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Pluto to be reinstated as a planet? An astrophysicist explains the controversy that refuses to die

Lately, an article appeared within the analysis journal Icarus, looking for a brand new cause to assault the definition of planets and argue for the promotion of Pluto once more. The researchers additionally observe that a number of satellites, together with our Moon, will be upgraded to planetary standing.

#PI_Daily This simply out, and I anticipate it to be a landmark paper in planetary science–unimaginable job, led by the unimaginable @DrPhiltill from @UCF Take a look! And RETWEETS please!!
— Alan Stern (@AlanStern) October 29, 2021
To grasp this confusion, let me rewind first and introduce the context.
I used to be a younger PhD scholar once I attended the Normal Meeting of the Worldwide Astronomical Union (IAU-GA) at Prague in 2006. I had no clue that I might have a ringside seat to a historic resolution. On this meeting, a committee appointed by the IAU offered a draft definition of the phrase “planet”, reclassifying Pluto as a ‘dwarf planet’, and all hell broke free.
So what’s a planet?
About 3,000 years again, Babylonians realised that a few of the brightest dots within the evening sky don’t stay connected to a given star sample (i.e. constellation) however transfer alongside a slim round band. The Solar and the Moon additionally transfer in the identical band. Thus, these seven shifting our bodies (i.e. Solar, Moon, Mercury, Venus, Mars, Jupiter and Saturn) have been known as planetai (which means wandering objects) by historical Greeks. Over the following 2000 years, all civilisations tracked motions of those seven ‘planets’.

With discoveries of Galileo, Copernichus and Kepler, it grew to become clear that the Solar is actually a star and is (virtually) on the centre of what we now name because the photo voltaic system. Therefore, the Solar was faraway from the checklist and the Earth (which was going across the Solar like different planets) was added to the checklist.
Subsequently, two extra big planets, Uranus (1781) and Neptune (1846) have been found. Between 1800-1840, 4 small our bodies have been additionally found to lie between Mars and Jupiter. All these have been dubbed as planets.
After 1840, it was proven that there are too many small our bodies between Mars and Jupiter’s orbit which are orbiting the Solar. They have been relabelled as ‘minor planets’ / ‘asteroids’, and the 4 authentic our bodies from the asteroid belt, together with Ceres, misplaced their ‘planet’ standing.
For the following 80 odd years, the settled variety of planets within the photo voltaic system was eight.
Enter Pluto!
In 1930, we found one other physique which was past Neptune’s orbit. This was the primary ‘planet’ found within the period of mass media (newspapers, radio) and likewise the primary planet found by an American astronomer.
It caught the creativeness of most of the people. With plenty of fanfare, it was named Pluto. All of the TV and radio reveals, together with Carl Sagan’s Cosmos, the traditional astronomy books we grew up studying, even the American house probes Pioneer 10 and 11’s iconic golden plates present this image of the photo voltaic system with 9 planets.

The difficulty for Pluto began brewing within the Nineteen Nineties as we began discovering extra our bodies with orbits overlapping that of Pluto’s. Was the Asteroid belt story repeating itself? Was there one other belt of minor our bodies past Neptune (as predicted by Kuiper)?
In 2003, American astronomer Mike Brown and his colleagues found a physique (now often called Eris), which was decidedly greater than Pluto and we may not duck a number of questions
*What number of planets are precisely on the market?
*Ought to we are saying 9, together with Pluto?
*However then what about Eris? Ought to we are saying 10?
*However who’s to ensure that there is no such thing as a quantity 11 or 12, and even quantity 20 inside the identical belt?
*Ought to we dethrone Pluto as Ceres was dethroned 150 years again and return to eight planets?
*Ought to this be an arbitrary / case-by-case resolution or can there be an goal solution to outline a planet?
IAU appointed a committee of astronomers, headed by American astrophysicist and historian of science Owen Gingerich, to search out solutions to those questions. This committee submitted its suggestions to the final meeting in Prague that I discussed earlier.
As per their suggestions, any physique “going round any star, spherical in form and never itself a star” needs to be known as a planet. This could have saved Pluto as a planet, upgraded Ceres again to the planet standing and added Eris and Charon to the checklist of planets.

About 70 per cent of the astronomers attending the final meeting strongly disagreed with virtually every bit of this definition.
Some identified that there are prone to be planetary sized spherical our bodies lurking within the galaxy which aren’t orbiting any star. Some argued that we don’t perceive the physics of planetary sizes effectively sufficient, whereas some argued that this definition meant that the checklist of planets within the photo voltaic system would maintain rising every year, finally even crossing 100.
Some even alleged that the committee had pre-decided that the one ‘planet’ found by an American (i.e. Pluto) should stay a planet and solely thought of definitions which glad that mandate.
So what’s the acceptable definition of a planet?
Realising a widespread disapproval for the proposed definition, IAU appealed to the astronomers current to carry discussions inside themselves and provide you with an appropriate definition.
In these discussions, it was agreed that as an alternative of a common definition of planet, one ought to solely concentrate on planets in our photo voltaic system. That manner, one may maintain apart controversial points similar to planetoids with out stars and an higher restrict on the scale of a planet. It was additionally agreed that Kuiper belt objects, together with Pluto, Charon and Eris, needs to be handled in the identical manner because the Ceres and different asteroid belt objects have been handled.
With these rules in thoughts, the next definition of the phrase ‘planet’ was proposed.
A planet is a physique in our photo voltaic system that
(a) Orbits across the Solar with out orbiting another planetary physique
(b) Is of a spherical form
(c) Would have cleared its orbit
The our bodies which fulfill the primary two standards however not the third one have been dubbed as ‘dwarf planets’. This definition ensures that the variety of dwarf planets might maintain rising however the variety of planets is unlikely to extend drastically.
Clearly, a number of American astronomers have been sad about this demotion of Pluto. Owen Gingerich saved citing this ‘unfair’ remedy of Pluto in a number of public talks and articles for quite a few years.

One 12 months earlier than this common meeting, NASA had launched ‘New Horizons’ mission to flyby Pluto in 2015. The New Horizons crew, together with the Precept Investigator of the mission Alan Stern, was dismayed by the brand new definition. Alan Stern is likely one of the authors of the brand new paper that desires Pluto again as a planet.
In hindsight, how would we take a look at the talk that occurred in 2006 and the compromises that have been reached? Actually it’s not an ideal definition. With Kepler mission information, we now know that the difficulty of uncertainties about higher restrict on planetary mass was most likely an overcautious stance. The ‘clearing the orbit’ criterion can be not as simple as folks thought then. As you go within the outer reaches of the photo voltaic system, orbital intervals turn out to be longer and therefore even Neptune-size our bodies might get categorized as ‘dwarf planets’.
We don’t know for positive what the ultimate variety of planets within the photo voltaic system goes to be. Even now, some astronomers are searching for our bodies greater than Earth past the Kuiper belt. However we all know for positive that the present manner of classifying the planets is the least unhealthy of all choices.
— The writer is an Affiliate Professor on the Homi Bhabha Centre for Science Schooling (HBCSE-TIFR), Mumbai.

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