Four “planets” that nearly made it into the solar system

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Newspapers around the world last week reported on the new evidence published that there could be a large but as yet unobserved ninth planet in  our solar system.

The secretive planet has been alluded to by an unexpected clustering of asteroids in the Kuiper Belt, an aggregation of material that lies beyond the reach of Neptune’s orbit. The perhaps optimistically named “Planet Nine” is thought to be around 5,000 times the mass of Pluto and may take up to 20,000 years to orbit the Sun.

planet_9_art_1_ CALTECH R. HURT (IPAC)
An artist’s impression of Planet 9, a potential new member of the solar system (Image: Caltech/R. Hurt (IPAC))

Despite the media frenzy surrounding Planet Nine, there is no certainty that it even exists, and, even if it did, whether it would be eligible for official planetary status. One of the researchers, Mike Brown, is very confident it will be – he stated in the press release it is “the most planet-y of the planets in the whole solar system,” but sadly, the decision isn’t up to him.

The final decision of what actually is a planet lies with the International Astronomical Union (IAU). We spoke to Piero Benvenuti, General Secretary of the IAU who explained how it is decided if an object really is a planet.

“Before a celestial object is named [as a planet], the most important thing is that the scientific community has carried out enough observations and research to be sure that the object actually exists, as well as what type of object it is and what its characteristics are,” he said.

So for Planet Nine, we can’t say for sure if it can yet be added to the solar system family: “It will take another few years and more work to confirm the existence [of Planet Nine]. Until then, the IAU will not take a position on whether this object is a planet or not.”

Those with an interest in astronomy would note that this sort of frenzy over potential new planets is nothing new, and there have been several objects touted as the potential newest members of the solar system, but didn’t quite make the final cut:

1. Ceres, located within the asteroid belt

Ceres at 84,000 kilometers (52,000 mi) away (12 February 2015) WIKI
Photos of Ceres at 84,000 kilometers (52,000 mi) away (Photo: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Ceres, an object discovered orbiting in the asteroid belt between Mars and Jupiter way back in 1801, was originally classified as a planet, but in the 1850s it became the first body to be named an asteroid.

Ceres was recognised again as a potential full member of the solar system by the mid nineteenth century, but now is officially classed as a minor planet – an astronomical object that is neither a planet nor an asteroid or comet.

2. Sedna, another Kuiper Belt resident

Generated by IJG JPEG Library
An artist’s impression of Sedna (Image: NASA/JPL-Caltech/R. Hurt)

Sedna, similar in size to Pluto but three times further away from the Sun, was discovered in 2004 and was described by the BBC to potentially be the Solar System’s tenth planet.  It’s since been classified as a minor planet too.

3. Pluto’s ‘moon’, Charon

nh-charon-neutral-bright-release NASA
Photo of Charon as taken by the New Horizons spacecraft in 2015 (Photo: NASA)

Pluto’s largest moon, Charon, was in the running to be considered a planet too for the briefest of periods. Charon is in a binary system with Pluto and not technically a satellite, meaning they both rotate around the same common centre of mass, but it was decided that Charon was to be considered a moon for the foreseeable future by the IAU.

4. Eris, located outside the Kuiper Belt

Artist’s impression of the dwarf planet Eris
Artist’s impression of the dwarf planet Eris (Image: ESO/L. Calçada and Nick Risinger)

In 2005, there was the suggestion by NASA that Eris, a planet originally touted as larger in size than Pluto, could be considered the tenth planet. It was actually Mike Brown, one of the two researchers behind the discovery of Planet Nine, who was the first to suggest that Eris could be eligible for solar system inclusion.

So how does an object pass for a planet in our solar system?

At one point, a model that included 12 planets in the solar system was suggested and eventually in 2006 the IAU pinned down the definition of a planet. The IAU initially proposed that a body could be considered a planet if:

  • It has sufficient mass for its own gravitational pull to force it into a spherical shape
  • Is in orbit around a star (such as the Sun)

This wasn’t a particularly straightforward decision to make, and for a while a model of the solar system that included Ceres, Pluto, Charon and Eris too was being considered as a result.

What our textbook diagrams might have looked like, if the plan for 12 planets in the solar system was to go ahead. (Photo: NASA)

Brown deemed this model “a mess”, and it also would have made more than 50 other celestial bodies eligible for planet status. After more discussions a third parameter was added: a planet must “clear the neighbourhood around its orbit,” which meant that Pluto, Ceres, Eris and Charon were no longer eligible for an esteemed place as a planet in the solar system.

So, could this Planet 9 live up to the potentially prematurely, optimistic branding and meet the planetary standards of the IAU? Brown and colleague Konstantin Batygin state that their research shows that it would should it exist, and the challenge now is to spot the object in space, which could take years.

So it certainly will be  while before textbook printers need to start redrafting their planet guides and the mnemonic fans have to return – again – to the drawing board to make new rhymes to help us remember all of members of this exclusive celestial club.

So for now, let’s not count our planets until they have been detected and remember that the battle to even be a planet is still a tough one to fight.

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Philippa Skett

Philippa Skett

Media Officer at Institute of Physics
Philippa was the IOP's media officer
Philippa Skett

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