Recently, an article appeared in the research journal Icarus, trying to find a new reason to attack the definition of planets and argue for the promotion of Pluto again. The researchers also note that several satellites, including our Moon, can be upgraded to planetary status. To understand this confusion, let me rewind first and introduce the context.
I was a young PhD student when I attended the General Assembly of the International Astronomical Union (IAU-GA) at Prague in 2006. I had no clue that I would have a ringside seat to a historical decision. In this assembly, a committee appointed by the IAU presented a draft definition of the word “planet”, reclassifying Pluto as a ‘dwarf planet’, and all hell broke loose.
So what is a planet?
About 3,000 years back, Babylonians realised that some of the brightest dots in the night sky don’t remain attached to a given star pattern (i.e. constellation) but move along a narrow circular band. The Sun and the Moon also move in the same band. Thus, these seven moving bodies (i.e. Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn) were called planetai (meaning wandering objects) by ancient Greeks. Over the next 2000 years, all civilisations tracked motions of these seven ‘planets’.
With discoveries of Galileo, Copernichus and Kepler, it became clear that the Sun is in fact a star and is (almost) at the centre of what we now call as the solar system. Hence, the Sun was removed from the list and the Earth (which was going around the Sun like other planets) was added to the list.
Subsequently, two more giant planets, Uranus (1781) and Neptune (1846) were discovered. Between 1800-1840, four small bodies were also discovered to lie between Mars and Jupiter. All these were dubbed as planets.
After 1840, it was shown that there are too many small bodies between Mars and Jupiter’s orbit that are orbiting the Sun. They were relabelled as ‘minor planets’ / ‘asteroids’, and the four original bodies from the asteroid belt, including Ceres, lost their ‘planet’ status.
For the next 80 odd years, the settled number of planets in the solar system was eight.
In 1930, we discovered another body which was beyond Neptune’s orbit. This was the first ‘planet’ discovered in the era of mass media (newspapers, radio) and also the first planet discovered by an American astronomer.
It caught the imagination of the general public. With a lot of fanfare, it was named Pluto. All the TV and radio shows, including Carl Sagan’s Cosmos, the classic astronomy books we grew up reading, even the American space probes Pioneer 10 and 11’s iconic golden plates show this picture of the solar system with nine planets.
The trouble for Pluto started brewing in the 1990s as we started discovering more bodies with orbits overlapping that of Pluto’s. Was the Asteroid belt story repeating itself? Was there another belt of minor bodies beyond Neptune (as predicted by Kuiper)?
In 2003, American astronomer Mike Brown and his colleagues discovered a body (now known as Eris), which was decidedly bigger than Pluto and we could no longer duck several questions
How many planets are exactly out there?
Should we say nine, including Pluto?
But then what about Eris? Should we say 10?
But who is to guarantee that there is no number 11 or 12, or even number 20 within the same belt?
Should we dethrone Pluto as Ceres was dethroned 150 years back and go back to eight planets?
Should this be an arbitrary / case-by-case decision or can there be an objective way to define a planet?
IAU appointed a committee of astronomers, headed by American astrophysicist and historian of science Owen Gingerich, to find answers to these questions. This committee submitted its recommendations to the general assembly in Prague that I mentioned earlier.
As per their recommendations, any body “going around any star, spherical in shape and not itself a star” should be called a planet. This would have kept Pluto as a planet, upgraded Ceres back to the planet status and added Eris and Charon to the list of planets.
About 70 per cent of the astronomers attending the general assembly strongly disagreed with almost each piece of this definition.
Some pointed out that there are likely to be planetary sized spherical bodies lurking in the galaxy which are not orbiting any star. Some argued that we don’t understand the physics of planetary sizes well enough, while some argued that this definition meant that the list of planets in the solar system would keep increasing each year, eventually even crossing 100.
Some even alleged that the committee had pre-decided that the only ‘planet’ discovered by an American (i.e. Pluto) must remain a planet and only considered definitions which satisfied that mandate.
So what is the acceptable definition of a planet?
Realising a widespread disapproval for the proposed definition, IAU appealed to the astronomers present to hold discussions within themselves and come up with an acceptable definition.
In these discussions, it was agreed that instead of a universal definition of planet, one should only focus on planets in our solar system. That way, one could keep aside controversial issues such as planetoids without stars and an upper limit on the size of a planet. It was also agreed that Kuiper belt objects, including Pluto, Charon and Eris, should be treated in the same way as the Ceres and other asteroid belt objects were treated.
With these principles in mind, the following definition of the word ‘planet’ was proposed.
A planet is a body in our solar system that
(a) Orbits around the Sun without orbiting any other planetary body
(b) Is of a spherical shape
(c) Would have cleared its orbit
The bodies which satisfy the first two criteria but not the third one were dubbed as ‘dwarf planets’. This definition ensures that the number of dwarf planets may keep increasing but the number of planets is unlikely to increase drastically.
Obviously, several American astronomers have been unhappy about this demotion of Pluto. Owen Gingerich kept bringing up this ‘unfair’ treatment of Pluto in several public talks and articles for a number of years.
One year before this general assembly, NASA had launched ‘New Horizons’ mission to flyby Pluto in 2015. The New Horizons team, including the Principle Investigator of the mission Alan Stern, was dismayed by the new definition. Alan Stern is one of the authors of the new paper that wants Pluto back as a planet.
In hindsight, how would we look at the debate that happened in 2006 and the compromises that were reached? Certainly it is not a perfect definition. With Kepler mission data, we now know that the issue of uncertainties about upper limit on planetary mass was probably an overcautious stance. The ‘clearing the orbit’ criterion is also not as straightforward as people thought then. As you go in the outer reaches of the solar system, orbital periods become longer and hence even Neptune-size bodies may get classified as ‘dwarf planets’.
We don’t know for sure what the final number of planets in the solar system is going to be. Even now, some astronomers are looking for bodies bigger than Earth beyond the Kuiper belt. But we know for sure that the current way of classifying the planets is the least bad of all options.
The author is an Associate Professor at the Homi Bhabha Centre for Science Education (HBCSE-TIFR), Mumbai.