How Many Rogue Planets Are In The Milky Way?

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According to new simulations, many, even most, planets get ejected from their star early in their history

As J.R.R. Tolkien wrote in The Fellowship of the Ring, ⁘Not all those who wander are lost.⁘ But in the case of planets, it⁘s possible that most of them are .

Still, these interstellar drifters are pretty interesting. Most have been found via microlensing : their gravity acts as a lens that boosts the light of a background star in a measurable way. These worlds tend to be so small, dark and far away that they⁘re otherwise invisible to us. Some, similar to Jupiter in mass, have been glimpsed in images; these likely formed directly from the gas and dust in a nebula , much as a star does, and may have thus always lacked a home star. But others, much lower in mass, are expected to have formed around a star only to subsequently be ejected from their planetary system. Now these outcasts slip silent and cold through the sunless spaces between the stars.

How are they ejected? While there are a handful of possible methods, the most common is likely via interactions with another planet around its host star. We know that planets don⁘t just orbit their star in the same place forever. Over time, planetary orbits can shift because of the combined gravitational influences of other planets in a system. If two planets get too close to each other, the interaction can cause one (usually the less massive of the two) to gain a lot of orbital energy, causing it to be flung out of the system.

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