Dr MacKinnon says in some cases, alien species have been introduced to a country by people with good motives in mind, but they lead to disastrous results.

麦克·金南博士说，在某些情况下，向一个国家引进外来物种的人的动机是好的，但导致了灾难性的后果。

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"Invasive alien species are non native species which are accidentally or intentionally introduced into new areas. They range from microbes to mammals," Dr MacKinnon says.

麦克·金南博士说，“侵入性外来物种是偶然或故意被引入新地区的非本土物种，其范围涉及从微生物到哺乳动物的多个物种。”

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Dr Baross said that looking for alien life has always involved using the Earth as a model.

巴诺斯博士表示，寻找外星生命时，总是使用地球作为参照物。

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To estimate the number of alien worlds, Dr. Cassan and his colleagues studied 100 million stars between 3, 000 and 25, 000 light-years from Earth with gravitational microlensing.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate

To compare alien planets, Dr. Mendez at the Planetary Habitability Laboratory and his colleagues are creating an Earth Similarity Index that ranks worlds by their resemblance to our own planet, on a scale from zero to one, based on their mass, radius, density and surface temperature.

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On Wednesday, Dr. Welsh and his colleagues announced that they have confirmed the existence of two more alien planets in distinctive double-star solar systems in the constellation Cygnus.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate