A new way of studying planets in other solar systems — by doing sort of an autopsy on planetary wreckage devoured by a type of star called a white dwarf — is showing that rocky worlds with geochemistry similar to Earth may be quite common in the cosmos.
In a study published on Oct. 17, researchers studied six white dwarfs whose strong gravitational pull had sucked in shredded remnants of planets and other rocky bodies that had been in orbit. This material, they found, was very much like that present in rocky planets such as Earth and Mars in our solar system. Given that Earth harbors an abundance of life, the findings offer the latest tantalizing evidence that planets similarly capable of hosting life exist in large numbers beyond our solar system.
“The more we find commonalities between planets made in our solar system and those around other stars, the more the odds are enhanced that the Earth is not unusual,” said Edward Young, a geochemistry and cosmochemistry professor at the University of California, Los Angeles (UCLA), who helped lead the study published in the journal Science. “The more Earth-like planets, the greater the odds for life as we understand it.”
Photo: REUTERS
照片：路透
The first planets beyond our solar system, called exoplanets, were spotted in the 1990s, but it has been tough for scientists to determine their composition. Studying white dwarfs offered a new avenue.
A white dwarf is the burned-out core of a sun-like star. In its death throes, the star blows off its outer layer and the rest collapses, forming an extremely dense and relatively small entity that represents one of the universe’s densest forms of matter, exceeded only by neutron stars and black holes.
Planets and other objects that once orbited it can be ejected into interstellar space. But if they stray near its immense gravitational field, they “will be shredded into dust, and that dust will begin to fall onto the star and sink out of sight,” said study lead author Alexandra Doyle, a UCLA graduate student in geochemistry and astrochemistry. “This is where that ‘autopsy’ idea comes from,” Doyle added, noting that by observing the elements from the massacred planets and other objects inside the white dwarf scientists can understand their composition.
The researchers observed a fundamental characteristic of the rocks: their state of oxidation. The amount of oxygen present during the formation of these rocks was high — just as it was during the formation of our solar system’s rocky material. They focused on iron, which when oxidized ends up as rock. “Rocks are rocks, even when they form around other stars,” Young said.
The closest of the six white dwarf stars is about 200 light-years from Earth. The farthest is about 665 light-years away.
(Reuters)
研究其他太陽系內行星的一種新方法──對遭到白矮星這類恆星吞噬的行星殘骸進行「驗屍」──顯示：由岩石構成、地質化學和地球相似的天體，在宇宙中可能相當普遍。
在十月十七日發表的一份報告中，科學家研究了六顆白矮星。它們強大的引力，曾把過去環繞著它們運行的行星和其他岩石星體破碎的殘骸吸入其中。科學家發現，這些殘骸和岩石行星──像是我們太陽系的地球和火星──上面可見的物質相當相似。有鑑於地球庇護著豐沛的各式生命，這項發現提供吊人胃口的最新證據，顯示像地球一般、能讓生命寄宿的行星，大量地存在於我們的太陽系之外。
加州大學洛杉磯分校的地質化學和宇宙化學教授艾德華‧楊格表示：「我們在太陽系裡製造的行星，和圍繞其他恆星形成的行星之間找到愈多共通性，地球並非如此獨特的可能性就會愈為增加。」楊格協助主持這份發表於期刊《科學》的研究。他進一步指出：「有愈多像地球的行星，我們了解的生命出現機率就愈高。」
位於我們太陽系外的行星被稱為「系外行星」，第一批是在一九九○年代被發現的。然而，科學家對於判斷它們的組成成分一直感到相當棘手。研究白矮星提供了一條新的途徑。
所謂的白矮星，是一顆像太陽的恆星，燃燒殆盡後留下的核心。在它步向死亡的掙扎中，恆星外層會發生爆炸，其他部分則會向內塌陷，形成一個極端高密度而體積相對小的星體，是宇宙密度最高的一種物質型態，僅次於中子星和黑洞。
曾經環繞它運行的行星和其他星體可能會在恆星爆炸時被彈飛到星際空間中。不過，這篇研究的主要作者亞莉珊德拉‧道爾指出，如果這些星體偏離軌道，飛近恆星巨大的重力場時，會「被撕裂成塵埃，而這些塵埃就會開始逐漸落在白矮星上，最後沉入其中，消失不見」。道爾目前是加州大學洛杉磯分校的地質化學和宇宙化學研究生，她補充說：「這也就是我們想到『驗屍』這個方法的由來」，同時她也指出，藉由觀察遭到屠殺的行星和其它星體在白矮星內留下的成分，科學家可以進一步了解它們的組成。
研究人員對這些岩石的一項基本特徵進行觀察：它們氧化的狀態。氧氣含量在這些岩石形成的階段顯得相當高，跟我們太陽系岩石物質形成時的情況相同。科學家遂聚焦於鐵，因為鐵經過氧化，最終會形成岩石。楊格表示：「岩石畢竟還是岩石，就算它們在別的恆星周圍形成，也不會有太大差異。」
這六顆白矮星中，距離地球最近的約為兩百光年遠，最遠的大約為六百六十五光年。
(台北時報章厚明譯)
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