Planet or dwarf planet—all worlds are worth investigating

March 20, 2017 by Tanya Hill, The Conversation
Pluto is a dwarf planet but that doesn’t make it any less worthy of our attention. Credit: NASA/JHUAPL/SwRI

Pluto's status as a "dwarf planet" is once again stirring debate. This comes as some planetary scientists are trying to have Pluto reclassified as a planet – a wish that's not likely to come true.

Pluto has been known as a dwarf planet for more than a decade. Back in August 2006 astronomers voted to shake up the solar system, and the number of planets dropped from nine to eight. Pluto was the one cast aside.

There was some outcry that Pluto had been destroyed in an instant and was no longer important, and the reverberations were most keenly felt across America.

After all, Pluto was "their planet", discovered in 1930 through the meticulous observations of American astronomer Clyde Tombaugh at the Lowell Observatory in Arizona.

At the time of the vote, NASA's New Horizons spacecraft was only seven months into its nine-year journey to Pluto. There was concern that when it finally arrived, would people even care about a dwarf planet?

For many , the demotion of Pluto was a defining moment. It wasn't a gesture of destruction and it wasn't aimed specifically at Pluto. What it signalled was a major leap forward.

In that moment the world's astronomers acknowledged significant progress in our understanding of the solar system, an achievement to be proud of – even if everyone was not entirely happy.

Earth became a planet too, once the ‘wanderers’ were understood. Credit: NASA/Reid Wiseman

What's in a name?

The first step to understanding a group of objects is to classify them. We group like with like to examine the aligned characteristics or any significant differences between groups. With this insight comes a deeper understanding of how things work, form or evolve.

The planets were originally grouped together because the ancient Greeks saw them as "the wanderers", travelling across the sky. Five bright objects – Mercury, Venus, Mars, Jupiter and Saturn – may have looked like stars, but while stars stayed fixed within their constellations, these planets moved independently from them.

The cause of this planetary motion was eventually established by the Polish astronomer Nicolaus Copernicus in the 16th century, bringing with it a new revelation. Planets were more than wanderers, they were objects in orbit about the sun and with this understanding Earth became a planet too.

Defining a planet in the 21st century

More than 400 years and many discoveries later, a new storm began brewing in our understanding of the solar system.

Since 1992, astronomers had begun to find objects orbiting the sun out in the realm of Pluto. Were they planets too?

Astronomers raise their yellow cards and Pluto becomes a dwarf planet. Credit: Martin George

Conversely, Pluto was a bit of an oddball. It was smaller than several moons of other planets, and it had a highly inclined orbit that made it stand out from the others. Was it truly a planet or was it part of a much larger family of objects?

With the discovery of Eris (originally known by its designation 2003 UB313) in 2003, a decision could no longer be avoided. Eris was about the size of Pluto and certainly more massive. Was Eris a planet? And if not, where did that leave Pluto?

Astronomers have a forum for such deliberations via the International Astronomical Union (IAU). Representing astronomers worldwide, the IAU is the recognised authority responsible for naming and classifying planetary bodies and their satellites.

The IAU formed a Planet Definition Committee to consider the scientific, cultural and historical issues at hand. A draft proposal was put forward, and during the 2006 IAU General Assembly in Prague, with the world's astronomers gathered together, the Committee's proposal was vigorously debated.

A revised proposal was presented to the IAU membership on the final day of the General Assembly and was passed with a large majority.

The Milky Way and its neighbouring dwarf galaxies, the Large and Small Magellanic Clouds seen in the lower left. Credit: ESO/C. Malin

For the first time, a planet was formally recognised as being "a celestial body that":

(a) is in orbit around the sun

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape

has cleared the neighbourhood around its orbit.

Since Pluto had not "cleared the neighbourhood around its orbit", it was not a planet but would be recognised as a "dwarf planet".

A colleague of mine, Martin George, director of the Launceston Planetarium, was there when the vote was taken and captured the excitement and the nuance of the event.

There was quite a buzz in the room and we knew we were about to make history. Did everyone agree on the exact wording? Perhaps not. However, I think it would have been worse to see media headlines reading 'Astronomers cannot decide what a planet is'.

The dwarf planets compared to Earth. Credit: NASA

Size matters and location too

The distinction of planet and dwarf planet brings a consistency to how objects are named across the universe. On the grand scale, there are galaxies and there are dwarf galaxies.

