What Is A Planet?

Astronomer Mike Brown lectures at the National Air and Space Museum, March 20, 2007. Video source: Smithsonian YouTube channel.

What is a planet?

That's the question Caltech astronomer Mike Brown posed to a friend in December 1999.

Seven years earlier, Massachusetts Institute of Technology astronomers David Jewitt and Jane Luu proved the existence of the Kuiper Belt, a vast population of small solar system bodies beyond Neptune.

After Clyde Tombaugh discovered Pluto in 1930, many astronomers suspected more might lie beyond, but lacked the technology to explore further.

Pluto itself was a bit of an oddball. It was much smaller than the other planets. It's smaller than Earth's moon. Pluto's eccentric orbit takes it within the orbit of Neptune for twenty years of its 248-year orbit around the Sun; the last time was 1979-1999.

In his 2010 book How I Killed Pluto, and Why It Had It Coming, Brown argues that Pluto never should have been categorized as a planet in the first place.

The definition of a “planet” has changed over the centuries.

The word “planet” derives from the ancient Greeks, who described as “wandering stars” the lights in the night sky that were not fixed, but slowly moved over time. Our word “planets” is descended from the Greek phrase for wandering stars, asteres planetai.

In the 19th Century, as the tools used by astronomers matured, many new “planets” were discovered. Brown writes about the discoveries of Ceres, Pallas, Juno, and Vesta in the years 1801-1807. They too were described as “planets.” By 1851, fifteen more of these asteroid planets had been discovered, as well as Neptune. Brown has in his collection an 1896 Rand McNally Atlas solar system map that lists Ceres, Pallas, Juno and Vesta as planets.

By the early 20th Century, common convention had dropped these as planets. The asteroids were no longer assumed to be “planets.” No international organization or body voted to change the definition. Popular opinion simply changed. An object no longer simply had to be a “wandering star” to be a planet. Size seemed to matter.

When Clyde Tombaugh discovered Pluto in 1930, it was bigger than the known asteroids, but it was much smaller than the other planets and had an eccentric orbit. The Kuiper Belt would remain undiscovered for another seventy years, so Pluto could not be evaluated in its context. Pluto became the ninth planet, simply because no other term seemed to fit.

The International Astronomical Union was founded in 1919. According to its web site, “Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation.”

On the naming of astronomical objects, its web site states:

Celestial nomenclature has long been a controversial topic. At its inaugural meeting in 1922 in Rome, the IAU standardized the constellation names and abbreviations. More recently IAU Committees or Working Groups have certified the names of astronomical objects and features ...

The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.

The discovery of the Kuiper Belt, and subsequent discoveries of Kuiper Belt Objects (KBOs), is perhaps one of the most fundamental and revolutionary astronomical advances of our time.

Until a few years ago, we all thought that, beyond Neptune, it was only Pluto and then interstellar space.

According to David Jewitt's web site, “There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU.”

An astronomical unit (AU) is the distance from the Sun to Earth, about 93 million miles or 150 million kilometers. Pluto's average distance from the Sun is about 40 AU.

KBOs have orbits of various eccentricities. Many of them have highly ellipitical orbits, influenced by the gravitational pull of large objects such as Neptune. Others seem to meander on their own.

In January 2005, Brown's Caltech team discovered a KBO that the IAU eventually approved naming Eris. It was 27% more massive than Pluto, although Pluto is slightly larger by volume. Eris' average distance from the Sun is about 68 AU; right now, it's about 97 AU from the Sun.

The discovery of Eris raised once again the question, “What is a planet?”

If only size matters, Eris is more massive than Pluto, so Eris must be a planet. But what about other KBOs that may be larger than Pluto?

Many of these objects don't sweep their orbit of debris. Many planets have craters on their surface, because their gravity attracted asteroids and other objects to impact. On Earth, most impacts have been erased over the eons by weather and seismic events. On planets with little or no atmosphere, the evidence is far more evident.

KBOs have impact craters, but lots of debris still barrel about their orbits. KBOs are so small, they have relatively little gravitational pull. They don't sweep their orbit like what we think of as planets. Their surfaces are rough and craggy, not relatively smooth and round like much larger celestial objects.

Taking all this into consideration, the IAU in August 2006 chose to define a planet. Planets and other bodies “except satellites” (i.e. moons) henceforth would be defined into one of three categories — planet, dwarf planet or small solar system body.

The difference between a planet and dwarf planet is that the latter “has not cleared the neighbourhood around its orbit.”

What do they have in common?

Both orbit around the Sun, have sufficient mass for gravity to force the body into a round shape, and are not a satellite.

If an object doesn't fall into either category ... it's a “small solar system body.”

Eris and Pluto became dwarf planets, along with KBOs Haumea (discovered in December 2004) and Makemake (discovered in March 2005). Ceres, in the asteroid belt between Mars and Jupiter, is the fifth dwarf planet.

