天文學一問

Main from the spectrum of the light of the star / planet. Different element / chemical will absorb different frequency of the light. When we received the ligth emmited / reflected from the star / planet, we extract the spectrum, to find which frequencies are missing. You can find of absorption spectrum of different chemicals / elements on web. The simplest way to extracted a spectrum, is to use a prism like in primary school to see the compose of sun light.

The astronomers get the information of the star / planet now, are the light come from the star / planet in past. eg, For a star is 60 ly (light year) away from us, the light received now is the information of that star at 60 years ago. The situation of that star at this moment need to travel 60 years to us.

We are in fact looking in the past of the stars.

當然是依靠望遠鏡來觀察行星的。

要分析行星的地質、大氣等資料，需要用到紅外線望遠鏡。
要知道行星的大小，只要計算距離就可以了。
要知道行星的質量，只要統合行星地質、密度、體積等資料。

而科學家就依靠這些數據，再觀察行星與主恒星的距離，主恒星的溫度等，來推算該行星生命存在的可能性。

當然，現在所觀察的系外行星，因為光速有限的緣故，無法有同步的資料，不過因為生命誕生是一個非常漫長的過程，因此如果行星的距離只有數千、數萬光年的話，基本上分別不會太大。