas in space starts to fall in on itself. This region becomes denser, so more and more material falls onto it (due to gravity) in a process dubbed gravitational collapse.
Eventually this ball of gas becomes dense and hot enough for nuclear fusion to start — hydrogen burning in the case of stars, deuterium (a type of hydrogen with an additional particle, a neutron, in the nucleus) burning for brown dwarfs. FFPs may form in the same way, but just never get big enough for fusion to start. It's also possible such a planet could start off life in orbit around a star, but at some point get kicked out into interstellar space.
Interstellar planets are mysterious objects, and researchers hope to shed light on their nature thanks to the next-generation telescopes that are being built right now. In particular, we want to answer the question: how do these planets form? Today there are two main hypotheses. These nomads may be planets formed within a star system and expelled by interactions with other planets. Otherwise, interstellar planets may form from clouds of interstellar gas that, once collapsed, do not have enough mass to activate the nuclear fusion of hydrogen and form a star.
Credit: ESO.
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