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Message from DPS Chair - What is a Planet?

Letter from the Chair: What is a planet?

On August 24, the IAU passed two resolutions that defined three categories of bodies in the solar system: planets, dwarf planets, and small solar system bodies. A majority of the IAU members gathered at the 2006 General Assembly in Prague voted that a planet is defined as a celestial body that (a) is in orbit around the Sun, (b) is round (i.e., is in hydrostatic equilibrium against rigid body forces), and (c) has cleared the zone around its orbit. (For the full text of the resolutions, see

Eight planets retain their planethood by this definition, and a new category of "dwarf planets" is defined: objects that are large enough to be in hydrostatic equilibrium but which have not cleared the neighborhood
around their orbits. Ceres gets elevated to the status of "dwarf planet" based on recent published results on its shape. Pluto is specifically recognized as a dwarf planet, and also as the prototype for all Pluto-like
planetary objects beyond Neptune. 2003 UB313 joins Pluto in this category, with additional dwarf planets in the "Plutonian" category likely to be announced soon.

Some controversy has arisen over the merit of the definition itself, and the fairness of the process by which the resolutions were passed. Opinions have ranged from "the DPS membership should be encouraged to support the IAU resolutions that were approved by an overwhelming majority" to "the IAU resolution defining the word 'planet' is fatally flawed and needs to be replaced by something better." This discord is not surprising, given the long history of foundered efforts to reach agreement on just what a planet is and the unwillingness of nature to be categorized into neat compartments.

Two years ago, the IAU appointed a committee of 19 planetary scientists (15 of whom were DPS members) to attempt to agree on the definition of a planet, but they could not reach accord. Then, a few months ago, the IAU appointed a seven-member panel of scientists, historians, authors, and educators (three of whom were nominated by the DPS Committee) to take up the task again, guided by the technical findings of the original 19-member committee but charged to take a broader view that accounted for historical, cultural, and educational issues as well.

Just prior to the IAU General Assembly meeting, this panel announced a resolution proposing that planets were celestial objects, in orbit around a star, that were massive enough to be round and that were not satellites. This definition, while subject to dispute, was firmly based on the physical properties of the objects themselves and was applicable to planets around other stars. Recognizing the authority of the IAU to render a decision, as well as the considerable input by DPS members in the process, the DPS Committee endorsed the definition, mindful of the fact that the final decision would be made at the IAU General Assembly after open discussion and debate.

As an eye-witness to the proceedings, I think it would be fair to describe the scene in Prague as intense, highly charged, and dramatic. The original proposal was quickly discarded, and after several iterations, it was replaced by a new set of resolutions (including the two that were finally approved), based not only on the shape of an object but also on its orbital zone. This second criterion tipped the balance against Pluto being classified with the other eight planets, and thus in effect the final vote was about Pluto's status. Of the approximately 400 voters present, a significant majority supported the new definition, leaving eight planets as the only "true" planets while naming Pluto as a dwarf planet and the prototype for its own new class of objects.

Proponents argued that the definition was practicable and that an eight-planet solar system was a sensible one, given what we know of the Kuiper Belt. Critics countered that changes in the resolutions had been made at the last minute, that the IAU had not allowed for sufficient discussion and review of the proposals by the full scientific community, and that basing a definition in part on celestial dynamics was not well-founded or clear. Combined with the loss of familiar Pluto as a planet on equal footing with the other eight, it is not hard to see why the issue is still contentious and opinions are polarized.

Could the IAU have been more open and inclusive? Probably. Would it have resulted in a different result? That is far less certain. Nearly every possible planet definition has been proposed and rejected many times, and there is no single right answer. What is definitely true is that the IAU has the authority to make such working astronomical definitions for its own purposes, that it established a procedure to define a planet in that context, and that it followed its own rules.

All possible definitions have a degree of fuzziness that requires intelligent application: what does "round" really mean? What does it mean to "control a zone"? These are technical issues to be addressed by Division III of the IAU, currently chaired by Ted Bowell, a fellow DPS member. There is still work to be done, too, in constructing a definition that is generally applicable to extra-solar planetary systems. These and other changes, radical or moderate, may well be addressed at the next IAU General Assembly in Rio de Janeiro in 2009. We hope that the DPS community will be involved in all stages of this process.

What is a planet? Ultimately, the true test will come in time through common and scientific usage. In the meantime, let's continue the scientific study of Pluto and all the other beasts in our celestial menagerie, planet, dwarf planet, satellite, asteroid, comet, or mote of dust. We'll surely then be in a better position to decide what it really takes to be a planet.

Richard G. French, DPS Chair

Notes from visitors::


Finally, got a message from DPS. That\'s still a long way to go for planet definition, but truly we can\'t be rest on the current definition, an open discussion is needed. For the ongoing 3 years, the whole community should work out the study.

by Sze-leung Cheung @ 2006-09-01 23:03 [Homepage]


Henry Tye 戴自海 -
The most important cosmologist you've never heard of

翻開剛收到的Astronomy (2006年10月號),看到一張熟悉的臉孔-戴自海教授!

兩個月前,在霍金熱潮時,戴自海教授 (Prof. Henry Tye)來港主持了一個宇宙的起源系列講座,當時全港的人只盲目追蹤霍金,但坦白說我覺得戴教授才是此系列講座中的皇者。


知道Prof. Tye是一個很強的人,一個很少聽聞,但很強的香港人。

早前也翻開Inflation Universe一書時見到Henry Tye...

