Every planet is completely round

Milky Way: 50 Billion Starless Planets?

Lonely Wanderers: In our galaxy there could be up to 50 billion lonely, starless planets - celestial bodies that were once ejected from their planetary system and are now floating unbound in space. This suggests a simulation in which around a quarter of the stars in an open star cluster lost at least one planet through collisions and nearby passages of other stars.

Most of the planets known so far are part of a system: They orbit one or more stars and are bound by its gravity. But there are also hikers - unbound planets that drift through space without a mother star. Astronomers discovered the closest known representative of these "loners" around 100 light-years away from us. CFBDSIR2149 has four to seven times the mass of Jupiter and could originally come from the AB Doradus star cluster.

Clutter in the star cluster

But how do such loners arise - and how many of them are there in our galaxy? “Young star clusters could play an important role in the creation of free-floating exoplanets,” explain Arjen van Elteren from the University of Leiden and his colleagues. Most stars and planetary systems are born in such star clusters - and thus in an environment in which collisions and close passages of neighboring stars can lead to disruptive effects of gravity.

To find out how often this happens, van Elteren and his team simulated the fate of planets in a typical open star cluster, the Trapezium Cluster in the Orion Nebula, some 1,300 light-years away. In the model, they equipped 500 of the 1,500 stars in this cluster with one to six planets. Then they followed the evolution of these 2,522 planets over eleven billion years of evolution.

Hardly any planet develops completely undisturbed

The result: Hardly any planet got away completely unscathed. Collisions with other planets and disruptive influences from nearby neighboring stars caused gravity turbulence that changed the orbit of the planets. "The majority of the planets - around 70 percent - experienced a change in their orbit of up to five percent," report the astronomers. “Another ten percent of the planets got a strongly eccentric orbit. 75 planets - three percent - collided with their parent star. "

The interesting thing, however, is that 357 of the 2,522 planets - 16.7 percent - were completely thrown out of their system and became starless loners, as the simulation showed. 80 percent of these unattached hikers not only lost their mother star, they even flew completely out of their native star cluster. According to astronomers, the probability for a planet to suffer this fate is independent of its mass and initial proximity to the star.

50 billion loners in the Milky Way

But what does this mean for the frequency of such lonely planets in our galaxy? “If every main sequence star had a planetary system, then there could be around 0.72 unbound planets for each of these stars,” estimate van Eltern and his team. Assuming that only every third main sequence star has planets, there could be a quarter as many unbound planets as there are stars in the Milky Way. Given the around 200 billion stars in our Milky Way, that would be around 50 billion lonely planets in our galaxy.

According to the researchers, our own solar system could also have experienced some adversity in its early days. The orbit of some comets in the Kuiper Belt suggests that around 70,000 years ago an alien star passed close to our system and threw some objects out of their orbit. Astronomers even suspect that our sun once belonged to a binary star system, but then lost its partner. (Astronomy & Astrophysics, 2019, in press)

Source: Astronomy & Astrophysics

March 11, 2019

- Nadja Podbregar