Astronomers Found a Rogue Planet 12 Times the Mass of Jupiter

It's the first time a rogue planet has been discovered from its radio emissions and points to a promising new method for exoplanet discovery.

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Aug 6 2018, 5:28pm

Astronomers discovered a massive rogue planet that is over 12 times the mass of Jupiter with an unrivaled magnetic field, according to new research published in the Astrophysical Journal . This marks the first time a rogue planet has been detected based on its radio emissions, which opens the door to an entirely new method of exoplanet discovery.

Rogue planets are celestial bodies that don’t orbit a star and instead orbit around the galactic center in interstellar space. There have only been a handful of rogue planets discovered to date, although some astronomers think there may be tens of thousands of rogue planets for every star in the galaxy.

The recently discovered rogue planet SIMP J01365663+0933473 was first detected in 2016, but was originally mistaken for a brown dwarf. These so-called “failed stars” are dense collections of gas that are too massive to be considered a planet, yet not massive enough to sustain the nuclear fusion that gives other stars their energy.

SIMP was a part of five brown dwarfs being studied by astronomers at the Very Large Array to learn more about how these wannabe stars produce such strong radio emissions. Yet when the original observational data was revisited by an independent team of scientists, it was discovered that SIMP was likely a massive gas planet, not a star.

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Trying to determine when a huge, dense ball of gas should be considered a brown dwarf rather than a planet is a bit like trying to determine how many grains of sand constitute a sand pile versus a sand heap. The boundary is an ongoing subject of debate among astronomers, but the general rule of thumb is that planethood starts below 13 Jupiter masses, which is known as the “deuterium-burning limit” because that is approximately the point when deuterium fusion stops.

SIMP is 12.7 times the mass of Jupiter, which puts it just this side of planethood.

Located only 20 light years away (about five times the distance to our closest stellar neighbor, Alpha Centauri), SIMP is estimated to be about 200 million years old and has a magnetic field 4 million times stronger than Earth’s. This strong magnetic field produces incredible auroras on the planet, much like the auroras seen at Earth’s poles, but on a far larger scale. These large auroras produce strong radio emissions, which led to the discovery of SIMP in the first place.

The presence of these auroras is something of a mystery, however. On Earth, auroras are produced when our magnetic field interacts with solar wind. Rogue planets don’t orbit a star, however, and thus are not exposed to solar wind. One possible explanation for the auroras is that they are caused by a moon in orbit around the rogue planet, much like Io creates auroral flares on Jupiter.

Importantly, the discovery of SIMP marks the first time that a rogue planet has been discovered based on its radio emissions and the first time that the magnetic field of a rogue planet has been measured. According to the astronomers involved in the research, this opens the door to a promising new method for detecting exoplanets and rogue planets.