Who decides what can be deifined as a planet?

Johns Hopkins University researcher, Kevin Schlaufman, sets the upper boundary of planet mass between 4 and 10 times the mass of Jupiter

Who decides what can be deifined as a planet?

Nailing down an exact definition of an exoplanet, or a planet that orbits a star outside of our solar system, is actually a topic of intense deliberation.

The latest planet definition was provided via NASA scientists, led by Alan Stern, in early 2017. The team’s planet definition says “a planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.” 

In other words, round objects in space that are smaller than stars. According to Stern’s planet definition, our solar system has not 8 major planets, but more than 100, including Pluto, of course, and including Earth’s moon.

However, in a paper published January 22, 2018, in the peer-reviewed Astrophysical Journal, a Johns Hopkins astronomer, Kevin Schlaufman, has set the upper boundary of planet mass between four and 10 times the mass of the planet Jupiter.

Narrowing down the limit to something in that range is now possible primarily because of improvements in the technology and technique of astronomical observation, Schlaufman said. We have recently discovered many more planetary systems, he said, and can now see patterns we could not before.

He came to his definition by using an unsupervised machine learning technique, essentially having a computer classify unlabeled data, to parse through 146 solar system worth of planets. He separated the planets (objects that form like Jupiter) from brown dwarfs or medium sized celestial objects in between a giant planet and a small star.

Schlaufman says you can know a planet not just by its size, but also by the company it keeps. Giant planets are almost always found orbiting stars that are metal-rich. Brown dwarfs are not so discriminating. 

His argument engages the idea of planet formation proposing that 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 while brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight.

“My proposed definition provides a solution to all of those problems by defining a planet as an object that formed via the same physical process as Jupiter, Saturn, Uranus, and Neptune in our own Solar System,” Schlaufman explains. “Astronomers have sought to apply such a definition in the past, but they were defeated because of the difficulty in uniquely determining how billion-year-old objects like exoplanets formed. The observations and analyses I presented in my paper overcome this problem and enable the practical use of my proposed definition for this the first time”, he concludes. 


Latin American Post | Luisa Fernanda Báez

Copy edited by Laura Rocha Rueda

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