Spaceflight Insider

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 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.

Reader Comments

Is Pluto completely spherical. ….

(( The new range is just west of the region within Pluto’s heart called Sputnik Planum (Sputnik Plain). The peaks lie some 68 miles (110 kilometers) northwest of Norgay Montes ))
At the same time NASA say” Pluto spherically Despite all those in surface that does not proportion to its size .
“But we don’t believe Pluto spherical because NASA wanted it spherical ”
Do?????? NASA hide a lot about Pluto’s surface details to prove Pluto is spherical to achieve spherical condition of IAU for returning it the ninth planet

Whether Pluto regains its status as a “planet” or remains a “dwarf planet” does not alter the fact that it has turned out to be a fascinating world with a unique history that will raise many new questions. Even before we have a chance to see all the data that New Horizons collected during its close encounter, now is a good time to start thinking about a follow on mission:

Considering how long it took to get New Horizons off the ground and reach Pluto, it could be another quarter of a century before we reach it again even with a repeat “fast flyby” mission like New Horizons. Such a mission could even be made part of a larger, fairly inexpensive program to send spacecraft to revisit Uranus and Neptune as well as other worlds in the outer solar system!

I indeed hope they are at least discussing the next pluto mission. Am I remembering correctly that New Horizon was perfect situation (orbital-mechanically speaking) that might not be repeated for a long time? Sort of a “now-or-never” back in 2006 for a launch window?

New Horizons’ 2006 launch actually used a sub-optimal window. Unfortunately the spacecraft could not be ready in time for the more energetically favorable 2005 window. The next launch opportunity for a repeat of a New Horizons “fast flyby” using a Jupiter Gravity assist is this year and 2016 (obviously we will miss these windows) and the next such launch opportunity is not until 2028/29. More details on available trajectory options are discussed in the link cited above.

I join the chorus in calling for the return to planetary status. Pluto should join the Jupiter/Saturn moon systems as a prime target for further exploration (orbiters, landers). There is much more to be learned this far out in our solar system.

To IAUTen Reasons Pluto is not a planet*

1- Does not has independent orbit around the Sun it cut the orbit
of Neptune.*

2- Pluto and Charon moving around the common center of gravity
which has location outside the bodies of Pluto and Charon. So we can’t say
“Pluto is orbiting Charon or Charon is orbiting Pluto ” also It is not fair
to name one of them planet ( Pluto) and the second (Charon) moon follow
Pluto where Charon has a powerful enough to move Pluto from outside its

3- Pluto is not completely spherically as revealed by NASA “The
new range is just west of the region within Pluto’s heart called Sputnik
Planum (Sputnik Plain). The peaks lie some 68 miles (110 kilometers)
northwest of Norgay Montes” In addition to a large terrain on the surface
of Pluto, with respect to its size is a very large, although NASA hide a
lot ofdetails about Pluto surface without gave us one **full picture
just which edited by painter ,that improve the condition of the
International Astronomical Union to re-Pluto a planet , also the same
thing about Charon .

4- as well as Pluto does not have the strong gravity that enables
it to be completely spherical because has two different faces ( the surface
of Pluto’s face that appears to Charon it has a different terrain in
comparison with surface of other face), So we can say Charon’s
gravity affect the attractiveness of the surface details of the Pluto.*

5- Pluto does not have enough gravity to clear its orbit there are
a lot of orbs in the Kuiper belt intervention in the planet’s orbit Pluto

6- Pluto only is not a planet because it is a binary with Charon.*

7- Pluto rotation around itself the day is equal to a month on
Pluto ,So we can say Charon control the Pluto rotation and Charon remains
free to turn around the itself

8- Pluto’s orbit has a great anomaly in the orbit in orbital
inclination around the sun and the big distance between the aphelion and

9- Other satellites orbiting around Pluto and Charon does not
around Pluto only or Charon only because the common center of gravity
control orbits of these satellites

10- Pluto and Charon the enters the dance which NASA called it compound
orbital dance without explain why those dance and her first interpretations
that Pluto’s mass is not concentrated in the center of the planet. In other
words, if we break it into halves, we will see that one half has larger
mass than the other half by over 20%, and the second interpretation,
according to the hypothesis of compound balance balls for the researcher
Adnan Alshawafi that orbital dance because Pluto and Charon not completely
spherical there are large terrain is the surface which working on
different area between Pluto and Charon opposite faces (Pluto’s surface
perspective from Charon and Charon’s surface perspective from Pluto)

The truth that must defend it and circulated among the members of IAU is

Pluto and Charon is a binary planet

Researcher : Adnan Alshawafi

Honestly, I don’t know what the big deal is!

Distance from the sun (our star) should not be a factor. There’s a lot of junk hanging out in our solar system at different distances from the sun such as the asteroid belt and the Kuiper belt. Icy objects (Kuiper belt) vs rocky objects (asteroid belt) shouldn’t matter.

We call both stars in a binary relationship stars. So we can still call both planets in a binary relationship planets.

The only consideration I have is this:
Is it round? Or round-ish? Even planet Earth is not perfectly round. Does it (singular) or do they (binary) orbit the sun or local star? If yes, then it’s a planet.
Does it orbit another major parent planet? If yes, then it’s a moon.

Once it’s decided if it’s a planet or not, then we can further classify planets and moons according to their composition. Rocky, Gas giant, Habitable (M-Class from Star Trek), Icy, etc.

There’s a lot of needless bickering going on about this.

Debra Giuliano

I grew up with Pluto being recognized as a planet. Should remain!

⚠ Commenting Rules

Post Comment

Your email address will not be published. Required fields are marked *