The objects in the asteroid belt and the Kuiper Belt, if not falling into the other two categories, are small solar system bodies.

Clear as mud?

Let's muddy the astronomical waters even more.

In January 2016, Caltech researchers Mike Brown and Konstantin Batygin announced “evidence of a real ninth planet.” The press release stated:

Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.

The researchers used mathematical models to infer the planet's existence, but it's so far away that it can't be observed directly. If it exists, it's about 700 AU from the Sun.

January 20, 2016 ... Mike Brown and Konstantin Batygin discuss their findings of a possible distant solar system planet. Video source: Caltech YouTube channel.

Their work was published in the February 2016 edition of The Astronomical Journal. Click here to download a PDF of the report.

In May 2016, science journalist Bruce Dorminey suggested at Forbes.com that the new James Webb Space Telescope (JWST) could be used to search for Planet Nine. Dorminey wrote:

“If a new planet is found, JWST will be able to fully characterize it,” Stefanie Milam, JWST deputy project scientist for planetary science at NASA Goddard Space Flight Center, told me. “Planet 9 is predicted to be fairly large but far, so most ground based facilities [would] barely be able to detect it.”

Milam says this would include being able to detect compounds like carbon dioxide in the putative planet’s tenuous, icy atmosphere.

According to NASA's JWST web site:

Webb can observe everything in our Solar System that is further from the Sun than the Earth is. Webb's sensitivity will be most useful in studying the faint rocky and icy objects in the far outer Solar System, including the dwarf planet Pluto and other Kuiper Belt Objects. Webb's studies of these objects will test theories of how the Solar System formed. Webb will also observe the moons of the gas giant planets, comets and asteroids and the planets Mars, Jupiter, Saturn, Uranus and Neptune.

The 20th Century saw humanity launch its first telescopes above the atmosphere.

The electromagnetic spectrum, from radio waves to gamma rays. Click to view at a larger size. Image source: NASA Goddard Space Flight Center.

The Hubble Space Telescope, deployed by the Space Shuttle in 1990, is the most famous but many preceded and followed. Space telescopes may be specialized to look at certain wavelengths within the electromagnetic spectrum, from radio waves to gamma rays. Humans see only visible light, but objects may be observed at other wavelengths. Planet Nine may be too far away to reflect visible light, but if it has any warmth JWST might be able to pick it up, because it's designed specifically to capture infrared light.

The 21st Century may reveal more planets, dwarf planets, small solar system bodies and other objects beyond the Kuiper Belt. If the century began with the question, “What is a planet?” it may end with the question, “Where does the solar system end?”

In 2013, NASA tried to answer this question when Voyager 1 entered interstellar space. The probe had moved beyond the Sun's bubble of electrically charged particles, called the heliosphere. Voyager 1 passed through the heliopause, the border where the bubble ends, and encountered interstellar wind.

A September 2013 NBC News illustration depicting a definition for the end of the solar system.

But Voyager 1 has yet to reach the Oort Cloud, a theoretical cloud of icy objects that are believed to be the source of comets and other debris that occasionally enter the solar system. The Oort Cloud is considered, for now, to be the outermost region of the solar system. The small solar system body, Sedna, discovered in 2003, may be the first object detected in the Oort Cloud. Sedna never enters the Kuiper Belt. It travels in a long elliptical orbit between 76 and 1,000 AU from the Sun.

The discovery of Sedna by the Caltech team led them to look for other objects with odd orbits. Their research resulted in the January 2016 paper suggesting the existence of Planet Nine.

The question “What is a planet” led Dr. Brown and the Caltech researchers beyond Pluto and beyond the Kuiper Belt. How much farther will they go?

Will “Where does the solar system end?” finally answer the question, “What is a planet?”

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UPDATE September 15, 2016 — The Los Angeles Times published an interview yesterday with Mike Brown and Konstantin Batygin about their upcoming search for Planet Nine.

What we have is a gravitational signature of the existence of this planet. This planet, just like every other major planet in the solar system, shepherds the small bodies that surround it. And the particular way in which the orbits of this debris are arranged can only be explained by existence of a planet. So it’s a little bit like being downtown and hearing an ambulance — you know it’s there, but you haven’t seen it yet. You even know the overall direction the sounds are coming from, but you haven’t seen what color the ambulance is.

In Pluto news, NASA announced yesterday the cause of a red splotch on Pluto's largest moon Charon.

... Charon’s polar coloring comes from Pluto itself — as methane gas that escapes from Pluto’s atmosphere and becomes “trapped” by the moon’s gravity and freezes to the cold, icy surface at Charon’s pole. This is followed by chemical processing by ultraviolet light from the sun that transforms the methane into heavier hydrocarbons and eventually into reddish organic materials called tholins.

A July 14, 2015 image of Charon taken by NASA's New Horizons probe. Click the image to view at a larger size.