一项研究表明，可能有许多类似地球的行星散布在宇宙周围，这增加了其他可居住的世界存在的可能性——并且生命可能已经在那里进化了。

第一颗系外行星——太阳系以外的行星——是在20世纪90年代发现的。根据美国国家航空航天局的数据，自那以后，数千颗行星被发现，超过4000颗被确认，另有4495颗候选系外行星。

这些宿主外星生命中是否有任何一个是未知的，科学家们目前正试图缩小可能具有类似地球的性质的系外行星的范围，从而使其具备适合生命的条件。这些包括成为一个不太热也不太冷的岩石星球，所以液态水可以存在。

在一项研究中发表于科学一组研究人员现在已经发现地球化学证据，表明宇宙中可能有比以前想象的更多的类地行星。由洛杉矶加利福尼亚大学的亚历山德拉·道尔领导，该小组观察了白矮星周围的太阳系岩石。这些是死星——大约50亿年后，当太阳耗尽燃料时，它会变成什么样子。

道尔说:“白矮星在研究系外行星时是独一无二的。”新闻周刊。“白矮星是类太阳恒星演化的最后一个已知阶段。当像太阳这样的恒星死亡时，它演变成白矮星。对于环绕恒星的行星系统来说，这种演化可能非常混乱，随着恒星的膨胀和收缩，围绕恒星运行的较小天体将改变轨道，这可能会将其中一些天体送向恒星，从而最终导致白矮星捕获岩石物质。”

她说，对系外行星地球化学的大多数研究都是通过观察质量/半径关系，或者通过观察它所环绕的恒星的化学性质来完成的。然而，对于白矮星，你可以更直接，因为测量的元素直接来自落在恒星上的岩石。

在这项研究中，该小组观察了六颗白矮星和曾经围绕它运行的行星上的岩石。道尔说:“系外行星内部的地球化学和地球物理学至关重要，因为它们可以确定行星可居住性的重要参数，例如，它是否有磁场、大气会是什么样子以及板块构造的存在。”。

他们评估了行星撞击白矮星后留在白矮星大气中的元素残骸。他们的发现表明，前行星的内部与地球或火星相似。道尔说:“其他恒星周围的岩石越像构成地球的岩石，就越有可能有像地球一样适合居住的行星。”。

“总的来说，对于多岩石的系外行星，我们发现在其他恒星周围形成的岩石在地球化学上与太阳系中的大多数多岩石天体相似。我们看到岩石就是岩石，即使它们是在其他恒星周围形成的。”

该小组现在希望对其他白矮星进行更多的测量，以便在他们的发现的基础上更进一步。道尔解释说，如果发现更多白矮星拥有类似地球内部的行星，统计意义就更大了。

该研究的合著者爱德华·杨在一份声明中表示，他们的发现“提高了许多岩石行星与地球相似的可能性——宇宙中有非常多的岩石行星。”

艺术家对外星的印象。研究人员称类地系外行星可能在宇宙中很常见。

UNIVERSE MAY BE FULL OF EARTH-LIKE EXOPLANETS

There may be many Earth-like planets scattered around the universe, a study has suggested, raising the possibility that other habitable worlds are out there—and that life may have evolved on it.

The first exoplanets—a planet beyond our solar system—were discovered in the 1990s. Since then, thousands have been revealed, with over 4,000 confirmed and a further 4,495 candidate exoplanets, according to NASA.

Whether or not any of these host alien life is unknown, and scientists are currently trying to narrow down the exoplanets that may have properties similar to Earth, therefore having conditions suitable to life. These include being a rocky planet that is not too hot or cold, so liquid water can exist.

In a study published in Science, a team of researchers has now found geochemical evidence to suggest there may be more Earth-like planets across the universe than previously thought. Led by Alexandra Doyle, from the University of California, Los Angeles, the team looked at the rocks of solar systems surrounding white dwarfs. These are dead stars—and what the sun will turn into in about five billion years, when it runs out of fuel.

"White dwarfs are unique when it comes to studying exoplanets," Doyle told Newsweek. "White dwarf stars are the last known phase of stellar evolution for sun-like stars. When a star like the sun dies, it evolves into a white dwarf. This evolution can be very chaotic for the planetary system surrounding the star, and as the star expands and contracts the smaller bodies orbiting the star will change trajectories, which can send some of them towards the star, thus causing eventual trapping of the rocky material by the white dwarf."

She said most studies of the geochemistry of exoplanets are done by looking at the mass/radius relationship, or by looking at the chemistry of the star it orbits. With white dwarfs, however, you can be more direct, as the elements measured come directly from rocks falling onto the star.

In the study, the team looked at six white dwarfs and rocks from the planets that once orbited it. "The geochemistry and geophysics of interiors of exoplanets are crucial, because they can determine important parameters for habitability of a planet, such as, whether or not it has a magnetic field, what the atmosphere will look like and the existence of plate tectonics," Doyle said.

They evaluated the elemental remains that had been left in the atmospheres of white dwarfs after the planets had crashed into them. Their findings showed the former planets would have had interiors similar to those of Earth or Mars. "The more rocks around other stars look like the rocks that made the Earth, the more likely it is that there are habitable planets like the Earth out there," Doyle said.

"For rocky exoplanets in general, we are finding that rocks forming around other stars are geochemically similar to most of the rocky bodies in the Solar System. We are seeing that rocks are rocks, even when they form around other stars."

The team now hopes to make more measurements of other white dwarfs in order to build on their findings. If even more white dwarfs are found to have had planets with Earth-like interiors, the statistical significance is greater, Doyle explained.

In a statement, Edward Young, co-author on the study, said their findings had "raised the probability that many rocky planets are like the Earth—and there's a very large number of rocky planets in the universe."

Artist impression of an alien planet. Researchers say Earth-like exoplanets may be common in the universe.