How many planets? #PlutoIsAPlanet? #TheMoonIsAPlanet?

Is it time to restore Pluto back to the planet club?

A new effort to restore Pluto to the so-called Planet club is underway.

As reported at

Rather than focusing on “external” factors such as whether a body has cleared its orbit (the portion of the IAU criteria that Pluto failed in 2006), the new geophysical definition instead brings to the forefront the intrinsic properties of the body itself. It takes into account the fact that many of the solar system’s worlds are physically complex and geologically active, from Ceres’ ice volcano to Pluto’s slushy heart.

In essence, rather than focusing on the orbit, this proposed new definition suggests we focus on the object itself. Questions like. Is it round? Is is complex on the surface? Is it active? Unfortunately this approach may suffer the same fate as many others, in that this new definition is too open. It would result in over 100 new ‘planets’ in our solar system alone. Even our Moon would now be a planet. As such, maybe this new definition is doomed to failure.

So, adopt this new approach of Pluto-as-a-planet, at the cost of labeling 100 other objects as planets? Or leave things as they are?

Move over, Eris – a new distant world takes your record #V774104 @profmcateer


The title of “most distant object in the Solar System” has a new champion.

Astronomers have used  Japan’s Subaru telescope to reveal a new icy body at 15.5 billion km from the Sun. This is over 100AU, nearly three times further away than even far-flung Pluto. The previously recognized most distant object was the dwarf planet Eris, which resides at about 12 billion km. The new distant object – cataloged rather unimaginatively as V774104 – is probably the same size as New Mexico, and its orbit remains a mystery The dwarf planet could eventually join one of two clubs. If its orbit brings it closer to our sun, it would become part of a more common population of icy worlds that interact with Neptune. But if its orbit continues to sling it away from the sun, it could join a rare club with only two other known members, Sedna and 2012 VP113.

Those two bodies are both actually currently slightly closer in than Eris, but their orbits will reach far deeper into space, out to 66 billion km and 140 billion km, respectively. Such erratic orbits are difficult to explain. Indeed it unlikely there were formed in these particular orbits. Several existing theories propose differing origins. In one theory, these bodies were perturbed gravitationally by some other planet and pulled on to their strange trajectories. The passing planet would then have been expelled far out of the solar system. A second theory proposes that such objects could be stolen from a sister star that formed from the cloud of gas and dust as our Sun. A third theory proposes that gravitational forces acting on the solar system when the protosun was surrounded by other stellar nurseries, could have provided the necessary nudges.

And it has personal touch for one American scientist

From Science magazine

Mike Brown, a planetary astronomer at the California Institute of Technology in Pasadena unaffiliated with the discovery, says that this is the allure of these extreme objects. “They carry the signature of whatever else happened,” he says. But until Sheppard pins down its orbit, V774104 may be interesting—or not, Brown says. “There’s no way to know what it means.” On the other hand, Brown acknowledges that he will have to give up the claim to having discovered the most distant solar system object, which came in 2005 when he found the dwarf planet Eris at a distance of 97 AU from the sun. “I have held the record for 10 years,” he says, jokingly. “I have to relinquish it. So I’m sad.”

And of course we should note that mankind is doing even better than V774104 – the Voyager 1 probe is even further away at 20 billion km from home and still going strong.