Within our Milky Way Galaxy, the sun is a yellow dwarf star that in billions of years will evolve to become a red giant before ending its life as a white dwarf.

These distinctions among galaxies and stars helps astronomers interpret and understand them, tracing their evolution.

Planets and are distinct because of their size and their location in the solar system. It provides a way to examine how planets and dwarf planets may have originated and evolved differently.

Planetary resemblance

At present, the IAU has officially recognised five dwarf planets. They are Pluto, Eris, Makemake and Haumea, which orbit the sun beyond Neptune, and Ceres, which is the only object in the asteroid belt massive enough to be spherical.

There are many solar system objects smaller than Earth. Credit: primefac

Detractors and also supporters of the standing planet definition can point to problems with it. For instance, it only applies to objects orbiting the sun. But what about exoplanets? And what is meant by "cleared its neighbourhood"? If Earth was located farther away from the sun, would it be able to clear its orbit?

But, as astrophysicist Ethan Seigal explains, minor qualifications to the planet definition can bring it in line with exoplanets and allows the definition to work with renewed clarity.

Whereas the latest proposal to reinstate Pluto, advocates a geophysical definition of planet. Namely, that a planet should be large enough to be round, but not so big that it is a star. This broad definition casts the net wide, and not only Pluto, but also the moon and more than 100 other solar system objects would become planets.

Now wouldn't that be a leap backwards in regards to structuring and understanding our solar system? How much of it is driven by the notion that nothing but a planet is worth exploration?

There's a plethora of "not-planets" in our solar system that are worlds worthy of attention. This includes the fiery volcanoes of Io, the icy geysers of Enceladus, the reddish surface of Makemake, the crazy spin of Haumea and the mystery of hundreds of worlds unknown orbiting beyond Neptune.

So let the official word on planets and dwarf be as passed in 2006 and let our exploration of the continue to amaze us.

Explore further: How many planets are in the solar system?

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antialias_physorg
1 / 5 (3) Mar 20, 2017
There was concern that when it finally arrived, would people even care about a dwarf planet?

Since those who would look at the data and actually work with it couldn't care less whether it's called a "planet" or a "dwarf planet" - what's the point? The public might go less 'oh' and 'ah' over images from a dwarf planet, but the public's opinion really doesn't matter one bit in any of this.
434a
5 / 5 (1) Mar 20, 2017
There was concern that when it finally arrived, would people even care about a dwarf planet?

Since those who would look at the data and actually work with it couldn't care less whether it's called a "planet" or a "dwarf planet" - what's the point? The public might go less 'oh' and 'ah' over images from a dwarf planet, but the public's opinion really doesn't matter one bit in any of this.


I think public opinion ultimately matters because the money being spent on missions is overwhelmingly a result of taxation. The consequence of the funders, sorry public's, failure to get behind mission spending in an era of competition for dwindling budgets may well mean the executive make decisions based on the potential headlines as much as the science.
antialias_physorg
not rated yet Mar 20, 2017
I think public opinion ultimately matters because the money being spent on missions is overwhelmingly a result of taxation.

Sure, but does the public even know what type of probes are in the pipe? The sun one? Or the Webb? Would the public be for/against NASA spending any differently if they were aware of these missions?

And don't forget: The Pluto mission was launched 10 years before it got there (and the budget was appropriated six years before that). The public doesn't have that kind of horizon that would give them a "yay" or "nay" attitude towards such a mission one way or another. There certainly wasn't any big PR going on in 2000 when the budget for this were planned
434a
5 / 5 (1) Mar 20, 2017

And don't forget: The Pluto mission was launched 10 years before it got there (and the budget was appropriated six years before that). The public doesn't have that kind of horizon that would give them a "yay" or "nay" attitude towards such a mission one way or another. There certainly wasn't any big PR going on in 2000 when the budget for this were planned


I understand what you are saying but I think It's not as specific as a mission by mission view, it's about public sentiment that includes debates about public funding for science, NASA budgets, value for money etc etc
In the current political and economic climate I would be certain to ask how a follow-up mission to Pluto may be perceived by the people I am doing this on behalf of.
I know you know that politics and sentiment are factors in any publicly funded organisation. I think that Pluto's planetary status is simply an additional factor that would need to take into account.
Steelwolf
5 / 5 (3) Mar 20, 2017
Something mis-represented here is while they did put the question to the Assembly on the last day of the Meeting, MOST of the astronomers had already left and so there was NOT a "Majority" of ALL the Astronomers voting at the time, so they can only clain that a majority of a TINY SLICE of the assemblage voted on it.

