In a new article published in the Astrophysical Journal (arXiv.org preprint), Johns Hopkins University researcher Kevin Schlaufman sets the upper boundary of planet mass between 4 and 10 times the mass of Jupiter.
An artist’s rendering of an extrasolar gas giant. Image credit: NASA.
“While we think we know how planets form in a big-picture sense, there’s still a lot of detail we need to fill in. An upper boundary on the masses of planets is one of the most prominent details that was missing,” Dr. Schlaufman said.
“The conclusions are based on observations of 146 solar systems,” he added.
“Almost all the data he used was measured in a uniform way. The data are more consistent from one solar system to the next, and so more reliable.”
Defining a planet, distinguishing it from other celestial objects, is a bit like narrowing down a list of criminal suspects. It’s one thing to know you’re looking for someone who is taller than 5-foot-8, it’s another to know your suspect is between 5-foot-8 and 5-foot-10.
In this instance, astronomers want to distinguish between two suspects: a giant planet and a more massive object called a brown dwarf.
“For decades brown dwarfs have posed a problem for scientists: how to distinguish low-mass brown dwarfs from especially massive planets? Mass alone isn’t enough to tell the difference bzween the two. Some other property was needed to draw the line,” Dr. Schlaufman said.
In his new argument, the missing property is the chemical makeup of a planet-hosting star.
“You can know your suspect, a planet, not just by itssize, but also by the company it keeps. Giant planets such as Jupiter are almost always found orbiting stars that have more iron than our Sun. Brown dwarfs are not so discriminating,” he said.
That’s where his argument engages the idea of planet formation.
“Planets like Jupiter are formed from the bottom-up by first building-up a rocky core that is subsequently enshrouded in a massive gaseous envelope. It stands to reason that they would be found near stars heavy with elements that make rocks, as those elements provide the seed material for planet formation. Not so with brown dwarfs,” Dr. Schlaufman added.
Brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight.
“My idea was to find the mass at which objects stop caring about the composition of the star they orbit,” he said.
“I found that objects more massive than about 10 times the mass of Jupiter do not prefer stars with lots of elements that make rocks and therefore are unlikely to form like planets.”
For that reason, and while it’s possible that new data could change things, Dr. Schlaufman has proposed that objects in excess of 10 Jupiter mass should be considered brown dwarfs, not planets.
_____
Kevin C. Schlaufman et al. 2018. Evidence of an Upper Bound on the Masses of Planets and Its Implications for Giant Planet Formation. ApJ 853, 37; doi: 10.3847/1538-4357/aa961c
