Astronomer proposes new definition of a planet

By adding these new constraints, astronomers should be able to differentiate between a large planet and a brown dwarf


This new definition states that an object with a mass of 10 times the mass of Jupiter should be considered a brown dwarf. Image credit: NASA/JPL-Caltech

A controversial question that has caused much debate throughout the years is: what is and what is not a planet? With the demotion of Pluto back in 2006, the decision to rewrite the definition of a planet has since been questioned. With the discoveries of massive objects outside our Solar System, including Jupiter-like giant planets, the line has become more blurred. John Hopkins University astrophysicist Kevin Schlaufman has proposed a new definition on the matter.

Schlaufman has suggested that the upper boundary of the mass of a planet is between four and ten times the mass of Jupiter. The reason for this is due to improvements in the technologies and techniques of astronomical observation. These improvements have led to more and more planetary systems being discovered beyond our Solar System, and showing robust patterns that lead to new revelations.

“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,” says Schlaufman. “An upper boundary on the masses of planets is one of the most prominent details that was missing.”

This conclusion was made after analysing 146 planetary systems beyond our own, and almost all of these observations were measured in a similar way. The data from these planetary systems are more consistent and much more reliable because of these technological improvements.

With these systems, it’s important to be able to distinguish between two celestial suspects: a giant planet or a low-mass star called a brown dwarf. A brown dwarf is a star that is more massive than a planet, but less massive than the smallest stars. However, they are thought to form the same way stars do.

Brown dwarfs are the mysterious bridge between planets and stars, which scientists have yet to define. Image credit: ESO

This celestial mystery, a brown dwarf, has been an enigma for decades for scientists, as they struggle to differentiate a brown dwarf and a large planet. Schlaufman says that the mass alone cannot separate the two, so other properties are needed to draw the line.

In Schlaufman’s case, the chemical makeup of a planetary system’s host star can be the missing property. He suggests that a planet can be deduced from its company, and not just by its size. Massive planets, such as Jupiter, are largely found orbiting stars that have more iron than our Sun, whereas brown dwarfs are not so discriminating.

This is where the idea of planet formation comes into the frame. Planets are believed to form via the ‘bottom-up’ theory, where they start with a rocky core and then subsequently become wrapped up in gaseous envelope. It would make sense that these planets would have formed near stars that have more heavy elements, like iron, which consequently provide the appropriate material for form the rocky core.

On the other hand, brown dwarfs and other stars originate from the ‘top-down’ theory, where gas clouds collapse under their own gravity. Schlaufman’s idea was to find the mass at which point objects stop caring about the composition of the star they orbit. From this research, he determines that objects more massive than ten-times the mass of Jupiter do not only follow stars with lots of heavy elements, and therefore form brown giants. This discovery is what led to the conclusion that objects in excess of 10 Jupiter masses should be considered brown dwarfs, and not planets.

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