Is “Planet Earth” really a planet?

Shutterstock | NASA

We call our home planet a planet, but is that the case? Aleteia speaks with two scientists who might have the answers to an ongoing debate.

Weird as it might sound, some members of the scientific community claim, for different reasons commonly not known by non-scientists, that Earth might not be exactly a planet. According to the definition defended by the International Astronomical Union (IAU) since August 2014, Earth would be, strictly speaking, a dwarf planet, as it has not yet “cleared” its orbit. This is precisely what happened with “planet” Pluto back in 2015. This definition states that, in order for a celestial body to be considered a planet, it should meet these criteria:

  1. It should be in orbit around the Sun
  2. It must have sufficient mass for its self-gravity to overcome rigid body forces so it assumes a hydrostatic equilibrium (nearly round) shape, and
  3. It must have cleared the neighborhood around its orbit.

Pluto failed to meet this third requirement, because it lacks what is commonly referred to as “gravitational dominance.” Being part of what is called the Kuiper Belt – a group of small and icy celestial bodies beyond the orbit of Neptune — Pluto got downgraded to the rank of “dwarf planet.”

And, according to some, the same applies to Earth, because of an asteroid. Meet Asteroid 2016 HO3, which orbits a bit too close to our planet to be considered a true satellite. This would be proof that Earth, like Pluto, has not yet cleared its neighborhood. All of a sudden, ours is a dwarf planet. But is that really the case?

According to Alexandra De Castro, PhD, a theoretical physicist with more than 15 years of experience as a scientific researcher and lecturer and former Research Associate in the University of Sydney, IAU’s definition, “which aimed towards more precision, ended up being more vague and confusing. What are we to do with celestial bodies not orbiting around our Sun, such as the newly discovered Trappist?”

“New Horizons (NS),” De Castro explains, “wonders if we should also call planets all other celestial bodies – besides stars — outside our solar system, belonging to other star systems or roaming in space, or whether IAU’s designation of ‘exoplanets’ is enough. I personally think this distinction between planets and exoplanets must be made, yet mainly to maintain the notion of which objects remain under the influence of the gravitational field of the Sun and which not.” But, also, she continues, “the New Horizons’ manifesto posits that the definition of a planet should adhere to the intrinsic characteristics of the objects instead of contemplating the complexities of what we know as celestial mechanics. In fact, they use geophysical criteria. For NS, a planet is ‘a massive sub-stellar object that has never undergone nuclear fusion and which has enough gravity to assume the shape of a spheroid regardless of its orbital parameters.’”

This means, basically, according to De Castro, “we would end up not having 8 or 9 planets in our solar system, but around 100 instead. I personally think both variables should be taken into account: the intrinsic characteristics of the object as much as its interaction with others.” As can be already seen, astronomical classification of celestial bodies is becoming more and more “a multidisciplinary issue.”

Are these issues, then, exclusively related to nomenclature? What consequences might there be if Earth is, then, no longer considered a “planet”?

“Although scientific societies have always engaged in debates,” says De Castro, “the world scientific consensus under strict rules is relatively new. The IAU gives guidelines, accepts proposals, discusses them and finally approves them or not, through democratic processes held among its members. In that sense, nomenclatures are indeed conventions, but approved by a group of experts. What is unquestionable is that classification is an essential tool that allow scientific studies to progress. Because of that, that nomenclature should characterize as best as possible the objects of study and their relationships between them. As for the consequences of considering the Earth a planet or not, those are not purely and entirely scientific: they are also related to the public’s understanding of its environment. This is indeed an important element to consider, and the IAU will definitely take it into account, before stripping Earth of its ‘planet-ness.’ In fact, NS is also aware of this, and although their definition of a planet would give Pluto back its status as a planet, adopting more than 100 bodies and calling them ‘planets’ all of a sudden can be even harder to digest.”

Asking why these apparently basic definitions are still the matter of discussion and even controversy is, then, fundamental. That is the opinion of Marcio Meléndez, PhD, a scientist working on the James Webb Space Telescope, at NASA’s Goddard Space Flight Center. For him, the fact that these discussions remain open “is always important, as science is not a monolith but is rather flexible and adapts and adjusts itself as human accumulated knowledge also grows.” However, in the case of Pluto, for instance, the researcher admits “the term ‘planet’ transcends the boundaries of scientific definitions and posits itself as a sentimental label. This is a feeling we are intrinsically and uncontrollably related as we are in our own house, orbiting the Sun, but also because for more than 80 years Pluto has always been ‘the last planet’ in our classrooms.” But also, he adds, “due to the growing complexity of our solar system, it is no surprise many astronomers do not agree, either totally or partially, with these newly adopted definitions.”

In fact, IAU’s definition and the aforementioned abc criteria “includes Mercury, Venus, Earth” — in spite, for the time being, of our dear Asteroid 2016 HO3 — “Mars, Jupiter, Saturn, Uranus and Neptune, Pluto being a ‘dwarf planet’ but having now Ceres and Eris joining it as dwarves instead of asteroids.” In fact, and precisely addressing the issue of the Asteroid 2016 HO3, Marcio explains “the ‘clearing the orbit’ requirement does not include small bodies, asteroids, comets, or remnants of the formation of the solar system, which can continue to orbit the earth (or any planet).”

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