This is a LONG way from A MAJORITY OF ASTRONOMERS voting for it. Besides, Neither Earth, nor Mars or Jupiter or Saturn have completely cleared their orbits, there are the Trojan points in the orbits, and they are heavily populated with asteroids as well. Thus not even Jupiter is fully a Planet by that factor.

Getting a vote from a depleted crowd like that is NOT Representative and is an insult to the intelligence of the rest of us to have it mischaracterized like that.
laurele
5 / 5 (2) Mar 20, 2017
NO, let the official word on planets and dwarf planets NOT be as wrongly passed in 2006! That vote was taken by just four percent of the IAU's members, most of whom are not planetary scientists but other types of astronomers; was done in violation of the IAU's bylaws, which prohibit a resolution from being placed on the floor of the General Assembly without first being vetted by the proper IAU Committee, and makes very little sense. It was opposed by an equal number of planetary scientists in a formal petition just days later.

Alan Stern is the person who coined the term "dwarf planet," but he did so to designate a third class of planets in addition to terrestrials and jovians, not to designate non-planets. The four percent of the IAU who voted misused his term. Dwarf planets are a subclass of planets just like dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies.

laurele
5 / 5 (2) Mar 20, 2017
Even Owen Gingerich, head of the IAU's Planet Definition Committee, admits the IAU overstepped its bounds trying to define "planet" and should never have attempted to do so. The IAU leadership has been asked on numerous occasions to reopen this discussion and has blatantly refused.

Meanwhile, the IAU definition has numerous other problems. First, no object fully "clears its orbit," as even Jupiter orbits with Trojans, and Neptune does not clear its orbit of Pluto. Second, by saying a planet has to orbit the Sun, the IAU definition automatically precludes any exoplanets from being considered planets. And because it puts primacy on an object's location rather than on its intrinsic properties, the IAU definition could result in the absurdity of the same object being classed as a planet in one location and as not a planet in another location. If Earth were in Pluto's orbit, it would not clear that orbit either.
laurele
5 / 5 (1) Mar 20, 2017
Even more importantly, science is not decided by decree of "authority." That is dogma.

Since you quote Ethan Siegel and provide a link to his article, it is only fair that you also provide a link to a rebuttal of that article, which can be found at http://laurelsplu...has.html
arcmetal
5 / 5 (1) Mar 21, 2017
"A revised proposal was presented to the IAU membership on the final day of the General Assembly and was passed with a large majority."

I wouldn't call less than 5% who show up for a vote a "large majority".

antialias_physorg
2 / 5 (2) Mar 21, 2017
Even more importantly, science is not decided by decree of "authority."

Good thing then that assigning a label is not science, eh?

I wouldn't call less than 5% who show up for a vote a "large majority".

If the attendance was really that low (especially amongst planetary scientists) then that gives you a clue as to how unimportant the issue is to all scientists concerned. They don't care whether you call it a planet, dwarf planet of a froopydum. It doesn't change science one bit what you call stuff.
fcsuper
5 / 5 (1) Mar 21, 2017
The only reason why we care so much about what is and is not a planet is due to the Astrological origins of Astronomy. The term "planet" isn't special, and things we call planets shouldn't be special either. The current definition by IAU doesn't make any sense since it pretty much invalidates all the planets of "planet" status. For example, Pluto is tied to Neptune's orbit. So, technically, Neptune's orbit hasn't been cleared of other objects (the biggest being Pluto's system), so Neptune isn't a planet. It's a mess. We need to stop treating the word "planet" like it's some special term. It's just a word and it should be used to describe a well defined class of objects that aren't determined by ancient superstitions.
arcmetal
5 / 5 (1) Mar 22, 2017
Even more importantly, science is not decided by decree of "authority."

Good thing then that assigning a label is not science, eh?

I wouldn't call less than 5% who show up for a vote a "large majority".

If the attendance was really that low (especially amongst planetary scientists) then that gives you a clue as to how unimportant the issue is to all scientists concerned. They don't care whether you call it a planet, dwarf planet of a froopydum. It doesn't change science one bit what you call stuff.

Incorrect. You should read up on what happened before commenting: http://news.bbc.c...3956.stm

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