Last updated January 24, 2018 at 12:10 pm
As scientific debate continues over what is and isn’t a planet, an American astrophysicist has decided it’s time to add one key parameter – size.
Astrophysicist Kevin Schlaufman has weighed into the debate over how to define planets with a deceptively simple suggestion – set the upper boundary of planet mass at between four and 10 times the mass of the planet Jupiter.
Bigger than that, he says, and it’s a brown dwarf.
It’s an important distinction. One of the difficulties in defining planets has been distinguishing them from other celestial objects. It was known that brown dwarfs are smaller than the smallest stars but bigger than planets, but not how much bigger.
“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,” said Schlaufman, an assistant professor in Johns Hopkins University Department of Physics and Astronomy.
“An upper boundary on the masses of planets is one of the most prominent details that was missing.”
Difference between a planet and a brown dwarf
Schlaufman says we can now set that boundary thanks mainly to improvements in the technology and techniques of astronomical observation. It is possible to discover many more planetary systems outside our solar system and therefore possible to see robust patterns that lead to new revelations.
The conclusions in the new paper are based on observations of 146 solar systems.
Schlaufman says mass alone is not enough to tell the difference between a planet and a brown dwarf and argues that the extra property needed is the chemical makeup of a solar system’s own sun.
Giant planets such as Jupiter are almost always found orbiting stars that have more iron than our sun. Brown dwarfs are not so discriminating.
That’s where his argument engages the idea of planet formation.
Planets such as 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, he says, 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.
Brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight.
Schlaufman’s idea was to find the mass at which objects stop caring about the composition of the star they orbit. He 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.
The paper was published by the Astrophysical Journal.