Press Release

CSEPR examines movement to set aside IAU planet definition ruling

By SpaceRef Editor
August 31, 2006
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CSEPR examines movement to set aside IAU planet definition ruling

On August 24, a session of the International Astronomical Union (IAU) General Assembly in Prague passed a resolution re-defining the planets of our solar system. Only 428 of the IAU’s nearly 10,000 members were involved in the vote. A proposal crafted over the previous year by the IAU Planet Definition Committee would have expanded the number of objects designated as planets in the solar system to 12, with the potential for additions in the future. At the assembly, however, the proposal was modified over the course of several days to define the term with the intent of excluding all but the eight largest planets. Neither definition was subjected to critical review by the broader planetary science community prior to the assembly.

As part of its role to examine the nature of scientific authority, the Center for Space Exploration Policy Research (CSEPR) is considering the role of the IAU and its findings, as well as a petition to reevaluate the principles for planet definition.

Just after the August 24 vote, members of the space science community pointed out serious technical and pedagogical flaws in the IAU’s definition of planets. As a consequence, a grass-roots petition was posted, stating:

“We, as planetary scientists and astronomers, do not agree with the IAU’s definition of a planet, nor will we use it. A better definition is needed.”

The statement was placed on the web at and circulated by e-mail to a small fraction of the world’s astronomical research community.

In less than five days, the petition was signed by 300 professional planetary scientists and astronomers. The list of signatories (posted at the website above) includes researchers who have studied every kind of planet in the solar system, as well as asteroids, comets, the Kuiper Belt and planetary interactions with the space environment. Many have been involved in the robotic exploration of the solar system from some of the earliest missions to Cassini-Huygens, missions to Mars, and ongoing missions to the innermost and outermost reaches of our solar system. Others are leading missions that are preparing for launch. The petition list includes prominent experts in the field of planet formation and evolution, planetary atmospheres, planetary surfaces and interiors, as well as international prize-winning researchers.

“This petition gives substantial weight to the argument that the IAU definition of planet does not meet fundamental scientific standards and should be set aside,” states petition organizer Dr. Mark Sykes, director of the Planetary Science Institute in Tucson, Az. “A more open process, involving a broader cross section of the community engaged in planetary studies of our own solar system and others, should be undertaken.”

“I believe more planetary experts signed the petition than were involved in the vote on the IAU’s petition,” adds co-sponsor Dr. Alan Stern, executive director of the Space Science and Engineering Division at Southwest Research Institute. “From the number of signatories that the petition received in a few days, it’s clear that there is significant unhappiness among scientists with the IAU’s planet definition, and that it will not be universally adopted by scientists and textbook writers.”

“A key public policy question is who has the social mandate to alter the definition of something as fundamental as a planet,” says Dr. Mark Bullock, director of the CSEPR. “Scientists have in the past vested the IAU with authority to name asteroids and other planetary objects. However, the word ‘planet’ has cultural, historical, and social meaning and as such requires much broader discussion and consensus than those required for the naming of astronomical bodies.”

The CSEPR is currently examining the nature of scientific authority, and its use and misuse in issues of fundamental concern to the public. The scientific and cultural value of the definition of planets, both within and outside our solar system, is of utmost significance. Accordingly, continues Stern, “To achieve a good planet definition that achieves scientific consensus will require more work.”

The Center for Space Exploration Policy Research seeks to stimulate discussion on the ethical, cultural and philosophic aspects of space policy. For more information on the CSEPR, visit


Dr. Mark Bullock

Dr. Mark Sykes

Dr. Alan Stern

SpaceRef staff editor.