Rejoice, for a rogue planet has been detected pottering about 20 light-years from Earth. Although we’ve detected worlds like this before, this is the first time that a planetary-mass object has been spotted beyond our Solar System using a radio telescope, the Very Large Array (VLA) in New Mexico.
At this point, I can hear you all asking: What the hell is a rogue planet? Referred to using all manners of monikers, from wandering planet to orphan planet, a rogue planet is simply one without a star.
There are a range of ways these take shape. From their original one or multi-star planetary systems, these worlds were dragged away by, say, another passing star, or perhaps they were ejected as a protoplanet during the system's early days of piecing itself together.
Although some estimates place the number of rogue planets in our galaxy as high as 100,000 for each star, a 2017 study concluded that there’s probably around one for every four stars. Many of them appear to be gas giants, and some are so huge that their mass lingers between that of Jupiter and the smallest stars.
Unlike their stellar furnace compatriots, these titans aren’t massive enough for a runaway nuclear fusion of their hydrogen and helium to take place. As such, they remain almost-stars and are termed “brown dwarfs”.
The fact that they are often still contracting under their own mass explains why they give off their own heat signature. Per The Astrophysical Journal, this new rogue world has a mass that just about allows it to build up a decent amount of gas pressure, which partly explains why its surface temperature is around 825°C (1,500°F)
SIMP J01365663+0933473, this rogue planet under the microscope, was first spotted using the VLA in 2016, along with four others. It was first thought to be both massive and ancient, but a separate research team re-checked the data in 2017 and found something remarkable.
Not only was it extremely young – 200 million years old, compared to the usual several billion – but nowhere near as big as other brown dwarfs. This latest discovery is still huge, mind you – 12.7 times the mass of Jupiter – but that just about puts it on the boundary of a gas giant and a brown dwarf.
In that respect, it’s a bit of a minnow, but what it lacks in mass it makes up for with its bonkers magnetic field.
Our own world gets its magnetic field thanks to the turbulence and convection in the iron-rich liquid outer core. Jupiter has a magnetic field that’s 20,000 times stronger than Earth’s; lacking a liquid iron core layer, it instead generates its field using hydrogen, which deep within the hazy world is in a highly compressed, metallic form.
The new study, led by the California Institute of Technology, found that SIMP J01365663+0933473’s own magnetic field is 200 times stronger than Jupiter’s. It’s potentially generated by the movement of charged particles somewhere within its hazy layers, similar to how the Sun gets its own magnetic field.
Weirdly, like other brown dwarfs without a star, it gives off a powerful auroral radio signature, which is what the VLA picked up on in the first place. Auroras, no matter where they are, seem to require not just a magnetic field, but solar wind – a stream of charged particles – to form.
It’s not clear, then, why starless brown dwarfs are able to produce their own. It’s thought that interstellar gas may play a role, as might a planet orbiting the brown dwarfs that are shedding particles as it does so – much like Jupiter’s Io – but it remains a tantalizing enigma at present.