差一點就成為Inflation Theory的始創者,在cosmic string等理論上的貢獻,還有近期的brane inflation theory,我真的迷上了Henry Tye的理論。




Pluto Love



1. [2006-08-25] 失去冥王星的一天 同時是新紀元的開始
2. [2006-08-24] 行星定義第三稿草案 降格冥王星為"矮行星"
3. [2006-08-16] 太陽系行星數目將增至十二顆
4. [2006-07-21] 再見「九大行星」講座





Astronomy Now
Sep 2006 Cover








冥王星的擁戴者,還有不少天文學家。包括可憐的Alan Stern。天文學家本不是一些固執的物體,一向以來,新的發現或新的論證都很容易令天文學家信服錯誤的概念需要更改,但今次冥王星是否行星的討論,令天文界分化,出現兩個極端。冥王星,位於行星的灰色地帶,有著其他八顆行星的類似大氣,也可能有冰火山活動,但也有著和主要行星相異的軌道,加上其他凱伯帶天體的發現,令冥王星比其他行星更奇特。

Alan Stern是冥王星的大力支持者之一,是數本我很喜歡的書的作者或編者,如Our WorldOur UniverseWorlds Beyond,也是New Horizons計劃的Principal Investigator,剛剛我也在American Astronomical SocietyDivision of Planetary Sciences的選舉中投了他神聖的一票,讓他成為DPS的Vice Chair。他就是那些為冥王星抱不平,力圖推翻IAU決定的重要人物。其實我很欣賞Alan Stern,看見他因為冥王星而痛心,我也痛心,是為痛心之二。

若你有留意New Horizons的網頁,IAU起初提出冥王星與查龍這對雙行星的建議,列出了在網頁的當眼位置,但冥王星被降為矮行星,隻字不提。多可憐的New Horizons,1月時,它還是帶著全世界的希望衝出大氣層,前往唯一未探索的行星,現在,冥王星降格了,New Horizons也默默地航行著。前往另一顆矮行星穀神星的Dawn,下年發射,但會早New Horizons數月到達穀神星,不得不承認,New Horizons發射時,我是多麼緊張和興奮。相比之下,我對Dawn的注意力卻沒有這麼高。

不止New Horizons如此,DPS也如此,當初12行星方案一出,DPS 以Committee命義聯署支持方案,但結果發展成8顆行星,同樣也是隻字不提。不用說,這事件在部份行星科學界的人士心目中有多大影響,多大的悲傷。


Pluto and Charon
U. of Arizona Press
728 pages, 1997
ISBN 0-8165-1840-8










因冥王星這個問題,致使天文界對立和分裂,看到也覺痛心,剛剛收到一封行星科學家之間內部傳閱向IAU提出反對的請願書,發起是次請願的全是有鼎鼎大名的行星科學界人物,美國高等學院的領導人,當中不乏人熟悉的名字如Shoemaker, Levy。當時布拉格的氣氛充滿火藥味,事後,相關人士的怒火仍未能平息,因為對立的兩派互相寸步也不讓,致成為現今的局面。



Shaw Laureates 2006


The Shaw Laureates in Astronomy 2006

The Shaw Prize in Astronomy 2006 will be awarded jointly to

Saul Perlmutter,

Adam Riess,


Brian Schmidt

For discovering that the expansion rate of the universe is accelerating, implying in the simplest interpretation that the energy density of space is non-vanishing even in the absence of any matter and radiation.

:: Link

今年又是UC Berkeley


Announcement of The Shaw Laureates 2006

The Shaw Prize will hold a press conference in Hong Kong on Wednesday, June 21, 2006 to announce the Shaw Laureates for 2006.

Highlights of the press conference will be released worldwide via satellite between HK time 1930-2000 (GMT 1130-1200).

The Shaw Laureates 2006 will receive their Awards in Hong Kong at the ceremonial prize-giving in September. The Shaw Prize, which was launched in November 2002, consists of three annual prizes: Astronomy, Life Science and Medicine, and Mathematical Sciences. Each prize carries a monetary award of US$1 million.

Established under the auspices of Mr. Run Run Shaw, the Prize honours individuals, regardless of race, nationality and religious belief, who have achieved significant breakthrough in academic and scientific research or application and whose work has resulted in a positive and profound impact on mankind.

The Shaw Prize is an international award managed and administered by The Shaw Prize Foundation based in Hong Kong. Mr. Shaw also founded The Sir Run Run Shaw Charitable Trust and The Shaw Foundation Hong Kong, both dedicated to the promotion of education, scientific and technological research, medical and welfare services, and culture and the arts.

Contact: Ms Daisy Chow The Shaw Prize Secretariat Email: daisychow@shawprize.org or press@shawprize.org Tel Nos: (852) 2994 4888 & 2335 5621 Fax No: (852) 2994 4881

:: Link

Notes from visitors::


Yes, really hope observations can be done in Hong Kong in the near future.

by Sze-leung Cheung @ 2006-09-01 23:04 [Homepage]


Dear all, How are you? The prize winners of the Shaw\'s prize began their their Supernova-Cosmology project many years ago. They got a lot of important and amazing data. The accelerating universe information is quite interesting under their smart system. In fact, several years before, in TVB pearl, the Supernova program was boardcasting. I just hope Hong Kong will have one similar system like that. However, Hong Kong do not have enough good clear sky night. Hope it will be changed. Cheers, Sing

by Leung Chun Sing @ 2006-08-19 12:37

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