Topic of the Moment – When a planet is not a planet

On the anniversary of the naming of Pluto, and shortly after the discovery of a new body in the outer solar system, we look at the planets that didn’t quite make the grade

On 1 May 1930, members of the Lowell Observatory voted for ‘Pluto’ as the name for the body discovered at the edge of the solar system by Clyde Tombaugh earlier that year.

Naming it after the Greco-Roman god of the underworld was suggested by an 11-year-old schoolgirl from Oxford, Venetia Burney. Pluto was considered to be the ninth planet from the Sun from its discovery until 2006.

In March this year, US astronomers discovered the furthest known object orbiting the Sun. The not-very-snappily named ‘2012 VP113’ is more than twice as far as Pluto, in a region where we wouldn’t actually expect to find it.

Both are now considered dwarf planets rather than true planets. But what’s the difference?

Classical planets

The closest five planets to the Sun, other than the Earth – Mercury, Venus, Mars, Jupiter and Saturn – have been known about since antiquity. They were distinguished from stars since they moved across the sky against the apparently fixed background of stars – the word “planet” comes from the Greek for “wanderer”.

Initially considered divine, as human knowledge progressed it was eventually recognised that the interior planets are broadly similar to our own, while Jupiter and Saturn are composed of gas – and that they orbit the Sun rather than the Earth.

New planets and the asteroid belt

Uranus was discovered in 1731 by Sir William Herschel , who initially mistook the gas giant for a comet.

Because the distances of the planets from the Sun appeared to follow a regular pattern, apart from the large gap between Mars and Jupiter, a search for another planet in that region was made. Ceres was discovered at the beginning of 1801, but astronomers lost track of it, only recovering its position at the end of that year.

Ceres was also initially classed as a planet, but further observations of the space between the orbits of Mars and Jupiter revealed a huge number of similarly sized bodies, and all were later reclassified as asteroids.

Starting from 1821, when orbital tables of Uranus were published, discrepancies between its observed and predicted position led astronomers to suggest that there was another unseen planet even further from the Sun.

That planet – Neptune – was discovered by Urban Le Verrier and Johann Galle in 1846.

Pluto and beyond

Continued observations of Uranus, however, suggested that Neptune couldn’t be solely responsible for the discrepancies in its orbit, and the hunt was on for a ninth planet. Pluto was discovered by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona in 1930.

It was found to be on a highly elliptical orbit that sometimes too it inside the orbit of Neptune, meaning the two periodically switched places as the furthest planet from the Sun.

But once Pluto’s mass was measured, it too was found to be too small to account for the discrepancies in the orbit of Uranus, and astronomers began to look for yet more bodies in the outer solar system.

The Kuiper Belt, similar to the inner solar system’s asteroid belt, was discovered in 1992. Eris, a body about 27% more massive than Pluto followed in 2005, and was originally considered the solar system’s tenth planet.

This led the International Astronomical Union to reconsider the definition of a planet, ultimately deciding that the only bodies worthy of the name are those which are too small to begin thermonuclear fusion, orbit a star (or stellar remnant), are rounded by their own self-gravity, and have cleared their neighbourhood of its orbit.

Pluto is rounded by its own gravity but has not cleared its neighbourhoods, and so lost its status as a planet, much to the chagrin of internet campaigners, being redefined as a dwarf planet along with Ceres, Eris and several other outer-solar-system bodies.

The newly discovered 2012 VP113 will likely also be considered a dwarf planet, but poses another problem. It lies between the Kuiper Belt and the Oort Cloud – the icy region nearly a lightyear from the Sun where long-period comets are believed to originate. One problem: there shouldn’t be a stable orbit in that region.

Further study of such bodies may shed light on how our solar system formed – whatever we choose to call them.

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