Exoplanet Definition

Although planets in our own solar system have to satisfy very specific criteria to qualify as a planet, the situation with respect to what should be a good exoplanet definition is somewhat murky. First, I would recommend that you read about the formal definition of a planet in our solar system.

We see from the formal definition of a planet that the first condition that must be met for an object to qualify as a planet is that its mass must be less than the critical mass at which thermonuclear burning of deuterium can begin. (Otherwise the object would become star-like: deuterium is heavy hydrogen, or hydrogen whose atoms have two neutrons instead of one.) At the lower mass end, the definition of a an exoplanet is consistent with that for our solar system: the object must be massive and compact enough that its self-gravity makes it “nearly round.” The condition of “clearing the orbit of debris” also carries over. However, in addition, there is the issue of whether an object that is not associated with a star (i.e., a “free-floating” object) should be called a planet (even if it has a mass of less than 12 to 13 Jupiter masses). For the moment, such objects are not labeled as planets, but are labeled as “subbrown dwarfs.” However, sometimes they are referred to as “planemos” (short for “planetary mass objects”).

You can get instant access to the book Exoplanets and Alien Solar Systems:

In practice, it may not be possible to measure the required observables for an exoplanet to unambiguously determine its formal qualification as a planet. In particular, only a small percentage of planets have yet been imaged, and it is not possible, in general, to say with confidence that an object is “nearly round,” and the issue really begs a more quantitative notion of what exactly is meant by “nearly round” anyway. There is a document that was produced by the “IAU Exoplanet Working Group” in 2006 that describes the ins and outs of the classification of exoplanets, and it is still quite an ongoing affair.

To sum up, the formal definition of a planet in our solar system would carry over to exoplanets, except that most exoplanets cannot be directly imaged. For those that can be imaged, the detail in the image is not yet good enough to determine if the object is “round enough” to qualify for planetary status (this is in fact not possible to do even for some of the smaller objects in our own solar system near Pluto and beyond). Even aside from these problems, the formal definition of a solar system planet does not accommodate free-floating objects.

Learn more about exoplanets with Exoplanets and Alien Solar Systems.

Follow on Twitter

Blog Book Tutorial Exoplanets FAQ Exoplanets Websites

File under: exoplanet observations, what is an exoplanet? Exoplanet and extrasolar planet definition.