Pluto flyby spurs new petition for planetary status
On July 14, 2015, the day that NASA’s New Horizons spacecraft conducted its historic flyby of the dwarf planet Pluto, the producers of Yahoo’s web TV show “Other Space” published a petition on the advocacy site change.org seeking the reinstatement of Pluto’s planetary status. This is just the latest step in a back-and-forth battle to determine what the definition of a planet is.
Titled “Declare Pluto a Planet” and featuring the hashtag #PlutoFlyby, the petition has received more than 4,270 signatures in the week since its placement online.
It is directed at the International Astronomical Union (IAU), the group that in 2006 adopted a controversial planetary definition that excludes Pluto. The petition asks for Pluto’s reinstatement at the organization’s triennial General Assembly, scheduled to take place next month in Honolulu, Hawaii.
The 2006 vote was motivated by the discovery of the Kuiper Belt, a large region of icy objects beyond the Solar System’s gas giants. Several Kuiper Belt Objects large enough to be spherical were discovered beginning in the 1990s.
In 2003, Eris – an object 1,445 miles (2,326 kilometers) in diameter and initially thought to be larger than Pluto – was discovered, reigniting the long-standing debate over Pluto’s status as a planet that began with the distant world’s discovery in 1930. By comparison, Pluto is estimated to be some 1,473 miles (2,370 km) in diameter.
That debate came about when Pluto was found to be much smaller than the gas giant that astronomers had initially been searching for beyond the orbit of Neptune.
Until 1978, when Pluto’s large moon Charon was discovered, astronomers continued to think Pluto was about the same size as Earth, primarily because they had not realized that they were looking at two objects very close to one another.
Eris is about 27 percent more massive than Pluto. Upon its discovery, many scientists began referring to it as the Solar System’s tenth planet.
Concerned that many more Pluto-sized objects might lurk undiscovered in the Kuiper Belt, some astronomers worried that the Solar System could end up with hundreds or thousands of planets.
In an attempt to resolve this issue, the IAU appointed a Planet Definition Committee to draft a new, more specific planet definition.
That definition, which included Ceres, Pluto, and Eris, was voted down by the General Assembly. On the last day of the conference, a small group of IAU members put a hastily drawn up alternate definition to a vote, bypassing a rule in the panel’s bylaws requiring resolutions to first be vetted by the appropriate committee before being put to the General Assembly floor.
Adopted that day by only 424 out of its 10,000 members, that resolution set three requirements for an object to be considered a planet. It must orbit the Sun, be massive enough to be rounded by its own gravity (a state known as hydrostatic equilibrium), and must clear the neighborhood of its orbit.
Objects that meet the first two requirements but not the third were placed in a new class called “dwarf planets”. Part b of the resolution that established these categories, which would have placed dwarf planets under the broad umbrella of planets, was voted down 333-91.
That means dwarf planets, which orbit in belts along with many smaller objects, are not considered planets but an entirely different class of objects by the IAU.
New Horizons had launched only seven months earlier on its nine-and-a-half-year journey to Pluto.
The IAU definition proved controversial from day one. Alan Stern, New Horizons’ principal investigator, collected the signatures of several hundred professional astronomers on a petition rejecting it within just a few days.
For nearly nine years, the debate has raged on. Questions were raised as to the meaning of “clearing an orbit” since most Solar System planets have asteroids in their orbital paths.
Many noted the definition precludes all exoplanets from being considered planets, as it requires an object to orbit the Sun rather than a star to be considered a planet.
The debate highlighted growing professional divisions between astronomers, who study the effects celestial objects have on other objects, and planetary scientists, who focus on the intrinsic properties of individual objects.
In this latest petition, Paul Feig, creator of “Other Space” and Trace Beaulieu of “Mystery Science Theater 3000” points out that the 2006 definition was crafted mostly by astronomers rather than planetary scientists (by comparison, one need only imagine the reaction if planetary scientists attempted to redefine what a nebula or galaxy was).
“By declaring that Pluto was no longer a planet, the IAU put into place a planetary definition that would have even declassified Earth as a planet if it existed as far from the Sun as Pluto does,” the petition reads.
“But this is about much more than planetary definitions. When bureaucratic bodies with little expertise in planetary science choose to create inaccurate definitions of planets, they threaten to suppress millions of dollars in funding for future generations of space exploration,” the petition states.
Just one day before the flyby, New Horizons data confirmed Pluto is marginally bigger than Eris, with a diameter of 1,473 miles.
Stern, who is not involved with the current petition, prefers the geophysical planet definition, according to which a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. That last provision, which establishes spherical moons as secondary or satellite planets, is also a point of controversy.
Some members of NASA’s Dawn mission, whose spacecraft entered orbit around Ceres in March of this year, believe spherical Ceres should be classed as a planet too.
“When we complete our observations, we will show that Ceres is every bit a planet as its terrestrial neighbors Mars, Earth, Venus, and Mercury,” said Dawn mission leader Christopher Russell.
The petition does not address the status of Ceres, Eris, or other dwarf planets, although many planetary scientists believe they too should be considered a subclass of planets.
Its organizers invoke the support of planetary scientist and blogger Philip Metzger, who recently retired from NASA’s Kennedy Space Center.
The petition argues that reclassification of Pluto as a planet by the IAU will build on excitement generated by the New Horizons flyby, potentially motivating a new generation of planetary explorers.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.