Why Extraterrestrial Life May Be More Unlikely Than Scientists Thought

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Why Extraterrestrial Life May Be More Unlikely Than Scientists ThoughtBy Marcus Woo, Live Science Contributor | April 9, 2018 06:41am ET

                               
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Scientists analyzed the Crab Nebula, which is expanding debris from the explosion of a massive star.

Credit: NASA; CXC (X-ray); STSCI (Optical); JPL-Caltech (Infrared)

Phosphorus is an essential element for life — but that there was enough of it for life to start on Earth might just have been a matter of luck, new findings suggest.

According to new observations of the Crab Nebula — the leftovers from an exploding star first seen by Chinese astronomers in 1054 — presented on April 5 at the European Week of Astronomy and Space Science in Liverpool, England, the abundance and distribution of phosphorus in the Milky Way galaxy may be more random than scientists previously thought. As such, some places in the galaxy may not have enough phosphorus to support life, even if they are home to otherwise hospitable exoplanets, the researchers said. [Extreme Life on Earth: 8 Bizarre Creatures]

Most of the universe's phosphorus was created during the last gasps of dying massive stars or during a supernova — when such a star exhausts its fuel and explodes. Phosphorus is difficult to observe, and only in 2013 did astronomers make the first measurements of the element in a stellar explosion, in the wispy remains of a supernova called Cassiopeia A. Surprisingly, they found up to 100 times more phosphorus than what's observed in the rest of the Milky Way.

But that might have been an outlier. Recently, astronomers Jane Greaves and Phil Cigan of Cardiff University in the U.K. pointed the William Herschel Telescope in the Canary Islands toward the Crab Nebula, located about 6,500 light-years away. Preliminary data, analyzed just two weeks ago, shows an amount of phosphorus more similar to the values found in the interstellar gas and dust of the Milky Way — a pittance compared with the abundance in Cassiopeia A. (The findings have not yet been submitted to a peer-reviewed journal.)

"It's not a guaranteed thing to have phosphorus abundant everywhere, ripe for the picking," Cigan told Live Science. "It seems to look like luck plays a bigger role in this."

Some of that luck may come down to size. The star that created Cassiopeia A is roughly twice as massive as the one that made the Crab Nebula. A more massive star could have generated different reactions that produce more phosphorus, the researchers said.

If the production of phosphorus varies widely across the galaxy, so might the likelihood of life on other planets. Even if a planet had every other condition required for habitability, it might still be bereft of life because it formed where there was a dearth of phosphorus, the researchers said.

But the observations are still preliminary; the astronomers were only able to measure parts of the nebula before clouds and a snowstorm spoiled the rest of their observing run. Still, Cigan said, the data they do have show significantly less phosphorus in the Crab Nebula than in Cassiopeia A.

The researchers have applied to use the Herschel Telescope to study the rest of the nebula. Future analysis will also include comparisons with computer models, Cigan said.

Ultimately, astronomers will need to measure phosphorus in other supernova remnants across the cosmos, Cigan said. "We really want to look at how it's spreading out from supernova remnants and falling back into the interstellar medium — that's the key."

Originally published on Live Science.

为什么外星生命可能比科学家想象的更不可能？

                               
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科学家们分析了螃蟹星云，它正在扩大巨星爆炸的碎片。信用：NASA; CXC（X射线）; STSCI（光学）; JPL-Caltech（红外线）

磷是生命中不可缺少的元素 - 但是新的研究结果表明，磷对于地球生命开始有足够的作用可能只是运气的问题。

根据对螃蟹星云的新观察 - 中国天文学家1054年首次观测到的一颗爆炸恒星的残骸 - 4月5日在英国利物浦的欧洲天文与空间科学周上发表的报告中，乳状液中磷的丰度和分布一路星系可能比以前认为的科学家更随机。因此，研究人员表示，即使星系中有其他好客的系外行星，星系中的一些地方可能没有足够的磷来维持生命。 [地球上的极限生物：8种奇妙生物]

宇宙中的大部分磷是在最后一批正在死亡的巨星或超新星期间产生的，当时这颗恒星耗尽了燃料并爆炸。磷很难被观察到，直到2013年，天文学家才在恒星爆发中首次测量了超新星Cassiopeia A超新星的纤细残骸。令人惊讶的是，他们发现磷的含量高出100倍银河系的其余部分。

但这可能是一个异常。最近，英国卡迪夫大学的天文学家简·格里夫斯和菲尔·奇甘将威廉赫歇尔望远镜指向加那利群岛，距离大约6500光年远的蟹状星云。仅在两周前分析的初步数据显示，磷的量与银河系星际气体和尘埃中的数值更为相似 - 与仙后座A的丰度相比，微不足道的数量（结果尚未提交给一篇同行评议的期刊）。

“不管是到处都有丰富的磷，采摘的时机已经成熟，这并不是一个有保证的事情，”Cigan告诉Live Science。 “看起来好像运气在这方面发挥了更大的作用。”

一些运气可能会降低到一定程度。制作仙后座A的恒星大约是制作蟹状星云的恒星的两倍。研究人员说，一颗更大的恒星可能会产生不同的反应，产生更多的磷。

如果磷的生成在整个银河系中变化很大，那么其他星球上的生命可能也会如此。研究人员说，即使一个行星有其他所有可居住条件所需的条件，它仍然可能会失去生命，因为它形成了磷缺乏的地方。

但观察仍然是初步的;天文学家只能在云层和暴风雪破坏了观测的其余部分之前测量了部分星云。尽管如此，Cigan说，他们所做的数据显示，蟹状星云中的磷含量要比仙后座A少得多。

研究人员已经申请使用赫歇尔望远镜来研究其余的星云。未来的分析还将包括与电脑模型的比较，Cigan说。

最终，天文学家需要测量整个宇宙中其他超新星残余物中的磷，Cigan说。 “我们真的很想看看它是如何从超新星遗迹中扩散出来并回落到星际介质中的 - 这是关键。”

最初发表于Live Science。