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[2006-09-01]

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
http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html.)

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::

1

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]


[2006-08-31]

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。

也是我第一次認識戴教授,以往一向很少聽到他的名字,就正如今期Astronomy所說。他在香港長大,就讀喇唦中學,是一個百折不扣的香港人,從他的演譯和與在場觀眾之間的對答,可見他是一個對弦理論認識非常深的人,香港竟然有如此的世界級專家,還要是有關弦論!實在太不可思議。

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。

沒想過今期Astronomy會有他的訪問和專題介紹,還達6頁篇幅,必看!

大家也該認識一下這任香港的宇宙論英雄。



[2006-08-27]

冥王星之愛
Pluto Love


冥王星,終於被天文學家奪去其行星資格,如果冥王星是有生命,不曉得它有什麼話要說。

這次文字上的改動,牽起世界的話題,這次冥王星被降級,相信比太陽系增加至12顆行星更為轟動,這次事件的來龍去脈,故事的峰迴路轉,可以參閱以下我數晚通宵寫的報導和預備的講座。

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

這堙A想書寫一點感受。

爭辯,表面上完結了兩天,但相信事件沒有這麼快能夠真正平息。因為這件事牽涉到一些大是大非的根本概念,因為天文學的進步,因為我們的太陽系已經不再簡單,要用一個簡單的概念形容整個太陽系已經不可能,因此才出現冥王星事件,行星的爭論也不可能是一朝一夕解決的。

有一點,相信都廣為被接納,就是冥王星是一顆矮行星,問題核心是,矮行星是否行星?

是,我們就有12顆行星;不是,就只餘下8顆。


Astronomy Now
Sep 2006 Cover

痛心

痛心的,不是冥王星被除名,而是為冥王星的支持者痛心。無論,冥王星是否被除名,它依然於天上,但76年的行星地位,的確很難一下子在人心中抹去。

我可算是一個清楚知道事件來龍去脈的人,對冥王星除名早有心理準備,但看到真正放上投票桌上的決議案最終定稿,也不免為冥王星流下幾滴淚,很多朋友還主動安慰我。

科學是科學,但情感歸情感,我也如此,更不用說,一般人對此的反應可想而知。「為什麼?!」無數人充滿著疑問,或許大眾對天文的認識仍停留在九大行星,或是壯麗的宇宙吸引了所有人的注意力,忘卻了冰冷的太陽系邊緣。

冥王星被視為凱伯帶和矮行星的一員,絕對是科學的表現。身邊有朋友不捨得,是因為「冥王星」這個名稱很好聽,的確,因為冥王星,人們對這位代表希臘十二主神之一的地府之神多了認識,但這原因不可能成為它不被貶的理由。

「以後可以少記一顆星名」不知聽過這說話多少次,是為痛心之一。那麼以後,什麼也不要學,不去記,沒有宇宙,沒有地球,沒有家,沒有知識,不是更好。

若果認為知識的倒退是可喜的事,可恥。

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。

其實,在整個行星定義的討論過程中,還有人提出一些走火入魔的方案,有人認為要有大氣層才是行星,那麼連水星也踢出局,但土衛六泰坦也會成為行星…這也該適可而止,不要過份地偏離「行星」的原意!


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

不要回到70年前

正如在有線電視的訪問中,我也說,這是科學的做法。縱然有點不捨得,但我們總要向前走,不能原地踏步。在蘋果日報的訪問中,我也是這樣說。

但,真是向前走嗎?

當初,IAU的初稿就是考慮到教育與歷史原因,決定不把冥王星除名,而跑出十二行星的方案。孩子會因為發現新行星而興奮,相反,奪去一顆行星,會令孩子失望。天文學家要這樣做,需要膽量。結果,天文學家果真不受文化、歷史、社會所影響,判了冥王星死刑。

記得,我曾看過,有人質疑除名一事會令社會不接納並需要多年時間消化,而反駁者稱科學家做的是科學的事,至於教育市民的責任,則落在教育工作者身上。當時,我很認同這說法,也登時覺得自己責無旁貸,對整個社會,對天文的認知,自己肩負著這重任。

但,當我看見真正的報導出街時,不免失望。除了有線電視外,其他全部傳媒都把重點只放左冥王星除名一事,而忽略了這次投票的其他意義。人們也只想著如何在太陽系介紹中刪去冥王星,教科書要九改八,但可想過,這麼,我們只是回到70年前,那麼,有什麼意義?

冥王星除了在行星之列除名,也代表著新一概念的誕生-矮行星,還有以冥王星為首的新一類海外天體,這些都是這次決議案中一同決定的事,我們應該放眼於這堙A教科書的更改也應該加入矮行星的概念,這才是代表時代進步的做法,單是抹去冥王星,只會讓知識回到70年前,這無助人們對太陽系的多姿多采有所認識。教育工作者,請緊記。

可能是我背後幫忙的關係,覺得有線電視的報導較全面。投票前一晚,有線特地來到拍攝穀神星,也跟我做了些訪問。之後,我和幾位扮演員的中學師弟去「看」冥王星,因為冥王星實在太暗,要借助儀器才可以在電腦中看到,這幾位小朋友看到冥王星多麼的興奮,一直到十一時離開才開始有餓的感覺。見到他們如此興奮,可知,其他人若也知道世界不只是有八大行星,他們或許有部分也有一樣的反應。

我自己本身年幼時喜歡天文,也是因為知道地球以外還有別的世界,有九大行星,我想信也會有小孩跟我一樣,所以越多的世界越能引起人的好奇心,因此千萬不要回到70年前,只介紹8顆行星。否則,我們只會讓歷史倒流,罪過罪過…

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。

無論怎樣,歷史新的一頁已經翻開,希望事件能早日平息,若冥王星真是這麼得天獨厚,有著行星的特質,不久的將來,當我們更為認識太陽系的邊界和太陽系外的行星時,冥王星自然會重歸行星的懷抱。


[2006-06-22]

Shaw Laureates 2006




Announcement

The Shaw Laureates in Astronomy 2006

The Shaw Prize in Astronomy 2006 will be awarded jointly to

Saul Perlmutter,

Adam Riess,

and

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



看到今年的得獎名單
很高興
一點也不surprise的結果
不過發現了一件事
今年又是UC Berkeley


[2006-06-21]

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::

2

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

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

 
1

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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