土星:太阳系新晋「卫星之王」

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如果老读者们还有印象的话,去年 7 月,卡内基科学研究所宣布新发现了 12 颗木星的卫星 [1]。至此,木星卫星数目一口气提升到了 79 颗,继续稳坐“拥有卫星最多的太阳系行星”宝座。

这个记录自 2002 年以来,就没有再被打破过,直到最近。

几天前(2019 年 10 月 7 日)卡内基研究所又宣布新发现了 20 颗土星卫星,将土星卫星总数推至 82 颗[2]。

几经胶着之后,土星终于再次赶超木星,成为新晋太阳系卫星之王

全地球最炫酷的追星小分队

毫不意外,这次又双叒叕是全地球最炫酷的追星团队——Scott Sheppard 小组发现的。

Scott Sheppard 自 2004 年夏威夷大学天文学博士毕业之后,基本上只专注于一件事儿——搜寻外太阳系新天体。这个追星小分队的成员除了 Sheppard,还有来自加州大学洛杉矶分校的 David Jewitt 和来自夏威夷大学的 Jan Kleyna。而他们行走江湖的利器则是世界上最大的单块主镜望远镜之一——位于夏威夷的日本国立天文台昴星团望远镜(Subaru)

在 2000-2011 年间,Sheppard 带领的团队一共发现了 40 多颗超级小的木星卫星,是同时期其他所有团队加起来的四倍多…此外,他们还发现过一众土星、天王星、海王星的卫星和其他外太阳系天体。简而言之,这哥们就是个“追星专业户”。

到了近些年,随着神秘的“第九大行星”猜想的横空出世,Sheppard 童鞋卓越的追星技巧更加大放光彩。

自从 2006 年冥王星被踢出行星之列,太阳系很长一段时间都只剩下了 8 颗“正牌”行星,最远的就是海王星了。而近几年,天文学家们又发现,海王星之外可能还有一颗更远的行星,这也就是传说中的新任第九大行星(Planet X)。

Sheppard 小组近些年来一直在致力于搜寻这颗传说中的行星。只不过,包括他们在内的所有天文学家至今都还没有找到这颗不知道是不是真的存在的新行星。

然而,Sheppard 小组在这漫长的搜寻过程中着实发现了不少好东西。

比如 12 颗木星新卫星:

轨道超级椭圆的矮行星 2015TG387:

一度成为人类发现的距离太阳最远的太阳系天体 2018 VG18:

绝对可以算得上是失之东隅、收之桑榆

之所以要再聊聊“第九大行星”,是因为很多读者误以为这次的土星新卫星又双叒叕是搜寻 Planet X 的副产品。所以这里必须要澄清一下:其实不是的

和去年木星新发现的 12 颗卫星完全不一样,这次的 20 颗土星新卫星和 Planet X 可没什么关系:它们全部都是早在 2004 年就被 Sheppard 小组首次观测到了,只是直到最近才得到正式确认而已

20 颗新卫星,长啥样?

答案是:不知道。

因为这些新卫星都太小啦!

卫星和行星一样都不发光,只能反射阳光,所以个头越大的卫星才越容易被观测到

这也不难理解为什么人类发现的第一颗土星卫星是土卫六了。

1655 年,荷兰天文学家克里斯蒂安·惠更斯首次发现土卫六 Titan,它是土星最大的卫星,也是太阳系中第二大卫星(仅次于木卫三 Ganymede)。土卫六有多大呢?它的平均直径约 5149 公里,是月球的 1.5 倍,比正牌行星——水星还要大。

此后的 1671-84 年间,意大利裔法国天文学家乔凡尼·卡西尼又陆续发现了 4 颗土星卫星:土卫八 Iapetus、土卫五 Rhea、土卫三 Tethys 和土卫四 Dione,个头也是出类拔萃的。至此,直径 1000 公里以上的卫星被一网打尽。但饶是直径有 1000 公里,在地球上也完全不可能看清它们表面的形貌——因为它们距离地球实在是太远了。

然而,随着观测技术的提升和旅行者号、卡西尼号探测器的近距离探访,不仅之前已经发现的大卫星们有了被我们一窥真容的机会,

越来越小的土星卫星们也被陆续发现。

但本次确认发现的 20 颗新卫星不仅个头都极小(直径约 5 公里),而且也无缘被已经结束任务的卡西尼号拍照,所以,我们能看到的,仅仅只能是一个个小亮点而已

不过我们也不是一无所知。虽然目前完全无法知道这些新卫星长什么样,但好歹还是可以知道它们是怎么飞的。

成群结队的不规则卫星

木星的不规则卫星相似,距离土星较远的卫星也往往个头很小,且有着不规则的轨道:有的公转方向和土星的自转方向相同(顺行),有的则相反(逆行),但公转轨道面都和土星的自转轨道面有较大的夹角(轨道倾角)。

