Presto change-o! Brown dwarf star is now a planet
When you're dealing with astronomical bodies that are light years away from you, sometimes classifications can be tricky. That seems to be the case with SIMP0136, an object located in a 200-million-year-old group of stars called Carina-Near. Once thought to be a brown dwarf, it now seems the celestial body is really more of a planet.
To be fair, it's a fine line between brown dwarf stars and giant gaseous planets. The former is created when a collection of dust and gas gets sucked closer and closer by gravity. In the case of regular stars, this compression of material eventually leads to the fusion of hydrogen which fuels the massive fireballs.
In the case of a brown dwarf, however, hydrogen fusion never kicks off. Instead, the system becomes stable and cools, although light is still generated because of the thermal energy produced by the collision of the gas and dust particles. Until they cool completely, this light is emitted in the red and infrared spectrum, which can make brown dwarfs hard to spot. In fact, they were considered strictly theoretical until the late 1980s.
In addition to giving off light, brown dwarfs can also have planets circling around them, which makes them seem like stars. They are more like planets on the other hand, because they can also have atmospheres with weather phenomena, such as the aurorae on Jupiter. Their temperatures can also climb from those of planets to as hot as stars depending on where they are in the cooling process.
Going to mass
Mass also plays a role.
"The mass of a star is defined as larger than 75 times the mass of Jupiter, and the mass of a brown dwarf is defined between 13 and 75 times the mass of a Jupiter," astrophysicist Jonathan Gagné told New Atlas. "The mass of a planet is defined as lower than 13 times the mass of Jupiter. Our measured mass (of SIMP0136) is about 12.7 times the mass of Jupiter, which places it right at the limit between planets and brown dwarfs."
It's that measurement of mass that led Gagné – Sagan Fellow at the Carnegie Institution for Science, Department of Terrestrial Magnetism – and his team to feel that SIMP0136 deserved planetary, instead of star, status. But getting to that measurement was a bit tricky.
"Basically, the age of SIMP0136 is the key to knowing its mass," he told us. "Brown dwarfs and planetary-like objects cool down as they age because they don't gain energy from nuclear fusion in their core. This means that a young planetary-mass object that is still warm from its formation could have the same temperature as a much more massive and old brown dwarf that had a lot of time to cool down. We thus need both their temperature and an age to get their mass. We previously had a pretty good idea of the temperature of SIMP0136, but we had no idea what its age was."
So to find the age, the researchers measured the speed at which it was traveling through space and realized that it was identical to a relatively nearby group of stars named Carina-Near, which is 200 million years old.
"This can only be explained by SIMP0136 being a member of Carina-Near," Gagné said. "This tells us that SIMP0136 is 200 million years old, which we combined with its temperature to get its mass." And that mass placed the body just on the side of the cosmic fence that makes it a planet.
Despite its new classification though, the body is still giving off light.
"SIMP0136 still gives off light mostly in the infrared wavelengths, as its temperature is now approximately 830° C (1,526° F)," Gagné told us. "It will take it approximately 10 billion years more before it reaches 0° C, and it will keep cooling down to the mean temperature of the universe, which is approx. -270° C, but that will take an enormous amount of time."
In a separate statement, Gagne says that the advantage of knowing that SIMP0136 is more planet-like than previously thought is that it allows scientists to better understand the atmospheres and evolution of giant gas planets. In other solar systems beyond our own that might have gas giants, the planets are often invisible to our instruments because their nearby stars are too blinding. Because SIMP0136 is floating by itself, observing it will be an easier task.
The dwarf-cum-planet now joins the ranks of just a few other free-floating planetary objects.
"This newest addition to the very select club of free-floating planetary like objects is particularly remarkable, because we had already detected fast-evolving weather patterns on the surface of SIMP0136, back when we thought it was a brown dwarf," added Étienne Artigau, co-author and leader of the original SIMP0136 discovery in 2009, in a statement.
The find has been published in Astrophysical Journal Letters (PDF).
Source: Carnegie Science