Is Alien Life Hiding Beyond Earth 2.0? 外星生命藏在地球2.0版以外嗎?

Ethan Siegel 伊桑·西格爾

  When we think about life out there in the Universe, far beyond the limits of Earth, we can‘t help but look to our own planet as a guide. Earth has a number of features that we think are extremely important — perhaps even essential — for enabling life to arise and thrive. For generations, humans have dreamed of life beyond Earth, striving to find another world similar to our own but with its own unique success story: our own Earth 2.0.

  But just because life succeeded here on Earth doesn‘t necessarily mean that life is likely to succeed on Earth-like worlds, only that it‘s possible. Similarly, just because life hasn‘t been found on non-Earth-like worlds doesn‘t mean that it isn‘t possible.

  There are lots of reasons to believe that looking for a world as Earth-like as possible, around a star as Sun-like as possible, might be the best place to look for life elsewhere in the Universe. Practically no one in the exoplanet or astrobiology communities thinks that looking for worlds similar to a proverbial Earth 2.0 is a bad idea. But is it the smartest course of action to invest the overwhelming majority of our resources in solely looking for and investigating worlds that have these similarities to our own, life-rich planet?

  If science has taught us anything, it‘s that we shouldn‘t assume we know the answer before doing the key experiments or making the critical observations. The Universe is full of surprises, and if we don‘t give ourselves the opportunity to allow the Universe to surprise us, we‘re going to draw biased — and therefore, fundamentally unscientific — conclusions.

  While we have every reason to believe that life might be ubiquitous — or at least have a chance — on worlds that are very similar to Earth, it‘s also very plausible that life may be more plentiful on worlds that aren‘t like our own.

  Perhaps exomoons orbiting large planets are even more conducive to life originating than a world like Earth is.

  Perhaps liquid water on the planet itself isn‘t a requirement for life, as perhaps the right kind of cell wall or membrane can enable water to exist in an aqueous state.

  Perhaps radioisotope decay, geothermal sources, or even chemical sources of energy could provide life with the external source it needs; perhaps rogue planets — without parent stars at all — might be home to alien life.

  Perhaps even super-Earths might be potentially habitable under the right circumstances. To examine a planet for hints of life, we can approach this puzzle with many different lines of inquiry. We can:

  ·wait for a planetary transit and try to perform spectroscopy on the absorbed light, probing the contents of an exo-atmosphere,

  ·we can try and resolve the world itself with direct imaging, looking for seasonal variations and signs such as the periodic greening of the world,

  ·or we can look for nuclear, neutrino, or techno-signatures that might indicate the presence of a planet being manipulated by its inhabitants, whether they are intelligent or not.

  It may be the case that life is rare in the Universe, in which case it will require us to look at a lot of candidate planets in order to reveal a successful detection. But if we search exclusively for planets that have similar properties to Earth, and we restrict ourselves to looking at parent stars and solar systems that are similar to our own, we are doomed to get a biased representation of what‘s out there.

  You might think, in the search for extraterrestrial life, that more is more, and that the best way to find life beyond Earth is to look at greater numbers of candidate planets that might be the Earth 2.0 we‘ve been dreaming of for so long. But non-Earth-like planets could be home to life that we‘ve never considered, and we won‘t know unless we look. More is more, but “different” is also more. We must be careful, as scientists, not to bias our findings before we‘ve even truly started looking.