What is a planet?

Detections of planets orbiting nearby stars naturally require establishing what is meant by a “planet” and how do we know a planet when we see one? This question is not as easy to answer as it is to pose. To understand the difficulties, it is useful to examine the answer to: “Is Pluto a planet?”—a simple question without a simple answer.

Astronomers like categorising objects of astronomical and astrophysical interest, giving them labels such as “planets”, “stars”, “asteroids” and many more. According to the decision of the International Astronomical Union (IAU), Pluto is classified as a “dwarf planet” because it is not the “gravitationally dominant body” on its orbit. Setting aside for a moment what that actually means, it is, however, pretty obvious based on the recent imagery from the NASAs New Horizons mission that Pluto is certainly a world in its own right – something that we could be very tempting to call a planet regardless of what the IAU has decided.

According to the IAU, a planet is defined as follows:

  1.   A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

  2.   A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

  3.   All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

Although Pluto is in orbit around the Sun and has sufficient self-gravity to assume a roughly spherical shape through hydrostatic equilibrium, it has not cleared the neighbourhood of its orbit by gravitationally tugging smaller objects out of its residential area in the Solar System. IAU therefore decided to classify Pluto as a dwarf planet rather than a full-blown member of the set of planets in the Solar System.

The situation gets more complicated when remembering that Jupiter, for instance, has also failed to clear the neighbourhood around its orbit, as there are thousands of trojan asteroids at and around Jupiter’s orbit. Yet, nobody disagrees whether we should classify Jupiter as a planet or not.

But with respect to extra-solar planets, the IAUs resolution is not valid – it has only been designed to be valid when classifying celestial bodies in the Solar System.

Astronomers consider extra-solar objects that orbit stars other than the Sun to be planets if they (1) are large enough to have reached hydrostatic equilibria, and (2) small enough such that they cannot sustain nuclear fusion in their cores and thus cannot be considered stars or even brown dwarfs. But that is only where the problems begin—it is not at all trivial to determine how large the objects orbiting other stars are when their very presence is difficult to observe.

It is reasonably straightforward to conclude that an object transiting a nearby star, such as the extraordinary haul of worlds found by the Kepler spacecraft, is large enough to have reached hydrostatic equilibrium if it is in fact large enough to be seen blocking the light coming from the stellar surface. But what about the larger objects that are comparable in size to Jupiter? Because the planetary transits can only reveal their radii in relation to their host stars, it cannot be known whether some of them are in fact more than roughly 13 times more massive than the Jupiter, which is sufficient for the fusion of deuterium into helium in their cores. Such objects would then be classified as brown dwarfs rather than planets.

The situation is even more complicated, sometimes frustratingly so, when observing exoplanets with the Doppler spectroscopy technique applied in the Pale Red Dot campaign. Because this technique can only be used to reveal the lower limit for the planetary masses, it is impossible to tell whether any individual discovery actually corresponds to a genuine planet rather than a small star or a brown dwarf even though, on statistical grounds, the vast majority of them are certainly small enough to be considered planets.

But whether extra-solar objects of suitable size to be classified as planets have cleared the neighborhoods of their orbits is beyond our observational capabilities. It is also less than certain what the possible free-floating planetary sized objects should be called as they do not revolve around stars of any kind.

It is quite possible that a general definition of a planet proves as elusive as that of a “continent” that no geographer dares to define—nor are they even interested in doing so. Similarly, biologists cannot produce a general definition for “life” but more often than not simply say that “they know whether it is alive or not when they see it”, which can be seen as an attempt to brush the problem under the carpet. It is probably a human trait to attempt classifying things into rigid categories even when nature has cynically decided that there is simply a continuum of objects and that any and all classifications are thus only subjective opinions without any deeper meanings. In such cases, the definitions do not help in understanding nature any better—and may even hinder scientific developments by providing a biased frame of reference.

And Pluto, as it – in my opinion – certainly is a world in its own right, deserves to be called a planet regardless of any subjective definitions individuals might consider appropriate. Although that might not be acceptable for all, one thing is clear. If an object resembling Pluto was found orbiting a star other than the Sun, I believe it would be called a planet.

Mikko Tuomi
Mikko Tuomi

About the author. M. Tuomi is working as an astronomer at the University of Hertfordshire, UK. His research interests include detection and characterization of low-mass planets and planetary systems around nearby stars, development of statistical models or Doppler spectroscopy data to understand variability caused by astrophysical and instrumental effects, study of the dynamical properties of tightly packed planetary systems, and exploring the
statistical properties of small planets orbiting the stars in the Solar
neighborhood. He has also worked as an environmental scientist at the Finnish Environment Institute. M. Tuomi is one of the editors of palereddot.org, and can be blamed for first spotting tentative evidence of ‘The signal’ in archival UVES and HARPS data.