Forget What You Knew About Planets, There’s A New Definition Of What Planets Are
In recent times, everyone’s favorite ‘planet’ Pluto monopolized attention in the debate on what should be a planet and what not.
Now, a scientist has considered the other extreme of the question and wondered how massive a planet could be to be labeled as such.
An astrophysicist at Johns Hopkins University has proposed that objects that exceed ten times the mass of Jupiter be considered brown dwarfs, and not planets.
In an article published in the Astrophysical Journal, Kevin Schlaufman, assistant professor in the Department of Physics and Astronomy at Johns Hopkins, says that it is now possible to establish an upper mass limit to planets, thanks to improvements in technology and astronomical observation techniques.
The paper is online in the journal at http://iopscience.iop.org/article/10.3847/1538-4357/aa961c. It is also archived at https://arxiv.org/abs/1801.06185.
These technological advances have allowed us to discover many more planetary systems outside our solar system and, therefore, it is possible to see stable patterns that lead to new revelations.
“While we believe we know how planets are formed in a broad sense, there are still many details that we must understand,” said Schlaufman.
“An upper limit on the masses of planets is one of the most important details that was missing.”
The conclusions in the new paper are based on observations of 146 solar systems, the systems, Schlaufman said, is the fact that almost all the data he used was measured uniformly. 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 reducing a list of criminal suspects. It is one thing to know that you are looking for someone who measures more than 5 feet 8 inches, another is to know that your suspect is between 5 feet 8 feet and 5 feet 10 feet, he argues.
In this case, researchers want to distinguish between two suspects: a giant planet and a celestial object called brown dwarf.
Brown dwarfs are more massive than planets but less massive than smaller stars. It is believed that they are shaped like stars.
According to Schlaufman, it has been an issue to distinguish low mass red dwarfs from, especially massive planets, Mass alone was not an option to differentiate the too, so another property was needed.
According to Schlaufman, the missing property is the chemical makeup of a solar system’s own sun. Schlaufman says you can know your suspect, a planet, not just by his size, but also by the company he 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. And it is there where his argument addresses the idea of planetary formation.
Planets like Jupiter are formed from the bottom up by first building a rocky core that is then wrapped in a massive gaseous envelope. It is logical that they are close to stars loaded with elements that form rocks since these elements provide the seed material for the formation of planets. Not so with brown dwarfs.
Schlaufman further notes how Brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight.
The idea of Schlaufman was to find the mass in which objects stop caring about the composition of the star in which they orbit. He discovered that the most massive objects, approximately ten times the mass of Jupiter do not prefer stars with many elements that form rocks and, therefore, are unlikely to be formed as planets.
This is why Schlaufman has proposed that objects in excess of 10 Jupiter mass should be considered brown dwarfs, not planets.
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