When is a planet not a planet – or more precisely, when should what is not regarded as a planet be a planet?
Right now, according to the International Astronomical Union (IAU), our solar system comprises eight formally recognised planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. That’s been the case since 2006, when the IAU opted to classify bodies orbiting the Sun in three ways:
- As planets – defined as a) celestial bodies that (a) are in orbit around the sun; b) have sufficient mass for their self-gravity to overcome rigid body forces so they assume a hydrostatic equilibrium (nearly round) shape; c) have cleared the neighbourhood around their orbit of other objects
- As Dwarf planets – defined as celestial bodies which a) orbit the sun; b) have sufficient mass for their self-gravity to overcome rigid body forces to assume hydrostatic equilibrium (aka “is nearly round” in shape); c) have not cleared the neighbourhood around their orbit; and d) is not a natural satellite
- As Small Solar System bodies: all other objects except satellites orbiting the Sun.
Thus, since 2006, Pluto has been a dwarf planet. However, moves are afoot to get things changed – and not just for Pluto.
In a paper authored by planetary scientists involved in the New Horizons mission which zipped through the Pluto system in July 2015, there is a call for the term “planet” to be redefined; if not by the IAU then at least in popular use. Should it happen, it could see the number of planets in the solar system leap from 8 to over 100.
The scientists argue that the IAU definition of “planet” focuses only on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. In fact, under the IAU’s definition, Earth, Mars, Jupiter and Neptune don’t actually qualify as “planets” as none meet the third criteria (c) – Earth, for example, has regular “close encounters” with asteroids which cross its orbit. Instead, the team offer a simpler definition:
A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters.
Such a definition would mean that Pluto could regain its planetary status – as would the proto-planet (or small solar system body) Ceres, the dwarf planets of 136199 Eris (discovered in 2005, and the trigger-point for Pluto’s “downgrading”) , 136472 Makemake, and 136108 Haumea, together with (possibly) 50000 Quaoar, 90377 Sedna, 90482 Orcus and a host of trans-Neptunian objects tumbling around the Sun. Nor is that all; the new definition would also mean that the likes of Jupiter’s Galilean moons, Saturn’s Titan and Enceladus, Neptune’s Triton and many other bodies we regard as “moons” would be lifted to planetary status – including our own Moon.
The paper proposing the change will be presented at the Lunar and Planetary Science Conference on March 20th to 24th, 2017 in Texas. And it has already come in for some criticism.
Mike Brown is the scientist largely behind Pluto’s demotion. Currently engaged in the search for the elusive “Planet Nine”, he (somewhat harshly) sees the efforts of the New Horizons team to get Pluto reclassified as being about them wanting the prestige of having run a planetary mission, more than anything else.
However, there are valid reasons for seeking some kind of change, even if it is only informal. One is as basic as gaining more public interest in efforts to explore and understand the many environments found on planets and moons alike within our solar system.
“Every time I talk about this [the science and data gathered about Pluto by New Horizons] to the general public, the very next thing people say is ‘Pluto is not a planet any more’,” said Kirby Runyon, the lead author of the paper. “People’s interest in a body and exploring it seems tied to whether or not it has the name ‘planet’ labelled on it.”
There are scientific reasons for the definition to be broadened as well. Places like Pluto, Ceres, Europa, Io, Ganyemede, Callisto and Triton all evidence geophysical, hydrothermal, atmospheric and other characteristics very much in keeping with bodies such as Earth, Mars, and Venus. They are thus of exceptional interest to planetary scientists the world over. In fact, many of them (like Pluto) are completely re-writing our understanding of “planetary bodies”.
Ultimately, the team behind the paper aren’t going to put their proposal before the IAU for a change in the “official” definition of “planet”. “As a geophysical definition, this does not fall under the domain of the IAU, Runyon notes, “[It] is an alternate and parallel definition that can be used by different scientists. It is “official” without IAU approval, partly via usage.”