不规则卫星通常认为是行星系统成型之后,一些小天体飞过附近时被行星巨大的引力“拐(捕)来(获)”的。

不过,和木星有点不同的是:土星有着太阳系最壮观的环系结构。由于土星环的影响,土星绝大多数不规则卫星都是位于土星环之外的。事实上,如果不考虑最外侧、也最微弱的菲比环,而是以次外侧的 E 环为界的话,“绝大多数”这几个字也可以去掉

也和木星的不规则卫星相似,土星的不规则卫星也会“抱团”活动。同簇卫星有相似的轨道,自成一派,这意味着同簇卫星很可能有着相同的来处——一颗大得多的卫星 / 小行星被撞碎之后幸存的碎片。也就是说,它们虽然“身体”分开了,但原本的轨道还是保留了下来,成为如今认亲的“指纹”之一。

本次新发现的 20 颗土星卫星全部都是不规则卫星,个头小(直径约 2-4 公里),距离土星也非常远,17 颗逆行,3 颗顺行

其中 2 颗顺行和 1 颗逆行的卫星距离土星稍近一些,大约每 2 年多可以绕土星转一圈,剩下 17 颗卫星离土星更远,至少需要约 3 年时间才能绕土星一圈。

按照各自的轨道特征,新发现的 20 颗卫星可以加入已有的3 个土星卫星小团体

1)因纽特族卫星:顺行轨道,公转平面相对于土星自转平面(土星赤道面)的夹角在 40-50°之间。

这一族群之前已有 5 个成员,本次新确认发现的2 颗顺行卫星倾角为 46°,属于因纽特族。

2)高卢族卫星:顺行轨道,公转平面相对于土星自转平面的夹角在 35-40°之间。

这一族群之前已有 4 个成员,本次新确认发现的1 颗顺行卫星倾角约为 36°,似乎应当属于高卢族。不过这颗卫星比之前发现的其他顺行卫星都要远离土星,所以也还说不好:既有可能是一颗向外迁移了的高卢族卫星,也有可能完全属于一个新的顺行族群。

3)北欧族卫星:逆行轨道,公转平面相对于土星自转平面的夹角在 136-175°之间。这也是土星的不规则卫星里数目最庞大的一个族群,而且还可以进一步细分成多个子族群。

这一族群之前已有 29 个成员,而本次新确认发现的17 颗逆行卫星也全部属于北欧族,可以说是愈发壮大了。

叫什么名字?由你来决定!

和去年木星新发现的卫星一样,这次的土星新卫星也采用了面向全世界公开征名的方式来命名——当然,也不能瞎取,是有规定的。

1)2 颗属于因纽特族的新卫星必须以因纽特神话中的巨人来命名,比如之前的 5 颗因纽特族卫星:

2)1 颗属于高卢族的新卫星必须以高卢神话中的巨人来命名,比如之前的 4 颗高卢族卫星:

3)17 颗属于北欧族的新卫星必须以北欧神话中的巨人来命名(比如雷神索尔 Thor?),唯一的例外是土卫九菲比——它是以希腊神话中的人物命名的。这是因为土卫九早在 1899 年就被发现了,那会儿还没有这么多复杂的命名规定呢。

当然,不能有商业目的,也不能和之前已经用过的天体名重名。

在满足这些要求的情况下,你可以尽情发挥自己的创造力和丰富的知识储备。举个栗子:去年木星卫星的征名结果里(要求以希腊 / 罗马神话里宙斯 / 朱庇特的恋人或后代的名字来命名),S/2017 J4 被最终命名为 Pandia——宙斯和月神赛琳娜的女儿。而背后的另一层涵义是…全世界都爱大熊猫呀~

那么,大家为这些太阳系的新成员们想到啥好名字了么?

扩展阅读

木星:一觉醒来多了 12 颗卫星是一种怎样的体验?

大家好,我叫“遥遥”,我也不知道这记录能保持多久…

出品:科普中国 @中国科普博览

制作:haibaraemily

监制:中国科学院计算机网络信息中心

“科普中国”是中国科协携同社会各方利用信息化手段开展科学传播的科学权威品牌。

本文由科普中国融合创作出品,转载请注明出处。

参考

[1] A Dozen New Moons Of Jupiter Discovered, Including One “Oddball”

https://carnegiescience.edu/node/2367

[2] Saturn Surpasses Jupiter After The Discovery Of 20 New Moons And You Can Help Name Them!

https://carnegiescience.edu/news/saturn-surpasses-jupiter-after-discovery-20-new-moons-and-you-can-help-name-them

[3] New Moon Alert: Saturn has 20 new Moons!

https://www.youtube.com/watch?v=0dNH-odX4qE&feature=youtu.be

[4] https://subarutelescope.org/Gallery/tele_dome.html

[5] https://carnegiescience.edu/news/new-extremely-distant-solar-system-object-found-during-hunt-planet-x

[6] https://sites.google.com/carnegiescience.edu/sheppard/farout

[7] https://sites.google.com/carnegiescience.edu/sheppard/home/newsaturnmoons2019?authuser=0

[8] https://sites.google.com/carnegiescience.edu/sheppard/moons/saturnmoons

发帖时间: news

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