What is a rogue planet?

It was recently announced that up to 50 billion rogue planets could roam the galaxy in interstellar space, without the orbit-generating attraction of a host star. For most of human astronomy, the planets have necessarily been linked to our own solar system and only in the past Twenty-five years since we’ve been able to detect planets orbiting other stars, so the idea of ​​a rogue planet is particularly, well, alien.

What is a rogue planet?

In 2003, the International Astronomical Union (IAU) – the same authority that demoted Pluto to its current status as a dwarf planet – declared that planetary-sized bodies that do not orbit a star should be called “dwarfs. sub-brunettes ”.


Brown dwarfs, objects about 13 to 80 times the mass of Jupiter but less than that of smaller stars, are often thought of as failed stars that cannot achieve nuclear fusion of hydrogen into helium in their nucleus, a step necessary to become a true Star.

Sub-brown dwarfs, or rogue planets as they are commonly referred to, are objects in interstellar space between 3 times the size of Earth and 12 times the size of Jupiter that broke free from their mother star, but did not just scratches the surface.

Source: NASA / JPL-Caltech

These bodies move without orbit in space and have been the source of much speculation and even the object of fascination by various apocalyptic cults, that is, the cataclysm of Niburi.

While a rogue planet traversing our solar system is not out of the question – there are theories that the current planet nine (or planet ten for us diehards of Pluto) could have been a rogue planet that was indeed captured by our Sun’s gravity – if we’ve made a substantial traverse of our solar system in the past billion years, we probably wouldn’t be here to speculate. The gravitational disturbance it would cause would be enough to disrupt the orbits of all the planets in its path, causing all kinds of disruption to our climate, possibly causing us to come out of our Goldilocks orbit around the Sun.

There could be billions of rogue planets out there

Star systems
Source: NASA, ESA and M. Kornmesser (ESO)

Various studies have been used to determine the number of rogue planets that could be in the galaxy.

The problem is, rogue planets on their own don’t emit any light. Without a star’s light to reflect, astronomers must find other ways to spot these wandering celestial masses.

Recently, researchers have used the microlens to identify rogue planets as they pass the light of distant stars. In 2011, the Microlensing Observation in Astrophysics survey published an article that made a stunning statement: There could be two rogue planets for every star in the galaxy. With around 200 to 400 billion stars in the galaxy, that would mean 400 to 800 billion rogue planets flying through interstellar space.

“Very few of us in the microlens business believed in the original MOA results, simply because they were so difficult to reconcile with other observations and theories,” says Ohio State University astronomer Scott Gaudi. “But it was difficult to know what was causing the apparent excess of events.”

Source: N. Metcalfe & Pan-STARRS 1 Science Consortium / Wikimedia Commons

A more recent survey produced results more consistent with other observations. Using the same microlens technique, the researchers used data they collected from more than 6 years of observing nearly 50 million stars, the recent article indicating that there could be up to a rogue planet the size of Jupiter for four stars in the galaxy. This would mean that there could be as many as 100 to 200 billion Jupiter-sized rogue planets flying through interstellar space.

“Our new microlens observations are in line with theoretical expectations on the frequency of free-floating Jupiters and with infrared readings for planetary mass objects in star-forming regions,” says Przemek Mróz, astronomer at the Observatory. from the University of Warsaw in Poland. and a lead author of the microlens study. “We have found that the planets of Jupiter’s mass are at least 10 times less frequent than previously thought.”

Orion nebula
Source: NASA, ESA, M. Robberto (Space Telescope Science Institute / ESA) and Hubble Space Telescope Orion Treasury Project Team

More recently, astronomers at the University of Lieden in the Netherlands published the results of a simulation of 1,500 stars in the Orion trapezoid to further refine this number.

By simulating 2,522 planets orbiting 500 of these stars, they determined that 357 of these stars freed themselves from the gravity of their mother star during the first 11 million years of evolution. “Of these,” University of Lieden astronomer Simon Portegies Zwart told Forbes last week, “281 are leaving the cluster, others remain linked to the cluster as intra-cluster planets floating freely “.

Portegies Zwart estimates that, given a conservative model of 200 billion star galaxies, about a quarter of those stars have lost one or more planets.

How does a planet go rogue?

Rogue planet
Source: NASA / JPL-Caltech

How did these planets break free from their stars?

TESS discovers a new planet three times the size of Earth

In many cases of the Portegies Zwart model where a star loses at least one planet, the planets tended to have more tilted orbits. Additionally, “collisions between planets and between planets and their host star are common,” he said. “It happens in over 3% of planetary systems. “

Other events that disrupt the gravitational stability of a solar system can also cause a planet or two to be lost. In many active stellar nurseries, stars can pass close enough to each other that their gravity can disrupt the orbits of another star’s planets, perhaps enough for a planet to launch completely out of orbit. of its mother star.

Ultimately, there is still a lot we don’t know about rogue planets, so much of it is still guesswork, but as astronomers increasingly look at these planetary nomads, we find that rogue planets offer real insight into the stars that once hosted them and how the planets in our solar system once evolved.

Arline J. Mercier

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